| 摘要 |
To control the earphone and microphone amplifiers as well as the balancing network of a subscriber's telephone terminal which is fed, through a subscriber line in communication systems, to the effect that the attenuations in subscriber lines of unequal lengths do not affect the transmission characteristics between the exchange and the subscriber, a circuit arrangement is provided for producing the necessary control function, which is simple, integratable and has minimum tolerances. Above a predetermined threshold value, a voltage proportional to the line current variations controls the emitter-to-base junction of two grounded-base transistors at the collectors of which the control current is available. The control currents produce their effect on the negative feedback of the microphone and the earphone amplifiers. An increasing line current causes the cotrol current to decrease, thereby effecting a stronger feedback and a reduced amplification. Further, the voltage proportional to the line current variations and available at the emitter resistor of another transistor, controls the balancing impedance through a current mirror, to attenuate the sidetone independently of the length of the subscriber line.
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