Well, LK would explain this better but:
There can be a huge variance in characteristics between two FETs of the same type, which is why you need to do the biasing.
This is what Don Tillman says about the subject:
"I should point out that FETs in general suffer from a serious lack of manufacturing consistancy. The FET VGS and IDSS, the parameters that determine the bias point, can be anywhere over a 5-to-1 range and still be within spec. That's pretty awful, but such is life. It's an engineering accomplishment to design a circuit that can function exactly the same over a wide variation of component parameters. But I can't guarantee that in this situation; there's not enough supply voltage headroom."
On the other hand, when it comes to regular transistors, the variance between units is not much. This makes it possible to design a circuit that will work with all transistors of the same type, without the need for re-biasing.
By the way, i started building the driver. As an experiment, i'm using tinplate from a tea tin for the core. It's easy to find, easy to cut and shape, and highly magnetic.
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