Telephone transmitter circuit

Claim 2

A telephone transmitter having terminals for connection to a telephone line having a direct current voltage applied thereto remote from the terminals, comprising: a self-polarized microphone; a semiconductor amplifier having a substantially constant output impedance and responsive to the output of the microphone to cause fluctuations in the current carried by the telephone line; a direct current voltage divider connected across said terminals and having a voltage tap, the voltage divider Including a plurality of serially connected diodes, the forward conduction characteristics of the diodes restricting current flow through the voltage divider below a particular operating voltage at the terminals, thereby providing a control voltage at said voltage tap which is proportional to the operating voltage above said particular operating voltage; a variable impedance alternating current shunt connected across the terminals and to the voltage tap, the impedance of the shunt being controlled by and in proportion to the control voltage; the impedance of the shunt and the output impedance of the amplifier co-acting to substantially duplicate the acoustical to electrical conversion efficiency of a typical carbon telephone transmitter over its operating current range.

Claim 3

A telephone transmitter as defined in claim 1 further comprising: a resistance in series between one side of the output of the amplifier and the terminal corresponding therewith, the resistance increasing the effective output impedance of the amplifier, and in which the alternating current shunt comprises: a first resistor one end of which is connected to one of the terminals; a diode connected in aiding current flow relationship between the other end of the first resistor and the other terminal; a second resistor and a capacitor connected in series between the one terminal and the junction between the first resistor and the diode; amplifying means responsive to the control voltage and having an output connected to the junction of the first resistor and the diode, the amplifying means operating to abstract current from the junction of the first resistor and the diode to reduce the voltage across the diode when the operating voltage exceeds said particular operating voltage, whereby the operating impedance of the diode is increased substantially in proportion to the operating voltage.

Claim 4

A telephone transmitter as defined in claim 1 further comprising: a resistance in series between one side of the output of the amplifier and the terminal corresponding therewith, the resistance increasing the effective output impedance of the amplifier, and in which the alternating current shunt comprises: a capacitive resistive combination; a transistor having base, emitter and collector electrodes, the collector electrode connected to the one terminal, and the emitter electrode connected to the other terminal via the capacitive resistive combination, a resistive alternating current conduction path between said one side of the amplifier and the base electrode; a voltage divider network connected between the terminals and including a first plurality of diodes connected in series aiding current flow relationship between the one terminal and the base electrode of the transistor via a resistance, the number of diodes in the first polarity determining a lower voltage limit which when exceeded, by the operating voltage, biases the transistor ON so that the signal transmitter to the base electrode via the conduction path causes the transistor to conduct an alternating current component in out-of-phase relationship with the alternating current component at the amplifier output, a second plurality of diodes connected in series aiding current relationship between the base electrode and the other terminal via a resistor, the number of diodes in the second plurality determining voltage limit at the base electrode beyond which the amount of alternating current component conducted by the transistor is reduced; the plurality of diodes in the direct current voltage divider being at least one greater than the first plurality of diodes in the voltage divider network, amplifying means responsive to the control voltage and having an output connected to the emitter electrode of the transistor, the amplifying means operating to inject current at the emitter electrode to increase the direct current voltage at the base electrode when the operating voltage exceeds said particulAr operating voltage.