Operational Amplifier Circuits

Transcription

1 ECE VIII. Basic 5 Operational Amplifier Circuits Lab 8 In this lab we will verify the operation of inverting and noninverting amplifiers constructed using Operational Amplifiers. We will also observe the frequency response of these circuits. The frequency response of a circuit answers the question, How does the gain of a circuit vary with frequency? All amplifiers have frequency limitations. Use the 74 OPAMP (and no other) in the lab and in PSpice. The UA74 is equivalent to the LM74. VIII.A. PreLab We will be using the circuits of Figure VIII and Figure VIII. These are inverting and noninverting configurations. We will be investigating each circuit for three different gains. You will have six different designs.. Design the inverting amplifier of Figure VIII to have gains of, 0, and 00. The smallest value of R F you should use is0 kω. You will have separate circuits. With the available resistor values you may not achieve these gains exactly. Come as close as possible while only using two resistors. Rf R 7 4 V V 74 VEE OS OS U 6 5 VEE DC = 5 DC = 5 Figure VIII: Inverting amplifier.. Design the noninverting amplifier of Figure VIII to have gains of,, and 0. The smallest value of R you should use is 0 kω. You will have separate circuits. Note that for a gain of, no resistors are required. A noninverting amplifier with a gain of is also referred to as a buffer. With the available resistor values you may not achieve these gains exactly. Come as close as possible while only using two resistors. U 7 V 74 4 V VEE OS OS 5 6 DC = 5 DC = 5 R VEE R Figure VIII: Noninverting amplifier.. Read Sections 5.E and 5.F in the PSpice manual. VIII

2 4. Obtain plots of gain versus frequency for all 6 circuits. Plot the gain in Decibels versus frequency for frequencies from Hz to 00 MHz. To plot gain in db, add the trace db(v()) instead of V(). Use the cursors or a goal function to find the low frequency gain and the db frequency for each value of gain. The db frequency is the frequency where the gain is down by db from its maximum value. Remember that gain is the ratio ut /, and gain specified in db is 0log 0 (ut / ). Fill in the tables below. Note than when calculating below, the bandwidth is the db frequency and the gain is not in db. Table VIII: NonInverting Amplifier PSpice Simulation Results PSpice Pspice ( / ) db ,584 Table VIII: Inverting Amplifier PSpice Simulation Results PSpice Pspice ( / ) db 0 00 Note that when you calculate, the gain is not in db. VIII.B. NonInverting Amplifier Laboratory Measurements Wire the circuit of Figure VIII. VIII.B.. Use the resistors you chose to achieve a gain of.. Let V IN be a khz sine wave. Set its amplitude so that is volt peaktopeak. Obtain a scope plot that shows that the gain is and that the output is in phase with the input. The plot should use as much space in the scope window as possible.. We will now measure the small signal frequency characteristics of this circuit. Make sure that is volt peaktopeak or smaller. If the output is not an undistorted sine wave, reduce the input until the output is an undistorted sinewave. is only measured with small signal outputs. Fill in the Table VIII. Note that Table VIII contains several rows where the frequency has not been specified. You must fill in all rows of this table and choose frequencies that will make a nice looking plot. Voltages can be measured as either peaktopeak or magnitude (center to VIII

3 peak). Make sure you specifically measure the db frequency and highlight it in the table. Plot the measured results using MATLAB. On this plot, use MATLAB to place a red star at the predicted db frequency. You can do this with the command: semilogx(x,y,e6,, r* ) This command plots the measured data contained in variables x and y, and then also places a star at coordinates y = db, and x = MHz. You will need to change the coordinates of the star for your calculations. Table VIII: NonInverting Amplifier 0 00,000 0,000 00,000 db,000,000 VIII.B.. Use the resistors you chose to achieve a gain of.. Let be a khz sine wave. Set its amplitude so that is volt peaktopeak. Obtain a scope plot that shows that the gain is and that the output is in phase with the input. The plot should use as much space in the scope window as possible.. We will now obtain the small signal frequency characteristics of this circuit. Make sure that is volt peaktopeak or smaller. If the output is not an undistorted sine wave, reduce the input until the output is an undistorted sinewave. Fill in Table VIII4. Voltages can be measured as either peaktopeak or magnitude (center to peak). Make sure you specifically measure the db frequency and highlight it in the table. Plot the measured results using MATLAB. On this plot, use MATLAB to place a red star at the predicted db frequency. VIII

4 Table VIII4: NonInverting Amplifier 0 00,000 0,000 db 00,000,000,000 VIII.B.. 0 Use the resistors you chose to achieve a gain of 0.. Let be a khz sine wave. Set its amplitude so that is volt peaktopeak. Obtain a scope plot that shows that the gain is 00 and that the output is in phase with the input. The plot should use as much space in the scope window as possible.. We will now obtain the small signal frequency characteristics of this circuit. Make sure that is volt peaktopeak or smaller. If the output is not an undistorted sine wave, reduce the input until the output is an undistorted sinewave. Fill in the Table VIII5. Voltages can be measured as either peaktopeak or magnitude (center to peak). Make sure you specifically measure the db frequency and highlight it in the table. Plot the measured Table VIII5: NonInverting Amplifier ,000 db 0,000 00,000,000,000 VIII4

5 results using MATLAB. On this plot, use MATLAB to place a red star at the predicted db frequency. VIII.B.4. Measured Bandwidth Based on your previous frequency measurements, fill in Table VIII6. Compare the results to PSpice (Table VIII). What do you notice about? Table VIII6: NonInverting Amplifier Bandwidth Measured Measured db ( / ) 0 00 VIII.C. Bandwidth By this time you should realize that is approximately constant for this OPAMP circuit. In general, for any OPAMP circuit you can assume that is constant. A number for the = Constant can be found in all OPAMP data sheets. It may sometimes be given different names but it is a number that is always available. Some common names are Unity Bandwidth, Small Signal Bandwidth, or Unity Crossover. This number is usually easy to find if you look for units of Mhz. We will use the product to find the db frequency of the circuit in Figure VIII. Note that the OPAMP is now a TL07. Find the specification of the Unity Bandwidth in the data sheets for the TL07. Calculate the gain for the circuit and use the Unity Bandwidth to find the db frequency. Measure the db frequency in the lab and compare the result to the calculated value. UA TL07 8 V 4 V VEE DC = 5 DC = 5 R R k k VEE Figure VIII: Noninverting circuit using a TL07 opamp. VIII5