The application of junction transistors to audio-frequency telephone line amplifiers

The alloy junction transistor is well suited to application in audio-frequency telephone line amplifiers, where it can supplant the thermionic pentode with advantages in power economy and perhaps in reliability. The application requires good linearity and constancy of gain, and the stability of the input and output impedances is important. The requirements are most simply met by using a common-base circuit without feedback, and a simple amplifier of this type is described; but the use of mixed feedback in a common-emitter stage enables higher gain and output power to be obtained from a given transistor. A single-stage push-pull amplifier using mixed feedback has a performance approaching that of the standard thermionic line amplifier, but it requires individual adjustment of the feedback paths. Nevertheless it has useful applications and is on field trial on a small scale. In order to duplicate the performance of the standard thermionic amplifier without recourse to individual adjustment, two stages are necessary, and two amplifiers of this type are described. The first has a common-emitter stage driving an output stage based on the common-collector configuration, but it requires the use of electrolytic capacitors. A second amplifier, having a common-collector stage driving a common-emitter stage, has only a single paper-dielectric capacitor.