A Voltage-Controlled Amplifier takes two inputs — a signal and a control voltage — and outputs the signal scaled by a gain that the control voltage determines. VCAs are the heart of audio compressors, automatic gain control (AGC) loops, synthesizer envelopes, and lock-in amplifiers. Unlike a PGA (which steps gain digitally), a VCA varies gain continuously and smoothly, which matters when you need to modulate amplitude without zipper noise.

The core trick: a current-controlled element. Most VCAs exploit the exponential I-V relationship of a BJT base-emitter junction, or a translinear multiplier cell. The classic Gilbert cell uses two cross-coupled differential pairs sharing a tail current. When the tail current changes, the transconductance (gm) of the pairs scales linearly with it — and since gm sets the gain, you've built a multiplier. The output voltage equals (signal × control current) / reference current.

OTA-based VCAs are simpler for hobbyist builds. An Operational Transconductance Amplifier like the LM13700 or CA3080 outputs a current proportional to input voltage times the bias current Iabc. Gain is set by:

• gm ≈ 19.2 × Iabc (in mA/V at room temp, for the LM13700)
• V_out = gm × V_in × R_load

Practical example — audio compressor sidechain: You want to reduce gain by 6 dB when input exceeds -10 dBu. Feed your audio through an LM13700 OTA. A precision rectifier detects peak level; an op-amp integrator with a 10 ms attack / 100 ms release time constant produces a control voltage. That control voltage drives a current mirror that sets Iabc. At nominal level, Iabc = 500 µA gives unity gain through a 10 kΩ load. To drop 6 dB, halve Iabc to 250 µA.

Watch out for these gotchas:

• Distortion: OTAs are linear only over ±30 mV input. Use input attenuators (1:100) and amplify the output to keep THD below 1%.
• Control-voltage feedthrough: Rapid changes in Iabc inject charge into the signal path. Trim with a small pot to null at DC.
• Temperature drift: The exponential junction is ~-2 mV/°C. Precision VCAs (THAT2180, SSM2018) include on-chip temperature compensation.

Rule of thumb: For audio-grade VCAs, budget at least 80 dB of control range and keep signal levels at ~10 mV peak at the OTA input. If you need better than 0.1% THD, skip discrete OTAs and use a dedicated chip like the THAT2180 — it's worth the $5.