FEEDBACK AMPLIFIERS

Presented By Harit MohanB-tech (CSE-CCVT) 3rd semUniversity of technology and ManagementShillong

OUTLINE

Definition Classification of Basic Amplifiers Amplifier without Feedback or with Feedback Basic Concept of Feedback General characteristics of negative-feedback amplifiers Effect of negative feedback on input-output resistance Method of identifying Feedback

Definition

Feedback is defined as the process in which a part of output signal (voltage or current) is returned back to the input.The amplifier that operates on the principle of feedback is known as feedback amplifier.

CLASSIFICATION OF AMPLIFIERS• Voltage Amplifier- An amplifier provides a voltage output

proportional to the voltage input and the proportionality factor does not depend on the magnitudes of the source and load resistance

• Current Amplifier- An amplifier which provides an output current proportional to the signal current .

• Transconductance Amplifier- An amplifier in which, the output current is proportional to the signal voltage, independent of the magnitudes of source and load resistance.

• Transresistance Amplifier- An amplifier in which output voltage is proportional to the signal current of the magnitudes of source and load resistance.

Amplifier Without Feedback

• The most important thing to understand from Fig. is that the output and input terminals of this amplifier are not connected to each other in any way.

• Therefore the amplifier of Fig. is an amplifier without any feedback,

Gain without feedback.

Amplifier With Feedback

• Refer to Fig. Here the same amplifier with a gain A is being used along with a mixer network, sampling network and a feedback network.

• The voltage gain of the feedback amplifier is given by,

BASIC CONCEPT OF FEEDBACK

• In each one of these circuits we may sample the output voltage or current by means of a suitable sampling network and apply this signal to the input through feedback two-port network.

• At the input the feedback signal is combined with the external (source) signal through a mixer network and is fed into the amplifier proper.

Types of feedback

1. Positive feedback 2. Negative feedback.

If the original input signal and the feedback signal are in phase, the feedback is called as positive feedback.

However if these two signals are out of phase then the feedback is called as negative feedback.

General characteristics of negative feedback amplifier

Stabilization of voltage gain Decreasing output impedance

Increasing input impedance Decreasing distortion

Effect of negative feedback on input-output resistance

• Input resistance- feedback amplifiers ,series mixing at the input tends to increase the input resistance and shunt mixing tends to decrease the input resistance.

• Output resistance feedback amplifiers, voltage sampling at the output tends to decrease the output resistance and current sampling tends to increase the output resistance.

Method od identifying feedback topologyStep1 -To find the type of sampling network• By shorting the output if feedback signal becomes

zero, then it is called “voltage sampling “• By opening the output loop if feedback signal

becomes zero, then it is called “current sampling”.

Step 2- To find the type of mixing network• If the feedback signal is subtracted from the

externally applied signal as a voltage in the input loop , it is called “series mixing”.

• If the feedback signal is subtracted from the externally applied signal as a current in the loop it is called “shunt mixing”.

Thus by finding the type of sampling network and mixing network ,type of feedback amplifier can be determined.

Thank you…