Predicted Small-Signal Gain of the HBT Distributed Amplifier with Practical Terminations

Recent interest in the heterojunction bipolar transistor (HBT) distributed amplifier (DA) has led to a need for an analytical expression predicting its small-signal gain. Previously published DA analysis has almost always assumed ideal characteristic impedance terminations as a basis. This assumption introduces inaccuracy into the prediction of the performance of a practically terminated DA, which has been found to be more significant in the case of the HBT DA than in the case of the MESFET DA. This paper describes an analytical expression which has been developed to predict the small-signal gain of an arbitrarily terminated DA. By removing the restriction of characteristic impedance terminations from the analysis, better agreement with the simulated performance of an HBT DA with practical terminations has been obtained. Emphasis has also been placed on basing theoretical analysis on a simplified equivalent HBT model which is, as far as possible, physically meaningful.