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

This is an update on cascode transistors in SPICE:

mike wrote:

[...]

> I tried stacking NPN's and PNP's in a complimentary emitter follower
> with a crossover reducing resistor between the bases and emitters.
> It looks like this may work. It could be extended to any desired
> voltage with very low power consumption and reasonable bandwidth.
>
> One problem was trying to model it. The version of SPICE I use does
> not like having the op amp supply voltages track the input signal,
> and it gave very wierd output signals. I tried several op amps with
> similar results and finally found the UA741 barely works in this
> circuit.

It turns out the problem was having the collector of one transistor feed
the emitter of the next in the stack. Apparently SPICE doesn't like
handling leakage currents in series. Adding a 1,000 megohm resistor
across each transistor solved the problem, and the simulation now runs
much better.

One thing I'll have to watch out for. Shorting the output to ground seems
to saturate all the transistors in the stack except one. This now has the
full supply voltage across it. It will blow, of course and probably
short. This would place the full supply voltage across the remaining
transistors, which would also likely blow. Like John says, it is probably
a good idea to add some series resistance to limit the resulting
shrapnel.

With a low value of load resistance in place of a short, the stack seems
to saturate in a peculiar manner. With three transistors in series, the
one closest to the op amp saturates first, then the middle one. This
leaves the ones connected to the supply voltage to handle the full load
current. I'll have to ponder a while to figure out why they saturate in
such a predictable sequence, but it seems to be real and not a SPICE
artifact.

> As usual, one big problem is protecting the inputs. Using a pair of
> fets in series with the input to limit the current only works to
> 500V or so.
>
> How do HP, Fluke and the others get above this limit?

Mike Monett
.

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