Feedback Amplifier :
In an ideal amplifier , mid -frequency region , provides an output signal , which is same as input signal . However , in practical amplifiers differs from this ideal output response due to non-linearity character of transistor , parameter variation ,temperature effect etc . To get desired degree of improvement , principal of feedback is used . The feedback is a process of injecting some part of output response to the input side . The amplifier which use the feedback principal , is called as feedback amplifier .
Principal of Feedback Amplifiers :
We know that voltage gain of open loop amplifier (without feedback) is given as Av = Vo / Vin . This Voltage gain (Av) is also known as open loop gain . Now in the feedback circuit of amplifier the values of amplifier gain , input and output voltages changes .
Let, Vo' be the output voltage of feedback amplifier.
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| Block diagram of a voltage series feedback amplifier. |
ꞵ be the fraction of output voltage fed to the input . It is known as feedback ratio
Av' be the voltage gain of feedback amplifier.
Av' be the voltage gain of feedback amplifier.
The Feedback amplifier sends a fraction (ꞵ) of output voltage Vo' and return it to the input voltage , which is in total become equal to Vin + ꞵ.Vo' , if feedback voltage is in phase with input voltage and Vin - ꞵ.Vo' , if feedback voltage is out of phase with input voltage. This input voltage is amplified by the circuit and gives output as :
Vo' = Av ( Vin + ꞵ.Vo' )
or
Av ' = Vo' / Vin = Av /( 1 - ꞵ.Av)
Av ' is called as voltage gain of feedback amplifier .
For positive feedback , the feedback ratio (ꞵ) is positive and for negative feedback , it is negative .
Therefore , for positive feedback Av' = Av / (1 - ꞵ. Av)
for negative feedback Av' = Av / (1 + ꞵ. Av)
Voltage-Series Feedback Amplifier :
In this model , a fraction of output voltage is applied in series with the input voltage through feedback network . While the input to the feedback network is parallel with output of amplifier. In the voltage series feedback connection the input resistance increases of feedback amplifier and the output resistance decreases. This characteristics helps to build desired cascade amplifier that is why voltage series feedback amplifier is preferred for cascaded amplifier .
The input resistance is given as
R1 = ( 1 + ꞵ.Av) . Ri
where Ri is input resistance without feedback
Av is voltage gain without feedback
and , the output resistance is given as
R2 = Ro / ( 1 + ꞵ.Av)
where Ro is output resistance without feedback.
Voltage series feedback circuit reduces the voltage gain of the voltage series feedback amplifier and it is given as :
Av' = Av / (1 + ꞵ.Av ).
Current-Series Feedback Amplifier :
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| Block Diagram of Current-Series feedback amplifier |
This feedback connection network increases both output resistance and input resistance by factor of ( 1 + ꞵ.Av )
The input resistance is given as
R1 = ( 1 + ꞵ.Av) . Ri
and , the output resistance is given as
R2 = ( 1 + ꞵ.Av) . Ro