Expt. No. 2 - Diode Clipper and Clamper Circuits

Indian Institute of Technology Gandhinagar Electrical and Electronics Laboratory Expt. No. 2

DIODE CLIPPER AND CLAMPER CIRCUITS

Aim: To learn about clipping and clamping of signals using diodes and passive elements.

Pre-lab work: 1. Read the theory of these circuits. One possible source is Microelectronics, by Millman and Grabel. 2. This Laboratory sheet gives only a brief exposition of the theory. 3. Design all the circuits given in this Laboratory procedure, before coming to the lab.

Background: •

Clipper circuits: Clippers remove either the positive or the negative parts of a waveform. This action is useful in signal shaping, circuit protection and communications.

Figure 1: Positive Clipper Figure 1 shows a positive clipper. When the input signal is positive, the diode is on and appears to be a short circuit at the output. Ideally, the output voltage is zero. When the input signal is negative, the diode is open, and negative signal appears across the output. The series resistor RS is kept much smaller than the load resistor RL by design (You need to reason out why this must be so. Also, think of what would happen to the output waveform if the diode were non-ideal)

•

Clamper circuits: Clampers add a dc voltage offset to the input signal. When a positive clamper has a sinewave input, it adds a positive dc voltage to the sine wave. The effect is to have an ac voltage centered on a dc level. This means that each point on the applied sinewave is shifted up by the dc level.

Figure 2: Positive Clamper Figure 2 shows a positive clamper. The capacitor is initially uncharged. On the first negative half cycle of the input voltage, the diode turns on. At the negative peak of the input voltage, the capacitor has fully charged and its voltage is Vp. Slightly beyond the negative peak, the diode turns off. The RLC time constant is deliberately made much larger than the period T of the signal. For this reason, the capacitor remains fully charged during the off time of the diode. Then the capacitor acts approximately like a battery with a voltage Vp. This adds an offset to the signal at the output.

Laboratory Procedure: Note: Your report should contain an explanation of the functioning of all circuits set up in this experiment. CLIPPER: a. Set up the circuit shown in Figure 3 on a breadboard. Apply a sinusoidal 1 kHz input voltage Vin (10V peak to peak)

Figure 3: Biased Clipper View the input voltage Vin on Channel 1 of the CRO, and the output voltage Vout on Channel 2. Observe and sketch in your report, the input and output voltage waveforms for (i) Vs = 0V (ii) Vs = +3V (iii) Vs = −3V . (What signal coupling mode would you set on the CRO for this part of the experiment?) Obtain the transfer characteristics by placing the CRO in the XY mode. Explain all your observations. b. Figure 3 shows a negatively biased clipper. Modify and set up the circuit for a positively biased clipper and repeat the procedure of (a) above. c. Design a circuit to implement the input-output characteristics shown in Figure 4. This is a combination clipper. (Hint: use both positive and negative clippers together)

Figure 4: Combination Clipper Transfer Characteristics CLAMPER: a. Set up the circuit shown in Figure 5. Apply a sinusoidal 1 kHz input voltage Vin (10V peak to peak)

Vin

+ 10 µF

+ RL

10 VPP 1 KHz

VOUT VS

Figure 5: positive Clamper

-

Figure 6: Voltage Doubler

View the input voltage Vin on Channel 1 of the CRO, and the output voltage Vout on Channel 2. Observe and sketch in your report, the input and output voltage waveforms for (i) Vs = 0V (ii) Vs = +3V (iii) Vs = −3V . (What signal coupling mode would you set on the CRO for this part of the experiment?) Obtain the transfer characteristics by placing the CRO in the XY mode. Explain all your observations. b. The circuit shown in Figure 5 is positive clamper. Make suitable modifications to the circuit to make it into a negative clamper. Set it up on the breadboard. Draw its transfer characteristics. c. Set up the voltage doubler circuit shown in Figure 6 on your breadboard. Use RL = 100k, and set Vin to be a 10 V peak-to-peak, 1 kHz sinewave. Observe the voltage across the diode D1 and the voltage Vout simultaneously on the CRO. Sketch these in your report. Explain how this circuit functions.