mas Module 4

DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS

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MODULE 4 : ELECTRONIC FUNDAMENTALS

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS

WARNING

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This document is intended for the purposes of training only. The information contained herein is as accurate as possible at the time of issue, and is subject to ongoing amendments where necessary according to any regulatory journals and documents. Where the information contained in this document is in variation with other official journals and/or documents, the latter must be taken as the overriding document. The contents herein shall not be reproduced in any form without the expressed permission of ETD.

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS TABLE OF CONTENTS

4.1 SEMICONDUCTORS.......................................................................................................................................................................................... 1 4.1.1(A) DIODES .........................................................................................................................................................................................................1 4.1.1(B) DIODES .........................................................................................................................................................................................................3 4.1.2(A) TRANSISTORS ..............................................................................................................................................................................................31 4.1.2(B) TRANSISTORS ..............................................................................................................................................................................................32 4.1.3(A) INTEGRATED CIRCUITS ..................................................................................................................................................................................55 4.1.3(B) INTEGRATED CIRCUITS ..................................................................................................................................................................................59

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4.2 PRINTED CIRCUIT BOARDS .......................................................................................................................................................................... 68

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4.3 SERVOMECHANISM ....................................................................................................................................................................................... 72 4.3(A) SERVOMECHANISM ..........................................................................................................................................................................................72 4.3(B) SERVOMECHANISM ..........................................................................................................................................................................................73

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

4.1 Semiconductors (DCAM Ref. 4.1) Level 2 4.1.1(a) Diodes Application of Semi-conductor P-N Junction Diodes – Diodes in Series

Diodes in Parallel Where current supplied by one rectifier would exceed its maximum forward current, or exceed its maximum operating temperature, it is possible to connect two or more diodes in parallel. The current, therefore, will be divided between the diodes.

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The voltage across each diode will be the same and the current distribution between the diodes will depend on the characteristics of the diodes (again, for further information on rectifiers see later notes in this series). Figure 1: DIODES IN SERIES

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When diodes are connected in series to a known load then it must be remembered that the current will be the same and the maximum forward current must not be exceeded for each diode. Because each diode has a small forward resistance there will be a volts drop across each diode, which will depend on each diode's characteristics. These individual volts drops will subtract from the supply voltage to leave a certain voltage across the load (see later notes on rectifiers).

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Figure 2: DIODES IN PARALLEL

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

TESTING OF DIODES It is essential the diode is connected the correct way round in a circuit, so a coloured band or spot usually marks the cathode (k) end.

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Figure 3: TESTING OF DIODES

If it is necessary to verify the connections in the absence of any marking then a test meter is used. Using the old AVO-meter it should be remembered, as with any ohmmeter, that the BLACK (NEGATIVE) terminal becomes the positive output and RED (POSITIVE) terminal is the negative. When a 'FLUKE' is used it has a switch selection to test diodes. The meter displays the forward voltage drop (VF) up to 2 volts and beeps briefly for one diode drop (VF < 0.7V) for the forward bias test. For reverse bias or open circuit the meter displays OL, and if there is a short circuit the meter emits a continuous tone.

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

4.1.1(b) Diodes SEMICONDUCTOR MATERIALS Figure 4 shows the structure of the germanium and silicon atoms, two very important elements in the manufacture of diodes and transistors

At room temperatures the atoms are vibrating sufficiently in the lattice for a few bonds to break, setting free some valence electrons, leaving a "hole" where the electron was. Free electrons are attracted to the hole as the atom, short of an electron is now positively charged.

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Figure 4: ATOMIC STRUCTURE

Bear in mind that the diagrams are only two-dimensional and that in reality the orbiting electrons do not rotate in perfect circles or rotate in a flat plane.

From figure 4 it can be seen that each atom has four electrons in its outer shell, these electrons are called VALENCE ELECTRONS, they are farthest from the nucleus and therefore are least tightly bound (less attractive force). It is the valence electrons that play the active part in electrical conduction.

Silicon and germanium are crystalline substances and the valence electrons of the individual atom link up and arrange themselves with the valence electrons in adjacent atoms to form CO-VALENT BONDS. Every atom has a half-share in eight valence electrons. This gives a very stable arrangement of a regularly repeating three dimensional structure called a crystal lattice. Figure 5 shows the two dimensional effect of the covalent bonding. Pure silicon and germanium are therefore very good insulators.

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Figure 5: CO-VALENT BONDS

If a battery is placed across a pure semiconductor, electrons are attracted to the positive terminal. These free electrons travel through the semiconductor topping' from one hole to another, and it therefore appears that the positive holes are moving towards the negative terminal. This current flow is very small and is called INTRINSIC CONDUCTION.

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

To understand the concept of electrons moving one way and holes moving the other is not easy but it can be likened to an empty seat at the end of a row in a cinema. Assume the vacant seat to be at the right hand end of the row. If the first person next to the seat moves into it, then he/she has moved to the right, but the vacant seat has moved one place to the left. If each person in the row does the same (i.e. moves to the empty seat to his/her right) as soon as it becomes empty, the vacancy (hole) appears to have moved along the row in one direction while the occupants (electrons) have move in the opposite direction. If the temperature is raised more bonds break down and conduction increases i.e., resistance decreases, this means more heat is generated, and more conduction occurs, resistance decreases further, more heat is generated - and so on. This is called thermal runaway and will eventually destroy the crystal structure.

Figure 6: N-TYPE SEMI-CONDUCTOR

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Semiconductors have a negative temperature coefficient. In other words their resistance decreases with an increase in temperature. We need now to look at how we can change the basic insulator into a conductor. This is achieved by mixing (doping) a very small quantity of a selected impurity atom into the 10 semiconductor material. (Typically 1 part in 10 ) The material now becomes an extrinsic semiconductor.

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Note: Although extra electrons have been inserted, it must be remembered that each impurity atom is itself neutral and so the whole of the N-type material is also neutral. MAJORITY CARRIER - ELECTRONS (NEGATIVE) [N = N-TYPE] MINORITY CARRIER - HOLES (due to intrinsic conduction)

There are two types of extrinsic semiconductors: 1. N-Type semi-conductor material. 2. P-Type semi conductor material. N-Type Semi-conductor Material

Doping impurities such as phosphorus or arsenic are used. These have five (pentavalent) electrons in the outermost orbit. When introduced into the basic material, four of the electrons join up with the co-valent bonding, whilst one electron is left 'free'. (The number of free electrons can be strictly controlled by this doping). The free electrons can migrate through the inter-atomic space and can therefore act as current carriers when a (very low) voltage is applied.

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Figure 7: ELECTRON FLOW IN AN N-TYPE SEMI-CONDUCTOR

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

P-Type Semi-conductor Material In this material, impurities such as Indium or Aluminium are used. These have three (trivalent) electrons in the outermost orbit. When introduced into the basic material, all three electrons link into the crystal structure but this leaves a 'hole' in the structure. This hole is looking for an electron to fill it and so it is a form of positive current carrier. If a (very small) voltage is applied, electrons will move to fill in the holes but this forms fresh holes and so there is a general drift of holes through the material from positive to negative (in the opposite sense to the electron flow in the N-type material). Again, the material is neutral.

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Figure 9: ELECTRON FLOW IN A P-TYPE SEMI-CONDUCTOR

Figure 8: P-TYPE SEMI-CONDUCTOR

MAJORITY CARRIER - HOLES (POSITIVE) [P = P-TYPE] MINORITY CARRIER - ELECTRONS (due to intrinsic conduction)

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

THE P-N JUNCTION Imagine a piece of N-type material being brought into contact with a piece of Ptype material. Both pieces are, up to the instant of contact, neutral. Remembering that the holes are looking for electrons to complete the lattice network, it can be seen that electrons will migrate across the junction to fill in the holes as soon as the two materials are brought together.

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Figure 11: P-N JUNCTION

The Barrier Potential is approximately 0.2V for Germanium and 0.6V for Silicon. It must be remembered that the barrier potential is always present at a P-N junction even if it is sitting in a storage bag on a shelf.

f o g y n r i r a t e e e i n r i p g o n r E P S A M Figure 10: P-N JUNCTION BEFORE CONTACT

As electrons leave the N-type material, it will become positively charged. As electrons fill holes in the P-type material, it will become negatively charged.

If an external supply is connected +ve to the P-type material and -ve to the N-type, it will oppose the barrier potential. If it is bigger than the barrier potential, the barrier potential will be overcome and current will flow, electrons moving from supply negative to positive and holes moving in the opposite direction, as shown in figure12. This is known as FORWARD BIASING the junction.

A BARRIER POTENTIAL is built up at the boundary, forming what is known as the Depletion Layer (figure 8). This build-up in potential will eventually be strong enough to stop further migration of electrons across the junction.

Figure 12: FORWARD BIAS P-N JUNCTION

The intrinsic conduction, (covalent bonds breaking down at normal temperature) produces minority carriers and thus small current flows in the same direction as the majority carriers i.e., it adds to it.

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

If the external supply is connected in the other sense, +ve to the N-type and -ve to the P-type, it will reinforce and increase the barrier potential and therefore no current will flow, except for any slight leakage current (see below). The depletion layer will be enlarged as shown in figure 13. This is known as REVERSE BIASING the junction.

RECTIFIER ACTION If an ac supply is applied to a P-N junction then when 'P' is made positive to 'N' then the positive half cycle will flow through the junction as it is forward biased. On the negative half cycle of the ac 'P' is negative to 'N'.

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This is the reversed bias mode and the junction will not conduct on this half of the cycle.

f o g y n r i r a t e e e i n r i p g o n r E P S A M Figure 13: REVERSE BIAS P-N JUNCTION

At first sight it might appear that there is no current flow, but due to intrinsic conduction, which produces minority carriers, which causes a tiny current to flow across the junction this is known as the LEAKAGE CURRENT.

Raising the temperature of the P-N junction causes a rapid increase in the generation of minority carriers, and therefore leakage current increases. At room temperature each 10°C increase roughly doubles the rate of generation for germanium.

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Figure 14: ACTION OF A DIODE

The junction passes current through R only when the P material is positive. Therefore an output voltage is produced only on the positive input half cycle.

For silicon the doubling rate is 5°C. It might appear from this that germanium would be used for higher temperature conditions, however, although the rate of increase is greater for silicon, its actual value is considerably less than that of germanium, so silicon is used where high temperatures are encountered.

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Figure 15: DIODE SYMBOL

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Figure 16: DIODES The P-N junction is acting as a rectifier and is known as a SEMICONDUCTOR DIODE. The symbol is as shown in figure 15.

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It is important to note that the arrow points in the direction of CONVENTIONAL current flow and the two connections are known as the ANODE (A) and CATHODE (K). The cathode (negative end) is often marked with a band as shown in figure 16. Diode Characteristics

Typical characteristic curves for silicon and germanium diodes at 25°C are shown in figure 17. When forward biased, a voltage is required to overcome the barrier voltage before the diode current increases; this is typically 0.2V for germanium and 0.6V for silicon. After this, current rises rapidly as the applied voltage increases.

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Figure 17: DIODE CHARACTERISTIC CURVES

The left-hand side of the origin of the characteristic curve is where the voltage is reversed, i.e. reverse biased. As can be seen the current is extremely small, this is the leakage current due to minority carriers. Note that the voltage scale is not linear, with the larger divisions on the negative axes of the graph. As the voltage is increased at a certain point the current increases rapidly to a high value. This is known as AVALANCHE BREAKDOWN and will cause permanent damage to the diode if it is allowed to occur. It occurs because as the reverse voltage becomes too great, the minority carriers are accelerated to a point where they heat up the diode and collide with atoms in the depletion layer. This will dislodge further electrons, thus creating more minority carriers and this effect 'avalanches' to cause a rapid rise in current. The breakdown voltage can have any value from a few volts up to 1000V for silicon and 100V for germanium depending on the construction of the diode and the level of doping.

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

Diode Parameters Diodes are manufactured in a wide range of voltage and current ratings. These must be taken into account when choosing a diode for a particular circuit. Typical parameters considered are: 1. 2.

Maximum forward current Peak inverse voltage

Depending on its use, frequency is also a parameter to be considered, but generally these are special diodes and will be discussed later. Single Phase Half Wave Rectifier

Maximum operating temperature

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The diode has a small forward resistance when it is conducting, so power must be dissipated as it conducts. This power dissipation causes heat at the junction, this local heating must be kept down, as excessive leakage current will occur. There is therefore a MAXIMUM FORWARD CURRENT so that the temperature is not reached which will cause deterioration of the structure of the diode. The PEAK INVERSE VOLTAGE (PIV) is the maximum operating voltage appearing across the terminals of the diode acting in the reverse direction, and therefore represents the maximum reverse voltage that may be applied to the diode without reverse breakdown occurring. This may be written as Maximum Reverse Voltage instead of PIV. MAXIMUM OPERATING TEMPERATURE is a maximum junction temperature above, which the structure of the diode deteriorates. The maximum forward current is so chosen that this temperature is not exceeded in the worst combination of circumstances.

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With reference to figure 18, when terminal A is positive with respect to B the diode conducts, this causes a current to flow around the circuit and a voltage will be developed across RL. When the input polarity reverses terminal A will be negative with respect to B and the diode will switch off.

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Figure 18: HALF WAVE RECTIFICATION

The voltage developed across RL is therefore half-sine-waves and is known as a half wave rectifier. The output being DC, albeit variable. The average value being half that of the supply, i.e. peak x 0.318. (assuming no losses). The output DC ‘ripples’ have a frequency equal to the input frequency of the AC supply, i.e. ripple frequency - supply frequency.

However, it should be remembered that the maximum forward current will also depend on the temperature in which the diode is operating; and maximum forward current is usually quoted at two or more ambient temperatures.

We know as the temperature rises the leakage current increases and as a guide the leakage current doubles in value for each 10°C rise in temperature.

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

Single Phase Full Wave Rectifier As the name implies this uses both half cycles of the input wave form. Figure 19 shows diodes D1 and D2 used with a transformer, which is centre tapped at C. The point C can be considered as neutral with terminals A and B swinging alternately positive and negative about it. When A is positive to C, Diode D1 conducts with D2 switched off. On the other half cycle of input, B is positive to C and D2 conducts with D1 switched off. The output is therefore undirectional, with both diodes alternately conducting, giving a full wave output across RL. The average output voltage is 0.637 x peak (assuming no losses), i.e. average of the supply.

This will act as a reverse voltage across D2 so the peak inverse voltage for the diodes must be twice the peak voltage on either half of the secondary of the transformer. Bridge Rectifier

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Figure 20 shows a bridge rectifier. Assume the top of the secondary winding of the transformer to be positive (positive half cycle), trace the current flow through the load using the arrows shown.

f o g y n r i r a t e e e i n r i p g o n r E P S A M Figure 19: FULL WAVE RECTIFICATION

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This is also a single phase full wave rectifier, and has advantages over the previous circuit in that the transformer does not need to produce twice the voltage required and the secondary is in use all the time. Unlike the previous circuit where only half the secondary winding was used at any one time.

Figure 20: BRIDGE RECTIFIER - FIRST HALF CYCLE

The output DC 'ripple’ is therefore twice the input supply frequency. Having to use the double winding on the transformer makes this component more bulky in size and therefore more expensive.

A point to note about this circuit is that when D1 is conducting, the voltage across the load resistor RL is the peak voltage. With D2 cut off the voltage across C-B is in series with this voltage, so these two voltages combine to give a total of twice the peak voltage.

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

Figure 21: BRIDGE RECTIFIER - SECOND HALF CYCLE On the next half cycle (figure 21) assume the bottom of the secondary is positive and trace the circuit through the load following the arrows. Note the direction of current through the load is the same during each half cycle, i.e. it is DC.

Figure 23 shows the waveform of the three-phase supply and the resultant supply voltage to the load.

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Note that in this circuit the two non-conducting diodes have twice the supply voltage across them, (load/supply voltage + supply voltage = twice supply voltage). However, this voltage is shared between the two non-conducting diodes in series, therefore the peak inverse voltage per diode is the supply voltage. As before the ripple frequency is twice the supply frequency.

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Figure 22: DELTA STAR TRANSFORMER

Typically all four diodes are available in one package. Three Phase Half Wave Rectification

In order to obtain three-phase half wave rectification a diode must be inserted into each of the supply lines to the load and the return from the load to the supply MUST be to the star point of the three-phase system.

Figure 23: WAVEFORMS - THREE PHASE RECTIFIER

Therefore this form of rectification can only be used where there is a star connection using a neutral line. Assume this star connection is the secondary of a three phase (DELTA-STAR) transformer as shown in figure 22.

Note that the ripple frequency of this rectifier output is three times the supply frequency, with three DC output voltage 'blips' for one sequence of the three-phase AC supply.

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

Three Phase Full Wave Rectification This form of connection does not require a neutral line, so can be used on either Star or Delta connected systems. Figure 24 shows the diode circuit diagram.

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Figure 25: THREE PHASE FULL WAVE WAVEFORM

Figure 24 FULL WAVE RECTIFIER CIRCUIT

The arrows show the time in the three phase cycle when phase A is maximum and passing peak current to the load (say 10 amps). After passing through the load, the current splits into two, of five amps each to return to the B and C lines back to the supply. The output ripple frequency is six times the supply frequency. We shall now look at some other uses of diodes.

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

CLIPPING OR LIMITING As the name implies it is the limiting' or 'clipping off of part of the voltage waveform that lies above or below a certain chosen level. This level is called the bias, or reference level. Some examples are shown in figure 26.

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Figure 27: SERIES NEGATIVE LIMITER

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In figure 28, assume the input is a sinewave of (say) +20 to -20 volts. When the diode is conducting (assuming negligible resistance) the voltage across it is negligible and the output voltage (VOUT) will be equal to VIN. When the diode is cut off the output voltage is practically zero. The circuit therefore clips the portion of the waveform, which goes negative.

Figure 28 WAVEFORM OF SERIES NEGATIVE LIMITER

If the diode was to be turned round we then have a series positive limiter and the diode only conducts on the negative going cycles and so the positive going portion of the input waveform is clipped.

Figure 26: EXAMPLES OF LIMITING

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

The resistance R must be some value intermediate between the two diode extremes of resistance. This means R is very large compared to the conducting resistance (almost zero ohms) and very small compared with the cut-off resistance (which is almost infinite). A typical value for R in practice will be between 10kΩ and 100kΩ. Figure 29 shows a shunt positive limiter with the diode in shunt (parallel) with the component (VOUT) and the resistor is in series.

below some reference voltage other than zero. This can be done using slightly modified versions of the basic limiting circuits already shown. Figure 31 shows a shunt negative limiter to -10V.

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Figure 29: SHUNT POSITIVE LIMITER

Figure 31: SHUNT NEGATIVE LIMITER

During the positive half cycles, with the diode conducting the voltage developed across it is practically zero, so output voltage is zero. When the diode is cut off on the negative half-cycles, practically the whole of the input voltage is across the diode and therefore VOUT = VIN. This circuit therefore clips the portion of the input waveform, which goes positive.

The waveform may be limited to any positive or negative value by holding the appropriate electrode of the diode at the required bias or reference level.

Figure 30: WAVEFORM OF POSITIVE LIMITER

If we wish to remove the negative cycles of the waveform all that is required us to turn the diode around; the circuit now becomes a shunt negative limiter. The circuits so far discussed have all 'clipped' or limited the waveform above zero volts. In practice it is often necessary to clip the portion of the waveform above or

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

On one half cycle of the input, the diode is cut off and practically the whole of the input voltage appears as VOUT. On the other half cycle the diode is cut off until it reaches above the bias level, up to this point VIN = VOUT, when the diode conducts the VOUT is equal to the bias level and clips the negative half cycle as shown in figure 32.

If the diodes are turned round then the reverse outputs will occur. The same principle can be applied to series limiters. Figure 34 shows a series positive limiter to -10V and figure 35 shows its waveform.

If the polarity of the bias was turned around the other way then the output would be as shown in figure 33.

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Figure 34: SERIES POSITIVE LIMITER

Figure 32: WAVEFORM OF SHUNT NEGATIVE LIMITER

Figure 33: REVERSE POLARITY WAVEFORM OF SHUNT NEGATIVE LIMITER

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Figure 35: WAVEFORM OF SERIES POSITIVE LIMITER

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If the (10V) battery at the bottom of the resistor was reversed then the output waveform would be as shown in figure 36.

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Figure 38: WAVEFORM OF COMBINED LIMITER

In practice, reference or bias levels are not provided by batteries, but by a potentiometer connected across a dc supply line.

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Figure 36: WAVEFORM OF SERIES POSITIVE LIMITER WITH REVERSE POLARITY BATTERY Again if the diodes were turned around the reverse outputs will occur.

Figure 37 shows the circuit where the two are combined. This 'combined limiter' can be used to take a 'slice' out of an input waveform, as shown in figure 38.

Figure 37: COMBINED LIMITER

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

Clamping These circuits are widely used in radar and communications equipment to change the reference level of a waveform without reducing its amplitude. Circuits which move waveforms up or down in this way are known as Clamping Circuits because their effect is to fix or clamp the top or bottom level of the waveform. Figure 39 shows the difference between a limiter/clipping circuit and a clamping circuit. The limiter circuit simply 'cuts off’ a part of the waveform, whilst a clamping circuit moves the whole waveform up or down.

The voltage to which the bottom ends of the resistor or diode are returned is again known as the bias or reference level. It may be of either polarity including zero volts.

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Figure 40: CLAMPING CIRCUIT

The circuit is clamped to this bias level. In the previous drawing the output waveform is clamped to zero volts. The two types of clamping circuits are:

1.

2.

Positive clamping - the bottom of the output waveform is clamped to the bias voltage, so the output waveform is positive to the bias level. Negative clamping - the top of the waveform is clamped to the bias voltage, so the output waveform is negative to the bias level.

Figure 39: LIMITING/CLAMPING

The simplest form of clamping circuit is a diode circuit that consists of a capacitor and resistor, forming a long CR circuit to the input waveform.

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Figure 41 shows a circuit with positive clamping to zero volts and figure 42 shows the waveforms.

A to B The input rises to 100V from zero. The capacitor is initially uncharged and cannot charge immediately. VR therefore rises instantly to 100V and since this voltage is applied to the cathode of the diode, the diode is cut-off. B to C With the diode cut-off, C charges on a long time constant CR seconds and VC (voltage across the capacitor) rises by a small amount. Thus VR falls by the same amount. C to D The input falls by 100V to zero and since VC cannot change immediately VR also falls to 100V to a small negative potential which causes the diode to conduct. D to E With the diode conducting, C discharges on a short time constant CRD seconds. RD is diode resistance. Both VC and VR quickly return to zero volts and the diode is cut off. E to F The input rises again by 100V and the cycle is repeated.

f o g y n r i r a t e e e i n r i p g o n r E P S A M Figure 41: POSITIVE CLAMPING CIRCUIT

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Except for small negative 'pips' the output VR is clamped to a base level of zero volts and is positive going from this level. A similar action takes place with a negative going square wave. Figure 43 shows a negative clamping circuit and figure 42 shows the waveforms.

Figure 42: WAVEFORM - POSITIVE CLAMPING

With reference to figure 41, since R and the diode are in parallel the output voltage always equals the voltage developed across R. In any CR circuit the input voltage VIN = VC+ VR at all times.

Figure 43: NEGATIVE CLAMPING CIRCUIT TO ZERO VOLTS

The description of the waveforms (figure 42) is as follows:

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C to D VIN changes instantaneously from +100v to zero volts and this step appears in full across R. Thus vr becomes immediately -100V, the diode is non-conducting and VC is unchanged. D to E The circuit is now a long CR and C discharges slowly, VR rises slowly towards zero volts. (In a very long CR circuit the change of D to E is only a very small proportion of the input waveform amplitude).

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E to F VIN instantly becomes 100V again, and this rise causes VR jump from -98V (say) to +2V, which causes the diode to conduct.

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After F C quickly charges back to +100V on the short CR circuit and the process repeats itself.

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Thus after the initial spike is over, the waveform VOUT is a very slightly distorted version of the input waveform, but negatively clamped to zero volts.

Figure 44: WAVEFORM OF NEGATIVE CLAMPING CIRCUIT TO ZERO VOLTS Assuming a square wave of 0V and +100V (figure 44).

Prior to A - the capacitor is initially uncharged and since VIN equals zero volts, VOUT equals zero volts.

A to B The input voltage rises from zero, and since C cannot change its state of charge instantaneously, the rise appears in full across R (VOUT). Since VR is the same as the voltage across the diode the diode conducts. B to C Capacitor C and the conducting diode form a short CR circuit and so the capacitor quickly charges to +100v. VOUT falls to zero volts.

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Figure 47 POSITIVE CLAMPING TO POSITIVE BIAS Figure 45: NEGATIVE CLAMPING TO NEGATIVE BIAS

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In the examples shown the output waveform is clamped to either positively or negatively to zero volts. If it was necessary, as in some radar circuits, to clamp to a level other than zero, then the bias voltage is placed in the resistor rectifier line as shown in figures 45, 47 and 49. The waveforms produced are shown respectively in figures 46, 48 and 50.

Figure 48: WAVEFORM OF POSITIVE CLAMPING TO POSITIVE

Figure 46: WAVEFORM OF NEGATIVE CLAMPING TO NEGATIVE BIAS

BIAS Figure 49: POSITIVE CLAMPING TO NEGATIVE BIAS

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

VOLTAGE DOUBLER Another application of a diode is in a voltage doubler circuit, which is typically used in a High Energy Ignition Unit, (HEIU). Figure 51 shows the basic principle of a voltage doubler circuit.

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Figure 50: WAVEFORM OF POSITIVE CLAMPING TO NEGATIVE BIAS

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Figure 51: VOLTAGE DOUBLING CIRCUIT - 1

On one half cycle of the supply capacitor C1 will charge up to V volts, on the other half cycle C2 will charge up to V volts. As the two capacitors are in series then the output is approximately 2V volts. Figure 52 shows another type of voltage doubling circuit.

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FLY WHEEL DIODE Sometimes a diode is connected across a relay coil. When the supply is switched off the collapse of current causes a self-induced emf in the coil which by Lenz's Law tries to keep the current flowing and may cause arcing across the control switch contacts. The diode allows a path for the dissipation of this voltage and prevents this possible arcing. This may also be called a free-wheel diode.

Figure 52 VOLTAGE DOUBLING CIRCUIT - 2

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With reference to figure 52, C3 is charged to V volts during the negative half cycle of the supply voltage. The potential between C3 now acts as a battery in series with the supply. In the positive half cycle of the supply, C4 is charged to a voltage equal to the sum of the peak supply voltage and the voltage across C3, i.e. approximately 2V. By connecting the output of one multiplying circuit onto the input of the next (cascading) the dc voltage output can be four times the ac input.

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Figure 53: FREE-WHEEL DIODE

ZENER DIODE

You will remember that, with a P-N diode under reverse bias conditions, the only current flowing is due to the minority carriers passing across the depletion layer. As can be seen from the graph if the reverse bias is increased, there is little effect on the flow at the minority carriers, if the reverse bias is continually increased the point of breakdown is reached and the current increases rapidly. In the rectifier diodes discussed so far we make sure we do not get anywhere near this value of reverse voltage because the diode would be destroyed. However, the zener diode makes use of this breakdown or avalanche condition. Just to look at the breakdown mechanism in a little more detail. As the reverse bias increases the acceleration of the electrons increases and they dislodge other electrons as they collide with the atoms.

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The zener diode symbol is shown in figure 55.

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Figure 55: ZENER DIODE SYMBOL

The zener diode can be used as a voltage stabilizer, i.e. to keep the voltage constant across a circuit irrespective of load current or supply voltage variations. With reference to figure 56: a)

b)

More electrons are now created to cause more collisions and so on, and a situation is reached which is uncontrollable (avalanche) and the diode is destroyed. However, if a resistor of a suitable value is placed in series with the diode the current can be limited which ensures no overheating and does not cause damage to the diode.

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If the load current IL increases, the zener current decreases by the amount, if IL decreases then the zener current increases by the same amount thus maintaining a constant voltage across the load at all times. If the supply voltage should increase, then the current through the zener increases while the increase in voltage appears across rd not across the zener. The zener voltage remains at breakdown value irrespective of the increase in current through it. If the input voltage falls, zener current decreases and the voltage across rd falls, but again the voltage across the zener and the load remains constant.

f o g y n r i r a t e e e i n r i p g o n r E P S A M Figure 54: GRAPH OF REVERSE BIAS

The zener diode is always connected in REVERSE BIAS, i.e. cathode to positive, anode to negative. At the required breakdown voltage, determined by the doping levels the zener will breakdown, but if the reverse voltage is reduced then the zener will again become a blocking diode.

If you look at the graph again you will see that the Voltage across the diode remains virtually constant at the breakdown voltage value even though the current through it can increase. The zener is therefore a CONSTANT VOLTAGE, VARIABLE CURRENT device. They are made in a wide range of breakdown voltages 2 - 200v being a typical and also a wide range of power ratings from half a watt to many watts.

Figure 56: VOLTAGE STABILISER CIRCUIT

The property of the zener means it can also be used as a reverse voltage switch, i.e. it can be arranged to breakdown at a certain reverse voltage to activate a switch, as used in aircraft transistorized regulators and protection systems.

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SILICON CONTROLLED RECTIFIER (THYRISTOR) The SCR is a P-N-P-N semiconductor switching device, which has three terminals ANODE, CATHODE and GATE.

If the two centre regions of the SCR are regarded as being split, diagonally as shown in figure 58. It becomes two interconnected transistors TR1 and TR2. TR1 is a PNP transistor and TR2 is an NPN transistor. With the anode positive to the cathode, the base collector junctions' (J2) are reverse biased and apart from a small leakage current no current flows.

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If a pulse of current is injected into the gate terminal this turns TR2 on, this base current produces a larger collector current in TR2 which also forms the conduction path for the base current of TR1, which increases its collector current and forms the base current of TR2. The SCR is now self-sustaining and the gate supply can be removed. Typically a few microseconds of a small current applied to the gate will turn the SCR 'ON'.

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The device will remain in its conducting state until:

Figure 57: SCR SYMBOL & CONSTRUCTION

An explanation of the operation of the SCR can be carried out using the twotransistor analogy.

1. 2. 3.

The device is reverse biased, i.e. positive to cathode, negative to anode. The supply is removed. The voltage across the device is reduced so that the current falls below its "holding value" (see characteristic).

Figure 58: SCR OPERATION

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The SCR can be made to carry a wide range of currents from 1A to 1000A. Figure 60 shows different types of SCR.

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Figure 59: GRAPH OF SCR CHARACTERISTICS

Figure 59 shows a graph of the characteristics for an SCR for different values of gate voltage. The points a, b and c represent values at which the junction reverse bias is overcome and the SCR conducts, known as 'breakover', 'a' represents the highest voltage and 'c' the lowest gate voltage. Once the SCR is conducting the voltage across it is typically 1 volt.

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Figure 60: SCRs

In aircraft systems, the SCR would be typically used in firewire control, windscreen-heating control, etc. In windscreen heat control, the SCR can be gated at the beginning or at any point through out the half cycle. The earlier it is gated then more current will flow to the windscreen, the later it is gated then less current will flow.

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An LED consists of a junction diode made from the semiconductor compound gallium arsenide phosphide. It emits light when forward biased, the colour of the light emitted is in direct proportion to the current flow. Light emission in the red, orange, green and yellow regions of the spectrum is obtained depending on the composition and impurity content of the compound.

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Figure 61: GRAPHS OF SCR INPUTS & OUTPUTS

The basic SCR, when fed with ac, will switch off after- one half cycle as the other half cycle will reverse bias the SCR. So it only allows half power through. A TRIAC consists of two SCR's connected in parallel but in opposition and controlled by the same gate. It is triggered on both half cycles and therefore one conducts on one half cycle and the other one conducts on the other half cycle. Figure 62 shows the symbol.

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Figure 63: LIGHT EMITTING DIODE

When a P-N junction is forward biased electrons move across the junction from the N-type side to the P-type side where they recombine with holes near the junction. The same occurs with holes going across the junction from the P-type side. Every recombination results in the release of a certain amount of energy, causing, in most semiconductors, a temperature rise. In gallium arsenide phosphide some of the energy is emitted as light that gets out of the LED because the junction is formed very close to the surface of the material. In applying this to aircraft displays either the 7 segment or dot matrix configurations may be used.

Figure 62: TRIAC SYMBOL

The TRIAC is therefore used in windscreen heat control and domestically as a lamp dimmer or motor speed control for an electric drill. LIGHT EMITTING DIODE (LED)

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

SCHOTTKY DIODE This diode is a rectifying metal to semiconductor junction. Several metals may be used, including gold and aluminum, which are fused directly to a semiconductor material.

f o g y n r i r a t e e e i n r i p g o n r E P S A M Figure 64: SEVEN SEGMENT LED DISPLAY

In the 7 segment display for numerical indication as shown in figure 64, each segment is an LED mounted within a reflective cavity with a plastic overlay.

When used on with an ac supply should be protected against reverse breakdown, this can be done with a conventional diode connected in shunt across the LED. On reverse voltage the diode will conduct at about 0.4v protecting the LED which would breakdown at about 3-11 volt reverse voltage.

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Since the mobility of electrons is greater than holes an N-type semi-conductor is used. Current flow in this diode differs from current flow in conventional P-N junction diodes in that the minority carriers do not take any part in the process. The diode has very low capacitance and high switching speeds, produces less noise and has a smaller forward conducting voltage (0.2 to 0.4v) then conventional P-N diodes.

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Figure 65: SCHOTTKY DIODE SYMBOL

The basic construction, as already mentioned, is a piece of aluminium fused to an N type semiconductor. Some of the aluminium atoms diffuse into the silicon because aluminium has a valency of 3. This makes a very small P region. The current carrier is almost 100% electrons due to free electrons in the N type semiconductor and the metal. The Schottky diode is used in the making of logic gates as the switching time is high.

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

VARACTOR DIODE

VARISTOR

Under reverse bias conditions, a junction diode can be regarded as a parallel plate capacitor having two plates (the P and N regions) that are separated by a dielectric (depletion layer). The capacitance will vary according to the area and width of the depletion layer. The narrow depletion layer gives a higher capacitance than a wider depletion layer.

The metal oxide varistor (MOV) is a semiconductor resistor made of zinc oxide semiconductor crystals. When the voltage across this specialised resistor becomes two high, the resistor breaks down and becomes a conductor. The action of the varistor can be compared to a pair of zener diodes wired back to back in series.

f o g y n r i r a t e e e i n r i p g o n r E P S A M Figure 66: SYMBOL - VARACTOR DIODE

If this reverse bias can be varied then we have a variable capacitor typically between 2-10pf. These diodes are used to tune TV and VHP radio sets in special circuits, which allow the set to lock on to the desired station automatically. Figure 66 shows the symbol for the varactor (varicap) diode.

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Figure 67: TYPICAL MOV VOLT-AMPERE CHARACTERISTIC GRAPH

They are used for transient voltage suppression, voltage stabilisation and switch contact protection. Figure 68 shows the symbol used in drawings and figure 69 shows how a varistor reduces noise spikes in an ac voltage.

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Figure 68: MOV SYMBOL

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Figure 69: VARISTOR NOISE SPIKE CLIPPING ACTION

The varistor is connected across the secondary of the transformer and at normal voltage has a very high resistance and takes a very small current. However when the voltage spikes exceed the breakdown voltage, it conducts and clips off the noise spikes. The varistor switches extremely fast, unlike zener diodes that are slow switching. The principle described here could also be used for switch contact protection.

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PHOTO CONDUCTIVE DIODE The photodiode is a P-N junction that is reversed biased in normal operation. Its case has a transparent window through which light can enter. As it operates in reverse bias there will be leakage current (minority carriers) which increases in proportion to the amount of light falling on the junction. The light energy breaks the bonding in the crystal lattice of the semiconductor and produces electrons and holes to increase the leakage current. Figure 70 shows the drawing symbol and figure 71 shows the characteristics of the photodiode.

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Figure 70: SYMBOL - PHOTO CONDUCTIVE DIODE

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Figure 71: CHARACTERISTICS OF THE PHOTODIODE

Typically silicon diodes are used, as their leakage current with no light (dark current) is much lower than germanium. The sensitivity lies between 10mA/lm to about 50mA/lm (lm = lumen which is the amount of light emitted from a light source 1 candela strong) and the spectral response covers the visible to the infrared range. Photodiodes used with laser systems can operate at very high frequencies. They are very fast operating and are used in laser gyros and as an optical receiver for laser systems.

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

4.1.2(a) Transistors

f o g y n r i r a t e e e i n r i p g o n r E P S A M Figure 72: TYPICAL TRANSISTORS

The transistor can be used as an AMPLIFIER circuit and also as a SWITCH. The amplifier action is based on applying a low current to the base-emitter with a higher current flowing through the collector-emitter.

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The switching action is the effect of applying a small current to the base for the unit (NPN) to 'switch on' allowing current to flow between the collector-emitter. Removing the base-emitter current will cause the unit to switch off. These switching times can be very fast (say 2ns or 2 x 10-9 seconds or 0.000000002 seconds) (ns = nano seconds). Fast switching times are needed in computing.

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4.1.2(b) Transistors TRANSISTORS Construction and Theory of Operation The bi-polar or junction transistor consists of two P-N junctions in the same crystal. If two P-N junctions were fused together so that the two 'N' regions form a very thin (0.1 to 1mm thick) lightly doped layer between the two more heavily doped 'P' regions a P N P transistor is formed. Figure 73 shows the layout of the transistor and its symbol. Note the electrodes are called COLLECTOR, BASE and EMITTER (emitter - the one with the arrow in the symbol). The emitter is more heavily doped than the collector.

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Figure 74: NPN TRANSISTOR

Note. For both the PNP and NPN transistors the arrows show the direction of conventional current flow.

Figure 73: PNP TRANSISTOR

Similarly if two heavily doped 'N' regions are separated by a very thin lightly doped 'P' region then an N P N transistor is formed. Figure 74 shows the layout and its symbol. The emitter is again more heavily doped than the collector.

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Action of NPN Transistor For transistor action to occur the BASE-EMITTER junction must be forward biased (POSITIVE to 'P', NEGATIVE to 'N') and the COLLECTOR-BASE junction must be reverse biased (POSITIVE to 'N', NEGATIVE to 'P'). It should be noted that the battery Ee is much smaller than the battery Ec, it must also be of sufficient voltage to overcome the barrier potential of 0.6v for silicon.

The small amount of electron-hole combination in the base gives it a momentary negative charge, which is immediately corrected by battery Ee supply holes, or can be considered as electron flow. Remember conventional current flow is in the opposite direction.

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So transistor action is the controlling of a large current in the high resistance (reverse biased) collector-base junction by a small current through the low resistance (forward biased) base-emitter junction.

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Figure 76: CONVENTIONAL CURRENT FLOW NPN TRANSISTOR

Figure 75: NPN OPERATION

Under the influence of the electric field due to battery Ee electrons cross the junction into the base. Only a small proportion (about 1 to 2%) of the electrons combine with holes in the base due to it being very thin and lightly doped. Most of the electrons (98 to 99%), under the very strong positive influence of the battery Ec, are swept through the base to the collector to Ec to form the collector current in the external circuit. Electrons are the majority carriers in the NPN transistor.

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Action of P N P Transistor Again the base-emitter junction is forward biased and the collector-base junction is reverse biased. Under the influence of the electric field due to battery Ee, holes cross the junction into the base. Only 1 to 2% of holes recombine with free electrons in the base due to it being very thin and lightly doped. The majority of the holes 98 to 99% are accelerated towards the very strong negative influence of battery Ec. Holes are the majority carriers in the P N P transistor.

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Figure 78: CONVENTIONAL CURRENT FLOW PNP TRANSISTOR

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Since the carriers in the NPN and PNP transistors originate at the emitter and distribute themselves between base and collector, the sum of the base and collector currents must always be equal to the emitter current, therefore:

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Ie  Ic  Ib

Figure 77: PNP OPERATION

Due to recombination of holes and electrons in the base, the base loses free electrons and will therefore exhibit a positive charge. The electrons will be attracted by battery Ec into the base to 'make-up' for those lost by recombining with holes. Figure 78 shows the conventional current flow through the transistor.

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Testing Transistors Using an analogue multimeter switched to the ohms range. On most analogue multimeters on the ohms range the negative (-) terminal has a positive polarity and the positive terminal (+) has a negative polarity. This is an important point with regards to identifying NPN and PNP transistors. If a digital multimeter is used then check the polarities of the terminals on the ohms range.

TRANSISTOR AS AN AMPLIFIER First of all we need to look at how the bias is applied in a practical circuit. In our previous discussions batteries were used for the bias.

Figure 79 shows the readings you would expect using an analogue multimeter.

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Figure 80: AMPLIFIER CIRCUIT - 1

If DC only is applied to the circuit shown figure 81 then R1 and R2 will divide the supply voltage into the same ratio as that of the resistors. So if the resistor values were 80kΩ and 20kΩ then with a supply voltage of 10V the voltages across R1 and R2 would be 8v and 2v respectively.

Figure 79: TESTING TRANSISTORS USING A MULTIMETER

LINEAR circuits are amplifying-type circuits. They will have analogue inputs and the output will vary continuously and be more or less an exact but amplified copy of the input, i.e. the output is a linear representation of the input. Many class A transistor amplifiers, e.g. audio frequency and radio frequency amplifiers, are linear circuits.

Figure 81: AMPLIFIER CIRCUIT - 2

The voltage across must be 0.6V to overcome the barrier potential. This could be achieved by removing RE and making R2 of such a value so that 0.6V is dropped across it, however, the problem here would be R2 would have to be quite low and the amplification would be restricted.

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The voltage across the base emitter junction (VBE) must be 0.6V and is the difference between the voltage across R2 and RE. VBE = VR2-VRE.

Figure 82: AMPLIFIER CIRCUIT - 3

We now need to look at applying a signal to the amplifier. This will be a small ac signal (which may be superimposed on a dc level), so only ac must be applied to the amplifier. Capacitor C1 will block any DC component, and also the output amplified AC value must only be passed onto the next stage if again C2 blocks a DC component. These capacitors are known as COUPLING CAPACITORS.

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So RE must be of a value that when the standing dc current is flowing 1.4v will be dropped across RE leaving VBE to be 0.6v. So in the static condition, i.e. DC only applied, a standing current (quiescent current) flows through the circuit and TR1, R1, R2 and RE provide the bias necessary to operate TR1 and allow current to flow.

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It is also essential that the voltage across RE remains constant, and therefore VBE remains constant so that the AC input signal adds to and subtracts from the steady VBE bias.

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Figure 84: AMPLIFIER CIRCUIT WITH COUPLING/ DECOUPLING CAPACITORS

To ensure this, a capacitor is connected across RE. This capacitor will have a capacitive reactance at the operating frequency very much lower than re This means that if the ac "bypasses" RE it will leave a steady DC across RE. This capacitor C3 is known as a DECOUPLING CAPACITOR.

Figure 83: AMPLIFIER CIRCUIT - 4

With current flow through RL and TR1 there will be a voltage drop across RL. Let us assume this voltage drop is 5v so that the standing voltage is 10 – 5 = 5V. This is the condition that when DC is applied to the amplifier, all bias voltages are applied and a standing voltage is at the collector of TR1.

For Training Purposes Only

Please note the figures quoted are purely explanatory, and actual values will depend on the individual circuits. Also, the transistor used is an NPN but everything applies equally as well when using a PNP transistor except the positive rail would be at the bottom.

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Action with an AC Signal

So you can see with an input voltage of 2.5mV we get an output swing of 0.25V so

Assume that with DC applied the voltage at the collector is 5V. If a 2.5mV signal is applied as the input then when the AC signal goes positive it will add to the DC bias. The transistor will switch on more and the current through the transistor will increase and the voltage drop across RL will increase, so the collector voltage will fall. Assume if falls to by 0.25V.

therefore there is a gain

output   0.25V input  2.5mV

 100

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Please note again the values used are for explanatory purposes only.

Also note the function of RL (load resistor) without it there would be no voltage changes at the collector and no amplification.

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Another purpose, (probably its more well known one) for RE, the resistor in the emitter lead, is as a temperature compensating resistor.

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If the temperature increases, the resistance of the transistor decreases, this causes greater current through the transistor and therefore a greater voltage drop across re. If you remember the voltage across the base-emitter junction is VR2– VBE and this will decrease thus reducing the forward bias, reducing the current, compensating for the original increase.

Figure 85: AMPLIFIER ACTION – 1

When the AC voltage goes negative, it opposes the bias and the transistor conducts less, the current through RL is less so the volts drop is less and the collector voltage rises.

This amplifier configuration is known as a COMMON EMITTER AMPLIFIER. As you have seen it has a VOLTAGE GAIN

It also has a CURRENT GAIN

Vout typically 100 – 600. Vin

I out I c  typically 50 – 300. I in Ib

So it is a current amplifier as well as a voltage amplifier.

Figure 86: AMPLIFIER ACTION – 2

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DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

If there is a current gain and voltage gain then there must be a power gain.

Power out typically several thousands. Power in The input impedance is Z in

Power gain V  in typically 600 – 2000 Ω

I in

And the output impedance is Z out 

Vout typically 10 – 50 kΩ. I out

Current gain I e I c  Voltage gain medium Input impedance Output impedance

less than 1, typically 0.98 typically 500 – 800 compared to common emitter low typically 50Ω to 200Ω high typically 100kΩ to 1MΩ

Also note the phase relationship between the input and the output is 180°.

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Common Collector Amplifier (Emitter Follower)

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The COMMON EMITTER amplifier is used for the majority of amplifier applications. There are two other amplifier configurations, the COMMON BASE and the COMMON COLLECTOR. Common Base Amplifier

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Input and Output signals are in phase. Because of their very low input impedance and high output impedance they are used as impedance matching devices.

When the input goes positive this will increase the bias, the transistor will conduct more and the volts drop across RE will increase and the top of RE will go more positive. When the signal goes negative the bias will decrease, the transistor will conduct less the voltage across RE will decrease and the top of R goes more negative.

With reference to figure 88; If the input goes positive then the emitter is positive to the base and this reduces the bias voltage and the current through the transistor falls. The volts drop across RL falls and the voltage at the collector rises When the input goes negative the emitter is negative with respect to the base and the bias increases, the current increases and the volts drop across RL will increase and the collector voltage falls.

Figure 88: COMMON COLLECTOR AMPLIFIER

Figure 87: COMMON BASE AMPLIFIER Other characteristics of the common base amplifier are:

For Training Purposes Only

Issue 1 Revision 0 Jan 2011 Page 38

DCAM PART 66 CAT B1.1 MODULE 4 ELECTRONIC FUNDAMENTALS SEMICONDUCTORS (DCAM 4.1 L2)

Other characteristics are: Current gain I e I c  Voltage gain Power gain Input impedance Output impedance

typically 20 – 200 low high low

less than 1 compared to CB and CE 20kΩ to 100kΩ 20Ω to 500Ω

Each amplifier has the word common in front. This means that the input and output signals are common to whichever electrode is stated. INPUT BETWEEN BASE & EMITTER OUTPUT BETWEEN COLLECTOR & EMITTER INPUT BETWEEN EMITTER & BASE OUTPUT BETWEEN COLLECTOR & BASE INPUT BETWEEN BASE & COLLECTOR OUTPUT BETWEEN EMITTER & FOLLOWER

Identify – SIGNAL OUT

WHATS LEFT IS WHAT ITS ALL ABOUT!! e.g. SIGNAL IN on base

What’s left is emitter

SIGNAL OUT on collector

i a r

COMMON BASE

COMMON COLLECTOR

CURRENT GAIN

20 to 200