Patent ID: 6972619

Claim:
An amplifier with a gain proportional to power source voltage comprising: first and second P-channel MOS field-effect transistors (hereafter abbreviated to MOS-FETs) formed such that respective back gates are electrically isolated from a semiconductor substrate and respective sources are connected in common, a first voltage source for outputting a voltage obtained by dividing a power source voltage, a second voltage source for generating a positive voltage having a potential difference almost identical to the threshold voltages of said first and second MOS-FETs with reference to the output voltage of said first voltage source, a third voltage source for generating a predetermined negative voltage with reference to the output voltage of said first voltage source, and an operational amplifier, to the positive input terminal of which the output of said second voltage source is applied as a bias voltage, wherein the sources of said first and second MOS-FETs, connected in common, are connected to the negative input terminal of said operational amplifier, the respective back gates of said first and second MOS-FETs are connected to either their respective sources or said second voltage source, the gate of said first MOS-FET is biased at the potential of a circuit ground, and its drain is connected to an input terminal to which a signal with no DC component is input, and the gate of said second MOS-FET is connected to said third voltage source, and its drain is connected to the output terminal of said operational amplifier.