An electronic hybrid circuit is provided for connecting a signal transmitting circuit and a signal receiving circuit to a both-way signal transmission circuit, said circuit comprising a constant current supply source providing current supply terminals for connection to said both-way signal transmission circuits, a transistor, the current path of which is connected across said current supply terminals in series with a line balance load impedance, and a signal summation circuit having inputs connected respectively at opposite sides of the current path of said transistor. The arrangement being such that with a signal receiving circuit connected to provide a control signal to a current control electrode of said transistor, and a signal transmitting circuit connected to receive an output from said signal summation circuit, signal voltages generated across the impedance presented to said supply terminals in response to receipt of signals by said both-way transmission circuit or transmission of signals from said signal receiving circuit are fed to said signal transmitting circuit via said signal summation circuit, the signal voltages generated by said signal receiving circuit being at least partially cancelled in the summation circuit by corresponding signal voltages generated in phase opposition across said line balance load impedance.
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