A voltage-coupling and loop current supply mechanism controllably turns on
an output transistor within the tip amplifier circuitry portion of the
speech amplifier to controllably apply a prescribed voltage to the
telephone line during on-hook/standby mode. This controllably applied
voltage serves as a source of loop current when the phone goes off-hook.
The ring lead portion of the tip/ring wireline pair is connected to a
controlled current sink, which is coupled to the output of a voltage
regulator referenced to the battery voltage. This controlled current sink
is used to sink a prescribed loop current from the tip/ring pair when the
phone goes off-hook. When the phone goes off-hook and loop current is
detected, the voltage-supply mechanism provided by the tip amplifier's
output transistor is terminated, and the speech amplifier is placed in
its normal speech amplifier mode.