Operational Amplifiers

Transcription

1 Operational Amplifiers Reading Horowitz & Hill handout Notes, Chapter 9 Introduction and Objective In this lab we will examine op-amps. We will look at a few of their vast number of uses and also investigate the limitations of these almost too good to be true devices. Op-amps are integrated circuits (IC): the transistors, resistors, diodes, capacitors, etc. that compose the op-amp are all formed on a single silicon chip. We will not worry about the inner-circuitry, although the basic circuits are not too difficult to understand and are discussed in section 9.5 of Simpson and chapter?? of Horiwitz and Hill. (Note: a possible final project is constructing opamps from transistors, etc. and comparing performance to an IC). We will treat the op-amps as black boxes. However, remember that op-amps are active devices since they are made of transistors, and they must be connected to power supplies to function properly. We will use the ±12 V supplies that we used last week. Every circuit that we have seen with the transistor the follower, current source, and voltage amplifier as well as a host of other circuits can be vastly improved using op-amps. The key idea behind op-amps is to use negative feedback to stabilize circuit performance. Negative feedback is perhaps the most important concept in modern electronics. Without it none of our electronic instruments from computers to stereos would work nearly as well. If you learn one thing only this whole class, it should be an appreciation of negative feedback. As with transistors we will introduce a very simple model to analyze op-amp circuits. But once again be mindful that the simple model is an approximation and has limitations in real circuits. Here are the Golden Rules for ideal opamps wired with negative feedback: 1. The output attempts to do whatever is necessary to make the voltage difference between the two inputs zero. 2. The inputs draw no current (i.e., they have huge input impedance). That s it. If we carefully use these rules, we will be able to analyze all the circuits in this lab and many others as well. The main objective of this lab is to see how this model explains several important op-amp circuits. But we will also learn about the limitations of the model. Try to view this lab not only as a learning experience about op-amps, but also as a practice exercise in making precise measurements with your oscilloscope. Think hard about each procedure and the function of each of the steps you are asked to carry out. Make sure that your lab notes demonstrate the insights that you glean from the lab.

2 Laboratory Exercises In the exercises below, use an LF411 op-amp unless otherwise directed. Its low offsets, high speed, and low cost make the LF411 an excellent all-purpose opamp. The pin diagram for the dual-in-line package (which we will be using) is shown on the data sheets included with this lab. Make sure that you understand how to connect the op-amp to the breadboard and how to properly power it. Note that the 741 op-amp, which used to be the industry standard, has the same pin diagram as the Inverting Amplifier Wire up an inverting amplifier (shown at right) with Gain=-10 (R 2 =10k, R 1 =1k). Apply a 5 khz sin wave input signal of 0.5 Volt amplitude. Sketch the output and verify the gain. Increase the signal to 2 V. Sketch the output and explain. V in R 1 1. Measurement of Slew Rate The slew rate is the maximum rate at which the output voltage can change. It limits the op-amp s capacity to perform ideally according to the Golden Rules. In a plot of voltage vs. time (such as an oscilloscope trace) the slew rate is the slope in units of Volts per microsecond). To measure the LF411 slew rate, use the unity-gain voltage follower circuit shown below. Drive the input with a square wave with amplitude < 1V. Use the oscilloscope to simultaneously observe the input square-wave and the op-amp output. Measure the slew rate by determining the slopes of the rising and falling edges of the output. Are these edges equally distorted? See what happens as the input amplitude is varied. Compare your measurements with the 411 specifications given on the attached data sheet. Repeat these measurements using a 741 op-amp. Since the pin connections for the 741 are the same as for the 411, you can simply exchange op-amps without changing anything else in your circuit. How does the 741 s slew rate compare to the 411? Which op-amp performs better? - + R 2 V out 2. Measurement of Offset Voltage

3 Build the inverting amplifier with gain of 1000 as shown on the next page. With both inputs grounded, the amplifier s large gain will amplify the input offset voltage to a measurable level. This offset voltage is another limitation to ideal opamp operation. Compare your measured offset voltage with the data found on the attached data sheet. Add the 10 k pot as shown in the data sheet s Typical Connection diagram and adjust the pot until the offset voltage is zero. This shows you how to zero out any input offset voltage (and the purpose of pin 5), but for most applications the effect is small enough to ignore. 3. Gain & Phase Shift as a Function of Frequency Build the inverting amplifier with unity gain and input impedance of 1 k (so as not to load the function generator). Input a sine-wave signal at 10 khz. Simultaneously observe the input and output waveforms on the scope, measuring the gain and phase shift. Do a reasonable job estimating the phase shift a little or a lot is not a sufficient estimate of the phase shift. Now vary frequency so that you can find the frequency regime in which the gain rolls off. Make measurements of the gain and phase shift at a few frequencies below and above the roll off frequency. Are your results consistent with the idea that to produce an undistorted sine wave of frequency f Hz and amplitude V 0 volts requires the op-amp s slew rate to be at least 2πfV 0 volts per second? Plot your data. Repeat the above with an amplifier gain of 10. Take care to choose an appropriate peak-to-peak input amplitude so that the output is not forced into saturation.

4

5 4. Photodiode Amplifier Construct circuit 1, a simple way to get the signal from a photodiode. Figure out which lead of the photodiode has to be connected to the positive voltage supply (if you connect it the wrong way no current will flow). Measure the voltage across the resistor with your X10 probe to verify the photodiode works. What makes the signal large, what makes it small? Roughly, what is the max output voltage? Now connect the photodiode to an op amp to amplify the signal, as shown in circuit 2. Wire up a low pass filter to the output of the op amp. Pick the R and C of the filter to effectively attenuate 100 khz signals. (There will be some high frequency noise on the signal that we d like to eliminate). Measure the DC value of V out? How does it compare to the first circuit? Let s make the photodiode mobile. Use alligator leads to attach the photodiode to the circuit. Point the photodiode at the lights. What s V out? Is it a steady DC value or is there some ripple? To focus on the ripple, switch the input coupling on the probe channel to AC this filters out the DC part. Now you can turn down the Volt/Div to magnify the signal. Also have your Time/Div knob set to 2 ms/div. What is the amplitude of the AC signal? Roughly what frequency is it? What s generating this signal? Return the input coupling to DC and set the volts/div to 0.5 and the time/div to 0.5 ms. Zero the trace near the top of the screen. Test how the photodiode responds to different colors of light (use LEDs from around the room). Which generates a larger response? Now hold the photodiode close to the screen of the oscilloscope and observe what happens. What s going on? Describe it and sketch a few traces in your lab book. The oscilloscope is now part of the feedback of the op amp circuit. 5. Driving a speaker. When building these circuits with a speaker, be sure to turn off the power on your protoboard while you build the circuits. Only after you have the circuit completely wired and checked should you turn on the power. This will protect the speaker (and also the op amp) from nasty signals. Build a unity gain inverting op amp circuit with 1k input impedance. Attach a speaker to the output. Find the resistance and power rating of the speaker and record them in your lab book. (You could verify the resistance with the help of a DMM). Use the Function Generator to input a 0.5 V amplitude, 1 khz signal. Monitor the input and the output voltage on the oscilloscope. Does the speaker work? Can you hear it? Play around with the frequency and the amplitude of the signal. Be careful not to increase the amplitude too much you might destroy the speaker! At what input voltage does the output voltage begin to clip? How does this make the speaker sound? Are you surprised that the op amp is clipping at this voltage? Disconnect the speaker from the output and monitor the output

6 voltage; does it still clip at the same input voltage level? What voltage can the output go to without the speaker connected? What do you think is going on here? Now connect the output of the op amp to a transistor, as shown in circuit 3. What kind of filter does the capacitor and resistor form, and what is f 3dB? At what voltage is the base of the transistor biased? Monitor the signal at the base and at the emitter. Again drive the circuit with an input signal with 0.5 V amplitude at 1 khz. How does the loudness of the speaker now compare to the circuit with just the op amp? Play around with the amplitude (again being careful not to drive it too loudly) and see what it sounds like. Compare the waveforms to what you hear. What appears to be the advantage of adding the transistor? Find a photoresistor from the rack of electronic components. Use a DMM to measure the resistance of the device as you cover and uncover it. What does a photoresistor do? Replace the 1k resistor at the input to the op amp with the photoresistor. Now you can control the loudness with your hand! 6. Constant Current Source Time to revisit the constant current source and see how an op-amp and negative feedback do a better job than the circuit with only a transistor that we built in the last lab. The circuit below is a typical design for a constant current source (note the PNP transistor). Do not use the values of the resistors shown. Instead, determine values of the divider resistors and collector resistor that would provide a constant current of 6.0 ma. Build and test how constant the current is by varying R Load. Calculate the maximum allowed value of R load. References Significant parts of this lab, including diagrams, are taken from a lab written by John Essick of Reed College.

7 A diagram for this lab is taken from Hayes and Horowitz, Student Manual for the Art of Electronics, Cambridge University Press (1989). Simpson, Introductory Electronics for Scientists and Engineers 2 nd Ed, Prentice Hall (1987).