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|.
Types of Feedback Connection:
1. Voltage Series Feedback Circuit: In the Voltage series feedback circuit the output voltage Vo is sampled by connecting the feedback network. Here the feedback network is parallel with the load.Although This makes feedback signal proportion to the output voltage. The feedback voltage Vf is added in series with an input voltage source. Thus it is an example of voltage-series feedback circuit.
2. Voltage Shunt Feedback Circuit: In this type of network feedback current is proportional to the output voltage and this feedback current is fed in parallel with the input current source. Thus it represents a Voltage-Shunt feedback.
3. Current Series Feedback Circuit: In this type of network the feedback circuit is connected in series with the load and it produces a feedback signal proportional to the output current io. Even So it is a current feedback. Since the feedback voltage is applied in series with an input voltage source it is a Current-series feedback.
4. Current Shunt Feedback: In this type of feedback current is proportional to the output current io and this feedback current current is fed in parallel with an input current source. Thus it is an example of Current-Shunt feedback.
Advantages and Disadvantages of Negative Feedback:
1. Negative feedback stabilizes the gain of an amplifier against circuit parameters and voltage supply variations.
2. It reduces nonlinear , phase and frequency distortions.
3. It can reduce internal noise in an amplifier.
4. It increases the bandwidth and thus improves the frequency response of an amplifier.
5. Input and Output impedances can be modified suitably by using negative feedback.
Inspite of having some advantages over feedback circuit there is also a disadvantage:
1. It reduces the overall gain of an amplifier.
Applications of Feedback Amplifier
1. The feedback circuit is widely used in Regulated Power Supply.
2. In the Oscillators circuit Positive feedback are used.
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