Kirchhoff's Laws ELECTRONICS LABORATORY Experiment 3: Diode Applications: Clipper Circuits Objective: The purpose of this lab is to investigate the use of diodes in a clipper (waveform shaping) circuit. Equipment: Analog Discovery Design Kit (module) Analog Discovery Component Kit 100 KΩ Resistor 2 small signal diodes (1N914) An external 1.5V battery and battery holder (if available) Theory: The clipper circuit limits an input voltage to certain minimum and maximum values. In the circuit in Figure 3-1, one can see that as long as VI (input source, let’s say input sinusoidal) is less than VB1, then the diode will be reverse biased (an open circuit). In this case, the output voltage will track the input voltage. If VI exceeds VB1 then the diode turns on and then output voltage VO is limited to VB1 (ignoring the diode cut-in voltage), thus this circuit limits the output voltage to be always less than VB1. In actual analysis, you may need to consider diode cut-in voltages. In our circuit then output max will be limited to VB1+Vγ. By rearranging clipper circuit components, variations on this circuit can be achieved. Figure 3.1 Schematic of a clipper circuit which eliminates top portion of input wave Figure 3.2 shows another circuit where output minimum is limited to -VB (ignoring diode cut-in voltage). As input sinusoidal VI becomes less than -VB, the diode turns on fixing output Vo level minimum to -VB. For non-ideal diodes with Vγ , this voltage is -VB1-Vγ Figure 3.2 Schematic of a clipper circuit which eliminates bottom portion of input wave We can also have a two-way clipper circuit by combining the two circuits as can be seen in Figure 3.3. In this case, both the top and bottom of the waveform are clipped. So the output waveform stays in between VB1 and –VB2 (assuming an ideal diode). Figure 3.3. A two-way clipper circuit Clipper circuits are useful for designing protection circuits. Pre-Lab: Review the clipper circuits from lecture notes. Multisim Assignment: Realize each circuit shown in Figures 3.1-3.3 using Multisim and use the same component values specified under Procedure section. Make sure to use 1N914 diode(s) in each circuit. Record output vs. input waveforms for each of these three circuits. Include these waveforms in Pre-lab section of your lab report along with your Multisim circuit schematics for each. You will later use these waveforms in comparison. Procedure (Analog Discovery): Connect the circuit in Figure 3.1. Use R = 100 K resistor and a 1N914 diode. For the input signal vI, we will use arbitrary waveform generator 1 (AWG1). Set sinusoidal on AWG1 and set its amplitude to 4V (8Vp-p) and frequency to 1 kHz as shown in Figure 3.4. VB1 DC source shown in Figure 3.1 is to be set using AWG2. For this, select the DC and set an offset value of 1V as shown in Figure 3.4. Figure 3.4. Waveform generators’ setup The breadboard configuration of this circuit can be seen in Figure 3.5. Figure 3.5. Breadboard picture of Circuit in Figure 3.1 You should obtain a figure similar to Figure 3.6 which is shown in black and white color. Remember, on your lab report, you should include the color version of this figure with correct date stamp on it (mine has 2013/11/12), otherwise no credit! Figure 3.6. Input and the output waveforms of Figure 3.1. The waveform in Figure 3.6 shows that the clipping occurs after a certain voltage value same as what is expected. Include the waveforms and breadboard picture in your lab report. Connect the circuit in Figure 3.2. Use again R = 100 K resistor and a 1N914 diode. Figure 3.7 shows the breadboard picture. Figure 3.7. Breadboard picture of Circuit in Figure 3.2 For the input signal vI, again use the arbitrary waveform generator 1 (AWG1). Set sinusoidal on AWG1 and set its amplitude to 4V (8Vp-p) and frequency to 1 kHz as before. VB2 DC source shown in Figure 3.2 is to be set using AWG2. For this, select the DC and set an offset value of -1.5V. Figure 3.2 is redrawn for your convenience in Figure 3.8. Also, the AWG2 window is shown in Figure 3.9. Obtain the input and the output waveforms. Include the waveforms and breadboard picture in your lab report. Figure 3.8. Figure 3.2 shown with parameters. Figure 3.9. Setting AWG2 Implement the 2 way clipper circuit shown in Figure 3.3. Use a 100k Ohm resistance and two 1N914 diodes. For the input signal vI, use the arbitrary waveform gen. 1 (AWG1). Set a sinusoidal with an amplitude of 4V (8Vp-p) and 1 kHz frequency. Use AWG2 to set VB1 DC source of 1V. For VB2 source, you need to use a 1.5V external battery. You can use a battery holder or use soldering to attach battery to the circuit. Although not recommended, you can try taping the wire to the battery. Note that – terminal of the external battery goes to anode terminal of Diode 2 or D2, the + terminal of the battery should connect to the black wire on the Analog Discovery module. Include the waveforms and breadboard picture in your lab report. Comment on your results for all three circuits. Are they as expected? Questions for Lab Report: For each of the three circuits, compare the output waveforms you measured to the one you obtained using Multisim simulation. Are there any discrepancies? Comment on possible reasons. Based on the output waveforms you obtained using your breadboard; determine the value of cut in voltage for your diode. Draw the output waveforms for a square wave input having 4V amplitude and 0V offset having the same frequency. Optionally you can repeat the experiment with this input rather than showing it using your calculation. 2