Christoph Maier wrote:
But to generate an AC waveform, you need to start with active devices,
i.e.,
MOSFET switches. A MOSFET ring oscillator is straightforward at 700mV,
a RAIL-TO-RAIL MOSFET oscillator that can source substantial current
isn't.
Huh? Why? I think you are thinking BJT, not MOSFET. Or, perhaps, you
are thinking of an ultra-fast ring oscillator composed of differential
stages. Too high tech. Think lower tech, lower frequency, old school.
First, MOSFET inverters go rail-to-rail even when they are stuck doing
it slowly. It's almost impossible to prevent them from doing so.
Consequently, a ring oscillator composed of an odd (normally prime)
number of inverters will go rail-to-rail if the chain is long enough.
That's the beauty of CMOS technology. Or, as I like to put it:
"CMOS is a foolproof technology. The proof is how many fools use it."
Second, I don't even need a ring oscillator for the task. A standard
single MOSFET, class-D oscillator with an external crystal (extremely
high Q) can blow out a VLSI MOSFET if you don't clamp the pins with
diodes. This is the standard circuit for the battery driven,
time-of-day oscillator in your computer or your wristwatch.
Third, a CMOS inverter can be ratioed in order to take an analog signal
and convert it to rail-to-rail. Combine that with the MOSFET w/ crystal
and you have a rail-to-rail oscillator for exactly 3 MOSFETs.
Fourth, unlike bipolar transistors, MOSFETs are very happy to be ganged
in parallel for more current drive. They don't have a thermal runaway
problem where one transistor winds up taking more and more current the
hotter it gets.
Now, I do agree that doing this with BJTs would be murder. And, you
would probably have to find a nice 300-500mV threshold MOSFET. The
standard discrete, metal-gate 1V MOSFETs might not cut it (but it might,
I'd have to do the simulations. If you get even a *slight* amount of
post-linear, saturation behavior it would probably work).
-a
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