An MMIC low-distortion variable-gain amplifier using active feedback

A new low-distortion variable-gain amplifier (VGA) is proposed and analyzed. The VGA's gain is controlled through changing the transconductance of a common-drain FET (CDF) in the negative feedback path. The analysis and prototype results indicate that the third-order intermodulation-distortion ratio is greatly improved at high input-power levels due to a drastic reduction in the VGA's input impedance and the CDF's unilateral characteristic. The gain in a wide temperature range maintains stability due to the combination effect of the transconductances of the CDF and the amplifying FET. This CDF feedback VGA is very useful for developing a wide-dynamic range front-end MMIC. The noise and thermal performances were also measured.