Experiment #3 Operational Amplifier Circuits

EE 312 Fall 2000

E8 Operational Amplifiers

Same As

EE101 Basic Electronics Lab

Experiment #3 Operational Amplifier Circuits

http://www.ee.buffalo.edu/~ee101

OBJECTIVES:

(a) To introduce the operational amplifier, and (b) to test the response of standard non-inverting/inverting configurations with feedback, and (c) to test a comparator response.

EQUIPMENT

DC Power Supply, Signal Generator, Counter and Digital Oscilloscope.

COMPONENTS

LM358AN Dual CMOS Operational Amplifier, Proto-Type Board (a small circuit board), and Resistors (240 W, 1 kW, 2.2 kW, 10 kW, 100 kW).

PROCEDURE

A. Integrated Circuit Checkout

1. Turn the DC Power Supply off. Connect a dual (15V) power supply as shown in Fig. 3.1. To ensure the supply is dual tracking pull out the appropriate knob on the DC Power Supply. Turn that knob fully CCW to obtain a minimum value. Turn the individual coarse & fine power supply controls fully clockwise to obtain a maximum value for the + 15 V & -15 V supplies. The current limiting controls should be set at midway at 12 o'clock. Note that the dual supply uses a common connection that is not a ground connection. Note that only one half of the LM358AN Dual CMOS Operational Amplifier is used. The IC pin connection schematic is given in Fig. 3.2. Note that the pin label "GND" in Fig. 3.2 is the pin to which the negative supply voltage is connected.

2. Check the voltage on the V+s and V-s pins (with the DMM) to verify that the voltages are +15 V and -15 V.

3. Ground the non-inverting input (Vi+) and connect the inverting input (Vi-) to -15 V through a protective 100 k resistor. The output (Vo) should be in positive saturation (~+14V). Be sure that the scope is dc coupled.

4. Change the connection to the inverting input (Vi-) from - 15 V to +15V through the 100 k resistor. Now the output should be negative saturation (~-14V).

If either test fails, either the IC is bad or the power supply and/or grounds are improperly connected.

Fig. 3.1 Op Amp Test Circuit

Fig. 3.2 IC Pinout

B. Non-inverting Amplifier

Note: All subsequent operational amplifier circuits will assume the power supply to be connected and will not be shown explicitly.

1. Connect the components as a non-inverting amplifier configuration (Fig. 3.3). Choose the resistors Ra & Rb such that the voltage gain is approximately 10. The voltage gain is given by the ratio ( Ra & Rb)/ Ra. The values Ra = 1 kW & Rb = 10 kW should give a voltage gain of 11 which is close enough to 10. Measure the values of the resistors with a DMM. Calculate theoretical voltage gain. Retain a precision of 1 significant figure beyond the decimal point, e. g. 11.4

2. Connect the non-inverting input to a square wave source (be sure the dc component of the square wave is near zero).

Fig. 3.3 Non-Inverting Amplifier

3. Set the input voltage (Vi) to 1.0 Vpp (peak-to-peak) at a frequency = 1 kHz. Measure the voltage gain (A = Vo/Vi) with the CRO. Check it with the DMM.

4. Compare the theoretical voltage gain to the two values of the experimental voltage gain. The DMM experimental value should agree with the theoretical voltage gain value within 1 to 2%. The CRO experimental value should agree with the theoretical voltage gain value within 5%.

5. Change the input to a sine wave (1 Vpp). Measure the voltage gain (A = Vo/Vi) with the CRO. Check it with the DMM. .

6. Compare the theoretical voltage gain to the two values of the experimental voltage gain.

7. Increase the input to 3 to 5 Vpp. Note clipping at the output Record the values of the positive & negative output voltage at which clipping occurs. These are the values for the op amp saturation voltages.

8. Synchronize the two-channel scope trigger to the input channel. Display the input and output signals together. Note that the output is in phase with the input (that is, amplifier is non-inverting).

9. Measure the voltage gain with a dc input voltage of ~0.5 V. You can do this using the 5 V supply and a voltage divider.

C. Inverting Amplifier

1. Connect the inverting amplifier configuration shown in Fig. 3.4. Choose the resistors Ra & Rb such that the voltage gain is approximately -10. The voltage gain is given by the ratio ( -Rb)/ Ra. The values Ra = 1 kW & Rb = 10 kW should give a voltage gain of -10. Measure the values of the resistors with a DMM. Calculate theoretical voltage gain. Retain a precision of 1 significant figure beyond the decimal point, e. g. -10.4

Fig. 3.4 Inverting Amplifier Circuit

2. Connect the non-inverting input to a square wave source (be sure the dc component of the square wave is near zero).

3. Set the input voltage (Vi) to 1.0 Vpp (peak-to-peak) at a frequency = 1 kHz. Measure the voltage gain (A = Vo/Vi) with the CRO. Check it with the DMM.

5. Change the input to a sine wave (1 Vpp). Measure the voltage gain (A = Vo/Vi) with the CRO. Check it with the DMM. .

6. Compare the theoretical voltage gain to the two values of the experimental voltage gain.

9. Measure the voltage gain with a dc input voltage of ~0.5 V. You can do this using the 5 V supply and a voltage divider.

D. Comparator

1. Connect the comparator circuit shown in Fig. 3.5. Note that a single +5 supply is used with a voltage divider to provide a voltage at the inverting input. The value of that voltage is called the threshold voltage Vthres. Adjust the resistor ratio (R1/R2) so that the Vthres = ~1.0 VDC.

Fig. 3.5 Comparator Circuit

2. Apply a sine wave to the non-inverting input. Initially set the ac value to 0.5 Vpp. Set the dc offset value to zero.

3. Slowly increase the sinewave input signal Vi while observing the output Vo. Determine the minimum peak voltage for Vi that produces a square pulse at the output. The square pulse can be viewed as a digital signal equal to logic 1. The minimum peak voltage for Vi, which produces logic 1, should be Vthres.

4. Add a dc bias or offset to the sinewave by adjusting the appropriate control on the function generator. Note how the output pulse changes.

REPORT

Write a two to three page summary in your lab notebook prior to leaving the lab.