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A SiGe BiCMOS mixed-signal adaptive controller-on-chip is presented that implements gradient descent of a supplied analog control objective. Eight analog variables controlling the external plant are perturbed in parallel using sinusoidal dithers, and their gradient components are estimated by parallel synchronous detection of the dithers in the control objective. Translinear all-NPN bipolar circuits achieve linear tuning of frequency and amplitude in the oscillators and synchronous detectors, covering a 4-kHz-600-MHz range in dither frequencies with - 30-dB/octave suppression of intermodulation products. Experimental results demonstrate adaptive optimization of a three-variable nonlinear plant within 1 ??s for dithers in the 100-200-MHz frequency range. The chip measures 3 mm ?? 3 mm in 0.5-??m SiGe and consumes 110 mW at 3.3-V supply.