<p>I&#39;m not even sure how to properly describe this. &quot;nonlinear modulating low-pass filter&quot;, I guess?</p><p>the core of it is that the MOSFET is a big power FET with a ton of Cgs. combined with the 220k resistor, you get a low-pass filter. the channel A voltage is adjusted by the opamps to keep the MOSFET sub-threshold.</p><p>except the really evil part of this is that the MOSFET&#39;s gate capacitance is dependent on Vds. so channel B modulates Vds, adjusting the filter&#39;s cutoff frequency.</p>