Abstract: A signal transmission device capable, for example, of use as a hybrid circuit in a voice frequency telephone transmission system, comprises a controllable substantially load independent current source shunting a signal path between input and output ports of the device. The shunt path is de-coupled from a signal source or load to be connected to respective ports of the transmission device by series connected impedances which are high in relation to those of the respective source or load, while the current source is arranged to respond to a voltage signal applied to the input port in order to drive corresponding currents through the impedances. A current steering circuit is provided to determine the relative magnitudes of the currents driven through the respective impedances, and thus to determine the virtual impedances presented to the signal source and load as well as the signal gain of the device.

Abstract: In a method for the enhancement of the A.C. performance of an inductor such as a transformer or choke, the tendency of the core of the inductor to become saturated by flux arising from low frequency A.C. or D.C. currents flowing in the inductor winding is reduced by detecting the magnitude of such currents and applying to a control winding wound on the core of the inductor a compensating current in order to generate a magnetic flux in the core of the inductor which opposes the component of flux tending to cause saturation. An inductor to which the method may be applied may accordingly comprise a main winding, a magnetic core, a control winding, means for sensing the presence in said main winding of a current having a frequency below a predetermined limit, and means for applying to said control winding a control circuit tending to balance the flux generated by the sensed current.