ENSC 220
ELECTRIC CIRCUITS I

Lab #2: Op Amps

OBJECTIVE:
Using the circuits below you can study op amps and characterize their behavior.

  • Comparator
  • Inverting Amplifier

    PREPARATION:

  • Read Lab Handbook 2.2.4 (Function Generator) and 2.2.5 (Oscilloscopes).
  • Read Appendix X (pp. 268 - 272) on op amp configurations.

    EQUIPMENT:

  • Basic lab tools and breadboard
  • Dual DC power supply
  • Digital multimeter (DMM), Fluke 8010A
  • Function Generator, Wavetek 182A or equivalent
  • Oscilloscope, Tektronix 2235 or similar
  • Operational amplifier, TL072
  • Resistors
  • Connecting wire.

    NOTES:

  • When making circuit changes, turn down the current on the power supply and turn off the power supply. Turn it on again only after checking the circuit carefully.
  • Check the multimeter setup before hooking it into the circuit. Improper setup can give incorrect readings and/or damage the meter.
  • Lay out your circuit neatly and logically on your breadboard to facilitate subsequent changes or troubleshooting.
  • When operating the op amp from a single supply, note that the other power supply input on the op amp must be grounded.
  • Be sure that DC offset on the function generator is set at "0".
  • Use maximum power supply voltages of +/- 12 volts.

    METHOD:

  • COMPARATOR:

    The comparator demonstrates the "open-loop" behavior of an op amp. You can design the reference voltage so that the output voltage will "switch" whenever the input voltage (at the inverting input Vn) reaches the reference voltage Vref.

    Remember that the output voltage Vo for an ideal op amp is:
    Vo = A(Vp - Vn)
    where A -> infinity .

    Determine how nearly "ideal" your op amp is by the following method:

    1. Choose appropriate R1, R2, and R3 and build the circuit.

    2. Use the function generator to connect a 1 kHz sine wave to the input. Measure the input and output on the oscilloscope.

    3. Calculate and then measure the switching voltage level. See what effect there is from putting a resistor (say 2K7) in series with the inverting input.

    4. Find the meaning of the term "slew rate" and measure it. (Slew rate is used to measure how fast an op-amp responds to the variation of the input signal.)

    5. Measure the peak-to-peak output swing of the op amp.

    6. Compare this peak-to-peak swing to the supply levels used to power up the op amp.

    7. Try operating the op amp from a single supply and observe the output.

  • INVERTING AMPLIFIER:

    The inverting amplifier demonstrates the "closed-loop" operation of an op amp because a portion of the output is "fed back" to the input. Design the amplifier with a gain of -10 [V/V], connect a 1 kHz sine wave and use RL = 1K at the output:

    1. Find the maximum amplitude of the input that doesn't distort the output.

    2. Decrease the load RL = 100, and note the effect on the output signal.

    3. See what happens when you operate this circuit from a single negative supply.

    4. Return to dual supply.

    5. Set power supply to +/- 12 volt independently. Vary each side from 12 volts to zero volts, noting the effect on the output. Restore to +/- 12 volt operation.

    6. Increase the input frequency until the output amplitude decreases by 1/2. Note the frequency at this point.

    7. Double the gain to -20, measure the 1/2-amplitude frequency again and compare.

    8. Connect the input to ground and measure the output voltage on the 200 mV scale of the DMM.

    DISCUSSION:

  • All of these "non-ideal" effects can be explained and analyzed using a more sophisticated op amp model. Try to come up with a model to explain some of your observations.
    Last modified: Wed Aug 12 23:15:55 PDT 1998.