VOLTAGE-SERIES FEEDBACK AMPLIFIER

Aim: To plot the frequency response of a Voltage series feedback amplifier and to study the

effect of feedback on gain and Bandwidth.

Apparatus:

Sl. No

Name of the component

Specifications or

range

Quantity

1

Transistors

BC547 or

BC548

2

2

Resistors

8.2KΩ,100KΩ,4.7KΩ,

, 820Ω, , 620Ω, 120Ω,

4,2,1,1,1,1

3

Capacitors

10µF,100µF ,1KPF

3,2,1

4

CRO

20MHz

1

5

Function generator

2MHz

1

6

Regulated power supply

(0-30)V,1A

1

7

Connecting wires and probes

-----

Required no

Theory: Negative feedback is defined as a process of returning a part of the output signal to

the input out of phase with the input signal. It reduces gain and increases bandwidth.

Negative feedback is employed in amplifier circuits to improve the stability of the gain,

reduce distortion and the effect of noise. It also helps in obtaining desired values of input and

output resistances.

A voltage series feedback amplifier samples output voltage and returns the feedback

signal to the input in series opposing. Feedback signal is a voltage signal

𝑉𝑓 = 𝛽𝑉𝑜

voltage series feedback increases input resistance and decreases output resistance.

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Circuit Diagram:

Model graphs:

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Experimental procedure:

1. Connections are made as per circuit diagram.

2. The switch must be connected to S2 (4.7KΩ is connected to ground) the circuit does

not have feedback.

3. Check the DC conditions.

4. Apply the input A.C.signal of peak to peak amplitude 3mV or 2mV. Vary the

frequency of input signal from 30Hz to 1000KHz in appropriate steps and note Vo in

each case calculate gain without feedback Av.

5. Plot the graph between gain in dB Vs frequency.

6. Now connect the switch to positionS1.adjust the input voltage such that the output

produced by the circuit with feedback must be distotionless.

7. Repeat the steps 3 to 5 and compare gain and bandwidth with and without feedback.

Observations:

Tabular form: without feedback with input voltage Vi = 3mV.

Sl.No

Output voltage

(pk-pk)

Vo

Voltage gain

Av = Vo/Vi

Gain in (dB)

𝐴𝑣 = 20 𝑙𝑜𝑔𝑉𝑜

𝑉𝑖

100 Hz

1000KHz

Tabular form: with feedback with input voltage Vi = 20mV.

Sl.No

Output voltage

(pk-pk)

Vo

Voltage gain

Av = Vo/Vi

Gain in (dB)

𝐴𝑣 = 20 𝑙𝑜𝑔𝑉𝑜

𝑉𝑖

100 Hz

2MHz

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Precautions:

1. All connections must be done carefully.

2. Switch off power supply before making connections.

Result: Thus the operation of current shunt feedback amplifier with and without feedback

is observed and the results are

Gain without feedback-------------- Gain with feedback--------------

Bandwidth without feedback-------------- Bandwidth with feedback--------------.

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