ECE2201 Lab Report 8

ECE 2201 : ELECTRICAL AND COMPUTER ENGINEERING LAB 1

EXPERIMENT 8 :

NON-INVERTING AMPLIFIERS DATE OF EXPERIMENT : 4/04/2013

GROUP MEMBERS :

DATE OF SUBMISSION : 19/04/2013

OBJECTIVES  To demonstrate the operation of the non-inverting amplifier.  To demonstrate the effect of resistor faults on the operation of the noninverting amplifier.

THEORY Voltage amplification is a main application of an operational amplifier. Considering these two main applications, this electronic device is termed as operational amplifier (Op-amp). Operational amplifiers can be constructed from discrete components, mainly transistors, which provide stable and high voltage amplification. But commonly they are available as monolithic integrated circuits. They are also used as video and audio amplifiers, oscillators and others. Because of their versatility, Op-amps are widely used in all branches of electronics in both digital and analogue circuits. One of the most common Opamp IC is CA 741. An Op-amp IC is activated by applying a dual DC power supply (approximately –15V and +15V). An operational amplifier or op-amp is an electronic circuit module (normally built as an integrated circuit) which has a non-inverting input (+), an inverting input (-) and one output. The output voltage is the difference between the + and - inputs multiplied by the open-loop gain: V out = (V+ − V−) x Gopenloop. Since op-amps have uniform parameters and often standardized packaging as well as standard power supply needs, they help in designing an application.

So the voltage gain of the amplifier is where there is no negative sign as its convention indicating that the output is not negated. Below shows the example of basic configuration of non-inverting op-amp.

CALCULATIONS  Calculated close-loop voltage gain of amplifier, ACL : A CL=

Rf Ri

+1

 Measured close-loop voltage gain of amplifier, ACL : A CL=

Value of Rf, kΩ

V out V¿

Calculated close-loop voltage gain of amplifier, ACL A CL=

10

27

+

2 .28 1. 16

¿2

¿ 1. 97

27 k A CL= 10 k

4 .24 A CL= 1. 16

+1

¿ 3 .7 39 k 10 k

¿ 3 .7

+

1

¿ 2−1 . 97∨ ¿ × 100 2 Error=¿

A CL=

47 k 10 k

1 ¿ 5 .7

A CL=

+

¿ 1. 5 ¿ 3 .7−3 . 66∨ ¿ ×100 3 .7 Error=¿ ¿ 1. 08

5 .60 1 .16

¿4.9

¿4.9

47

A CL=

Percentage Error

1

A CL=

39

10 k 10 k

Measured closeloop voltage gain of amplifier, ACL

6 . 48 A CL= 1 .16 ¿ 5 .59

¿ 4 . 9−4 .83∨ ¿ ×100 4.9 Error=¿ ¿ 1. 43 ¿ 5 .7−5 .59∨ ¿ ×100 5.7 Error=¿ ¿ 1. 93

A CL=

82

82 k 10 k

1 ¿ 9 .2

+

10 .6 A CL= 1 .16

¿ 9 .2−9 . 14∨ ¿ ×100 9.2 Error=¿

¿ 9 .14

¿ 0 .65

DISCUSSION In this experiment, the non-inverting operational amplifier is being investigated. This is known as an non-inverting amplifier because the output waveform has a 0 degrees phase shift from the input waveform but the wave is amplified. This is achieved by connecting the input to the positive terminal of the op-amp, while the negative terminal is grounded. The operational amplifier also amplifies the signal, so the output voltage is also scaled. The amount of scaling is equal to the ratio of the impedances plus one from the feedback to the input. In this experiment, the input resistance and feedback resistance are both equal to each other: 10 kΩ. This means the scaling factor is 1.98, i.e. double scaling. In other words, this can only demonstrate the non-inverting property of this operational amplifier. As calculated, the experimental voltage gain was found to be 2. The output waveform had a double increased. The percentage error of 0% to nearly 1% is quite interesting. It is extremely rare to acquire a percentage error of 0% for any electrical experiment. This shows that the practical situation in the experiment was very close to the theorized ideal model. There may have been differences, such as the resistor values not being exactly equal to each other, and the op-amp not being ideal practically, but such sources of error may have canceled each other out until the overall error was not large enough to affect the limited sensitivity of the oscilloscope. The feedback resistor is then replaced with resistors of other given values. The amplification is calculated and compared with the theoretical. Here, there are two aspects of these results that are interesting. The first is that the percentage error seems to decrease as the resistance value increases, and the

second is that the percentage errors are quite significant compared to the 0% error of 10 kΩ. The percentage value decreasing with increasing resistance can be easily explained. As the feedback resistance increases, the amplification also increases. The variations between the theoretical and calculated aren’t a lot, but with increased amplification, these errors are a small proportion of the whole amplification and thus, they have a lower percentage error with increased resistance. As for why the produced error is much more significant compared to the original 10 kΩ, it’s possibly because the decade resistance box was used instead of actual resistors, so faults in the accuracy of the resistance box were all manifested in step H but not step G. In Step I, a load resistance was added. The value of the load resistance did not alter the amplification whatsoever. This shows that in an inverting amplifier, the output voltage is truly just the ratio of the feedback impedance to the input impedance and is independent of load impedance. This raises the issue of power consumption, since increased resistance with the same voltage would consume more power. Since the output voltage is independent of load resistance, it means the power consumed by the load resistor is not obtained from the ac input signal. Rather, it is supplied by the dc voltage supplied at terminals 4 and 7 of the operational amplifier. After all, it doesn’t logically make sense for a signal to be amplified without an extra supply of power, so the DC biasing is essential for the amplification, even though the shape of the output waveform and its peak values depend only on the input signal. This concept can be used to explain the distortion that is observed in step J. Although the output voltage is a scaled (and inverted) version of the input voltage, the power supplied to the load is from the dc supplies. Therefore, as the potentiometer’s resistance decreased while the output voltage remained the same, the power consumed by the potentiometer increased (P = V2/R). Eventually, the power consumed reached the maximum power supplied by the dc supply, and so the output waveform was distorted, and the voltage gain was less, to reduce the power consumption of the load. By the end of the experiment, we can see that the value obtain is not as accurate as the theoretical value. This is due to some errors such as the additional resistance that exist in the connecting wire and other electrical

devices, and also the malfunction on devices and apparatus. The reading on oscilloscope also fluctuated and this harden us to record the exact value. In order to prevent these errors, we need to reduce the number of wire used in construction of circuit and also check for the functionality of each device before starting the experiment. Repeated readings were also taken to find the average value. CONCLUSION The non-inverting op-amp was investigated and its characteristics were identified. It doesn't inverts the input voltage (0 degrees phase difference) which is the opposite of inverting op-amp and scales it with a scaling factor equal to the ratio of feedback resistance to input resistance plus one. Therefore the bigger the ratio, the smaller the effect of plus one. The output waveform, however, relies on the dc supplies of the op-amp to receive its power. Therefore, even if the scaling is set to infinity (by removing the feedback resistance), the output will still be clipped to the dc values.