BASIC APPLICATIONS OF OPERATIONAL AMPLIFIERS

Objective:

This lab session is intended to familiarize the students with some of the basic characteristics and applications of operational amplifiers (op amps). Some of the most widely used applications will be tested and characterized. Also the students are encouraged to investigate any discrepancy between test results and the results that might be expected from theoretical analysis.

Introduction:

Operational amplifiers are very popular integrated circuits that are available from various manufacturers to cover a wide range of operations and speed. In spite of the differences between various op amps available for different applications, they have many common properties. For example, op amps usually have a differential input with each input exhibits very high input impedance. In some op amps, the differential input impedance is in the order of few hundreds mega Ohms. Op amps are also characterized by their very high differential open-loop gain. In many off-the-shelf op amps, the open-loop differential gain is in the order of 60 to 80 dB. Additionally, the output impedance of the op amp is known to be low, perhaps in the order of few tens of Ohms or less.

The above mentioned general characteristics of op amps make them ideal for various buffering purposes as well as some other linear and non-linear applications. However, op amps are not without limitations. Some of the critical limitations are expressed in terms of slew rate which is a large signal parameter, and the gain-bandwidth product which is a small signal parameter.

Lab work:

  1. Non-inverting unity gain buffer:
    The simple amplifier configuration is as in Figure 1. Apply -/+ 14V supply voltages to pin 4 and 7 respectively.
     A simple unity gain buffer amplifier
    Figure 1 - A simple unity gain buffer amplifier

    1. Measure the frequency response of the buffer by applying 1V sinusoidal signal at the input. Vary the frequency of the signal, keeping its level constant. Measure the corresponding level of the output signal. The buffer (amplifier) gain is the ratio between output to input signal levels. Plot the amplifier gain versus frequency of the input signal. Take enough readings until the amplifier gain drops to less than one tenth its nominal values.
    2. Place a 10 μF capacitor between the signal source and the amplifier input (pin 3). Apply a 10 kHz signal. Monitor the output waveform. Compare the output with that obtained without a capacitor. Comment on the differences and explain why this capacitor makes a difference? Will the capacitor value impact the results?
    3. Apply a 1V (peak-to-peak), 100 kHz square wave at the buffer input. Monitor the output waveform. Change the input voltage to 2 V and then to 5 V. monitor the changes in the output waveform and the differences with the input signal. For a unity gain buffer, is the output signal meet expectations?
      Explain the differences between the input and output waveforms.
  2. Inverting amplifier:
    1. Connect the 741 operational amplifier as shown in Figure 2. Apply a 200 mV sinusoidal signal at the amplifier input and measure the frequency response. Compare the input and output waveforms at 1kHz frequency. Capture the oscilloscope display of the waveforms. Repeat the measurement at 100kHz. Compare the two sets of results, comment and explain the differences.
      Inverting Amplifier
      Figure 2 - Inverting Amplifier

    2. Change R2 to 50 kΩ and measure the frequency response of the amplifier. Repeat the measurement with R2 = 100 kΩ. Plot all three curves on the same chart in which the X-axis is the signal frequency in kHz, while the y-axis is the amplifier gain (G) expressed as:

      Where; Vin is the applied input Voltage, Vo is the amplified output voltage and G is the gain in decibel (dB).
    3. From the measurement and the plot obtained in 2.b, what is the amplification bandwidth of the amplifier for each of the three measured cases?
    4. Find the gain-bandwidth product for each of the three responses measured in 2.b.
  3. Difference amplifier:
    The basic connection diagram is as shown in Figure 3.
    1. Connect input 2 to ground. Apply 100 mV, 10 kHz signal to input 1, measure the output. Repeat the measurement by connecting input 1 to ground and applying the signal to input 2. Calculate the amplifier gain for each input port. Comment on the difference.
    2. Apply 100 mV 1 kHz signal to both inputs simultaneously, measure the output signal level. Compare measured result with the expected value, explain the differences if any.
       Circuit diagram of a difference amplifier
      Figure 3 - Circuit diagram of a difference amplifier

      In Figure 3, the general expression for the amplifier gain is given as;

      In the special case when R3/R4 = R2/R1, then Vo = (Vi2 - Vi1) R2/R1 .
  4. Comparator:
    In this application, the op amp is operating in a very high gain mode without any form of feedback, as shown in Figure 4.
     The operational amplifier  in a comparator circuit
    Figure 4 - The operational amplifier in a comparator circuit

    1. Apply 100 mV DC to the non-inverting input of the comparator (input 2).
    2. Apply 1 V (peak-to peak), 1 kHz sinusoidal signal to the inverting input of the comparator (input 1). Monitor both input and output waveforms on the oscilloscope simultaneously. Capture the displayed waveforms. Repeat the measurement and observations by changing the input signal level from 20 mV (peak-to-peak) to 4 V peak-to-peak. Are the obtained results in agreement with expectations?
    3. Connect input 2 to ground (0 V). Apply a 100 mV, 10 kHz sinusoidal signal to input 1 and monitor the waveforms. Repeat the measurement by changing the waveform to triangular. Capture the measured signal patterns.
    4. Measure the rise and fall time of the waveform at the comparator output. What determines or influence these two quantities?
  5. Active integrator:
    Connect the operational amplifier as shown in Figure 5.
    1. Apply a 3 kHz square waveform at the input. Monitor and measure the time parameters and voltages of the output waveform. Does the output signal have a DC component? Comment on the result.
    2. Disconnect the resistance R2 and measure the output waveform. Explain the results obtained.
       Circuit diagram of an active integrator
      Figure 5 - Circuit diagram of an active integrator

  6. Active differentiator:
    Connect the op amp as shown in Figure 6.
     Op amp as an active differentiator
    Figure 6 - Op amp as an active differentiator

    1. Measure and plot the frequency response for the differentiator shown in Figure 6. Monitor the phase difference between the input and the output waveforms at various frequencies. Plot the results.
    2. Apply a 1 kHz, 100 mV peak-to-peak saw-tooth waveform to the differentiator input. Capture the input and output waveforms on the oscilloscope simultaneously. How do you relate the output waveform to the input? What determines the voltage of the output waveform?
    3. Change the frequency of the input saw-tooth to 100 kHz and repeat the observations performed in 6.b.
    4. Apply a square wave (or rectangular waveform) to the differentiator input. Set the frequency to 10kHz. Monitor and capture the input and output waveforms simultaneously. Is the output waveform in agreement with the differentiator equation displayed in Figure 6?