Patent ID: 7518453

Claim:
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 said sum and difference currents and for developing an output signal proportional to said gain factor multiplied by said current through said resistor element; further comprising an interface for selectively providing a signal proportional to a variable across said inputs of said differential amplifier to drive said output signal to an output value; and a gain setting circuit responsive to said output value that produces a gain setting signal to adjust the gain of said variable gain amplifier.