## Wednesday, October 5, 2011

### VOLTAGE-SERTES FEEDBACK AMPLIFIER Electronic devices and circuits lab manual

VOLTAGE-SERTES FEEDBACK AMPLIFIER
AIM: To study the effect of voltage series feedback on Gain of the Amplifier.
APPARATUS:
Transistor BC 107
Regulated Power Supply(0-30V,1A)
Function Generator
CRO(30 Mhz,dualtrace)
Resistors 33kΩ,3.3k Ω,1.5kΩ,1kΩ,2.2k Ω,4.7k Ω,330Ω
Capacitors   10µF                                           - 2Nos
100µF                                        - 1No

CIRCUIT DIAGRAM:

THEORY:
When any increase in the output signal results into the input in such a way as to cause the decrease in the output signal, the amplifier is said to have negative feedback.
The advantages of providing negative feedback are that the transfer gain of the amplifier with feedback can be stablised against varations in the hybrid parameteresof the transistor or the parameters of the other active devices used in the circuit. The most advantage of the negative feedback is that by propere use of this , there is significant improvement in the frequency respponse and in the linearity of the operation of the amplifier.This disadvantage of the negative feedback is that the voltage gain is decreased.
In Voltage-Series feedback , the input impedance of the amplifier is decreased and the output impedance is increased.Noise and distortionsare reduced cosiderably.

PROCEDURE:
1.    Connections are made as per circuit diagram.
2.    Keep the input voltage constant at 20mV peak-peak and 1kHz frequency.For different values of load resistance, note down the output voltage and calculate the gain by using the expression
Av = 20log(V0 / Vi ) dB
3.    Add the emitter bypass capacitor and repeat STEP 2.And observe the effect of  Feedback on the gain of the amplifier
4.    For plotting the frquency the input voltage is kept constant at 20mV peak-peak and the frequency is varied from 100Hz to 1MHz.
5.    Note down the value of output voltage for each frequency. All the readings are tabulated and the voltage gain in dB is calculated by using expression                                      Av = 20log(V0 / Vi ) dB
6.    A graph is drawn by takung frquency on X-axis and gain on Y-axis on semi log graph sheet
7.    The Bandwidth of the amplifier is calculated from the graph using the expression Bandwidth B.W = f2 – f1.
Where f1 is lower cutt off frequency of CE amplifier
f 2 is upper cutt off frequency of CE  amplifier
The gain-bandwidth product of the amplifier is calculated by using the expression
Gain-Bandwidth Product = 3-dB midband gain X Bandwidth.

OBSERVATIONS:
Voltage Gain:

 S.NO Output Voltage (Vo) with feedback Output Voltage (Vo) without  feedback Gain(dB) with feedback Gain(dB) without feedback

Frquency Response:  Vi = 20mV

 S.NO Frequency (Hz) Output Voltage (Vo­) Gain A = Vo­/Vi Gain in dB20log(Vo­/Vi)

MODEL WAVEFORMS:

PRECAUTIONS :
1.    While taking the observations for the frequency response , the input voltage must be maintained constant at 20mV.
2.    The frequency should be slowly increased in steps.
3.    The three terminals of the transistor should be carefully identified.
4.    All the connections should be correct.

RESULT:
The effect of negative feedback (Voltage -Series Feedback ) on the amplifier is observed. The voltage gain and frquency response of the amplifier are obtained.Also gain-bandwidth product of the amplifier is calculated.