Method of Identifying Feedback Topology and Analysis of a Feedback Amplifier

Method of Identifying Feedback Topology and Analysis of a Feedback Amplifier

• To analyse the feedback amplifier it is necessary to go through the following steps.

Step 1 : Identify topology (Type of feedback )

a. To find the type of sampling network

1. By shorting the output i.e. V_o = 0, if feedback signal (x_f) becomes zero then we can say that it is "Voltage Sampling".

2. By opening the output loop i.e. I_o = 0, if feedback signal (x_f) becomes zero then we can say that it is "Current Sampling".

b. To find the type of mixing network

1. If the feedback signal is subtracted from the externally applied signal as a voltage in the input loop, we can say that it is "series mixing".

2. If the feedback signal is subtracted from the externally applied signal as a current in the input loop, we can say that it is "shunt mixing".

• Thus by determining type of sampling network and mixing network, type of feedback amplifier can be determine. For example, if amplifier uses a voltage sampling and series mixing then we can say that it is a voltage series amplifier.

Step 2 : Find the input circuit

1. For voltage sampling make V_o = 0 by shorting the output

2. For current sampling make I_o = 0 by opening the output loop.

Step 3 : Find the output circuit.

1. For series mixing make I_i = 0 by opening the input loop.

2. For shunt mixing make V_i = 0 by shorting the input

Step 2 and step 3 ensure that the feedback is reduced to zero without altering the loading on the basic amplifier.

Step 4 : Optional. Replace each active device by its h-parameter model at low frequency.

Step 5 : Find the open loop gain (gain without feedback), A of the amplifier.

Step 6 : Indicate X_f and X_o on the circuit and evaluate β = X_f X_o.

Step 7 : From A and p find D, A_f R_if, R_of, and R_of 

Review Questions

1. Compare voltage series and current shunt feedback amplifier for the following points :

i) Block diagram ii) R_i with R_s iii) R_o with R_L iv) Feedback gain v) Feedback factor.

2. Explain the typical methodology used for feedback amplifier analysis.