Join Date: May 2005
Location: Canoga Park, California
I suppose, because above a certain level of drain current power MOSFETs have a fairly constant transconductance, whereas bipolars mantain the quasi-exponential characteristic for quite a while. Then there is also the appeal of essentially zero d.c. gate current compared to the substantial bipolar requirement.
There is also the relative freedom from secondary breakdown effects in the FETs.
However, were one to use some of the new parts from On Semi which incorporate local diodes on chip with the big bipolar, it might be practical to use the predistortion technique and linearize the current-to-current transfer function. You won't quite be there with a minimalist circuit since the transistor will be seeing a bunch more volts than the diode (or diode-connected transistor---I'm not sure what is exactly inside the new parts). But at least the thermal coupling is pretty good. If the drive current itself is temperature-compensated then the stage current would be fairly stable as well. Perhaps a cascode structure would be a practical approach, limiting the collector voltage(s) of the initial device and passing the heavy voltage-lifting to the common-base stage---the tempco of alpha is much lower than that of beta.