EE 2212

Spring 2013

7 and 14 February 2013

Experiment 2: Operational Amplifier Circuits

Report Due: 21 February

Note 1: This will be a two-week experiment (7 and 14 February). I will suggest a natural break point but you are encouraged to proceed at your own pace. It will be graded on a 40-point scale with double the values on the 20-point rubric.

Note 2: The afternoon lab section on 7 February will end at about 2:30 because I will have to leave. Sukriti will be at the UM Career Fair in the Cities and will be unable to be present after I leave at 2;30.

Note 3: I will provide an overview of the op amp SPICE models at the beginning of the lab.

PURPOSE

Week One

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.

You can continue into Week 2 tasks as appropriate.

Week Two

To implement the designs of:

Ø An active analog Low-Pass Filter (LPF)

Ø An active analog High-Pass Filter (HPF)

Ø A Wien Bridge Oscillator

Ø A Phase Shift Oscillator

PRELAB FOR WEEK ONE

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 with standard value components and use the exact numbers in your circuit and simulation.

PROCEDURE

Refer to the mA741 data sheet on the class WEB page uA741.pdf, also distributed in class. 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 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 Use a transient 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 input voltage to observe 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 transient 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.

To Think About:

Did the basic operational amplifier model work well in your SPICE simulations. Do the transient and AC simulations agree with measurements?

PRELAB FOR WEEK TWO

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, the corner frequency is computed from and the low frequency voltage gain is given by and for the High-Pass Filter, 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 1, Problem Sets and the class notes. Include the derivations in your notebook.

PROCEDURE

Refer to the mA741 data sheet. Observe, again that you are using the 8-pin DIP. You do not need to 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, High Pass and Band Pass filter designs should be supported analytically and by SPICE simulations. Use the library model for the mA741. Always look at your output waveforms experimentally to insure you are not clipping. Explain why you will observe clipping when you use the mA741 while performing a transient simulation and you will not observe clipping when you use the generic op amp model which consists of only a voltage-controlled generator.

1. 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 3 to 5 kHz. Do not use series and parallel capacitor combinations or series and parallel resistor combinations . Use standard values that yield a corner frequency and voltage gain reasonably close to the specifications. The theory of operation was discussed during the 4 February class.

Ø Experimentally verify your design and simulation results.

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

2. Design and test a high-pass filter with a high-frequency voltage gain of 20 dB and a 3 dB corner frequency in the range of 100 Hz to 500 Hz. Do not use series and parallel capacitor combinations or series and parallel resistor combinations. Use standard values 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 q(jf) and compare your results with the SPICE AC simulation over a similar range.

3. Construct the following circuit which is similar to what is shown in Figure 12.45 on page 755 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 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 we get to 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 . I like to compare an oscillator starting with the howling noise you have all heard in a public address system when the microphone is in the speaker sight range. To show this in a SPICE simulation, add an initial condition of several volts to each of the capacitors and then use a .TRAN analysis that extends for several periods of the expected frequency output. The signal growth is kind of cool to watch during the simulation.

4. Construct the following circuit similar (but not exactly like) to what is shown in Figures 12.47 and 12.48 on page 756 of the text. Monitor V_o(t) using your oscilloscope. Observe there is no input signal. This is called a Phase Shift Oscillator. Explain why this is a useful circuit. (Note depending upon the resistor tolerances, you may have to increase your value of R1). Compare the observed frequency of operation to the equation, and the voltage gain required setting established by As with the Wien Bridge oscillator SPICE simulation, add an initial condition of several volts to each of the capacitors and then use a .TRAN analysis that extends for several periods of the expected frequency output. Again, it is interesting and fun to watch the signal growth as a function of time.

Observe an unsafe duplex outlet-no ground pin!

Some suggestions for writing laboratory reports.

For those of you who are “trekies” i.e. fans of Star Trek