On Wed, 2007-05-30 at 16:05 -0700, Andrew Lentvorski wrote: > 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 >
Unlike digital watch logic, which consumes next to no quiescent current, a voltage multiplier that drives a LED draws substantial current. So the rds(on) of the MOSFETs is a serious problem, and you can't just use commercial off the shelf low voltage CMOS inverters. Of course, it's always possible to design a little full custom CMOS ASIC for a science project. After all, you shouldn't spare any expenses if it is for your children's education. Christoph -- [email protected] http://www.kernel-panic.org/cgi-bin/mailman/listinfo/kplug-list
