Download ECE 322L Experiment 1: Negative and Positive Feedback in Op

P. R. Nelson
1
ECE 322L Experiment 1:
Negative and Positive Feedback in
Op Amp Circuits
1
Objectives
The objectives of this lab are to:
• construct and characterize op amp circuits
• display the voltage transfer characteristic of a circuit on an oscilloscope
• examine the effect of frequency on op amp circuit performance
• develop record-keeping skills
• analyze experimental data
• explore the differences between negative and positive feedback
2
Preparation
Bring the following to class:
• an op amp in a dip package (the stockroom should have these) and the
datasheet. You may use an electronic copy of the datasheet, but be
prepared to show it to the instructor.
• the other parts, wires, and cables required to construct and test each
circuit
• a sketch of the graph of Vout as a function of Vin that you expect for each
circuit, including approximate values for any important points on these
curves
• the definition of the “output voltage slew rate” of an op amp
3
Procedure
The power supply voltages for this lab are ±8 V.
322L-lab1.tex
Fall 2011
P. R. Nelson
3.1
2
Voltage Amplifier
Build the circuit of Figure 1 with 1 kΩ ≤ R1 ≤ 10 kΩ and R2 ≃ 10R1 . Using a
low-frequency sine wave at the input, display Vout as a function of Vin on the
oscilloscope. (In order to ensure that you have a “low-frequency,” you might
try a range of orders of magnitude of the frequency.) Determine the voltage
gain Vout /Vin and the dynamic range of your circuit. Compare your measured
results with theory. (Think carefully about what this comparison requires you
to measure, and what you need to record so that you could repeat your work!)
R1
R2
−
Vin
Vout
+
Figure 1: Voltage amplifier circuit
Test your understanding by changing R1 so that R1 ≃ R2 and make the
same measurements. Discuss your results.
With R1 at the original value, increase the frequency of the input voltage and
note any resulting changes in the shape of the voltage transfer curve. (Change
the frequency by orders of magnitude.) Make any additional measurements that
you need to explain how frequency affects the performance of this circuit. (Hint:
look at the time-dependent input and output waveforms.)
3.2
Schmitt Trigger
Build the circuit of Figure 2 with R1 ≃ R2 ≃ 10 kΩ. Use a low-frequency sine
wave at the input to display Vout as a function of Vin on the oscilloscope. Set the
amplitude of the input voltage so that your results display all of the important
aspects of your predicted curve. Explain how you chose both the amplitude and
the frequency of Vin . Compare your measured results with theory.
R1
R2
+
Vin
Vout
−
Figure 2: Schmitt trigger circuit
Increase the frequency of the input voltage (by orders of magnitude) and
discuss any changes that you observe in the shape of Vout vs. Viin . Make
any additional measurements that you need so that you can explain how the
frequency affects the performance of this circuit.
322L-lab1.tex
Fall 2011