EC2257 FEED BACK AMPLIFIER anna university lab manual - Computer Programming

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Monday, April 18, 2011

FEED BACK AMPLIFIER

AIM:
To design and test the current series and voltage shunt Feedback Amplifier and to calculate the following parameters with and without feedback.
1.   Mid band gain.
2.   Bandwidth and cutoff frequencies.
3.   input and output impedance.

APPARATUS REQUIRED:

 S.NO ITEM RANGE Q.TY 1 TRANSISTOR BC 107 1 2 RESISTOR 1 3 CAPACITOR 4 CRO (0-30 )MHz 1 5 RPS (0-30) V 1 6 FUNCTION GENERATOR (0 – 1 )MHZ 1

CURRENT SERIES FEEDBACK

DESIGN:  (Without Feedback ):

Given data :  Vcc = 15V , β = 0.9,  fL = 1kHz, Ic=1mA.
Stability factor = [2-10], Rs = 680W,
Av = 50dB , IE = 1.2mA .
Gain formula is given by
Av = -hfe RLeff / Z i
Assume,         VCE = Vcc / 2
RLeff = Rc    RL
re = 26mV / IE
hie = β re where re is internal resistance of the   transistor.
hie = hfe re
VE = Vcc / 10
On applying KVL to output loop,
Vcc = IcRc + VCE + IERE
VE = IERE
Rc = ?
Since IB is very small when compared with IC
Ic approximately equal to IE
RE = VE / IE = ?
VB = VBE + VE
VB = VCC . RB2 / RB1 + RB2
S = 1+ (RB           /RE )
RB = ?
RB = RB1    RB2
Find
Input Impedance , Zi = ( RB   hie )
Coupling and bypass capacitors can be thus found out.
Input coupling capacitor is given by , Xci = Z i / 10
Xci =   1/ 2pfCi
Ci = ?
output coupling capacitor is given by ,
Xco=(Rc   RL) / 10
Xc0 =   1/ 2pfCo
Co = ?
By-pass capacitor is given by ,XCE = 1/ 2pfCE
CE = ?
Design ( With feedback ) :
Remove the emitter capacitance ( CE )
β = -1 / RE
Gm =   - hfe/ [(hie + RE )      RB]
D = 1+ β Gm
Gmf =  Gm / D
Zif = Z iD
Zof = ZoD
CIRCUIT DIAGRAM: WITHOUT FEEDBACK:
+VCC

R1                   Rc         Co
Cin
B

BC107
E                        RL
Vo
CE
Vin               R2        RE
F = 1 KHz

WITH FEEDBACK:

+VCC

RB1                   Rc        Co

Cin
BC107
B
RL

Vin           RB2
RE

Voltage shunt DESIGN:  (Without Feedback ):

Given data :  Vcc = 15V ,  fL = 1kHz, Ic=1mA.
Stability factor = [2-10], Rs = 680W,
Av =40 dB .
Gain formula is given by
Av = -hfe RLeff / Z i
Assume,         VCE = Vcc / 2
RLeff = R c | | RL
re = 26mV / IE
hie = β re where re is internal resistance of the   transistor.
hie = hfe re
VE = Vcc / 10
On applying KVL to output loop,
Vcc = IcRc + VCE + IERE
VE = IERE
Rc = ?
Since IB is very small when compared with Ic
Ic approximately equal to IE
RE = VE / IE = ?
VB = VBE + VE
VB = VCC . RB2 / RB1 + RB2
S = 1+ RB /  RE
RB =?
RB = RB1|| RB2
Find
Input Impedance, Zi = (RB || hie )
Coupling and bypass capacitors can be thus found out.
Input coupling capacitor is given by , Xci = Z i / 10
Xci = 1/ 2pf Ci
Ci = ?
output coupling capacitor is given by ,
X co=(Rc | | RL) / 10
Xc0 = 1/ 2pf Co
Co =?
By-pass capacitor is given by, XCE = 1/ 2pf CE
CE =?
Design ( With feedback ) :
Connect the feedback resistance (Rf) and feedback capacitor (Cf) as shown in the figure.
Xcf = Rf / 10
Cf = Rf / 2πf x 10
Assume,   Rf = 68 KW
β = -1 / Rf
Trans – resistance Rm = - hfe (RB| | Rf ) (Rc | | Rf ) / (RB| | Rf ) + hie
D = 1+ β Rm
Avf = Rmf / Rs             Rmf = Rm / D
Zif = Zi / D
Zof = Zo / D

CIRCUIT DIAGRAM: Voltage shunt feedback
WITHOUT FEEDBACK:

+VCC

R1                   Rc         Co
Cin
B

BC107
E                        RL
Vo
CE
Vin               R2        RE
F = 1 KHz

WITH FEEDBACK:
+VCC

RB1    RF          Rc        Co

RS Cin
BC107
B
RL

Vin           RB2
RE

MODEL GRAPH(WITH & WITHOUT FEEDBACK)

Without feedback

3 dB

GAIN
(db)                          3dB                          With feedback

f3      f1                          f2         f4       f(Hz)

f2 – f1 = Bandwidth of without feedback circuit
f4 – f3 = Bandwidth of with feedback circuit
THEORY:
An amplifier whose function fraction of output is fed back to the input is called feed back amplifier. Depending upon whether the input is in phase or out of phase with the feed back signal, they are classified in to positive feed back and negative feed back. If the feed back signal is in phase with the input, then the wave will have positive gain. Then the amplifier is said to have a positive feed back.
If the feed back signal is out of phase with the input ,then the wave will have a negative gain. The amplifier is said to have a negative feed back. The values of voltage gain and bandwidth without feed back.

PROCEDURE:

The connections are made as shown in the circuit. The amplifier is checked for its correct operation .Set the input voltage to a fixed value. Keeping the input voltage Vary the input frequency from 0Hz to 1MHz and note down the corresponding output voltage. plot the graph : gain (dB) vs frequency .Find the input and output impedances. Calculate the bandwidth from the graph. Remove RE and follow the same procedure.

OBSERVATION:

WITH OUT FEEDBACK

Vin = ------------ Volts
S.NO
FREQUNCY
O/P voltage Vo

S.NO
FREQUNCY
O/P voltage

Av=20 log Vo/Vi

RESULT:

Theoritical                           Practical
 With F/B Without F/B With F/B Without F/B Input Impedance Outputimpedance Bandwidth Transconductance (gm)