Design considerations for multi-channel telephone line systems using transistors

The paper first considers the repeater spacing which produces the minimum noise per channel for a given route length, cable type and repeater-amplifier output power. The optimum spacing is shown to be equivalent to a repeater section of 1-neper loss if the repeater amplifier has a flat gain characteristic. The case where the amplifier gain follows the cable-loss characteristic is also discussed. The relationship between the technical optimum and the best economic spacing is considered: the transistor repeater is shown to permit a closer approach to the technical optimum spacing than is economically possible with a valve repeater. This is because of the lower initial cost of the repeater and its housing and of its lower power consumption. The lower section loss also permits relaxation of the near-end crosstalk requirements for the cable, permitting economies in its construction. The design of transistor repeater amplifiers is discussed, comparing common-base stages in tandem without overall feedback with connections which give considerable current gain but which require overall feedback to obtain the linearity and gain stability required in a repeater. The straightforward common-base connection appears to yield better linearity and stability than the more sophisticated arrangements which give equal gain in a broad-band repeater. If the difficulties in the feedback amplifiers can be overcome, it should be possible to design the amplifier gain to follow the cable loss with consequent improvement in noise performance, whereas it is difficult to produce a common-base amplifier with anything substantially different from a flat gain/frequency characteristic.