E11 Lab #2
2007

Introduction to Op Amps

Laboratory Procedures

The room where you will be doing this (and other) labs has computers equipped with web browsers, so you need not print this out (though you certainly may, if you would like to).  Since this document is fairly long, you should look through it at least once before coming to lab.


This laboratory exercise will give you some more practice with resistive circuits, as well as introducing you to Operational Amplifiers, one of the fundamental building blocks of analog circuitry.  The lab is fairly short, but the lab report may take a long time.  Start the report early, and come to me with any questions if you have them - none of the derivations is very hard if you approach it in the right way.

 

The lab exercise is split into three parts

  1. Procedure for testing the op amp circuit in the inverting configuration.

  2. Procedure for testing non-inverting programmable-gain configuration.

  3. Finishing up


Procedure for testing the op amp circuit in the inverting configuration.

  1. Put the op amp in the breadboard and connect +Vcc and -Vcc to the chip. Set the magnitude of Vcc to 12 volts (you will have to check this with a voltmeter).
  2. Set the function generator on the breadboard to a 1 kHz sine wave of about 1 Volt peak-to-peak (neither value needs to be exact) and connect it to the input of the circuit above with R1=10kΩ, and Rf=20kΩ.   Use small discrete resistors, not the resistor boxes.  You may need to learn to read resistor values.  Measure and record the values of the resistors using an ohmmeter. 
  3. After building the circuit, adjust the controls on the function generator and the oscilloscope so that you get a nice image showing both Vin and Vout.
    741 pinout
  4. To do this, select the channel you want to measure (e.g., for Channel 1, hit the "CH 1" button). Then hit the "Measure" button and cycle through the possible measurements (bottommost button on right of screen) until "Pk-PK" appears (on screen 4 of 6). Select this measurement. Do this for both channel 1 and 2, and then repeat the procedure to measure the frequency of the signal (on screen 1 of 6).  When the frequency and the amplitude of the two channels are displayed get a screen shot of the oscilloscope.  What is the gain of this circuit (i.e., the ratio of the magnitude of the output voltage to the magnitude of the input voltage)?  Is this what you expected? 
  5. Repeat part 3, but set the amplitude of the input voltage to be high enough to show distortion in the output, called "clipping."  Record a screenshot.

Procedure for testing non-inverting programmable-gain configuration.

  1. Put the op amp in the breadboard and connect +Vcc and -Vcc to the chip. Set the magnitude of Vcc to 12 volts (you will have to check this with a voltmeter).
  2. Set the function generator on the breadboard to a 1 kHz sine wave of about 1 Volt peak-to-peak (neither value needs to be exact) and connect it to the input of the circuit above with Ra=20kΩ, and Rb=10kΩ and RF=20kΩ.  Measure these resistances before using them in the circuit.
  3. We won't use switches in the circuit, we'll just connect and disconnect wires on the breadboard.  After building the circuit, adjust the controls on the oscilloscope so that you get a nice image showing both Vin and Vout.

  4. Close Sa and Sb (i.e., include both Ra and Rb in the circuit).  Have the scope measure the amplitude of the input and the output, as well as their frequency of the input.  Record these and also save a screenshot.
  5. Repeat part 3 with Sa open and Sb closed (i.e., include only Rb in the circuit). 
  6. Repeat part 3 with Sa closed and Sa open (i.e., include only Ra in the circuit). 
  7. Repeat part 3 with both Sa and Sb open (i.e., remove both resistors from the circuit). 
  8. Fill in the table below and let me see it before you leave lab (note that the rows in the table are not in the same order as those in the procedure):
Sb Sa Gain
open open  
open closed  
closed open  
closed closed  

Finishing up

Make sure you have all the information you will need for your report


email me with any comments on how to improve the information on this page (either presentation or content), or to let me know if you had any particular difficulties with this lab.

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