High precision, high bandwidth variable gain amplifier and method

摘要

A variable gain amplifier includes a differentially-connected pair of NFETs for receiving differential input voltage signals. The source- drain channel of each NFET is coupled to a respective PFET operating in linear mode as a variable load resistance in a g.sub.m ×R gain stage, the gain of which is determined by a gain- controlling signal applied to the gates of the PFETs. For broad dynamic range of voltage amplification, the drain of each NFET is coupled to a respective current source that is referenced to a supply voltage having a higher voltage level than the supply voltage for biasing the PFETs. Multiple stages of such variable gain stages are cascaded and independently gain controlled in successive stages to yield substantially exponential variation in overall gain as individual gain stages are adjusted. A control circuit controls the gains of each stage for obtaining a desired total overall gain for the multi- stage amplifier. Improved overall gain-bandwidth performance driving a heavy output load at high gain is achieved by increasing the voltage gain of an output stage from lowest gain last in succession in a procedure for adjusting the overall total voltage gain of the multi-stage amplifier.