EN1O8/1



ELEC3304/ELEC4344: On Use of Negative Feedback Using 741 IC Operational Amplifier

Name: University Number:

Objectives

1. To study the gain and phase shift of a negative feedback amplifier.

2. To study the square waveform response.

3. To study the low-pass filter.

Introduction

An amplifier is said to have a negative feedback when an increase in the output signal results in a feedback signal into the input in such a way as to cause an output signal which opposes the original change. The negative feedback has the advantages of (i) modifying the input and output impedances, (ii) stabilizing the transfer gain, (iii) reducing the distortion, and (iv) widening the bandwidth of an amplifier.

Apparatus:

1. Protoboard

2. 741 operational amplifier

3. Twin 15V DC power supply

4. Sine-square signal generator

5. Oscilloscope

References

Sedra & Smith, “Microelectronic Circuits”, Holt-Saunders I.E., 1982, PP. 648-655.

[pic]

Procedure:

1. a) Connect the circuit as shown in Figure 1 (ignore the setting of RV1).

b) Plot the frequency response (phase and magnitude response) of the amplifier for 10 Hz to 10 MHz.

[pic]

[pic]

[pic]

Figure 1

c) Adjust the signal generator to a frequency of 1 kHz, with a sinusoidal voltage of 1 volt peak-to-peak amplitude as measured on the oscilloscope.

d) Sketch the input and output voltage waveforms showing their magnitude and phase relationships.

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e) Sketch the output waveform with an input 1 kHz square waveform of 1 volt peak-to-peak.

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2. Repeat Step 1 by using R2 (5 kΩ) instead of R1.

a) Plot the frequency response (phase and magnitude response) of the amplifier for 10 Hz to 10 MHz.

[pic]

[pic]

b) Sketch the input and output voltage waveforms showing their magnitude and phase relationships.

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c) Sketch the output waveform with an input 1 kHz square waveform of 1 volt peak-to-peak.

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3. a) Repeat Step 1 with a capacitor of 100 pF across RF.

b) Plot the frequency response of the amplifier for 10 Hz to 10 MHz.

[pic]

[pic]

c) With an input of 1 kHz and 10 kHz sinusoidal voltage of 1 volt peak-to-peak, sketch the input and output waveforms.

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d) Change the input to 1 volt peak-to-peak square wave. Sketch the output waveform at 1 kHz and l0 kHz.

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4. Repeat Step 3b – e with a capacitor of 300 pF across RF.

a) Plot the frequency response of the amplifier for 10 Hz to 10 MHz.

[pic]

[pic]

b) With an input of 1 kHz and 10 kHz sinusoidal voltage of 1 volt peak-to-peak, sketch the input and output waveforms.

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c) Change the input to 1 volt peak-to-peak square wave. Sketch the output waveform at 1 kHz and l0 kHz.

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5. Repeat Steps 3 and 4 using R2 instead of R1.

1) With a capacitor of 100 pF across RF

a) Plot the frequency response of the amplifier for 10 Hz to 10 MHz.

[pic]

[pic]

b) With an input of 1 kHz and 10 kHz sinusoidal voltage of 1 volt peak-to-peak, sketch the input and output waveforms.

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c) Change the input to 1 volt peak-to-peak square wave. Sketch the output waveform at 1 kHz and l0 kHz.

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2) With a capacitor of 300 pF across RF

a) Plot the frequency response of the amplifier from 10 Hz to 10 MHz.

[pic]

[pic]

b) With an input of 1 kHz and 10 kHz sinusoidal voltage of 1 volt peak-to-peak, sketch the input and output waveforms.

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c) Change the input to 1 volt peak-to-peak square wave. Sketch the output waveform at 1 kHz and l0 kHz.

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Analysis of Results

Correlate your measured results with what you expect theoretically.

Problems for further study

1. Explain the output waveform of the amplifier (square input waveform) for the case of having a capacitor across RF?

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2. What application can you think of which uses an amplifier with a capacitor across RF?

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3. If R1 is replaced by a series combination of R and C, what frequency response can you expect from the amplifier? Give one application of such a circuit.

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4. If Rf is replaced by a parallel RC circuit and R1 is replaced by Figure 2, what frequency response can you expect from the amplifier? Give one application of such a circuit.

[pic]

Figure 2

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5. Comment on the problems that you have encountered in this design, construction and measurement exercise.

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