| 摘要 |
A variable gain amplifier comprising a differential input amplifier comprising a pair of transistors each having an input across which an input voltage is provided, the transistors being coupled such that each transistor is provided in series with a respective current source providing a reference current and whereby a current is developed across a resistor element coupling the transistors that is proportional to the voltage between the inputs; further comprising further transistors each coupled in series with a transistor of the transistor pair, and wherein the further transistors are arranged such that a current is developed in each further transistor due to the voltage provided across the inputs that is substantially equal to, in one further transistor, a sum of the reference current and the current in the resistor element, and in the other further transistor, a difference between the reference current and the current in the resistor element; further comprising a gain stage for developing currents equal to a gain factor multiplied by the sum and difference currents and for developing an output signal proportional to the gain factor multiplied by the current through the resistor element; further comprising an interface for selectively providing a signal proportional to a variable across the inputs of the differential amplifier to drive the output signal to an output value; and a gain setting circuit responsive to the output value that produces a gain setting signal to adjust the gain of the variable gain amplifier.
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