Chapter 4
Operational Amplifiers

m4.3 Summing Amplifier

1. Design an op amp summing circuit that performs the operation v_o = -(2.14v₁ + 1.00v₂ + 0.47v3). Use not more than four standard-value resistors with values between 10 kΩ and 100 kΩ. Refer to the resistor parts list in Appendix A.
2. Draw the output waveform v_o for the input waveforms v₁ and v₂ shown in Figure m4.3 and v₃ = 4.7 V.
3. State the minimum and maximum values of v_o.

NI Multisim Measurements

1. Enter the op amp summing circuit that you designed earlier. Use the following components and instruments:
   • Virtual three-terminal op amp model OPAMP_3T_VIRTUAL
   • Piecewise-linear voltage source PIECEWISE_LINEAR_VOLTAGE for v₁
   • Pulse voltage source PULSE_VOLTAGE for v₂
   • Four-channel oscilloscope
2. Plot v_o and the three inputs v₁ to v₃ with the four-channel oscilloscope.
3. Use the oscilloscope display cursors to identify the minimum and maximum values of v_o.

Additional Multisim tips for this problem:

• Specify the PWL voltage source waveform by entering endpoints of straight lines as time-voltage pairs. The triangle waveform of Figure m4.3 requires only three entries to specify a complete period. Select “Repeat data during simulation” to create a periodic triangle waveform.
• Three fields need to be adjusted for the pulse voltage source to make it match the required square waveform shape: “Initial Value,” “Pulse Width,” and “Period.”

NI myDAQ Measurements

1. Build the op amp summing circuit that you designed earlier. Use the following myDAQ signal connections:
   • AO0 (Analog Output 0) for v₁,
   • AO1 (Analog Output 1) for v₂,
   • AI0 (Analog Input 0) to display either v₁ or v₂; connect AI0+ to the input voltage of interest and AI0- to ground,
   • AI1 (Analog Input 1) to display v_o; connect AI1+ to the output voltage and AI1- to ground,

   Create the triangle and square waveforms with the NI ELVISmx Arbitrary Waveform Generator; use 50 kS/s as the sampling rate.

   Important: The two waveform files must of the same length (10 ms).

2. Plot v_o and v₁ with the NI ELVISmx Oscilloscope. Repeat with v₂.
3. Use the oscilloscope display cursors to identify the minimum and maximum values of v_o.

Additional tips for this problem:

• Use the Texas Instruments TL072 op amp described in Appendix C. Follow the pinout diagram of Figure  C.1 for either of the two available op amps in the package. You may also use an 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.
• Implement the constant voltage source v₃ according to the circuit diagram of Figure B.2 in Appendix B. Adjust the potentiometer until the measured voltage is as close to 4.70 volts as possible.

Further Exploration with NI myDAQ

Investigate the effect of the gain constants for waveform inputs v₁ and v₂. You can quickly and easily vary a resistor value by placing another resistor in parallel with it, thereby reducing the effective resistance. Place a 10 kΩ in parallel with the source resistor associated with waveform v₁ and observe the impact on the output voltage waveform. Plot the new output waveform, summarize the difference from the original waveform, and explain why reducing the resistor value causes this change in appearance.

Repeat the experiment with the source resistor associated with waveform v₂.