Re: eliminating CMRR problems -- was, 3 dB bandwidth

Mike Monett wrote...
>
>Winfield Hill wrote:
>
> [...]
>
>> If so, did you bench-vet your MOSFET's subthreshold model? The
>> standard models are completely masssively defective, but in a way
>> that makes the FET's transconductance appear to be very high at
>> low currents, say under 100mA, which improperly wipes out much of
>> their high Ciss problem, which can then make linear circuits
>> appear to work much better than in real life.
>
> Win,
>
> Everything is in SPICE to evaluate different configurations. ...
>
> I searched google groups and downloaded most of the threads on high
> voltage amplifiers, so I am familiar with your discussions about
> mosfet subthreshold characteristics.
>
> 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
.

Quantcast