A low-power method adding continuous variable gain to amplifiers

A method adding continuous gain control to high performance amplifiers is suggested. Any high performance, fixed gain amplifier can adopt the proposed method without deteriorating the performance of the core amplifier. The method is based on the idea of introducing an additional, adjustable feedback path in parallel with a fixed feedback path. The additional, adjustable feedback path can be an active feedback cell compatible with the amplifier architecture used. A low noise trans-impedance amplifier would for example require an additional adjustable trans-conductance cell with proper phase characteristics in parallel with the fixed feedback element used. By making the feedback factor of the adjustable feedback path proportional to the power consumption, extra power is only consumed when gain reduction is necessary. This feature is especially valuable in applications were gain reduction is necessary for a short period of time. One application having this feature is medical ultrasound imaging. The proposed method was originally developed for real-time 3D ultrasound imaging but is compatible with all systems requiring continuous gain control. A prototype circuit was designed were the dynamic range was increased 12dB for an amplifier having 58dB SNR over a bandwidth of 20MHz with a peak penalty of 46% increase in the power consumption from 412 mu W to 663 mu W. The prototype amplifier was design to work at frequencies in the low NMz range.