PHYSICS 330 LAB Operational Amplifier Frequency Response Objectives: To measure and plot the frequency response of an operational amplifier circuit. History: Operational amplifiers are among the most widely used devices in electronics. They were first conceived in the 1940's as calculation elements for analog computers. The name operational amplifier derives from the concept of an extremely high gain, differential-input DC amplifier, whose characteristics are determined by the feedback elements used with it. By changing the types and arrangement of the feedback elements, different analog operations could be implemented -- addition, subtraction, multiplication, integration and differentiation, to name just a few. Today, they are used in almost every type of instrumentation and amplifier circuit. Because of their high performance, low cost and wide availability, they have almost completely replaced discrete-component transistor (and vacuum tube) circuits in most applications, within their limitations of frequency response and power output. Introduction: This lab explores the frequency response of a simple operational amplifier circuit. Procedure: I. Inverting Amplifier Construct the basic inverting amplifier circuit, using a 10 kω resistor for R 1 and a 100 kω resistor for R f. (See Figure 1) Verify that the voltage gain for this circuit is -10.0 by applying at least three different (both positive and negative) DC voltages to the input and recording the output voltage. Record the input and output voltages and the corresponding voltage gain. Compare this with the theoretical value. Input Voltage (V) Output Voltage (V) Av % Error
Figure 1 B. Amplify an AC signal and Measure the Frequency Response In parts B and C we will measure the frequency response of inverting amplifiers, and see how the amplifier bandwidth changes with gain. As the amplifier voltage gain goes up by a factor of 10, the half-power (-3 db) frequency should decrease by a factor of 10. Procedure: Connect the function generator to the input terminal and ground. Set the input signal to a sine wave of approximately 100 mv peak to peak at a frequency of 100 Hz. Vary the frequency in steps from 100 Hz to 1 MHz, keeping the input voltage at a constant value - you may have to adjust the function amplitude knob as you go to higher frequencies. Measure the output voltage and make a table of input voltage, output voltage, voltage gain, voltage gain in decibels and frequency. In the table below, frequencies have been chosen with geometric spacing, so that there are 4 frequencies per decade. In those regions where the gain is changing significantly with frequency, you may wish to take more data points. Make two plots: one with the voltage gain, A v versus frequency, and a second with the voltage gain in db versus frequency, using a log scale for the frequency. The second graph is called a Bode Plot.)
Frequency Input Voltage Output Voltage Voltage Gain db Gain 100 180 320 560 1000 1800 3200 5600 10,000 18,000 32,000 56,000 100,000 180,000 320,000 560,000 1,000,000
C. Change the feedback resistor, R f to 1.0 MΩ. What is the new voltage gain? Record at least three different input and output voltages and then calculate the corresponding voltage gain. Compare this with the theoretical value. Input Voltage (V) Output Voltage (V) Av % Error D. Measure the Frequency Response for this Modified Amplifier Change the feedback resistor, R f to 1.0 MΩ. What is the new voltage gain? Repeat part B) and again plot the gain in db versus frequency, using a log scale for the frequency. Frequency Input Voltage Output Voltage Voltage Gain db Gain 100 180 320 560 1000 1800 3200 5600 10,000
18,000 32,000 56,000 100,000 180,000 320,000 560,000 1,000,000 Questions: What is the half-power bandwidth of the amplifier in each case? What is the -6 db down frequency for each amplifier? What is the gain-bandwidth product for each amplifier? How well does this match the model given in the text?