Electronic network for telephone set

Claim 1

In an electronic telephone network, for connecting a microphone, a receiver and a line powered tone generator to a line powered telephone line, comprising: a line terminating resistor and a line terminating capacitor connected in series across said telephone line; a line powered transmit amplifier, one input of which is connected to said microphone, another input of which is connected to the line powered tone generator, and the output of which is connected across said telephone line; a line powered receive amplifier, one input of which is connected through an impedance network to the telephone line, another input of which is connected to the transmit amplifier at a point where the transmitter signals are out-of-phase with those on said line; and the output of which is connected across said receiver; the impedance network controlling the side-tone level of the transmitter signals coupled to said receiver; a d-c clamp connected across said telephone line; switch means actuated substantially concurrently with actuation of said tone generator, for concurrently disabling said d-c clamp to reduce the loading across said line, disconnecting said microphone from the input to said transmit amplifier and connecting said tone generator to said input; the improvement comprising: a first resistor and capacitor serially connected across said line capacitor, the line voltage drop across said first capacitor controlling the d-c clamping voltage of said d-c clamp; the first resistor and capacitor coacting with the line terminating resistor and capacitor as a first two-stage R-C low pass filter; a second resistor and capacitor serially connected across said line capacitor, the line voltage drop across said second capacitor being the d-c bias voltage of the input to said transmit amplifier, the second resistor and capacitor coacting with the line terminating resistor and capacitor as a second two-stage R-C low-pass filter.