Feedback amplifier pdf in hindi

There are four types of feedback topology

Current series feedback amplifier
Voltage series feedback amplifier
Current shunt feedback amplifier
Voltage shunt feedback amplifier

Let’s discuss each of the feedback amplifiers briefly

Current series feedback amplifier

In this feedback amplifier, both the input and output impedance are increased.
The feedback circuit is placed in series with the input and output.
Here a fraction of the output voltage is applied in series with the input voltage in the feedback circuit.

Voltage series feedback amplifier

The feedback circuit is connected in shunt with the output in such a way that it decreases the output impedance and increases the input impedance.
In this circuit, it is placed in a shunt with the output but in series with respect to the input signal.

Current shunt feedback amplifier

It increases the output impedance and because of connecting the feedback circuit in parallel with the input, the input impedance is decreased.
Here, the feedback circuit is placed in series with the output and in parallel with the input.

Voltage shunt feedback amplifier

Here the feedback circuit is placed in a shunt with respect to output and input as well.
It decreases the input and output impedance.

Advantages and Disadvantages of Feedback Amplifier

The followings are the advantages and disadvantages of feedback amplifiers

Advantages

Improves stability gain of the circuit by negative feedback amplifiers.
Input resistance can be increased by selective configurations.
Similarly, output resistance can be decreased.
Reduces noise and distortion by negative feedback.

Disadvantages

Increased noise and distortion in positive feedback.
Can reduce gain.
Has an increased output gain for current series and current shunt feedback amplifiers.

Application of Feedback Amplifier

The following are the applications of feedback amplifiers

They can be used in regulated power supplies.
They are generally used in amplifiers where large bandwidth is required.
Generally used in electronic amplifiers.

Concept of Feedback

A feedback amplifier uses feedback from the output of the amplifier back to the input to enhance its performance. This feedback is achieved through a feedback loop and it can be either positive or negative.

Lets discuss each block of feedback amplier:

Signal Source : The signal source is generally a voltage source, represented by the symbol \(V_s\) , with source current \(I_s\) . The signal source can be connected either in series or in parallel with the resistor in the electronic circuit.
Feedback network : It is a linear two port network which contains different components of the electronic circuit, like resistors, industors, capacitors etc.
Sampling Network : A sampling network extracts the output signal from a feedback amplifier and “feeds” it back to the input signal.

Sampling networks are of two types: current sampling network and voltage sampling network.

Mixer : A mixer mixes the source signal and feedback signal to produce either a positive feedback or a negative feedback. It is also known as a comparator and can be connected in series or parallel.

Gain Stability Relation

The gain of a feedback amplifier is defined as the ratio of output voltage with respect to the input voltage of a circuit.

The gain stability of a feedback amplifier refers to the ability of the amplifier to maintain a constant gain irrespective of the changes in the operating conditions.

For an ideal system of a feedback amplifier, the gain of an amplifier is always infinite.

For a smaller input signal, the value of output signal is much higher. Hence a large value of the gain is not desirable in the required circuit.

The system only becomes stable when the value of gain is much smaller by employing a negative feedback into the loop.

For a negative feedback amplifier,

The loog gain margin can be mathematically expressed as,

A = open loop gain of the amplifier

\(\beta \) is the feedback gain factor.

Unity Feedback system

Unity feedback is a type of feedback in a feedback amplifier system, where the output feedback is fed to the input signal with a gain of 1.

Refer to the image above,

According to the definition,

The gain of feedback = 1, hence,

For a positive feedback amplifier,

For a negative feedback amplifier,

Negative feedback gain provides a much better value for the unit feedback system.

Summary of Feedback Amplifiers

Summarizing the concept of feedback amplifiers in the following points:

Voltage series feedback : It is generally used in both inverting and non-inverting amplifier circuits. This type of feedback is useful to improve the performance of amplifier circuits.

It is a voltage-controlled voltage source type of amplifier (VCVS).

The formula for input impedance: \(r_i(1+A\beta )\)

The formula for output impedance: \(\frac<(1+A\beta )>\)

\(r_i\ \&\ r_o\) are the input and output resistances.

A = open loop gain of the amplifier

\(\beta \) is the feedback gain factor.

Current shunt feedback : This increases the output impedance by connecting the feedback circuit in parallel to the input impedance.

It is a current-controlled current source type of amplifier.

The formula for input impedance: \(\frac<(1+A\beta )>\)

The formula for output impedance: \(r_o(1+A\beta )\)

Current series feedback : Here, both input and output impedance is increased and is a voltage-controlled current source type of amplifier.

The formula input impedance: \(r_i(1+A\beta )\)

The formula output impedance: \(r_o(1+A\beta )\)

Voltage shunt feedback : This circuit decreases the input and output impedance and is a current-controlled voltage source type of amplifier.

The formula input impedance: \(\frac\)

The formula output impedance: \(\frac\)

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