Voltage controlled attenuator for speakerphones

Amplifiers – With semiconductor amplifying device – Including differential amplifier

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Details

330257, 330284, H03G 330

Patent

active

053878779

ABSTRACT:
An integrated circuit voltage controlled attenuator which reduces control voltage feedthrough by employing NPN transistors for both sets of current steering elements is disclosed. A pair of emitter coupled NPN transistors are provided, connected to a common current source. The collector currents of these transistors form input currents of two precision PNP current mirrors. Output currents from the mirrors are coupled to a second pair of emitter coupled NPN transistors, having a current source connected to their emitters, whose value is adjusted by feedback to exactly match the sum of output currents of the precision mirrors. A single set of control voltages is supplied to both sets of emitter coupled NPN transistors. Signal input and output current is coupled from the precision current source output legs and made available in voltage form through operational amplifiers which perform input voltage to current, and output current to voltage conversions, respectively. An integrated control voltage generation technique is also disclosed, which provides accurate gain settings for a voltage controlled attenuator without the need for absolute value external components or signal feedback between the voltage controlled attenuator and control circuit.

REFERENCES:
patent: 4264873 (1981-04-01), Kominami et al.
patent: 4727335 (1988-02-01), Yokoyama
patent: 5157350 (1992-10-01), Rubens

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