## Vermont Technical College Electronic Applications ELT-2060 Lab 05: DC characteristics, input offset voltage and input bias current Reference: Operational Amplifiers with Linear Integrated Circuits Fourth edition William D. Stanley, pages 154-155 (Problems 3-21, 3-22 and Lab exercises LE 3-1 to LE 3-4) For the following exercises, make sure to record all calculations, estimations and measured results. Components: 2 741 Op Amps, 10k Ω Potentiometer, 4-10kΩ, 1kΩ , 100kΩ , 100Ω , 560kΩ , 5.6M Ω, resistors Objectives: a. Voltage offset Null Circuit and Closed-loop Differential Circuit b. Measurement of dc Input Offset Voltage c. Measurement of dc Bias and Offset Currents a. Voltage offset Null Circuit and Closed-loop Differential Circuit In this exercise, investigate the use of a null circuit to reduce the output dc offset to its minimum possible value. Refer to the “Voltage Offset Null Circuit” describe in the 741 op amp data sheet from Appendix C of your text book. Although there are no specific closed-loop configurations shown, use a closed-loop differential Amplifier shown in Figure 1. The differential nature of this type of circuit makes it particularly sensitive, therefore well suited, to illustrate the concept dc voltage offset. 1. Connect the closed-loop difference amplifier of Figure 1 with R=10k Ω and A=1. Using a 10kΩ potentiometer connect the “Voltage Offset Null Circuit” between nodes 1 and 5 as shown in the 741 data sheet. Keep in mind that a potentiometer is a three terminal device. You will need to connect the potentiometer wiper terminal to the lowest potential in the circuit -VCC. 2. Connect the two external circuit inputs (v1 and v2) to ground, measure the dc voltage. From the data sheet the expected value of offset voltage at room temperature is 2mV typical and 6mV maximum. Voltages at these levels will be hard to measure with the laboratory multimeter. 3. Adjust the potentiometer until the dc output magnitude is either zero or it’s minimum possible value. Record the minimum value of voltage attained. 5. Do not break down you difference amplifier. Next, build the non-inverting amplifier as shown in figure 2 with Ri=1k Ω and Rf =100k Ω. Attach the output of the difference amplifier to the input of the non-inverting amplifier. This will amplify your offset by 101. 6. Adjust the potentiometer until the dc output magnitude is either zero or it’s minimum possible value. Record the minimum value of voltage attained. 7. In effect we amplified the voltage offset from the difference amplifier by 101. Please describe any possible flaws in using this approach. Compare this result to what was measured in step 2. 8. Write an equation that expresses the expected output voltage Vo in terms of the two input voltages V1 and V2. 9. Apply dc input voltage for the following six combinations, compare the results to the expected values you calculate with the equation from step 8 a. V1=10V, V2=0V b. V1=0V, V2=10V c. V1=V2=10V d. V1=10mV, V2=0 e. V1=0, V2=10mV f. V1=V2=10mV b. Measurement of dc Input Offset Voltage ( Stanley Problem 3-21 page 151) A circuit and equation to measure the input offset voltage Vio is show in figure 3. With the proper selection of resistors Ri, Rf, and Rc the effects of offset due to input bias currents can be neglected. When the input terminals are both held to ground the resulting output voltage should be a direct measurement of Vio. 1. Build the circuit in Figure 3 with Ri=100 Ω and Rf=10k Ω measure and record Vo. Compare your results with the specification of input offset voltage provided in the data sheet. 2. Increase the value of Rf to 100k Ω, and measure Vo again. Did the output increase by approximately 10x the value recorded in step 1, if so explain how that validates the assumption the input bias currents are negligible. 3. Be sure to include a comparison of the measured values in steps 1 and 2. Include a discussion on how there relationship demonstrates that neglecting input bias current was a valid assumption. c. Measurement of dc Bias and Offset Currents (Stanley Problem 3-22 page 152) Consider the three circuits of figure 4 .The resistance R is chosen large so that the contribution to the output from bias currents is considerably larger than the contribution from the input offset voltages. The accompanying equations will predict the values of Ib+, Ib- and Iio. 1. Start with setting R=560k Ω and build each circuit in figure 4 one at a time. Going from one configuration to the next configuration should be quick, all that is changing is the placement of the resistors. Measure Voa, Vob and Voc for each circuit and calculate Ib+, Ib-, and Iio, compare your measurements to the values in the data sheet. 2. Increase the value of R to 5.6M Ω. Measure Voa, Vob and Voc for each circuit and calculate Ib+, Ib-, and Iio, compare your measurements to the values in the data sheet and to the results in part 1.Did the output increase by approximately 10x the value recorded in step 1, if so explain how that validates the assumption the input offset voltage effect is negligible. 3. Be sure to include a comparison of the measured values in steps 1 and 2. Include a discussion on why neglecting input offset voltage was a valid assumption. LAB write up: This lab requires a semi-formal lab report. Record all calculations, estimations, and measured results. No MultiSim will be required for this report. Please include a written English language paragraph for all lab steps that required an explanation.

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