Amplifier

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               Amplifier 


An amplifier is a circuit which is used for increase the voltage, current or power of alternating form by using  transistor. 
                          

                               Note

(1). In this blog we will use n-p-n transistor  to show you guys how amplifier works. 
(2). Amplifier can not amplify direct current (D.C). 

Circuit diagram


Working

W. K. T
I(E) = I(B) + I(C) 

W. K. T for n-p-n transistor
I(E) = I(B) + I(C) 

Now as we can see from circuit diagram 
Voutput = V(CE) - I(C) R(L)       .... (1) 

As I(E) increases
For first positive half cycle emitter base become forward biased, offers low resistance and hence I(E) increases also I(C) increases and finally "I(C) R(L)" Also increases. 
Therefore acc. To equation (1) 

V output become more negative

For negative half cycle emitter base become reverse biased  offers high resistance, I(C) decreases  and hence, I(C) R(L)  also decreases. 
Therefore, 
Voutput become more positive. 

Finally, we get an amplified output of 180° phase reversal. 


                                       Note

The same type of working process we will get with  the use of p-n-p transistor in amplifier. 



 Current, voltage and power gains of a common emitter amplifier

(1). AC current gain 

       It is defined as the ratio of small change in collector current ( Del.IC) to the base current (Del.Ib), when collector emitter voltage remains constant it is denoted by [ (Beta)ac ] and given as

   (Beta)ac = [(Del.Ic) / (Del.Ib) ]

(2). DC current gain

        It is define as the ratio of collector current 'Ic' and base current 'Ib' when collector emitter current is constant which is denoted by (Beta)dc and given as
                         (Beta)dc = [(Ic) /(Ib) ]

(3). AC voltage gain 

        It is define as the ratio of small change in output voltage to the small change in input voltage
Given as             
                      Av = Voutput/Vinput or  Av =(Del. Vce)
                                                                          /(Del.Vbe)

(4). AC power gain

        It is define as the ratio of small change in  the output power to the input power. Denoted by Ap
Given as 
                 Ap = output power/input power

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