EE 100 Electrical Engineering Concepts I. Lab 9 Revision 11/01

Transcription

1 EE 100 Electrical Engineering Concepts I Lab 9 Revision 11/01 Name: Partner: Date: TA: Op Amp Basics Gain/Bandwidth Relationship Op Amp Personalities Square Wave Response Now that s an amplifier I can trust! Would you say the gain is 5 here?

2 EE 100 Lab 9 Grading Sheet Lab Grade: (90 maximum) Presentation Grade: (10 maximum) (organization, clarity, neatness) Total: (100 maximum) Grader s Comments: Hand in all lab and work sheets, either stapled securely or in a folder.

3 EE 100 Lab 9 Page 1 1 Laboratory Exercise 1.1 Purpose We will build some op amp amplifiers using both a BJT and a MOSFET op amp, and compare ideal to actual performance as we increase input voltage and frequency. 1.2 Related Reading Textbook: Ch Equipment 1) Knight board 2) Oscilloscope 3) Function generator 4) Digital multimeter 5) Instrument leads and jumper wires 6) Resistors: 10 kω, 100 kω (Qty. 2) 7) One 1458 Dual Op BJT Amp 8) One TL082 Dual FET Op Amp 1.4 Introduction In our remaining labs, we will be working a variety of integrated circuit (IC) chips. To make connections to the IC pins, we set the IC into the breadboard so that it bridges the gap as shown in Fig The IC s will also require power; for the op amps the +12 and 12 VDC supplies to the left of the breadboard will provide the power. Fig. 9-1 In this lab, we re using two dual op amp IC s: the 1458, which uses BJTs and which is similar to the classic 741 op amp; and the TL082, which is a more modern MOSFET op amp. The pinout of both devices is identical, and is shown in Fig. 9-2.

4 Page 2 Lab 9 EE100 Fig. 9-2 To find pin 1, orient the device so that the leads are pointing down into the breadboard. The op amp will either have a round dot in the corner next to pin 1 or an indentation at the top between pins 1 and 8. Fig. 7-3 can then be used to locate the other pins. Remember, when hooking up op amp circuits, don t forget to hook up +12 VDC to V+ (pin 8) and 12 VDC to V- (pin 4). The op amp output will be quite boring without power! 1.5 Experimental Data and Results Inverting Amplifier Bandwidth Set up the following circuit USING THE 1458 OP AMP. You may want to make your op amp connections in the holes away from the IC, to allow space for your fingers when we remove the chip later in the lab. Figure 9-3

5 EE 100 Lab 9 Page 3 The 100 KΩ and 10 KΩ resistors supplied have a 5% tolerance, so you should measure yours to get a more accurate fix on what your theoretical gain should be. 100K resistor measures 10K resistor measures What is your theoretical gain? Set your FG to generate 1 khz sine waves, 2 V p-p. Set your oscilloscope to trigger off Ch. A1 and display about 4 cycles on the screen. Set both channels for the same V/div so the amplifier gain will be easy to see. Save this comparison of Ch. A1 and A2, and hand in as a plot labeled Plot 9-1. Using your o-scope cursors, determine the actual voltage gain of your amplifier. Gain = Now, set Ch. A2 (op amp output) to 5 V/div and increase the FG amplitude until you begin to see clipping in the output. Save the display of the clipped signal, and hand in as Plot 9-2. Explain why the clipping occurs.

6 Page 4 Lab 9 EE100 Now, let s see how the gain varies with the frequency of the input signal. Return your FG signal to 2V p-p, and fill in the following table using the scope cursors to measure V P-P. V in should always be about 2V p-p. If your high frequency displays seem jumpy with a Normal Trigger, switch to Single Trigger mode. Frequency, khz 1 Vin, p-p Vout, p-p Amplifier Gain The bandwidth of your amplifier is defined as the frequency at which the power level drops in half. We ve seen that since the power into a resistive load on the output of the amplifier is proportional to the square of the voltage, this corresponds to a drop in voltage gain of At what frequency does your amplifier output drop to of its value at 1 khz? This is your amplifier bandwidth: khz Above this frequency, your output waveform loses amplitude and shape. Set your FG frequency to twice the amplifier bandwidth recorded above. Reset your scope sample rate so 4 or 5 cycles are displayed. Save the display and hand in plot labeled as Plot 9-3.

7 EE 100 Lab 9 Page 5 Now, the bandwidth of the amplifier is primarily limited by how fast its output can swing. The required swing speed, or slew rate, of the amplifier goes up with either frequency or gain. Let s see if we can increase amplifier bandwidth by decreasing gain. We ll cut the 100 kω resistor to 50 kω by simply putting another 100 kω resistor in parallel with it. First, measure the second 100 kω resistor to get an exact value: Second 100 kω resistor measures What should the new amplifier gain be once this resistor is in parallel with the first resistor? HINT: First compute the equivalent feedback resistance presented by the two 100 kω resistors. Expected Gain Install your second 100 kω resistor in parallel with your first, and reexamine gain versus frequency. Make sure your FG signal is still 2V p-p, and fill in the following table: Frequency, KHz 1 Vin, p-p Vout, p-p Amplifier Gain

8 Page 6 Lab 9 EE100 Now what is your amplifier bandwidth? Amplifier bandwidth: KHz Comment on the statement Gain-Bandwidth is Constant for an op amp. Now, let s use an op amp with a little more performance. DON T UNPLUG WIRES FROM YOUR BREADBOARD. Since the device pinouts are the same, simply power the Knight board down, and carefully unplug the 1458 op amp. Plug the TL082 op amp into the same holes, keeping pin1 in the same orientation as you did earlier. Power the Knight board back up and you should be in business with an inverting amplifier with a gain of around -5. The TL082 is a FET op amp, and has a faster slew rate than the 1458, so it should have higher bandwidth. Let s reexamine gain versus frequency. Make sure your FG signal is still 2V p-p, and fill in the following table: Frequency, KHz Vin, p-p Vout, p-p Amplifier Gain

9 EE 100 Lab 9 Page 7 Now what is your amplifier bandwidth? Amplifier bandwidth: KHz Comment on the TL082 performance relative to the Square-wave Performance If we believe Fourier, square waves contain much higher frequencies than a simple sinusoid at the same frequency. The loss of high frequencies in square waves causes overshoot, undershoot, and ringing. Let s explore these effects a bit. Switch your FG to generate square waves at 2V p-p. You can stick with the TL082 op amp with a gain of about 5. On the next page, sketch the input and output waveforms at the indicated frequencies. Explain what is going on here. What bandwidth would you say your amplifier has for square waves?

10 Page 8 Lab 9 EE100 Frequency Input/Output Sketch 10 KHz 20 KHz 50 KHz 100 KHz 200 KHz