# Nyquist Criterion

The Nyquist criterion states that the amplifier is unstable if the Nyquist plot encloses the point (â€“1+*j*0), and the amplifier is stable if the curve does not enclose the point (â€“1+

*j*0). The Nyquist criterion for positive and negative feedback is represented in the complex plane as depicted in Fig.

**Locus of |1 + ÃŸ A|**

**Locus of Î² A in the complex plane for a feedback circuit which acts as negative feedback from f = 0 to f = f_{1} and acts as positive feedback form f = f_{1} to f = âˆž**

**Nyquist plot of feedback amplifier:(a) Unstable (b) Stable**