FET amplifier and bias power supply circuit for commonly amplifying multi-radio frequency (RF) signals

A radio frequency FET amplifier comprising:

• an amplifier circuit having an FET capable of commonly amplifying a plurality of radio frequency signals;
• and
• a bias power supplying circuit for varying bias power supplied to said FET according to a number of inputted radio frequency signals,
• wherein said bias power supplying circuit varies a drain voltage supplied to a drain terminal of said FET according to the number of inputted radio frequency signals and also varies a gate voltage of said FET so as to hold a drain current flowing in said FET substantially constant irrespective of changes in the number of inputted radio frequency signals, and
• wherein said bias power supplying circuit comprises:
• a DC/DC converter for generating, from a DC voltage supplied, a voltage corresponding to the number of inputted radio frequency signals at a converter output terminal;
• and
• a constant current circuit having a drain current detecting means inserted between the converter output terminal and the drain terminal of the FET for detecting the drain current, and a gate voltage supplying means for monitoring the drain current detected by said drain current detecting means and varying a gate voltage of the FET so as to hold the drain current substantially constant.