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Operational Amplifier Applications Lab

Objectives:

To provide examples and experience in the use of an operational amplifier.

Materials:
1.
741 Op‑Amp
2. General Purpose Diodes
3. Assorted Resistors
4. Meter Movement
5. Test Equipment


Procedure:

A. High‑impedance, analog DC voltmeter


Figure 1: High‑impedance, analog DC voltmeter

1. Select R such that the meter movement in Figure 1 will read full scale when Vin = 10VDC

2. Construct the circuit shown in Figure 1 and verify its operation. Record the position of the meter movement
for Vin = 0V, 2V, 4V, 6V, 8V, and 10V.


B. Differential Amplifier


Figure 2 Differential Amplifier

1. Write an expression for Vo in terms of Vx , Vy , RF and R1.

2a. Calculate the input impedance with a signal applied at the x‑input and the y‑input grounded.

2b. Calculate the input impedance with a signal applied at the y‑input and the x‑input grounded.

2c. Calculate the differential input impedance with a differential signal applied between x and y inputs.


3a. Construct the circuit shown in Figure 2.

3b. Measure and record the output voltage and input impedance with a 100mVpp 500Hz signal applied to the x‑input and the y‑input grounded. Compare to your predictions.

3c. Repeat b with the signal applied to the y‑input and the x‑input grounded.

3d. Measure and record the output voltage with simultaneous signals at Vx and Vy, Set Vx = 100mVpp at 500Hz. Use a potentiometer to voltage divide Vx to obtain the following values of Vy. Complete the table below.

Table B3: Difference Amplifier Output Voltage



C. Super‑diode


Figure 3: Super Diode

1. Explain the operation of the circuit shown in Figure 3. Sketch the expected transfer function Vo/Vin for positive and negative values of Vin.

2. Construct the circuit shown in Figure 3. Set Vin to a 2Vp sinewave at f = 100Hz. Predict, measure and record the waveform at Vo.

3. Reduce the amplitude of Vin to 0.5Vp. Predict, measure and record the waveform at Vo.

4. Reduce the amplitude of Vin to 0.1Vp. Predict, measure, and record the waveform at Vo.


Procedure & Data:

Part A:
For the given experiment three specific configurations of the 741 op‑amp were examined assuming ideal conditions. First of the three was a high‑impedance analog DC voltmeter (Figure 1). A value for the resistance 'R' was selected to set the meter movement to read full‑scale at an input voltage Vin = 10VDC. It was determined that R = 10kΩ where the current through 'R' was equal to 1mA(max). Next the circuit was constructed (Figure 1) to examine the devices characteristics. Once constructed specific input voltages were selected and the meters output voltage was measured and recorded (Table 1).

Table 1: Measured Values for Ideal 741 High Impedance DC Voltmeter


Part B:
In the second part of the experiment a differential amplifiers behavior was examined (Figure 2). First an expression was calculated for the output voltage Vo in terms of Vx, Vy, RF, R1. To simplify the calculations super position was used in determining the output expression for Vo, again assuming ideal conditions for the op‑amp. The voltage input at Vx was set equal to zero and the output voltage, Vo was calculated in terms of Vy (E.1), (E.2) by applying KVL at the negative input terminal of the device.

(Vy - V) / R1 = (V - Vo) / RF (E.1)

Voy = (-RF / R1) Vy (E.2)

Once equations were derived for the output voltage in terms of Vy, the input voltage Vy was set equal to zero and the output voltage Vo with respect to the input voltage Vx was derived. Again KVL was written at the negative input terminal of the device utilizing the ideal op‑amp principals (E.3), (E.4). Therefore the voltage at the positive input terminal of the device equals the voltage at the negative input of the device there by creating a virtual ground from the input terminals.

Vox / V = (RF / R1) + 1 (E.3)

Vox = (RF / R1) Vx (E.4)

Now by the rules of super position the sum of the voltages from each source, Vx and Vy, equals the total output voltage Vo. We have the following (E.6).

Given RF = 10kΩ , R1 = 10kΩ

Vo = (-RF / R1) ∙ (Vx - Vy) → Vo = 10 ∙ (Vx - Vy) (E.6)

Continuing with hand calculations the input impedance was calculated in much of the same was as above. Using the tools of super position the input impedance for a signal applied to the x‑input, y‑input and both x and y input terminals was determined (Table 2).

Table 2: Calculated Impedance's for Ideal 741 Differential Amplifier


Next the circuit was constructed (Figure 2) to verify the predicted calculations. The output voltage and input impedance was measured given the following conditions. A 100mVpp input sin‑wave at 500Hz was connected across the x‑input with the y‑input going to ground. Then the same input signal connected to the y‑input with the x‑input going to ground. Measurements were taken for each case (Table 3) and compared to the hand calculated results. Finally the input signal was connected to both x and y input terminals and again the input impedance and output voltage were measured and recorded (Table 3).

Table 3: Measured Values for Ideal 741 Differential Amplifier


Part C:
In the last part of the experiment a voltage follower op‑amp with a diode connected to the output terminal of the device was constructed (Figure 3). This configuration is better known as a super diode. First a sketch of the expected output transfer function was created and predictions were made prior to the construction of the circuit (Figures 4 & 5). The circuit was then constructed (Figure 3) setting the input voltage equal to 2Vp sinewave at a frequency of 100Hz. Then the output voltage Vo was examined, measured (Table 4 & Figure 6), and compared to the expected output sketch (figure 5). Next the input signal was reduced to Vin = 0.5Vp and measurements were taken (Table 3 & Figure 7). Finally the amplitude of Vin was once again reduced to Vin = 0.1Vp and the output voltage Vo measured (Table 3 & Figure 8).

Table 4: Measured Values for Ideal 741 Super Diode Configuration


Conclusion & Discoveries:

The benefit of a high impedance DC voltmeter is very beneficial when accurate measurements are required on virtually any load.

The technique of superposition is very effective in circuit analysis when dealing with multiple sources. For the differential amplifier superposition helped in predicting the circuits behavior.

By cascading a diode at the output of the 741 operational amplifier, the signal loss do to the diode is eliminated. Therefore, output rectified signals less then 0.7 volts can be achieved by using the super diode configuration.

In closing, by examining the effects of the 741 operational amplifier used with other devices and multiple input signals, allows one to grasp the benefits and many uses the operational amplifier have in circuit design.


Figures and Graphics
Graph 1: Predicted Transfer Function, IN4004 Diode
Graph 2: Predicted Transfer Function, Super-Diode
Graph 3: Scope Data 1
Graph 4: Scope Data 2
Graph 5: Scope Data 3

Lab Notes
Lab Notes Page 1 of 5
Lab Notes Page 2 of 5
Lab Notes Page 3 of 5
Lab Notes Page 4 of 5
Lab Notes Page 5 of 5

Electrical Engineering lab key words: feedback resistor, mixed signal, earth ground, input impedance, operational Amplifier, op-amp, differential circuit, super diode, high impedance voltmeter, terminal, test signal, 741, VDC, analog voltmeter, test equipment, circuit analysis.

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