Telephone circuit for sidetone balance and automatic transmission level adjustment

Claim 2

A telephone circuit comprising amplifier means constructed to produce a current which is proportional to the input voltage but not related to the load, a telephone line connected to the input terminals of said amplifier means, receiving means connected to the output terminals of said amplifier means, transmitting means connected across the input and output terminals of said amplifier means, a first variable impedance element connected in parallel with said receiving means, and a second variable impedance element connected in parallel with said transmitting means, said first and second variable impedance elements varying their impedances in accordance with the DC voltage condition of said telephone line.

Claim 3

The telephone circuit according to claim 1 wherein said amplifier means comprises a first transistor with its base and collector electrodes connected to the input terminals of said telephone line, a second transistor with its collector and base electrodes connected to the input terminals of said receiving means and a balancing network connected between the emitter electrodes of said first and second transistors, the collector electrode of said first transistor and the base electrode of said second transistor being interconnected.

Claim 4

The telephone circuit according to claim 1 which further includes a constant current circuit responsive to the DC voltage across said telephone line for supplying to said receiving means a constant DC bias current independent of the length of the telephone line.

Claim 5

The telephone circuit according to claim 1 which further includes a constant current circuit responsive to the DC voltage across said telephone line for supplying to said transmitter means a constant DC bias current independent of the length of said telephone line, and bias means biasing said constant current circuit for applying a constant voltage to said amplifier means.

Claim 6

The telephone circuit according to claim 1 wherein said first variable impedance element comprises a field effect transistor having a drain terminal and a source terminal which are connected across said transmitting means, and a gate terminal connected to means responsive to the DC voltage across said telephone line.

Claim 7

The telephone circuit according to claim 1 wherein said second variable impedance element comprises a field effect transistor having a drain terminal and a source terminal which are connected across said receiving means, and a gate electrode connected to means responsive to the DC voltage across said telephone line.

Claim 8

The telephone circuit according to claim 4 wherein said bias means includes at least one Zener diode connected to said constant current circuit.

Claim 9

The telephone circuit according to claim 5 wherein said means responsive to the DC voltage comprises a resistor connected across said telephone circuit.

Claim 10

A telephone circuit comprising an electromagnetic receiver having first and second terminals ; a source of DC current including a constant current circuit and two Zener diodes which are connected in series between said first and second terminals of said electromagnetic receiver; said constant current circuit including a diode with its cathode electrode connected to said first terminal of said electromagnetic receiver and a first transistor with its base electrode connected to the anode electrode of said diode, emitter electrode to the cathode electrode of said diode and collector electrode to the cathode electrode of a first one of said Zener diodes; a potentiometer resistor connected between the anode electrode of said diode and said second terminal of said receiver, said potentiometer resistor having an intermediate terminal ; amplifier means including a second transistor with its collector electrode conNected to said first terminal of said receiver and the base electrode to the cathode electrode of said first Zener diode, a third transistor with its base electrode connected to the first terminal of a telephone line and collector electrode connected to the second terminal of said telephone line and to the second terminal of said receiver, and a balancing network connected between the emitter electrodes of said second and third transistors ; the base electrode of said third transistor being connected to the juncture between said two Zener diodes ; a transmitter connected between said first terminal of said receiver and said first terminal of said telephone line ; a first field effect transistor with its drain and source terminals connected across said transmitter ; and a second field effect transistor with its drain and source terminals connected across said receiver ; the gate electrodes of said first and second field effect transistors being connected to said intermediate terminal of said potentiometer resistor.

Claim 11

A telephone circuit comprising an electromagnetic receiver having first and second terminals ; a source of DC current including a constant current circuit and two Zener diodes which are connected in series between said first and second terminals of said electromagnetic receiver; said constant current circuit including a diode with its cathode electrode connected to said first terminal of said electromagnetic receiver and a first transistor with its base electrode connected to the anode electrode of said diode, emitter electrode to the cathode electrode of said diode and collector electrode to the cathode electrode of a first one of said Zener diodes; a potentiometer resistor connected between the anode electrode of said diode and said second terminal of said receiver, said potentiometer resistor having an intermediate terminal ; amplifier means including a second transistor with its collector electrode conNected to said first terminal of said receiver and the base electrode to the cathode electrode of said first Zener diode, a third transistor with its base electrode connected to the first terminal of a telephone line and collector electrode connected to the second terminal of said telephone line and to the second terminal of said receiver, and a balancing network connected between the emitter electrodes of said second and third transistors ; the base electrode of said third transistor being connected to the juncture between said two Zener diodes ; a transmitter connected between said first terminal of said receiver and said first terminal of said telephone line ; a first field effect transistor with its drain and source terminals connected across said transmitter ; and a second field effect transistor with its drain and source terminals connected across said receiver ; the gate electrodes of said first and second field effect transistors being connected to said intermediate terminal of said potentiometer resistor.