RE: The long history of Cinematronics and Vectorbeam

> I looked into using MOSFETs once, the problem I had was how to control
> them in the analog mode. They really want to act like a switch. And
> the level at which they are in analog mode is very narrow and varies
> all over the place even between the same parts off the same batch.
>
Yep. The switching emphasis is what I'm bashing my head against now.

> I found a schematic for an audio amplifier once where they used
> MOSFETs in there analog mode as the output devices, there was a lot of
> circuitry to control them, and they were unable to come up with a
> designed that worked without tuning. Each device (there was two, it
> was a complimentary output driver) required a trim pot to be adjusted
> to the proper "analog" voltage level, to work.
>
There's an application note from IR or Vishay or someplace that has
something like what you describe. Did you ever see that MOSFET
deflection amp design I had from a book that I posted on my old webpage?
It used the high-power L165 (I think National's is the LM675?) op-amp as
source for the MOSFETs. I didn't have any luck finding the old MOSFETs
in the design to see what their characteristics were like though. :-(

> It makes you wonder if you could not use them in a PWM mode. That
> would be very efficient heat wise, but I'm sure the PWM frequency
> would have to be very high. But I know they've built audio amplifiers
> this way!
>
That was the first thing one of the guys where I work suggested. I
don't know-- I suppose it's do-able, but like you said the operating
frequency might be tough to synthesize at the required speed (and I
don't know what settling time and transitional behavior would "look"
like). Ears are pretty forgiving, but eyes might "see" something
undesirable?

-Clay
Received on Wed May 26 19:51:23 1999