Chapter 4
Operational Amplifiers

m4.2 Noninverting Amplifier

The circuit in Figure m4.2 uses a potentiometer whose total resistance is R₁. The movable stylus on terminal 2 creates two variable resistors: βR₁ between terminals 1–2 and (1 - β)R₁ between terminals 2 and 3. The movable stylus varies β over the range 0 ≤ β ≤ 1.

1. Obtain an expression for G = v_o/v_s in terms of β.
2. Calculate the amplifier gain for β = 0.0, β = 0.5, and β = 1.0 with component values R₁ = 10 kΩ and R₂ = 1.5 kΩ.
3. Let v_s be a 100-Hz sinusoidal signal with a 1-V peak value. Plot v_o and v_s to scale for β = 0.0, β = 0.5, and β = 1.0.

NI Multisim Measurements

1. Enter the circuit of Figure m4.2 into NI Multisim. Use these specific components: OPAMP_3T_VIRTUAL, AC_VOLTAGE, and virtual linear potentiometer; see the video tutorial below to learn how to search for parts by name. Set the AC voltage source frequency to 100 Hz.
2. Observe v_s and v_o with the oscilloscope. Vary the potentiometer value over its full range of 0 to 100%, and then use the oscilloscope cursors to measure the circuit gain for β = 0.0, β = 0.5, and β = 1.0.
3. Print screen shots of the oscilloscope for β = 0.0, β = 0.5, and β = 1.0. Use the same Channel A and Channel B vertical scale (volts per division) for all three screen shots.

Additional helpful tips:

• Use the Simulate → Instruments → Oscilloscope with v_s on Channel A and v_o on Channel B. Run an interactive simulation until several cycles of oscillation appear on the oscilloscope display.
• Use the cursors to measure the peak values of the input and output signals, and then calculate the amplifier gain as the output value divided by the input value.

NI myDAQ Measurements

1. Build the circuit of Figure m4.2 with the given component values. Use the following myDAQ signal connections:
   • AO0 (Analog Output 0) for v_s,
   • AI0 (Analog Input 0) to display v_s; connect AI0+ to the source voltage and AI0- to ground, and
   • AI1 (Analog Input 1) to display v_o; connect AI1+ to the output voltage and AI1- to ground.

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

2. Observe v_s and v_o with the NI ELVISmx Oscilloscope. Vary the potentiometer value over its full range of 0 to 100%, and then use the oscilloscope cursors to measure the circuit gain for β = 0.0, β = 0.5, and β = 1.0.
3. Print screen shots of the oscilloscope for β = 0.0, β = 0.5, and β = 1.0. Use the same Channel 0 and Channel 1 vertical scale (volts per division) for all three screen shots.

Additional helpful tips:

• 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.
• The potentiometer terminals of Figure m4.2 follow the standard pinout used by single-turn trim potentiometers. If your potentiometer does not label the pins, the stylus pin (Pin 2) is normally placed between Pins 1 and 3.

Further Exploration with NI myDAQ

Signal amplifiers apply a gain G ≥ 1 to increase the amplitude of weak signals, thereby making the signal information easier to use elsewhere in the system. Use a voltage divider circuit (known as an attenuator in this context) to create a “weak” signal from a portable audio player or computer audio output and then investigate how well the amplifier you built in this problem can restore the original signal strength.

1. Add the two-resistor voltage divider circuit to your amplifier as shown in Figure m4.2a.
2. Connect one plug of the 3.2 mm stereo cable supplied with your myDAQ kit to your audio player. Connect the other plug to the attenuator input v_m as shown in Figure m4.2a to apply the left channel of the stereo audio signal to the attenuator input.
3. Play some music and observe the signal v_s with the oscilloscope. Confirm that signal is indeed “weak” – its amplitude should be well under one volt peak.
4. Observe the amplifier output signal v_o with the oscilloscope. Confirm that you can adjust the circuit gain to strengthen the music signal’s amplitude.
5. Connect your earphones to the circuit output and listen as you vary the circuit gain.
   
   IMPORTANT – PROTECT YOUR HEARING!
   Do NOT disturb your circuit connections while you are wearing earphones. Accidently shorting together circuit connections can produce a very loud and unexpected noise. Alternatively, use a speaker to listen to the amplifier output or hold the earphones some distance from your ears.