A CMOS variable gain amplifier for a wideband wireless receiver

The variable gain amplifier (VGA) embedded in an automatic gain control (AGC) loop is an essential component of a linear receiver. The AGC builds up the received signal to some specified peak output so that it fully loads the fixed full-scale of, say, an A/D converter in the IF section. The VGA described here is for use in a low-IF wideband receiver. An active analog filter with programmable bandwidth selects the channel of interest. Active filters suffer from a high input noise level, which can easily dominate the receiver noise figure. The VGA must therefore amplify the received signal enough to overcome filter noise. The circuit reported here is designed to give a continuously variable gain from 0 to 70 dB. Unlike the GSM receiver, the high-order QAM modulation being detected in this receiver cannot tolerate discrete gain jumps. It is more of a challenge to design an amplifier with continuously variable gain, than one whose gain changes in steps. Furthermore, as the VGA bandwidth is fixed at 20 MHz, it must be able to handle much larger signals in adjacent channels before they are removed by the filter. To obtain the best overall performance, the AGC assigns gain to one VGA placed before the filter and possibly another one after it. A good gain assignment algorithm is essential to useful AGC design.