High-Performance, Automatic Level-Controlling Amplifier
George A. Hellwarth, David H. Beetle · The Journal of the Acoustical Society of America · 1963
In the laboratory analysis of speech signals, there is often a critical need to hold the amplitude function constant over intervals as short as a pitch period or as long as words or sentences. To fill this need, an all solid-state variable-gain amplifier has been developed that automatically controls the peak amplitude of these signals. The gain-controlling element is a symmetrical transistor. By operating this nonlinear device with a constant ratio of signal current to bias current, signal distortion and thump are maintained at very low and nearly constant levels over a 60-dB range of gain. A sudden increase in input level can result in a full 60-dB gain reduction in less than 1 msec. A decrease in input level results in a logarithmic gain recovery at a constant dB-per-second rate, which is manually adjustable over a very wide range. Component selection and bias adjustments are not critical for high performance of this circuit. Gain ranges exceeding 120 dB are feasible, using the same basic techniques.