EE 2212

Fall 2021

30 September and 7 October

Experiment 3: Operational Amplifier Circuits

Note 1: This will be a two week experiment.

Note 2: The report will be due Thursday, 14 October

Note 3: The two week experiment will be evaluated on a 40 point rubric (double all values on the 20 point rubric document.

Note 4: You are limited to six pages, besides the title and abstract page.

Note 5: I will provide an overview of the op amp SPICE models for both time and frequency domain simulation.

Note 6: I have divided this experiment into a non-frequency dependent and frequency dependent set of circuits

Note 7: Circuit wiring neatness will really pay off in getting quality and consistent experimental results.

PURPOSE (Non-Frequency Dependent)

To implement the designs of:

Ø Two versions of an inverting operational amplifier (Figures 1 and 3)

Ø A non-inverting operational amplifier (Figure 2)

GENERAL COMMENTS

Run the SPICE time-domain simulation with a VSIN generator and the frequency-domain simulation with a VAC generator. Use the μA741 model in the eval.slb library. Print the waveforms of the inputs and outputs on the same set of axes. You will need the following information from your SPICE simulations in order to complete this lab:

Ø TRANSIENT analysis for a sinusoidal input

Your hardware realizations designs should minimize the use of series and parallel resistors to meet the voltage gain specifications. It is more desirable to come close with standard value components and use the exact measured numbers in your circuit simulation.

PRELAB

Ø Specify the component values to meet the indicated specifications for Circuits 1 and 2 . You should come to the lab with a list of the components you will need to meet the specifications.

Ø The derivation, in your notebook, of the voltage gain Vo/Vs for Circuit 3 using summing point constraints. This is also a good exercise in the use of nodal analysis. (Look at the R2, R3, R4 node)

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. This package is also sometimes called the MINIDIP. Also note that the mA741 has certain requirements with respect to allowed resistance values that includes all resistors in your design must be greater than or equal to 2 kW. Do not include the 10 kW offset voltage potentiometer.

Use ± 12 volts for the power supplies. Verify that the polarities are correct or you will create a classic embarrassing odor not correctable with Old SPICE (pretty good pun!) body wash.

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

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

Ø Start with a 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 occurs.*

Ø Observe the resultant transfer characteristic. The transfer characteristic is a plot of Vout versus Vin. In order to see the transfer characteristic on the oscilloscope, you will need to change the display to “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. Note the negative slope is indicative of the low frequency 180°of phase shift in an inverting amplifier.

Ø Measure and plot the voltage gain in dB 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 10 kHz or so. Plot the results as you take your measurements. Note that if the Greek (Theta) q(jf) printed out as q(jf), your WEB browser and/or word processing program does not translate symbol font correctly.

*Go slow in increasing the amplitude of Vs! Do not overdo the input voltage to observe clipping because if your input becomes too large, you will damage the mA741 and create embarrassing odors.

Figure 1 Inverting Operational Amplifier Circuit

2. For Figure 2. Design and test a non- inverting amplifier with a low-frequency voltage gain of 14 dB.

You are essentially repeating the procedure for Figure 1.

Ø Start with a 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.*

Ø Observe the transfer characteristic. The transfer characteristic is a plot of Vout versus Vin. In order to see the transfer characteristic on the oscilloscope, you will need to change the display to “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. Note the positive slope indicative of the low frequency 0° of phase shift.

*Go slow in increasing the amplitude of Vs! Do not overdo the input voltage to observe clipping because if your input becomes too large, you will damage the mA741and create embarrassing odors.

Figure 2 Non-Inverting Operational Amplifier Circuit

3. Another Inverting Amplifier Configuration. Refer to Figure 3.

Figure 3 Another Inverting Operational Amplifier Circuit

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. Frequency response measurements are not required.

Hint: The voltage gain should be -3 (About 9.54 dB) from your PRELAB derivation that I expect to see in your notebook.

Purpose (Frequency Dependent-Analog Active Filters)

To simulate and implement the designs of:

Ø An active analog Low-Pass Filter (LPF)

Ø An active analog High-Pass Filter (HPF)

Ø An active Band-Pass Filter (BPF)

Ø A Wien Bridge Oscillator

GENERAL COMMENTS

Run SPICE frequency domain simulations with a VAC generator programs for the LPF, HPF, and BPF. Use the μA741 model in the eval.slb library. You will need the following information from your SPICE program in order to complete this lab:

Ø AC analysis including amplitude as a function of frequency from around 10 Hz to at least 10 kHz.

Ø TIME DOMAIN ANALYSIS IS NOT REQUIRED!

PRELAB

Use your design for the inverting operation amplifier from Figure 1, as a basis to implement your designs of the LPF and a HPF. Design the Low Pass and High Pass Filters to meet the indicated specifications. You should come to the lab with a list of the components you will need to meet the specifications. For the Low-Pass Filter, Figure 4, the corner frequency is computed from and the low frequency voltage gain is given by and for the High-Pass Filter, Figure 3, and the high frequency voltage gain is given by . The derivation of the corner frequencies follows that of the passive RC filter circuits from Experiment 2 and in-class discussions. We will also discuss more at the beginning of the lab period. Include the derivations in your notebook.

PROCEDURE

Refer to the mA741 data sheet. Observe, again that you are using the 8-pin DIP. Do not include the 10 kW offset voltage potentiometer. All resistors must be at least 2 kW. Use ± 12 volts for the power supplies. Your Low Pass and High Pass designs should be supported analytically and by SPICE simulations. Use the SPICE library model for the mA741. Adjust your input levels to avoid clipping.

1. ANALOG ACTIVE LOW-PASS FILTER

Design and test an low-pass filter with a low-frequency voltage gain of 20 dB and a 3 dB corner frequency in the range of 2 to 4 kHz, Figure 4. Do not use series and parallel capacitor combinations or series and parallel resistor combinations . Use standard values from the parts cabinet that yield a corner frequency and voltage gain reasonably close to the specifications.

Ø Experimentally verify your design and simulation results.

Ø For verifying low-pass filter operation, measure 20 log|A(jf)| and compare your results with the SPICE AC simulation over a similar range.

Figure 4 Low Pass Filter

2. ANALOG ACTIVE HIGH-PASS FILTER

Design and test a high-pass filter, Figure 5 with a high-frequency voltage gain of 14 dB and a 3 dB corner frequency in the range of 50 Hz to 200 Hz. Do not use series and parallel capacitor combinations or series and parallel resistor combinations. Use standard values from the parts cabinet that yield a corner frequency and voltage gain reasonably close to the specifications

Ø Experimentally verify your design and simulation results.

Ø For verifying high-pass filter operation, measure 20 log|A(jf)| and compare your results with the SPICE AC simulation over a similar range.

Figure 5 High Pass Filter

3. ANALOG ACTIVE BAND-PASS FILTER

Now cascade the output of the HPF with the LPF (Figure 6) and note the band pass characteristic. Measure 20 log|A(jf)| and compare your results with the SPICE AC simulation over a similar range. The center of your filter design will peak near 34 dB or about |50|. You will have to adjust your input level to avoid clipping.

Figure 6 Band Pass Filter

4. WIEN BRIDGE OSCILLATOR

So far, all of the circuits we have studied employ negative feedback. The following circuit, Figure 7, employs positive feedback; and as mentioned in class, an audio example of positive feedback is the “howl” observed when the microphone and speaker are not placed well in an auditorium and you have constructive (additive) signals. Construct the following circuit which is similar to what is shown in Figure 12.43 on page 741 of the text. At first glance, the circuits look different but they are the same. You are generating a signal source, that is you are demonstrating the operation of an oscillator. Observe that there is no external signal generator!!!! Monitor v_o(t) using your oscilloscope. Observe there is no input signal. This is called a Wien Bridge Oscillator. Explain why this is a useful circuit. (Note depending upon the resistor tolerances and circuit losses, you may have to increase your value of R2 somewhat; perhaps as high as 33 kΩ). Lead dress has an impact on the circuit performance. Compare the observed oscillating frequency of operation to the equation, and the voltage gain required setting established by.The SPICE simulation approach is interesting and I will demonstrate this when your group reaches that part of the lab. In a real circuit, an oscillator starts through random noise which provides an initial signal with the correct phase shift to obtain positive feedback . To show this in a SPICE simulation, add an initial condition of several tenths of a volt to each of the capacitors as an initial condition and then use a transient analysis that extends for several periods of the expected frequency output. The exponential signal growth is kind of cool (at least I think so) to watch during the simulation. The simulation makes you a believer in exp(+αt) DFQ solution from EE 2006!