Chapter 4
Operational Amplifiers

m4.4 Signal Processing Circuits

1. Design a two-stage signal processor to serve as a “distortion box” for an electric guitar. The first-stage amplifier applies a variable gain magnitude in the range 13.3 to 23.3 while the second-stage amplifier attenuates the signal by 13.3, i.e., the second-stage amplifier has a fixed gain of 1/13.3. Note that when the first-stage amplifier gain is 13.3 the overall distortion box gain is unity. The distortion effect relies on intentionally driving the first-stage amplifier into saturation (also called “clipping”) when its gain is higher than 13.3.

   Use a 10 kΩ potentiometer and standard-value resistors in the range 1.0 kΩ to 100 kΩ; see the resistor parts list in Appendix A. You may combine two standard-value resistors in series to achieve the required amplifier gains.

2. Derive a general formula for percent clipping of a unit-amplitude sinusoidal test signal; this is the percent of time during one period in which the signal is clipped. The formula includes the peak sinusoidal voltage V _P that would appear at the output of the first-stage amplifier with saturation ignored and the actual maximum value V _S due to saturation.
3. Apply your general formula to calculate percent clipping of a 1-V peak amplitude sinusoidal signal for the potentiometer dial in three positions: fully counter-clockwise (no distortion), midscale (moderate distortion), and fully clockwise (maximum distortion). Assume the op-amp outputs saturate at ±13.5 V.
4. Apply a 1-volt peak amplitude sinusoidal signal with 100-Hz frequency to the distortion box input and plot its output for the potentiometer dial in the same three positions as above. State the maximum and minimum values of the distortion box output.

NI Multisim Measurements

1. Enter your design for the distortion box into NI Multisim. Use the virtual five-terminal op amp model for both stages. Connect the power supply terminals ±13.5 volts. Apply the AC (sinusoidal) voltage source as the signal input; configure the source for 1-volt peak amplitude and 100 Hz frequency.
2. Observe the distortion box input and output signals with the oscilloscope and vary the potentiometer value over its full range of 0 to 100%, and then use the oscilloscope cursors to measure the percent clipping for the potentiometer settings 0%, 50%, and 100%.
3. Print screenshots of the oscilloscope display to show the distortion box input and output signals for the three potentiometer settings in the previous step.
4. Measure the maximum and minimum values of the distortion box output.

Additional helpful tips:

• Use these specific components: OPAMP_5T_VIRTUAL, AC_VOLTAGE, and virtual linear potentiometer.
• Remember that the five-terminal op amp symbol when initially placed has its positive power supply connection on top; applying a vertical flip to the symbol also flips the positive power supply connection to the bottom.
• Place the “CMOS Supply (VDD)” as the op amp positive power supply connection and “CMOS Supply (VSS)” as the negative power supply connection.
• Use the basic two-channel oscilloscope with the distortion box signal input on Channel A and its output on Channel B. Run an interactive simulation until several cycles of oscillation appear on the oscilloscope display.
• Take cursor measurements to determine the time duration of clipping for a half-cycle of the sinusoidal signal. Divide this time by the duration of the entire half-cycle and multiply by 100%.

NI myDAQ Measurements

1. Build your distortion box circuit and use the following myDAQ signal connections:
   • AI0 (Analog Input 0) to display the input signal; connect AI0+ to the source voltage and AI0- to ground,
   • AI1 (Analog Input 1) to display the output signal; connect AI1+ to the output voltage and AI1- to ground,

   Create the 100-Hz sinusoidal waveform with the NI ELVISmx Function Generator.

2. Observe the distortion box input and output signals with the NI ELVISmx Oscilloscope and vary the potentiometer over its full range. Use the oscilloscope cursors to measure the percent clipping with the potentiometer dial in three positions: fully counter-clockwise (no distortion), midscale (moderate distortion), and fully clockwise (maximum distortion).
3. Print screenshots of the oscilloscope display to show the distortion box input and output signals for the three potentiometer settings in the previous step.
4. Measure the maximum and minimum values of the distortion box output.

Additional helpful tips:

• Use the Texas Instruments TL072 op amp described in Appendix C. Follow the pinout diagram of Figure C.1 for the two available op amps in the package. You may also use a pair of equivalent dual-supply op amp.
• Power the op amp with myDAQ +15V to V _CC+ and -15V to V _CC-. Use AGND for the circuit ground.
• The TL072 op amp and similar devices saturate at approximately 1.5 volts under the supply voltage, consequently the actual saturation levels are about ±13.5 volts. If you use a different type of op amp with rail-to-rail outputs then you should expect to see the output saturation levels match the measured values of the myDAQ 15-volt dual power supply.
• Should you need to observe the output of the first-stage amplifier with the oscilloscope for troubleshooting purposes, you must consider the ±10 volt input range limitation of the myDAQ analog input channels. This range limit effectively makes you blind to any signal activity between 10 volts and the myDAQ power supply of 15 volts.
• Take cursor measurements to determine the time duration of clipping for a half-cycle of the sinusoidal signal. Divide this time by the duration of the entire half-cycle and multiply by 100%.