Digital exchanges require receive and transmit signals on separate two-wire circuits. This calls for two-wire to four-wire conversion. Such a conversion is normally required for trunk transmissions in analog exchanges. The circuit that performs 2- wire to 4-wire conversion is called Hybrid. A transformer based hybrid circuit is shown in Fig. The main function of a hybrid is to ensure that there is no coupling of signal from the input to the output in the 4-wire circuit. The operation of the circuit is as follows: The input signal is coupled to the B and F windings equally. Through the C winding, the input is coupled to the 2-wire circuit. The same signal when it flows through the balanced 2-wire couples the signal to winding D through winding C. The signal induced in B flows through E and induces a current in D that opposes the current induced by F. If the impedance ZB exactly matches that of the 2-wire circuit, the effect of input signal on the output winding D is completely nullified. In a similar way, the input signal from the subscriber end is completely nullified from coupling into the winding A.