Figure 1: Positive Feedback Amplifier.
When the feedback signal (Xf) is in phase with the source signal (Xs), then such type of feedback is referred to as positive feedback or regenerative feedback amplifier. A feedback amplifier circuit is illustrated in figure 1.
In positive feedback amplifier, the total effect of feedback increases the input signal of the amplifier (i.e., Xi = Xs + Xf) which intern increases the input voltage of the amplifier. As a result, the output voltage also increases exponentially. The gain of the amplifier with positive feedback is,
\[{{A}_{f}}=\frac{{{X}_{o}}}{{{X}_{o}}}\]
\[=\frac{{{X}_{o}}}{{{X}_{i}}-{{X}_{f}}}\text{ }\left[ {{X}_{i}}={{X}_{s}}+{{X}_{f}}\right]\]
\[=\frac{1}{\frac{{{X}_{i}}}{{{X}_{o}}}-\frac{{{X}_{f}}}{{{X}_{o}}}}\]
\[=\frac{1}{1-A\beta }\]
\[{{A}_{f}}=\frac{A}{1-A\beta }\]
Where,
Af – Gain of the feedback amplifier
A – Gain of the amplifier
Aβ – Loop gain.
In an amplifier with positive feedback, |Af| > |A|
If |Aβ| = 1, then from equation (1) becomes,
\[{{A}_{f}}=\frac{A}{1-1}\] \[{{A}_{f}}=\infty \]
As the gain of the feedback amplifier is infinite, the amplifier gives A.C output without A.C input signal. Hence, the amplifier behaves as an oscillator.
Advantages of Positive Feedback Amplifier
The main advantage of positive feedback is that it produces high gain i.e., gain is increased by a factor of (l + Aβ).
Disadvantages of Positive Feedback Amplifier
- It decreases the stability of the circuit.
- Distortion is more in this circuit.
- It is highly immune to noise.