Intermodulation distortion in an experimental group amplifier using transistors

Line and group amplifiers used in multi-channel carrier-telephony systems must be highly linear; hitherto, with only thermionic valves available the use of negative feedback has been essential. The common-base connection of the transistor (without negative feedback) is shown to be sufficiently linear in an experimental group amplifier for output levels up to ¿6dBm. The factors affecting the intermodulation distortion in the common-base circuit are analysed. In the group amplifier described, the major source of non-linearity is shown to be due to the dependence (a) of input resistance on emitter current, and (b) of the collector capacitance on the collector voltage and current. The effect of the variation of current gain on collector current and voltage is relatively small. Theoretical and experimental curves show the dependence of the intermodulation distortion on the collector voltage, emitter current, generator resistance and load resistance. The design procedure based on the analyses is illustrated by that for a single-stage amplifier, leading to the conclusion that a twostage design, described in detail, is necessary to meet the full performance required.