EE 2212

Fall 2014

18 and 25 September 2014

Experiment 2: Operational Amplifier Circuits

Report Due: 2 October

Note 1: This laboratory extends over two weeks. I will suggest a natural break point after Week 1 but you are encouraged to proceed at your own pace. The laboratory will be graded on a 40-point scale with double the values on the 20-point rubric. This also means you are allowed a maximum of six pages not including the cover page.

Note 2: I will provide an overview of the two op amp SPICE models at the beginning of the laboratories on 18 September.

PURPOSE

Week One

To implement the designs of:

Ø An inverting operational amplifier

Ø A non-inverting operational amplifier.

You can continue into Week 2 tasks as appropriate. You do have enough background to proceed.

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

GENERAL COMMENT

Run SPICE time and frequency domain programs for both circuits. Refer to Experiment 1. Use both the μA741 model in the eval.slb library and the linear model generic OPAMP in the analog.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 program in order to complete this lab:

Ø TRANSIENT analysis for a sinusoidal input

Ø AC analysis including amplitude and phase as a function of frequency.

Your designs must 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 measured numbers in your circuit simulation.

PRELAB FOR WEEK ONE

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. You might refer to your EE 2006 notes and labs since you have worked with op amps in that course.

Ø Derive the voltage gain Vo/Vs for Circuit 3 using summing point constraints.

PROCEDURE

Refer to the mA741 data sheet on the class WEB page uA741.pdf, also on the class WEB page. 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. Do not need to 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.

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 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 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 few tens of kHz. Plot the results as you take your measurements.

Ø Reset your input frequency to 1 kHz and observe the transfer characteristic. The transfer characteristic is Vout vs. Vin. In order to see the transfer characteristic on the digital oscilloscope, you will need to change the display to “XY” mode. Select 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. Note the negative slope indicative of the low frequency 180°of phase shift. 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 Set the input frequency to a 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), the voltage gain in dB, | and q(jf) and compare your results with the SPICE AC simulation. Extend your measurements to several tens of kHz. Plot your results as you collect the data.

Ø Observe the transfer function and verify the voltage gain and low frequency phase shift from the slope at 1 kHz.

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 also a good exercise in the use of nodal analysis. 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. Frequency response measurements are not required.

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 and the class notes from Monday, 15 September. 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 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 and High Pass 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 were discussed during the 15 September 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. So far, all of the circuits we have studied employ negative feedback. The following circuit 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. 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 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 and then use a transient analysis that extends for several periods of the expected frequency output. The signal growth is kind of cool to watch during the simulation.

Some suggestions for writing laboratory reports although not part of the rubric.

For those of you who are “trekies” i.e. fans of the vintage Star Trek television series and have a “smartphone”. NETFLIX has all of the original episodes which beats my pile of vintage VHS video cassettes. I also noted an article on CNN http://www.cnn.com/2014/09/03/tech/innovation/tricorder-x-prize-finalists/index.html that I thought was interesting. I suppose the newly announced Apple Watch is heading in the same direction.