Variable gain amplifier with constant input referred third order intercept

A method for amplifying a differential input signal with a variable gain amplifier, the method comprising:

• providing a substantially constant current to a differential transistor pair, wherein the substantially constant current is provided by a current source that is coupled with the differential transistor pair;
• adjusting a gain of the variable gain amplifier, wherein the gain is adjusted by establishing a resistance of a variable resistance circuit that is coupled with (i) the differential transistor pair and (ii) a power supply terminal;
• communicating the differential input signal to the differential transistor pair;
• and
• generating a differential output signal based on the differential input signal and the adjusted gain of the variable gain amplifier.

A variable gain amplifier circuit comprising:

• a differential transistor pair for receiving a differential input signal, the differential transistor pair, in operation, conducting a substantially constant current over a linear operating range of the variable gain amplifier circuit;
• a constant current source coupled with the differential transistor pair, the constant current source, in operation, providing the substantially constant current to the differential transistor pair;
• a variable resistance circuit coupled with the differential transistor pair, wherein, in operation, a resistance of the variable resistance circuit is adjusted such that a gain of the variable gain amplifier circuit is adjusted, the variable resistance circuit including:
• a first mixed resistance circuit, the first mixed resistance circuit being coupled with (i) a power supply terminal and (ii) a drain terminal of a first n-type field effect transistor (FET) of the differential transistor pair;
• a second mixed resistance circuit, the second mixed resistance circuit being coupled with (i) the power supply terminal and (ii) a drain terminal of a second n-type field effect transistor (FET) of the differential transistor pair;
• and
• a control signal terminal, wherein, in operation, a signal applied to the control signal terminal establishes respective resistances of the first and second mixed resistance circuits, the respective resistances corresponding with a desired gain of the variable gain amplifier circuit,
• wherein, in operation, the variable gain amplifier circuit produces a differential output signal, the differential output signal being an amplified version of the differential input signal.