Circuit arrangement for an electronic two wire - two-wire - fork, in particular for use in a telephone set

摘要

An electronic telephone network suitable for use with a two-wire telephone line includes a low-output dynamic microphone coupled to a preamplifier. The output signal of the preamplifier is coupled through a first equalization network which is responsive to an equalization signal which is related to the DC line current to equalize the frequency and amplitude spectrum of the transmitted signal irrespective of the telephone line length or loss. The output of the equalization network is applied to a line-driver amplifier, and as a first input to an electronic signal separator or hybrid. The output of the line-driver is coupled to the two-wire line, and as a second input through the separator by way of an attenuator. The output of the separator is coupled to a receive amplifier by way of a second equalization network which is also responsive to the equalization signal to equalize the frequency spectrum of the receive signal as a function of line length. The loss of the attenuator is selected to be equal to the gain of the line-driver thereby to isolate the transmitted signal and to pass the receive signal to the receive amplifier. In a preferred embodiment, the equalization signal is the DC supply voltage derived from the telephone line itself. The output impedance of the line-driver is lowered during transmit to reduce the sensitivity of the separator isolation and, therefore, the sensitivity of the sidetone signal. The various circuits of the network utilize active loads and other circuitry to provide maximum transmit signal dynamic range even at low line terminal voltages.