Highly linear amplification with transistors

Highly linear amplification can be obtained with HF diffused-base transistors in the common base configuration. It is the purpose of this paper to 1) present experimental results to demonstrate this, 2) demonstrate the causes for this high degree of linearity and derive expressions for optimum bias voltages and currents (these results can be extended with certain restrictions to the common emitter and common collector configurations), and 3) present a general method for analytically treating nonlinearity in all transistor configurations. A push-pull amplifier using 2N509- or 2N1195-type transistors provided 13 db of gain, with signal-to-distortion ratios of 70 to 80 db for broad-band white noise signals with +3 dbm of average output power or sine wave signals with +15-dbm peak output power. The stability of this gain and linearity with temperature and interchange of transistors is high. This high degree of linearity is shown to be due to 1) cancellation of current sensitive nonlinearity in the emitter and collector resistances, 2) collector voltage bias adjusted so that the derivative of the collector conductance with respect to collector voltage is zero, and 3) the use of a high alpha cutoff frequency transistor. An expression derived for optimum bias was in good agreement with experiment. The nonlinear distortion in transistors is shown to be different from that found in most devices, in that there are frequency-sensitive and frequency-insensitive nonlinearities. A general analysis carried out including these terms was in good qualitative agreement with experiment.