Graphical approach to analysis and design of gain-boosted near-fmax feedback amplifiers

It is a challenge to design amplifiers operating near the transistor unity-power-gain frequency fmax. This paper presents a graphical chart (MAG-K chart) and associated analytic equations for analysis and design of near-/max gain-boosted feedback amplifiers. The graphical approach provides an efficient means of finding feedback networks that give the highest possible maximum available gain (MAG) for a given stability (K) at a target frequency. This is somewhat analogous to the Smith chart providing an intuitive means of synthesising matching networks. Each of the designs thus found can subsequently be checked for bandwidth and realisability. It is shown with simulated design examples that a better feedback network could be found that offers higher gain over a broader bandwidth than successful state-of-the-art designs. Feedback configurations taken up in the examples include not only the simple shunt-shunt and series-series feedback, but also more complex, tapped transformer feedback, which was recently successfully used in an InP millimetre-wave amplifier.