M
Mike Monett
- Jan 1, 1970
- 0
Winfield said:Mike Monett wrote...
[...]
In this case, the device is being used as a follower, so I believe a
better model would only change the gate voltage a small amount. The
low impedance of the complimentary driver seems to eliminate the
problems with high input capacity. Is this a reasonable assumption
to go on?
No, because the transconductance of the FET is still very important
to the performance, and the bad model is using a value that could be
20 to 100x too high. That's got to have a very serious effect.
I described how you can fix your models. What parts are you using?
Thanks,
- Win
Promise you won't laugh
2N6802 NMOS
2N6804 PMOS
2N2222 NPN
2N2906 PNP
SPICE doesn't care about breakdown voltage on these devices. Again, the
goal at this stage is simply to evaluate different architectures. The
mosfets show a very dramatic difference with and without the bipolar
driver, so the driver seems a necessary part of the design. The actual
parts selection and detailed design is still way downstream. There's
still lots of work to look at short circuit protection, etc.
For example, short circuit protection by current limiting doesn't seem to
be possible in this type of follower. If the output were designed to
limit at some fixed current, the output voltage would be limited to Imax
* Rload. However, the input voltage could still go anywhere, which means
the problem is transferred to the input stage, which now has to handle
the voltage difference between input and output.
Given that very low leakage diodes may be impossible to find, and series
current limiting mosfets only go to about 500V, this creates a rather
difficult issue. Perhaps limiting the input current with a large series
resistor might work, but this increases the noise. And I still don't know
how the op amp would cope with 1mA or so current at the input. Some of
the datasheets show the input driven well past the supply rails, but I
still need to test with the actual device.
As far as fixing the SPICE models, there's several ways to go. Jim
proposed that Level 7 might solve the problem. Also, measurements on
working hardware might show the performance is more than adequate for the
requirements and no further SPICE work is needed. Or bench work might
uncover different problems that SPICE can't see.
For example, is the op amp really capable of 1 ppm performance, and where
do you get high value resistors with 0 ppm voltage coefficient? Will the
oscillations show up, and will they be impossible to kill without adding
too much resistance in series with the gates?
So there are lots of issues. If improving the SPICE model would help
solve these problems, then that would be the way to go. But right now, I
don't think it would make that much difference, and I can work on the
other problems while you and Jim sort out the modeling issue
Mike Monett