Stability in Small Signal Common Base Amplifiers

This paper presents an analytical study of parasitic oscillation in video bandwidth common base amplifiers. We develop a series of progressively more complex small signal equivalent circuits based on the hybrid-π model [1], [2]. These circuits are developed such that the effect of each new parasitic element on circuit performance is evident. The conditions required for negative input resistance are developed for each equivalent circuit in turn. A general design `rule' of τ
<sub>L</sub>
<; τ
<sub>T</sub>
for stability is proposed. The common base amplifier is also treated as a Colpitts oscillator. Conditions required for stability are developed by assessing the conditions necessary for instability. The effect of employing `base stopper' resistors is also explored. Finally some general guidance on the successful layout of video bandwidth circuits is given.