low-cost high-voltage electrometer amplifier

Not necessarily. I consider the capacitive divider idea interesting. Well, it is a sampling solution. Bigger caps are a bit hard on the switches, and smaller caps are sensitive to effects of semiconductor switches. Beside that the switches need a supply range at least covering the signal range.

Rene

Le Wed, 10 Oct 2007 18:08:18 -0700, Winfield a écrit:

Nothing to take care of stability issues? I mean bootstrapping the supply rails from the output defeats the opamp's compensation capacitor and will almost surely get you into trouble if this is not taken care of.

Did you actually build this or is it just a back of the envelop idea?

Yes, like I said, it actually has 40 parts, including active pulldown. Sorry for the over-simplification, I hadn't wanted to make a big ASCII drawing, but with these subsequent posts, some of the detail is coming out bit-by-bit anyway. feel free to ask away.

Here's the opamp compensation part. As you can see the opamp has its own local HF feedback control, but at a slower rate it can do what it wants to control the MOSFET, which in turn positions the stack.

As before, there are missing parts, like the MOSFET resistors and beads to suppress RF oscillation, etc.

Real relays are still neat for a lot of things, and there are some very good ones around, telecom rated for 1500 volts. A little series resistance will protect the contacts from arcing when the cap is switched, without affecting charge distribution.

John

Yes, indeed. Then we're rather close to DC. A reed relay is specified for 10^7 cycles. I admit a relay is a part I try to avoid.

Rene

..and relays have their own problems...

Then there's the "output compensation", which provides for rolling-off the huge effective gain of the output MOSFET stage in a controlled way, with capacitor poles and series-resistor zeros.