Feedback amplifier

Feedback amplifier

The propose of injection of some part of output of output voltage to the input supply for an amplifier is called as feedback amplifier in which it forms the part of input signal which determines the output. The amplifier in which a fraction of output voltage is feedback to it’s input is called as feedback amplifier. Thus the feedback amplifier consist of two different parts 1) amplifier circuit 2)feedback path . The fraction of output voltage that is given to input from source is called as feedback voltage (\(v_f\)). The ratio of this feedback voltage to the output voltage of amplifier is called as feedback ratio ‘\(\beta\)’.i.e. for open loop ,

$$\beta=\frac{v_f}{v_\circ}$$Depending upon type of feedback given to the feedback amplifier of two type:

1. Positive feedback amplifier

The feedback amplifier in which the feedback voltage given to input is in sane phase to that of input signal from source is called as positive feedback amplifier. Thus the gain of amplifier increases during positive feedback doe to increase in current. The positive feedback amplifier is seldom used due to excessive distortion. However, it is used in oscillator circuit.

2. Negative feedback amplifier

The feedback amplifier in which feedback voltage which is given to input is in output of phase to that of input is called as negative feedback amplifier. The gain of negative amplifier decreases due to decrease in input current. However, It is commonly used in different circuit due to large number of significant.The applied negative feedback improves performance (gain stability, linearity, frequency response, step response) and reduce sensitivity to parameter variation due to manufacturing or environment. Because of these advantages, many amplifier and control system use negative feedback.

Advantages:

1. Can increase or decrease input impedance (depending upon the type of feedback)
2. Can increase or decrease output impedance(depending upon type of feedback)
3. Reduce distortion(increase linearity)
4. Increases the bandwidth
5. Desensitizes gain to component variations
6. Can control step response of amplifier

Disadvantages:

1. May lead to instability if not designed carefully
2. Amplifier gain decreases
3. Input and output impedance of negative feedback amplifier (closed loop amplifier) become sensitive to the gain of an amplifier without feedback(open loop amplifier)-that exposes these impedance too variation in the open loop gain, for example, due to parameter variations or nonlinearity of the open-loop gain.
4. It becomes noisy with passage of long time
5. It provides poor impedance matching between the stages.

Principle of feedback amplifier

Let A be the open loop gain of amplifier having ouput voltage \(v_\circ\) and input voltage \(v_{in}\). Then,

\(A=\frac{v_\circ}{v_{in}}\dotsm(1)\)(without feedback)

Let ud take feedback path in which some fraction of output voltage is given to inp[ut voltage in which the feedback ratio ‘\(beta\)’ as in the circuit. Then the feedback ratio is given by,

$$\beta=\frac{v_f}{v_\circ}$$

$$v_f=\beta v_\circ\dotsm(2)$$Nowe the total input voltage (\(v_{in}\)) is given by,

$$v_{in}’=\pm v_f$$Where ‘+’ sign for positive feedback and ‘-‘ sign for negative feedback .

$$v_{in}’=v_{in}\pm \beta v_{\circ}’\dotsm(3)$$

Let A be loop gain of amplifier with input voltage \(v_{in}’\) and output voltage \(v_{\circ}’\) then,

$$A=\frac{v_{\circ}’}{v_{in}’}$$

$$\Rightarrow v_{\circ}’=A v_{in}’\dotsm(4)$$Using (3) in (4) we get,

$$v_{\circ}’=A(v_{in}\pm \beta v_{\circ}’)$$

$$v_{\circ}’(1\pm A\beta)=A v_{in}$$

$$\therefore \frac{v_{\circ}’}{v_{in}}=\frac{A}{1\pm A\beta}$$this is the overall voltage gain of the feedback amplifier which is also called as closed loop gain. The factor A\(\beta\) is called as feedback factor.

Here,\(A’=\frac{A}{1-A\beta}\) , for positive feedback

\(A’=\frac{A}{1+A\beta}\), for negative feedback

There are different forms of negative feedback amplifier such as,

1. Voltage shunt feedback
2. Voltage series feedback
3. Current shunt feedback
4. Current series feedback

Applications:

1. R-C coupled amplifier provides excellent audio-fidelity over a wide range of frequencies.
2. Therefore they are widely used as voltage amplifier e.g.in the initial stages of public system.

References:

(1)Theraja, B.L. Basic Electronics. N.p.: S.Chand, n.d. Print.

(2)C.L.Arora. Refresher Course in Physics. Vol. II and III. N.p.: S.Chand, 2006. Print.

(3)Malvino. Electronic Principles. N.p.: Tata McGraw-Hill, n.d. Print.

(4)N.Nelkon and P.Parker. Advanced Level Physics. 5th ed. N.p.: Arnold Heinemann, n.d. Print.

(5)Priti Bhakta Adhikari,Diya Nidhi Chaatkuli, Ishowr Prasad Koirala. A Textbook of Physics (2nd Year). N.p.: Sukunda Pustak Bhawan, 2070. Print.