Patent ID: 7768349

Claim:
A transconductance amplifier for supplying output current in proportion to input voltage, said transconductance amplifier comprising: a differential pair including source grounded first and second MOS transistors operating in triode region; a third MOS transistor that operates in saturation region with a source terminal thereof connected to a drain terminal of said first MOS transistor; a fourth MOS transistor that operates in saturation region with a source terminal thereof connected to a drain terminal of said second MOS transistor and a gate terminal thereof connected to a gate terminal of said third MOS transistor; a voltage generating circuit for outputting tuning voltage to be input to gate terminals of said third and fourth MOS transistors, and common voltage for generating first voltage and second voltage to be input to said differential pair in such a manner that a ratio between the tuning voltage and the common voltage is constant; and a differential pair input voltage generating circuit that receives the common voltage, outputting the first voltage to the gate terminal of said first MOS transistor and the second voltage to the gate terminal of said second MOS transistor, wherein the second voltage equals 2×(the common voltage)−(the first voltage); the input voltage equals the difference between the first voltage and the second voltage; and the output current equals the difference between a first drain-to-source current of said first and third MOS transistors and a second drain-to-source current of said second and fourth MOS transistors.