Chapter 4
Operational Amplifiers
m4.3 Summing Amplifier
- Design an op amp summing circuit that performs the operation vo =
-(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.
- Draw the output waveform vo for the input waveforms v1 and v2
shown in Figure m4.3 and v3 = 4.7 V.
- State the minimum and maximum values of vo.
NI Multisim Measurements
- 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
- Plot vo and the three inputs v1 to v3 with the four-channel oscilloscope.
- Use the oscilloscope display cursors to identify the minimum and
maximum values of vo.
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 Multisim video tutorials:
NI myDAQ Measurements
- 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 vo; 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).
- Plot vo and v1 with the NI ELVISmx Oscilloscope. Repeat with
v2.
- Use the oscilloscope display cursors to identify the minimum and
maximum values of vo.
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.
NI myDAQ video tutorials:
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.