biasing amplifiers
I have used an op-amp feed-back circuit to stabilize class AB bias. With the high gain of an op-amp you can make the drain sense resistor very small (~ 0.1 ohms). While it worked great and holds the bias rock-steady I never used it in production since a thermistor based circuit is simpler, more reliable (no active parts), and truly compensates for temperature effects. However this does not use feed-back. You must measure FET quiescent drain current over temperature and design the thermistor network to compensate.
The biggest variation in lots is threshold voltage, and that is easily adjusted with a potentiometer. Once quiescent bias is adjusted at room temperature, the drain current vs. temperature curves are nearly identical, so you only need to tweak your thermistor network once.
One problem you have to watch for in active bias is gate current. As RF drive increased so does drain current, so the active bias reduces gate voltage to counteract. This increases breakdown between gate and drain thus increasing gate current. Too much gate current can degrade and damage your FET. So a "stiff" active bias, such as an op-amp connected directly to the gate, is bad since there is no current limiting. Adding resistance between the gate and driving source will "soften" the bias and limit gate current, of course then you loose active bias. It's a trade-off.