Subscriber's line equipment for a telephone exchange

Claim 1

A subscriber's telephone line equipment for connection to a telephone exchange comprising: (a) a direct current supplying circuit which comprises: (i) a first, constant direct current source; (ii) a second direct current source; (iii) a chopper circuit arranged in a bridge configuration, having four symmetrical arms; (iv) voltage-doubling rectifying means connected to the ends of one diagonal of said chopper circuit; (v) a decoupling capacitor shunting the ends of the other diagonal of said chopper circuit, said capacitor also being connected in series to said first and second current sources; (vi) control means for supplying said chopper circuit control signals in the form of rectangular pulses having a duty factor of 0.5 and a radio frequency repetition frequency; (vii) a first resistor connected in series between said second current source and said chopper circuit; (viii) a current-limiting circuit connected in series between said first current source and said chopper circuit; and (ix) a capacitor shunting said subscriber's line; (b) a first circuit for transmitting from said telephone exchange to said subscriber's line which comprises: (i) an emitter-follower connected transistor whose emitter-collector circuit is connected in series between said first resistor and said second current source; and (ii) a first amplifier connected to the base of said emitter-follower connected transistor; (c) a second circuit for transmitting voice signals from said subscriber's line to said telephone exchange which comprises: (i) a second, differential amplifier; and (ii) an anti-sidetone circuit connected between the inputs of said second amplifier and the terminals of said first resistor; (d) a second resistor connected in series between said second current source and said emitter-collector circuit of said emitter-follower connected transistor; and (e) a threshold circuit connected to the terminals of said second resistor for deliverying a signal, the level of which represents the state of the subscriber's loop.