- Op Amp Pin Conventions are as Follows:
Figure 1.
Note that pin number 1 is adjacent to the dot impression on the top of the IC (Integrated Circuit.) There may also be a notch cut out of the top of the IC on the end where pin 1 is located.
Insert the op amp across the groove in the breadboard so that each pin is inserted into a unique connector. Be careful, the pins are easy to bend.
- DC Power Supply Setup
Two DC power sources are required to insure proper operation of the op amp. Select the +25 output on the DC supply. Adjust the voltage from the + side of the +25 output to COM to be +15 volts. Check to insure the voltage is +15 by using the DMM as voltmeter. Select the -25 output on the DC supply. Adjust the voltage from COM to the - side of the -25 output to be +15 volts. Check to insure the voltage is +15 by using the DMM as voltmeter. The COM terminal will be the circuit ground for all parts of this experiment. Be sure to make proper circuit ground connections for each circuit before connecting the power lines to pins 4 and 7. Failure to do this will almost certainly cause the op amp to burn out. See Figure 2.
Figure 2.
- Signal Source:
Turn on the signal generator, and adjust its AC output to minimum with the output amplitude knob. Adjust the DC offset of the signal generator to zero. Check to insure the DC offset is zero by using the DMM as a DC voltmeter for accuracy.
In this part you will set up an op amp as a linear amplifier with a gain of 10, and inspect the input and output to see how well or poorly it behaves as such a device. Operational amplifiers must be treated with care; they are powerful but can be destroyed by abuse. In particular it is very abusive to apply voltages to the input terminals before fully powering up the opamp, or to exceed certain maximum limits. Therefore, you will (a) set up the signal source but with zero output; (b) set up the rest of the circuit; (c) have your instructor check the circuit; and THEN (d) power the circuit up for the experiment.
Inverting Amplifier Circuit: wire up the circuit in Figure 3 below, checking carefully to see that it is correct, but with ALL POWER OFF (no connection to pins 4 and 7 yet) and the signal generator disconnected from the rest of the circuit. Connect VS1 to CH1 and VO to CH2 of the scope. Set the scope to display both of them simultaneously.
Note: Set the scope to trigger off of CH1 for all parts of this experiment. Your instructor will show you how to do this.
Figure 3.
Have your instructor check your circuit before any power is turned on. Power up the op amp by applying the 15 volt sources - be sure the polarities are correct. Set the function generator to a 1KHz sinusoidal function. Now gently increase the amplitude of VS1. You should see an inverted and amplified version of VS1 at VO. Adjust VS1 to have a peak-to-peak voltage of 2 volts. Set the vertical scales for CH1 to 1V/D and CH2 to 5V/D. Sketch one cycle of both VS1 and VO on the same set of axis (just as you see on the scope.) Be sure to note the scales. Is the amplifier working as expected. Is the gain correct. Is the output inverted with respect to the input? Repeat the above using a triangle input voltage of 2 volts peak-to-peak. Be sure to sketch the results.
Experiment with the amplitude of the input signal to see the effect of overdriving the op amp with a signal too big for it to amplify faithfully. Set the amplitude of the triangle wave to 4 volts peak-to-peak. What happens to VO? Sketch the signals.
Reduce the input to 2 volts peak-to-peak and experiment with the effect of the DC offset of the input signal. Is the DC offset amplified? Set the DC offset to 0.5 volts and sketch the signals.
Set up circuit in Figure 4, using the same precautions as before to protect the op amp from damage. In this circuit the output should be a linear addition of the two input signals VS1 and VS2. Use a triangle wave with 4 volts peak-to-peak amplitude for VS1 with the DC offset set to zero. Use the Sync output of the function generator as VS2. Display VS1 and VS2 on the scope. Set the vertical scales of both channels to 1V/D. Sketch one cycle of each function. Keeping VS1 connected to CH1 display VO on CH2. Sketch VO. Figure out just what the relationship should be between VO and the two inputs, and see if what you observe is what you would expect on the basis of the theory. If you have time, experiment with the sine and the square wave for VS1.
Figure 4.
Set up the circuit in Figure 5 with the scope set to display VS(t) and VO(t) on CH1 and CH2. Your instructor will assign values for R and C. Set VS1 = 4cos(10000pi*t) volts. Be sure that the DC offset is set to zero. Figure out the theoretical relationship between VS(t) and VO(t) for this circuit ignoring the current through the 100K resistor, and figure out what the output should be if the input signal is (1) a sinusoid (as above); (2) a square wave; (3) a triangle wave. Then apply these signals to the circuit and check your predictions.
Note: Try setting the coupling to AC (for both channels) if the images are not centered on the display. Your instructor will show you how to do this.
If the current through the 100K resistor is very small compared to the current through the capacitor, your analysis will be accurate. This will be true for signals at the frequency your instructor suggests. The 100K resistor is provided to avoid saturation of the op amp due to DC offset - a technical matter you can ignore for the time being.
Figure 5.