Positive Feedback and Oscillators

Positive Feedback and Oscillators

• Consider a noninverting amplifier with voltage gain A as shown in the Fig. 10.2.1.

• A sinusoidal input Vs is applied to noninverting amplifier.

• Due to noninverting nature, Vs and Vo are in phase.

• The part of output is fed back through feedback network which does not introduce any phase shift. But the amplitude of feedback signal is decided by the feedback factor p such that Vf = βVo.

• Thus the input and feedback voltage i.e. Vs and Vf are in phase hence it is called positive feedback.

1. Gain with Positive Feedback (A_f)

• The expressions for the gains are,

• For a constant A, for various increasing values of β the gain A_f increases. At β = 1 / A, 1 - A β = 0 and A_f → ∞. This indicates that circuit produces output without input i.e. Vs = 0. The circuit gets driven by the feedback signal and produces the oscillations. Thus circuit stops amplifying and starts oscillating.

• The positive feedback increases the closed loop gain and at β = 1 / A , the circuit works as an oscillator.

• It must be noted that P is always fraction and P < 1 and circuit adjusts itself such that AP = 1 when it produces sinusoidal sustained oscillations, working as an oscillator.

Review Question

1. Explain the concept of positive feedback and derive the expression for gain with feedback. When the circuit works as an oscillator ?