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Monday, April 18, 2011

EC2257 FEED BACK AMPLIFIER anna university lab manual


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        
Oval: CRO                                                  E                        RL
                                                                                                          Vo
                                                              CE
           Vin               R2        RE                       
    F = 1 KHz


















 



WITH FEEDBACK:

                                                                           +VCC  







 

                             RB1                   Rc        Co
                                                                                       
                        Cin    
                                                            BC107   
                                           B
Oval: CRO                                                                          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        
Oval: CRO                                                  E                        RL
                                                                                                          Vo
                                                              CE
           Vin               R2        RE                       
    F = 1 KHz


















 




WITH FEEDBACK:
                                                          +VCC  







 

                             RB1    RF          Rc        Co
                                                             
                        RS Cin    
                                                                           BC107                  
                                           B
Oval: CRO                                                                          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

     Gain

Av=20 log Vo/Vi






                            WITH FEEDBACK

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




Output
impedance




Bandwidth




Transconductance (gm)






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