Cause of instability of power amplifier with parallel-connected power transistors

Abstract

Power amplifiers with parallel-connected power transistors tend to be less stable than similar ones which use only one power transistor. Some designers believe that the reduced stability results from the larger collector-base capacitance C/SUB cb/ of the paralleled transistors making a larger degree of output-input feedback or making a larger nonlinear C/SUB cb/, thereby increasing the tendency toward parametric oscillation. Neither of these views is correct, as is shown here. The increased tendency toward oscillation results from the creation of additional modes of possible oscillation; these do not exist in the amplifier, which has only one transistor in a given stage of the amplifier. A preferred method is shown of connecting transistors in parallel, to minimize inequality of sharing input drive and output load and to minimize the tendency toward some modes of oscillation.