A low-distortion CMOS IFVGA with linear-in-dB control and temperature compensation

A CMOS intermediate-frequency (IF) variable-gain amplifier (VGA) is presented in this paper. A transconductance linearization scheme is proposed for the VGA core based on a signal-subtracting structure to achieve low distortion. Temperature-independent decibel-linear gain control characteristic is achieved by an exponential voltage generator based on transfer characteristics of differential pair. The whole VGA, including a highly-linear output stage, is fabricated in 0.25 mu m CMOS technology. Measurements show that the VGA provides a total gain control range of 43 dB with less than 1.2 dB error over 0-80 degrees C, and a constant 3-dB bandwidth of 100 MHz. The third-order intermodulation (IM3) distortion at differential output of 2 V-PP is better than -55 dB. The VGA dissipates 22.6 mA averagely from 3.3 V supply, and occupies approximately 0.53 mm(2).