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University of California, San Diego
Department of Electrical and Computer Engineering
ECE65, Spring 2006
Lab 3, OpAmp as Linear Amplifiers
Experiment 1: Inverting Amplifier
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PSpice Simulation 1: Set up an inverting amplifier circuit as shown with R 1 = 10 kĪ© and R 2 = 100 kĪ©. Use the device library in PSpice for uA741. Set the input signal amplitude to 0.1 V.
a) Use PSpice to generate the frequency response of this amplifier for the frequency range of 10 Hz to 1 MHz. Attach the Bode plots (both amplitude and phase). For plotting the data, choose decade or octave scaling with sufficient number of points, e.g., decade with 21 points/decade. What is the low-frequency gain of this amplifier. Does it match analytical formulas?
b) The cutoff frequency for this circuit is defined similar to that of filter circuits, i.e., difference between the maximum gain and gain at cut-off frequency is 3 dB. Find the cut- off frequency from your simulations. What is bandwidth of this amplifier? What is the phase shift in the output at the cut-off frequency?
c) Plot the input resistance (= Vi/Ii) of this circuit as a function of frequency. Does it match analytical formulas?
PSpice Simulation 2: Set up an inverting amplifier circuit with R 1 = 1 kĪ©. Set the input signal amplitude to 0.1 V. Use family of curve options of PSpice to obtain Bode plots for this amplifier for R 2 = 10, 100 , and 1,000 kĪ© in the frequency range of 10 Hz to 1 MHz (this option will plot all three plots in one page for comparison). Mark the cut-off frequency for each case. Note that as the gain is reduced (by using smaller R 2 ), the bandwidth is increased. Does it match Af = constant formula?
ECE65 Lab 3 Exercises Page 2
PSpice Simulation 3: Use PSpice to simulate an inverting amplifier with R 1 = 10 kĪ© and R 2 = 100 kĪ©. Set the input to be a square wave with a frequency of 10 kHz and amplitude of 0.5 V (You need to use VPULSE function, see Class Web site). Run a transient analysis for about 5 periods. Make sure that Vo has reached its āsteady-stateā waveform. If not, run longer transient analysis and plot only the last 5 periods. Plot both Vi and Vo. Examine Vi to ensure that it is correct. Note that the output signal is not a square wave due to the slew rate of the OpAmp. Use your simulation to calculate the slew rate of the OpAmp.
Lab Exercise: Set up the circuit with R 1 = 10 kĪ© and R 2 = 100 kĪ©. The pin arrangement of the IC is available in the Lab. Note that you have to supply power to the OpAmp (+ V to V +^ pin and -15 V to V ā^ pin). The ground pin of the IC should also be connected to your circuit ground (or common).
a) Set the function generator to produce a sinusoidal wave with an amplitude of 0.5 V and connect it to the input. Attach Scope channel A to the input and Scope channel B to the output. Vary the frequency and at various points, measure the output voltage and the phase shift between input and output. Generate the Bode plots for this amplifier circuit and compare with your PSpice simulations. As you increase the frequency look at the shape of the output signal. Stop your measurements if the shape departs significantly from a sin wave and becomes triangular (why does this happen?). In this case, reduce the amplitude of input signal until the output signal is again sinusoidal and continue your measurements.
b) Set the frequency of the input signal to be 1 kHz. Measure the input resistance of the amplifier circuit (see lecture notes, page 34).
c) Set the frequency of the input signal to be 1 kHz. Measure (or try to measure) the output resistance of the amplifier circuit (see lecture notes, page 34-35).
d) Set the frequency of the input signal to be 1 kHz. Start with an input amplitude of 0. V and slowly increase the amplitude of input. Watch the output signal (which is growing proportionally). Observe that at some point, the top and bottom of the output signal starts to flatten out. This is because OpAmp is leaving its linear region and becoming saturated. Measure the saturation voltage and compare it to the bias voltage of the OpAmp (Ā±15 V). Note this voltage, we will use it in experiment 2.
e) Set the frequency of the input signal to be 1 kHz. Attach a 200 Ī© resistor to the output. Start with an input amplitude of 0.5 V and slowly increase the amplitude of input. Watch the output signal (which is growing proportionally). Observe that at some point, the top and bottom of the output signal starts to flatten out. Note that the output voltage below the amplifier saturation voltage. This is because of OpAmpās maximum current limit. From this experiment deduce the maximum output current limit of the inverting amplifier configuration as well the OpAmp chip itself. Remove RL from the circuit after you are finished with this part.
f) Set the function generator to produce a square wave with an amplitude of 0.5 V and
