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Operational Amplifier Input Offset Parameters Lab

Objectives:

To become familiar with the DC input offset parameters of an operational amplifier and compare measured parameters with manufacturer’s specifications.

Equipment:
1. 741 Op‑Amp, 2 each
2. 10kΩ Potentiometer
3. 10MΩ Resistors
4. Assorted Resistors
5. Function Generator
6. Dual Channel Oscilloscope
7. Digital Multimeter


Introduction:

Ideal operational amplifiers have zero output voltage when the differential input voltage is zero. However, real operational amplifiers do not meet this ideal goal. A small DC voltage must be applied to the input in order to drive the DC output voltage to zero. This small differential input voltage is called the input offset voltage, Vos, and is a measurable parameter on real op amps and can be found on a specification sheet.

Also, the DC current flowing into the input terminals of an ideal operational amplifier is zero. However, the input differential stage of real op amps require some small input bias current to flow into both the inverting and non inverting input terminals, even at zero input voltage, to establish the DC Q point of the input stage. The average of the two input currents is called the input bias current, IB, and the difference of the two currents is called the input offset current, Ios. Both IB and los are measurable quantities and can be found on a specification sheet. It should be realized that both Vos and los are random quantities caused by slight mismatches in the operational amplifier and can be positive, zero, or negative quantities. A simplified model of a real op amp, which includes these input DC parameters is shown in Figure 1.


Figure 1: Simplified Model of an Operational Amplifier that Includes the Input Offset Parameters


Procedures:

A. Input Offset Voltage, Vos

1. Using the model shown in Figure 1 and circuit analysis, show that Vo = Vos for the real op amp connected as shown in Figure 2.

2. From the specification sheet, what is the expected value that you might measure and what is the maximum value for Vos(Vos typical = ?, and Vos max = ?)

3. Assemble the circuit in Figure 2, paying attention to carefully match the ± 15V power supply values. Measure and record the output voltage (Do Not connect a voltage offset adjustment potentiometer). Record the magnitude and polarity of Vos. Compare to the specification sheet values.


Figure 2: Circuit for Measuring Input Offset Voltage, Vos




B. Voltage offset adjustment range

1. Construct the circuit as shown in Figure 3. Measure and record the range in value of Vo as the 10kΩ potentiometer is varied from the full counter clockwise to the full clockwise position. Where Vo = ? (at full counter clockwise) and Vo = ? (at full clockwise)


Figure 3: Circuit for Measuring Voltage Offset Adjustment Range

2. Adjust potentiometer so that Vo = Vos = 0.0V.


C. Input Bias and Offset Currents (IB+, IB-, IB, Ios)

1. From the specification sheet, determine typical and maximum values expected for the input bias current and input offset current. Determine, IB typical = ?, IB max = ?, Ios typical = ?, Ios max = ?

2. Assemble the circuit as shown in Figure 4.


Figure 4: Circuit for Measuring Input Bias Currents

3. Connect jumpers between A and B and C and D. Adjust the offset null potentiometer for Vo = 0V. (i.e. set Vos = 0V)

4. Remove the jumper between C and D. Measure and record Vo, calculate IB+ using the model in Figure 1. (Vo = ? and IB+ = ?

5. Replace the jumper between C and D and remove the jumper between A and B. Measure and record Vo, calculate IB using the model in Figure 1. (Vo = ? and IB- = ?)

6. Remove the jumper between C and D. Measure and record Vo, calculate los using the model in Figure 1. (Vo = ? and Ios = ?)

7. Calculate IB = (IB+ + IB-) / 2 = ?

8. Compare measured and specification sheet values for IB and Ios.


D. Input Offset Parameters / Effects on Circuits

1. Reassemble the circuit shown in Figure 3 and adjust the potentiometer for Vo = 0VDC (i.e set Vos = 0.0V)

2. Leaving the voltage offset adjustment circuit set as determined in step (1) above; assemble the circuits shown in Figure 5a and 5b. Using your measured values for IB and Ios (realizing that you have adjusted Vos to be zero) and the model in Figure 1, predict the output voltages for Vi = 0V for the circuits in Figure 5a and 5b. Measure the actual output voltages and compare to your predictions. Fill in the table below, and comment on results in your report.


Figure 5: Circuits for Measuring Output Offset Voltages

3. Construct the circuit shown in Figure 6, but leave off the voltage offset adjustment potentiometer. Using superposition and previously measured values, predict the output offset voltage due to IB only, los only, and Vos only with Vi = 0V. Then predict and measure the total output offset voltage resulting from all three and compare.

Output Offset Voltages
Predicted due to IB Only
Predicted due to los only
Predicted due to Vos only
Predicted due to IB, Ios, and Vos
Measured due to IB, los, and Vos


Figure 6: Circuit for Measuring Output Offset Voltages



Lab Notes

Lab Data


Electrical Engineering lab key words: operational amplifier, op-amp, input offset parameters, DC circuit analysis, AC circuits, percent error calculation, voltage offset, circuit assemblies, potentiometer, variable, spec sheet, null, voltage range, board jumpers, terminals matched pair.

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