ECE 2212

Spring 2012

20 September 2012

Experiment 2: Operational Amplifier Circuits 1

NOTE: I will be checking during your that you are using a notebook as discussed earlier this semester. All information for Experiment 1 as well as any pre-lab for this experiment must be in a patent acceptable notebook.

PURPOSE

Ø To implement the designs of inverting and non-inverting amplifiers using an operational amplifier.

Ø To modify the SPICE frequency-independent model to simulate the measured frequency response of the inverting operational amplifier configuration.

Ø To compare the SPICE model discussed in class with the SPICE Schematic capture library model which will include frequency effects.

PRELAB

Design the circuits to meet the indicated specifications. You should come to the lab with a list of the components you will need to meet the specifications. You might refer to your ECE 2006 notes and labs since many of you have worked with op amps in that course. Write and run SPICE time and frequency domain programs for both circuits. Use the .LIB 741 model if you have it in your version of SPICE and use the linear model presented in class or the generic “op amp” in the .LIB file. Print the waveforms of the inputs and outputs on the same set of axes. You will need the following information from your SPICE program in order to complete this lab:

Ø 3 dB BW, key amplitudes, and times

Ø .AC analysis of frequency and phase

Ø .TRAN analysis

Ø Derivation of the voltage gain for Circuit 3.

Your designs should not incorporate series and parallel resistors to meet the voltage gain specifications. It is more desirable to come close and use the exact numbers in your circuit and simulation.

PROCEDURE

Refer to the mA741 data sheet on the class WEB page uA741.pdf. Observe, you are using the 8-pin DIP (Dual-Inline Package) Second package style from the top. Also note that the mA741 has certain requirements with respect to allowed resistance values. All resistors in your design must be greater than or equal to 2 kW. You do not need to include the 10 kW offset voltage potentiometer initially in your circuits for the first three circuits. Use ± 12 volts for the power supplies. Verify that the polarities are correct or you will create a classic embarrassing odor.

Your designs should be supported analytically and by SPICE simulation results. You should record all key oscilloscope waveforms on your flash drive for possible inclusion in your laboratory report.

1. For Figure 1. Design and test an inverting amplifier with a low-frequency voltage gain of 20 dB.

Ø Use a .TRAN analysis with a 500 mV zero-peak, 1 kHz sinusoidal input voltage. The input voltage level is not critical as long as you do not observe clipping on your output waveform.

Ø Experimentally verify your design and simulation results in the time domain.

Ø Experimentally determine the input signal level when “clipping” of the output waveforms occur.* Does the simulation using a linear circuit operational amplifier show this clipping? Explain. Compare to the simulation using the library model in SPICE.

Ø Measure and plot 20 log|A(jf)|, voltage gain as a function of frequency, and q(jf), which is the phase shift as a function of frequency, through the amplifier circuit, and compare your results with the SPICE .AC simulation. Extend your measurements to a few hundred kHz if you can. Plot the results as you take your measurements.

Ø In your SPICE .AC simulation using the linear model or the generic op amp model in the .LIB file, place a capacitor between the inverting node and the voltage-controlled generator node of such a value that the simulation matches the experimental measurement of the 20 log|A(jf)| plot reasonably well. How does your model compare with the SPICE library 741 model? As we have discussed in class, the mA741 model simulation we did in class was frequency independent because there was no capacitor or inductor in the model or the rest of the circuit.

 * You should do this in the time domain and observe the transfer characteristic. In order to see the transfer characteristic on the digital oscilloscope, you will need to change the display to “XY” mode. Push the “Display” key and select “XY Display” from the menu. Switch to “Triggered XY” mode. You may use the scale controls to adjust the axes accordingly. Also verify your voltage gain and phase shift measurements using the transfer characteristic. Do not overdo the clipping because if your input becomes too large, you will damage the mA741.

Figure 1 Inverting Operational Amplifier Circuit

2. For Figure 2. Design and test a non- inverting amplifier with a low-frequency voltage gain of 20 dB.

Ø Use a .TRAN analysis with a 500 mV zero-peak, 1 kHz sinusoidal input voltage. The input voltage level is not critical as long as you do not clip your output waveform.

Ø Experimentally verify your design and simulation results in the time domain.

Ø Measure 20 log|A(jf)| and q(jf) and compare your results with the SPICE .AC simulation. Extend your measurements to several hundred kHz. Plot your results as you collect the data.

Figure 2 Non-Inverting Operational Amplifier Circuit

3. Refer to circuit diagram given below

Figure 3 Another Inverting Operational Amplifier Circuit

(a) Derive the voltage gain Vo/Vs transfer function using summing point constraints. This is best done as part of your prelab.

(b) Use all 10 kW resistors. Verify experimentally and using SPICE, the voltage gain at 1 kHz . Use both a time domain and transfer characteristic representation of your work.

4. Offset Voltage Measurement

(a) Construct Circuit 1 as shown below such that the R2/R1 is nominally 100. Use the DMM to measure the dc voltage at the output terminal. The dc offset voltage for the operational amplifier is computed by Vo/100 = Vos from your measurements. Compare your results with the data sheet distributed in class. As before, use ±12 volts for the power supplies and all resistors > 2 kW. Use good lead dress and wiring practices.

(b) Refer to Circuit 2. Connect the potentiometer between Pins 1 and 5 with the center terminal to -12 volts; same as the negative power supply voltage. Adjust the potentiometer such that Vos is zero or as close to zero as you can obtain. How low can you go and how does this compare to the specifications? Why is this ability to “zero out” the offset voltage useful in a system design?

To Think About:

Did the basic operational amplifier model work well in your SPICE simulations. Do the .TRAN and .AC simulations agree with measurements? This lab seems long but if you come prepared with your circuit designs all ready to go, it proceeds well.

Observe an unsafe duplex outlet-no ground pin!

Some suggestions for wring reports.

And in due deference to the iPhone5 release last week.