[Amps] MOSFET amp article

[Steve Thompson]

Dan Sawyer wrote:
> All,
>
> I have a general question regarding the 'bias' connection on this amp.
> It is the same circuit a is used in the 4x MRF150 and the 2x MRF154
> amp. One of those references a separate bias control circuit, the
> other does not. That said, what would happen if Vcc were applied to
> the bias input?
You'll almost certainly kill any transistor that bias point connects to.

As an aside, I've alwys thought the 723 bias circuit used in the Motorola
apps to be needlessly complex, and an invitation for rf to get in where it's
not needed. I think it's a hangover from the same circuit used with bipolar
amps. A 78L12 (or even a zener stabilised supply) feeding a multiturn pot
(or pots if there's more than one bias feed) is fine in virtually all MOSFET
applications unless the amplifier circuit has some low resistance from gate
to ground already built in. It's good to include some series resistance
between the device and the pot (typ. 10k) so that if a device fails
drain-gate short you don't burn out the pot, and you don't put high voltage
on the bias feed to other devices/amplifier circuits. It's also good to put
a fixed resistor (typ. 100k) from the device gate to ground, so the gate
can't float, even if there's intermittent wiper contact in the bias pot.

Steve



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