Noise Gating and Adjustable Gain

The circuit shown here is a complete microphone signal conditioning system based on a single SSM2166 integrated circuit. Designed primarily for voiceband applications, this integrated circuit provides amplification, RMS detection, limiting, variable compression, and downward expansion. An integral voltage-controlled amplifier (VCA) provides up to 60dB of gain in the signal path with approximately 30kHz bandwidth. Additional gain is provided by an input buffer OPAMP circuit that can be set anywhere from 0 dB to 20 dB, for a total signal path gain of up to 80 dB. The project operates on a single +5 V supply, accepts input signals up to 1V_rms, and produces output signal levels > 1 V_rms (3V_p-p) into loads > 5 kΩ. The internal RMS detector has a time constant set by an external capacitor. The SSM2166 contains an input buffer and automatic gain control (AGC) circuit for audio and voiceband signals. Circuit operation is optimized by providing a user-adjustable time constant and compression ratio. A downward expansion (noise gating) feature eliminates circuit noise in the absence of an input signal. Onboard trimmer potentiometer allows the user to set the downward expansion threshold, the limiting threshold (rotation point), input buffer fixed gain, and the internal VCA’s nominal gain at the rotation point.