Patent ID: 7489195

Claim:
A variable gain amplifying circuit comprising: a gain stage receiving a pair of input signals and amplifying the input signals by a variable gain, the gain stage comprising: a first transistor and a second transistor constructing a differential amplifier for amplifying the input signals then outputting a pair of amplified signals; a first resistor connected between the first transistor and a steady voltage source; a second resistor connected between the second transistor and the steady voltage source; a first voltage controlled current source and a second voltage controlled current source for providing variable currents respectively through the first transistor and the second transistor; and a variable resistor for providing a variable resistance between the first transistor and the second transistor wherein the variable resistance is controlled by a control voltage; and a voltage-to-current converter for controlling the variable currents respectively flowing through the first voltage controlled current source and the second voltage controlled current source according to the control voltage.