The p-n-p-n diode as a cross-point for electronic telephone exchanges

A telephone exchange built on a basis of spatial switching of the speech paths uses a large number of cross-points, the nature of which is therefore a vital factor in the final assessment of a spatial switching system. The paper discusses the use of a semiconductor diode as a crosspoint. A p-n-p-n diode has a high `off¿ impedance and a low `on¿ impedance. It is triggered from its `off¿ to its `on¿ state by applying a large voltage across it, and is maintained in its `on¿ state by passing a maintaining current through it. The germanium p-n-p-n diode has a lower forward voltage drop in the `on¿ state than has the silicon diode, but has a lower `off¿ impedance and is more sensitive to temperature variations. A description is given of the way in which a speech connection through a single switching stage is set up using coincident voltage pulses. This method can be developed to allow the connection of tandem switching stages. Unbalanced switching is used, a transformer in the subscriber´s line circuit converting from 2-wire to a single-wire speech path, with earth return. Voltage and current clamps are used to prevent excessive current surges switching off the cross-points. The overall insertion loss of the network is low, and the contribution of the cross-points to this loss is neglible. The reduction in size of the switching network that the p-n-p-n cross-point allows reduces the crosstalk problem of unbalanced switching although it does not eliminate it.