Verifying Rollett’s Proviso on Active Devices Under Arbitrary Passive Embeddings

Some results implicit in Ohtomo’s original paper are reappraised and built upon to help high-frequency circuit designers avoid common misconceptions. An analysis of the small-signal stability of an intrinsic device is presented, based on a closed-form evaluation of the network determinant in admittance representation and on the relevant Nyquist plot. Since the intrinsic is studied as a three-port, the proposed method represents a rigorous tool for checking the fulfillment of the inherent-stability proviso under arbitrary passive embeddings. Conversely, the proviso is often simply assumed to hold, which can in principle lead to mistakenly predicted circuit stability, or even unconditional stability. Examples of the proposed graphical analysis are provided both for the intrinsic circuit of an actual device and for a modified version thereof, i.e., suitably tweaked for illustrative purposes. The latter example is then expanded to give the reader a full picture of the small-signal stability analysis process, both in a flawed and a correct form.

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