Design Theory of Balanced Transistor Amplifiers

Abstract

This paper discusses the expected characteristics of balanced transistor amplifiers with symmetrical directional couplers. Provided that pairs of transistors with similar characteristics can be selected from a given distribution, the input and output matches obtained with the balanced configuration are satisfactory over a ±10 per cent bandwidth with simple one-section lumped-constant LC directional couplers and over a ±40 per cent bandwidth (1.2 octaves) with one section distributed λ/4 couplers. For single-stage amplifiers, the decrease in gain is less than 0.1 db and the phase nonlinearities introduced by the couplers are about ±0.16° and ±0.6°, respectively, over the same bandwidths. The requirements on the terminations which are connected to the couplers to absorb the transistor reflections are not stringent: VSWR's less than 1.4 should be acceptable. The noise measure of balanced amplifiers is calculated to be a weighted average of the noise measures of the two component amplifiers, plus a small term which vanishes when the couplers have 3-db coupling and the component amplifiers have identical gains. Gain compression takes place at a 3-db higher signal level compared with conventional single-ended designs, and the expected improvement in the third-order intermodulation is 9 db on the average. In the final section, the cascade connection of identical balanced amplifiers is discussed. With typical microwave transistors, the input and output return losses for a multistage amplifier should be about 4.5 db worse than those for the individual single-stage amplifiers of which it is composed. The gain ripple introduced by the interactions between stages is also investigated in detail.