feedback amplifiers

The connection circuit in a closed loop such that a portion of the output is fed back to the input is called a feedback network.The feedback in which the output when applied to the input diminishes the output is called as negative feedback.



















A feedback amplifier therefore may be positive feedback or negative feedback depending on whether the output when fed back to the input enhances or decreases the output.

Classification of feedback amplifier :\

1.Voltage amplifiers:\
The voltage amplifiers have voltage output proportional to the input voltage signal and the proportionality factor is irrespective of the input source resistance and load resistance.

Voltage series amplifier has input terminals across a voltage source and output is drawn across the load resistance.



2. Current series feedback:
The output voltage is directly proportional to the input currenthe proportionality factor is irrespective of the source and load resistances.



3.Voltage shunt feedback:
The output cuurent is directly proportional to the input voltage.


4.Current shunt feedback:
The input and output current are in direct proprtion and the proptionality factor is independent of Rs and Rl.



An important table is given below:

frequency response of the transistor

The transistor symbol and characteristics have been discussed in a previous post.The transistor behaviour is different under low frequency and high frequency conditions.The transistor has two capacitances Cbe and Cbc between the base emitter and the base collector terminals.
1.Under low frequency conditions the reactance is high because of the low values of capacitance and frequency both.So it can be considered as open circuit.
2.On the other hand under high frequency conditions the capacitance and frequency values cause a considerable reactance,leading to a voltage divider configuration.Hence the gain of the circuit is impacted.

For low frequency analysis the two port equivalent circuit is used.
The h parameters are used to relate the voltage and current in output and input of a transistor. 
 v1=h11*i1+h12*v2
i2=h21*i1+h22*v2

h11=(v1/i1)at v2=0{input resistance at output short circuit}
h12=(v1/v2)at i1=0{reverse transfer ratio}
h21=(i2/i1) at v2=0{forward transfer ratio}
h22=(i2/v2)at  i1=0 {output conductance at input open circuit }

The transistor equivalent circuit at low frequency is as below:
 

The transistor model at high frquencies is shown below:


 

The Miller effect occurs only in inverting amplifiers –it

is the inverting gain that

magnifies the feedback capacitance.

 

Hi