An Amplifier is an Electronic circuit which can enlarged the strength of the signal. We can classified amplifiers in many ways.Depending on the operating condition of transistor or other active element used, frequency range of operation we can classified. In this topic we will discuss about What is Feedback Amplifier circuit.
What is Feedback Amplifier?
Feedback Amplifier is a part of the output signal which is returned on the input signal.In simple words an amplifier using such feedback is known as feedback amplifier. If the feedback signal is in phase with the input signal it increases the strength of effective input signal. It consequently increases the overall gain.
Feedback Types:
1. Positive Feedback: Positive feedback is a special type of feedback where feedback current or voltage is used to increase the input voltage.When we connect directly the output to the input then it is called Positive feedback.
2.Negative Feedback: Negative feedback is a special type of feedback where feedback current or voltage is used to reduce the input voltage or current.
Principles of a Feedback Amplifier:
This figure shows the general block diagram of a generalized feedback amplifier. On the other hand In the absence of the feedback network the input signal Vs is amplified to an output signal Vo=A.Vs .Where A is the intrinsic gain of the basic amplifier. A is called the open loop gain. In the presence of the feedback network a fractiion B of the output voltage Vof is feedback to the input such that the effective input becomes Vi=Vs+BVof. Where B is known as the feedback ratio.
Now the output of the amplifier with feedback is
Vof=A.Vi=A(Vs+B.Vof)
or, Vof(1-AB)=A.Vs
Therefore,overall gain with feedback is
Af = Vof/Vs = A/(1-A.B)
The quantity A.B is known as the loop gain. Af is sometimes called closed loop gain. If A.B is real , positive and less than unity.
|1-A.B|<=1 and |Af| > |A|.
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