TABLE OF CONTENTS By recognizing potential RF instability, you can protect yourself from unpleasant surprises. This article reviews RF circuit stability concepts and offers practical ways to guarantee stable operation.
By Les Besser
From APPLIED MICROWAVE & WIRELESS, VOL. 7, NO. 2, SPRING 1995
At low frequencies, the Nyquist criteria provides a safe indication of stability. As frequencies increase circuit and system designers face a more difficult and tedious task. A thorough stability analysis should be performed through a wide range of frequencies, possible terminations, and power levels. Since true broadband nonlinear models are generally not available for active devices, stability is evaluated at individual frequencies, based on small-signal device parameters.
A common mistake is to examine only the pass band of the system, which is not always sufficient. When out-of-band instability, particularly at low frequencies, is neglected, it may show up in unwanted oscillation. Then, the first incoming signal, even noise, can turn an intended amplifier into a comb generator.
While a complete nonlinear stability analysis is more than we can cover here, the following discussion, discussing linear small-signal circuit applications, also can be used as a start for large-signal analysis.
By using the appropriate feedback, an active two-port always can be turned into an oscillator at frequencies up to f max, the frequency at which the matched unilateral gain of the two-port is unity. Feedback may have been included by the engineer in the original design scheme or it may be completely unintentional. Oscillator...
