# Classification (or) topologies of feedback Amplifiers

There are 4 different combinations possible with negative feedback in Amplifiers as given below

1. Voltage-Series.
2. Current-Series.
3. Voltage-Shunt.
4. Current-Shunt.

The first part represents the type of sampling at the output .

• i.e ,  Voltage- Shunt connection.
• Current-Series connection.

and the second part represents the type of Mixing at the input

• Series- Voltage is applied at the input.
• Shunt-Current is applied at the input.

For any Amplifier circuit we require

• High Gain
• High Band Width
• High Input Impedance
• and Low Output Impedance.

Classification of feedback Amplifiers is also known as feedback Topologies.

Voltage-Series feedback Connection:-

at i/p side connection is Series and at o/p side connection used is Shunt  since o/p is collected is voltage.

Series connection increases i/p impedance and Voltage at the o/p indicates a decrease in o/p impedance.

i.e, $Z_{if}&space;=&space;Z_{i}(1+A\beta&space;)$    and    $Z_{of}&space;=&space;\frac{Z_{o}}{(1+A\beta&space;)}$.

Current-Series feedback Connection:-

Series connection increases i/p impedance and Current at the o/p indicates an increase in o/p impedance.

i.e, $Z_{if}&space;=&space;Z_{i}(1+A\beta&space;)$    and    $Z_{of}&space;=&space;Z_{o}(1+A\beta&space;)$.

Voltage-Shunt feedback Connection:-

In this connection, both i/p and o/p impedance decreases .

i.e, $Z_{if}&space;=&space;\frac{Z_{i}}{(1+A\beta&space;)}$    and    $Z_{of}&space;=&space;\frac{Z_{o}}{(1+A\beta&space;)}$.

Current-Shunt feedback Connection:-

Shunt connection decreases i/p impedance and Current at the o/p indicates an increase in o/p impedance.

i.e, $Z_{if}&space;=&space;\frac{Z_{i}}{(1+A\beta&space;)}$    and    $Z_{of}&space;=&space;Z_{o}(1+A\beta&space;)$.

Effect of negative feedback on different topologies:-

 Type of f/b Voltage gain Band Width with f/b i/p impedance o/p impedance Voltage-Series decreases increases increases decreases Current-Series decreases increases increases increases Voltage-Shunt decreases increases decreases decreases Current-Shunt decreases increases decreases increases

Similarly negative feedback decreases noise and harmonic distortion for all the four topologies.

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p style=”text-align: justify;”>Note:-  for any of the characteristics in the above table, increase ‘s shown by multiplying the original value with $(1+A\beta&space;)$ and decrease ‘s shown by dividing with $(1+A\beta&space;)$.

## Author: Lakshmi Prasanna Ponnala

Completed M.Tech in Digital Electronics and Communication Systems.

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