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ECE 220 Lab 4
Operational Amplifiers
Difference Amplifier, Light Meter
This lab introduces you to the μA741 op-amp. Two op-amp circuits will be constructed and tested: (1) difference amplifier; (2) light meter. The PSpice exercises, which form the bulk of the prelab, examine the inverting amplifier in addition to the above circuits.
Instrument skills to be reinforced are:
- Measuring dc voltage using the oscilloscope
- Measuring the amplitude and frequency of ac voltage waveforms using the oscilloscope.
Prelab & PSpice Exercises (30 pts) – PRINT ALL PSPICE SCHEMATICS
Sinewave measurements : Definitions of peak value, peak-to-peak value, and rms value are shown below.
-1 0 10 20 30 40 50 60 70 80 90 100
-0.
-0.
-0.
-0.
0
1
RMS value = Peak ÷√ 2
Peak-to-peak value in this case is 2V.
Peak-to-peak = 2 × Amplitude
Peak = Amplitude = 1V
RMS = 0.707 V
PSpice note : You will use transient analysis to view ac waveforms, as described in Part 1 below.
Space for notes:
0
0
R
12k
R
V
_
10k
TransientAnalysis
(^) Vi AMPLITUDE = 7V FREQUENCY = 100
V
0
0
Vo
Vneg -15V
U
uA
3
2
7
4
6 1
+^5
V+
V-
OUT OS
OS
0
13k
39k 43k
Fig. 1
Inverting amplifier. The PSpice schematic has been set up to do a transient analysis. Running PROBE will produce oscilloscope-like plots of the input and output voltages.
Part 1: Inverting amplifier with sinewave input
Create the PSpice schematic^1 shown in Fig. 1. Print the schematic. The two voltage markers can be placed using the small “V” icon at the top of the screen and will automatically invoke PROBE, the PSpice plotting program, and will plot the waveforms at these nodes after the circuit simulation. Before simulating, select PSPICE/NEW SIMULATION PROFILE from the PSpice toolbar. Name the simulation profile. Once in the profile select TIME DOMAIN (Transient) under Analysis Type. Check GENERAL SETTINGS if it is not checked. Enter RUN TO TIME 20ms and enter MAXIMUM STEP SIZE 1us. The input sinusoid vi is created using the part VSIN. After placing this part double click AMPLITUDE= and FREQUENCY= and enter 1V for amplitude and 100 (this is in Hz) for frequency. Run the simulation (four times) with the following values of AMPLITUDE: 1, 3, 5 and 7V. (These are peak values.) Explain the form of the output voltage waveform across RL for each value of the input peak AMPLITUDE. Print the plots for the cases AMPLITUDE = 1 and AMPLITUDE = 7V, but first use TRACE/CURSOR/DISPLAY from the PROBE toolbar to invoke the cursor and PLOT/LABEL/MARK to mark peak values of the output waveform on the plots. (You can switch between waveforms by clicking on the icon next to the waveform label at the bottom left part of the screen below the graph. If you run into difficulties estimate these values on the printouts. Indicate which is the output and which is the input using PLOT/LABEL/TEXT.
Space for notes:
(^1) It is more conventional to draw the op-amp with the inverting input (labeled -) on top. Select the op-amp
component and type EDIT/MIRROR/VERTICALLY.
vo
vb
va
TransientAnalysis
(^) V AMPLITUDE = 0.5V FREQUENCY = 200
V
R
1k
R
10k
V
0
AnalysisTransient V
AMPLITUDE = 100mV FREQUENCY = 500
0
R
100k
R
10k
V
V+
V-
R 8
R
b
R 8
U
Ideal_OPAMP
100k
a
c
d
vo
vb
va
1 3k 130k
= 4 = 30
Microphone
Hz
00mV 0Hz
50 0mV
Interference Source
13k 130 k
Fig. 3: Difference amplifier.
Part 3: Difference Amplifier
The purpose of the difference amplifier in Fig. 3 is to amplify a wanted AC signal and reject an unwanted AC interference signal. These AC signal sources are identified in Fig. 3 as:
{V2: 200mV peak-to-peak, 500Hz, 8-Ω Thevenin resistance}: simulated signal generated from output of an 8-Ω microphone. This is the useful or wanted signal.
{V1: 1V peak-to-peak, 300 Hz, 1kΩ Thevenin resistance}: simulated interference signal inadvertently picked up by both input wires (acting as antennas) from some nearby instrument. This is the unwanted signal.
As shown in class, the output voltage is given by:
v (^) o = 10 (vb−va )
where va and vb shown in Fig. 3 are with respect to ground. The gain of 10 results from the resistor ratio 130k/13k. This ratio occurs twice in the circuit, and in practice care must be taken to match these ratios exactly.
Draw the schematic of Fig. 3. Use the Ideal Op-Amp found in the CLASS library (or the part called OPAMP/ANALOG). This part requires no power rails. Connect the three Voltage/Level Markers as shown, plus one pair of Voltage Differential Markers as shown. Set up a TIME DOMAIN (Transient) simulation as in Part 1. Run the simulation for 20ms with a step size of 1us. Running the simulation produces a plot with four waveforms. Print this plot. Label the waveforms on your plot and briefly explain the shape and amplitude of each one in terms of the action of the difference amplifier. Why is the V2 signal amplified and the V1 signal suppressed (in the output vo)?
Run the simulation with R1 changed to 12kΩ, and observe the four waveforms, particularly vo. What is the circuit doing in this case?
Part 4: Light meter (using an inverting amplifier and an op-amp voltmeter)
The light meter is designed to provide a visual indication (using the 1mA panel meter from Lab 2) of the ambient light intensity. The light meter employs two op-amp ckts. The first ckt uses a photo-sensitive resistor to vary the gain of an inverting amplifier over the range 0 to -1. Because the input to the amplifier is a dc voltage of -10V, the amplifier output is a dc voltage in the range of 0 to 10V. The amplifier output voltage is proportional (although not in a linear sense) to the intensity of light striking the photo-resistor.
The second op-amp ckt is a voltmeter, shown in Fig. 4. This provides a visual display of the output voltage from the first op-amp ckt, which is proportional to the light intensity.
The voltmeter works as follows. The panel meter provides negative feedback from the output to the inverting input; hence, each input terminal is at the same voltage relative to ground. i.e. the inverting input is at vin relative to ground. Consequently, there is a voltage drop of vin – 0 across the resistor Rv. This voltage drop pulls a current to ground of value vin ÷ Rv. This current must come from the op-amp output: it is shown as im in Fig. 4. The voltmeter is designed by selecting Rv so that this current equals 1mA when vin is at the desired full-scale reading. Notice that the design does not depend on the resistance of the panel meter (nominally 128Ω).
Fig. 4:
Analog voltmeter combining high- input impedance op-amp with 1mA panel meter display.
VOLTMETER
d'Arsonval panel meter
vin Rv^ im
Assuming the op-amp is ideal, what is the theoretical input resistance of this voltmeter? Compare this to the input resistance of the d'Arsonval voltmeter designed in Lab 2.
The complete schematic for the light meter is shown in Fig. 5.
Fig. 5:
Light meter
V 10V
V 10V
0
R 11k
0
VCC
VEE U uA
3
2
7
4
6
1
+^5
V+
V- OUT
OS
OS
VCC
R
128ohms
I
VEE
U
uA
3
2
7
4
6 1
+^5
V+
V-
OUT OS
OS
0
VEE
R 620 VCC
VEE
R
910
R
{Rphoto}
VEE
1mA Panel Meter
P ARAM ET ERS: Rphoto = 400
R
1k
R 8.2k
OP-AMP Voltage Rails
v 1
v 2
Experiments (40 pts)
Part 3.
- Build the difference amplifier of Fig. 3. (Use a real 741 op-amp, with +/- 15 V power rails.)
- For the AC source V2 use the 8-Ω horn loudspeaker (which we will use as a microphone). R6 is not needed. (In the prelab, this represented the resistance of the microphone.)
- For the interference source V1 use the output of the function generator, set to the amplitude and frequency values shown in Fig. 3.
- Replace R3 (13kΩ) using a 10kΩ pot (connected as a variable resistor) in series with a 5.1kΩ resistor. R7 (8Ω) can be omitted.
- With both sources connected, and with the scope connected to the op-amp output, adjust the pot until the 300-Hz interference produced by the function generator seen at the op-amp output is minimized. Use the scope to measure the amplitude of the waveform at this pot setting.
- Whistle (or hum!) into the horn speaker, and observe the amplified speech signal on the scope. Measure, as best you can, the average amplitude of the speech signal.
- Additional noise can be introduced by touching (or gripping) the nodes labeled a,b,c and d in Fig. 3. (Your body behaves like an antenna and picks up stray electromagnetic radiation.) With the scope connected to the op-amp output, grip each node in turn and each time make a note of the approximate average noise amplitude produced on the scope.
- Finally, take R1, R2, R3 (resistor-pot combination), R4 and the horn loudspeaker out of the circuit and measure the resistance of each one. (Be careful not to disturb the pot setting.)
Question 3: (a) Compute the following Figure-of-Merit for your difference amplifier:
IN
OUT SNR
SNR
FOM =
where:
SNRIN = Amplitude of input horn signal ÷ Amplitude of input interference from function generator (The numerator can be figured out from the amplifier gain and your measured value from bullet 6. The denominator is simply the function generator amplitude.) SNROUT = Amplitude of output horn signal ÷ Amplitude of output interference (Numerator was measured in bullet 6. Denominator was measured in bullet 5.)
Express your answer in linear form (using the above equation), and also compute 20log 10 [FOM] (dB). Also, show that the FOM does not depend on the measurement made under bullet 6.
(b) Referring to bullet 7, explain why touching different nodes resulted in different amounts of amplified noise at the op-amp output. (c) Referring to bullet 8, compute the measured resistor ratios, and comment on how closely they match. (d) Include any additional observations on the performance of your difference amplifier.
Part 4.
- Build and test your light meter designed in the prelab. At full light the meter should read ~1mA, and ~0mA when your hand is cupped over the photo-resistor. Troubleshoot your ckt by making measurements of v 1 and v 2 if needed. Adjust your prelab resistor values if needed.
- Measure v 1 and v 2 , and record the panel meter reading (in the range 0-1mA) at full light and maximum darkness. Measure the resistance Rphoto at each of these two conditions. (It may be difficult to replicate illumination conditions when you make these two measurements. Your TA will guide you.)
- Ask your TA to verify that your meter needle swings from ~0 to ~1mA as you vary the light intensity.
Question 4: Provide calculations to show that your measured Rphoto values predict the meter readings at the two illumination extremes. (Use your prelab equations from (c) and (d) on page 6.)
