1 Lab 3: Limiting and Clamping Circuits using Diodes (20 points) 1. Objectives Limiter (Clipper): The purpose of a limiter (or clipper) is to shape an input waveform and clip or cut off its top half, bottom half or both halves together. Fig. 1 Purpose of a Clipper Circuit Clamper: The purpose of a clamper is to change the DC level of a signal to a desired DC value. Fig. 2 Purpose of a Clamper Circuit Our objectives are to study diode-based clipping and clamping circuits by: 1) Analyzing and simulating the circuits, and 2) Assembling and testing the circuits. 2. Materials 1) Parts: breadboard, diodes (1N4003), zener diodes, resistors (1 kΩ, 10 kΩ, 100 kΩ), and capacitor (47 μF or smaller if required) 2) Tools: wiring kit, wire cutter, and wire stripper 3) Equipment: function generator, oscilloscope and multimeter 3. Part 1: Simulations (10 points) First, simulate the circuits shown in Fig. 3 by using either Multisim or LTSpice. 2 Fig. 3: (a) Diode Limiter 3.1 Diode limiter: D1, D2 = 1N4003 (or similar), R = 1 kΩ, VI: frequency=100 Hz, amplitude=2.5 V (5 Vpp), DC=0 V. Use small time steps for finer resolution. Note the highest and lowest voltage values. Note how abruptly the diodes cut off the input waveform. This is known as hard-limiting. Try playing with the input voltage amplitude, input voltage frequency and resistor values. For frequencies, try 100 Hz, 1kHz, 10kHz, 100 kHz, 1MHz, 10 MHz. How does the output change? L1: Provide the plot of VI vs time and VO vs time. What are the highest and lowest voltage values? Why do we get these values? L2: Provide the plot of VO vs VI ; use frequency=100 Hz, amp=2.5V (LTSpice: click on x axis->change Quantity Plotted, Experiment: on the o-scope, press Display, then change YT to XY). L3: What happens to the waveform if you change the resistor value or input voltage amplitude or the input voltage frequency? L4: Keep D1 and Remove D2 from the circuit and simulate. What is the result? Next, keep D2 and remove D1 from the circuit and simulate. What is the result? Fig. 3: (b) Zener Diode Limiter 3.2 Zener diode limiter: Z1, Z2 = 1N4733A (or similar), R = 1 kΩ. VI: frequency=100 Hz, amplitude=7 V (14 Vpp), DC=0 V. 1) In LTSpice, you will need to replace the generic zener with a zener that has low breakdown voltage (In the experiment, we will use the zener- 1N4733A that has breakdown rating of 5.1 V). In LTSpice, first place the generic zener, then right click->pick new diode->sort by type. You will see lots of zener diodes. Pick the one with breakdown rating closest to 5.1 V. Note how the circuit cuts of the input waveform gradually. This is known as soft-limiting. 3 2) Note the min and max values of the output voltage. 3) Increase and decrease the resistor value. Note the change in the output voltage. Where is it smoothest? L5: Provide the plot of VI vs time and VO vs time. L6: Provide the plot of VO vs VI. L7: How does the output voltage change while changing the resistance, R? D1 Fig. 3: (c) Clamper: Use square wave for vI 3.3 Clamper: D1= 1N4003 (or similar), R = 10 kΩ, C = 47 μF, VI: square wave, frequency=100 Hz, amplitude=1 V (2 Vpp), DC=0 V. In LTSpice, right click on the voltage source and type in the following values: Note: Larger the capacitor, better the shape of output voltage. L8: Provide the plot of VI vs time and VO vs time. L9: What are the highest and lowest voltage values in the output waveform? 4 Clamper Peak detector Fig. 3: (d) Voltage doubler: Vout is taken across C2 or R1 3.4 Voltage doubler: D1, D2 = 1N4003 (or similar), R = 100 kΩ, C1 ,C2 = 47 μF, VI: frequency=100 Hz, amplitude=2.5 V (5 Vpp), DC=0 V; Note how a voltage doubler (and hence multiplier) can take AC voltage, and multiply that voltage by some factor giving a larger DC voltage (the output voltage should be 2 diode drops lower than twice the peak input voltage). Probe/Measure the voltage across D1. Note the peak value. Now probe/measure the output voltage. L10: Provide the plot of VI vs time and VO vs time. 4. Part 2: Experiments (10 points) Now assemble the circuits shown in Fig. 3 on the breadboard. Connect the function generator and the oscilloscope. Demonstrate the operation for each of the circuits. L11-L20: Provide the plots of VI vs time and VO vs time for each of the circuits and the pertinent answers based on the experimental results. Please make sure to include the following in your report: 1) Short description of the objective and usefulness of the circuits 2) Results of the Spice simulations 3) Experimental results 4) Short conclusion containing lessons learned, problems encountered, possible solutions, etc. 5 FYI: Applications of Clipping and Clamping Circuits Q: What are the applications of the circuits you tested? 1) Diode limiters: This circuit can be used in op-amps to limit the voltages across the input terminals or any circuit that requires soft limiting or hard limiting. Audio amplifiers within the headphones often employ soft limiting circuits which have smaller number of higher harmonics and hence less distortion. 2) Clamper: Often in a given circuit, an AC coupled digital logic signals (where DC has been removed) appear. A clamper can restore the DC value. An important application of DC restorer (clamper) is that the final DC value can be used to determine the duty cycle of a signal with Pulse Width Modulation (PWM). PWM is used in communication, servo control and power delivery. 3) Voltage Doubler: There is always a demand for higher voltage level. An example includes the microwave oven. The power supplies, on which such devices run, can not produce such high voltage level. Here a voltage-doubler or voltage multiplier can be used. One such example is the Villard Voltage Multiplier.