fast ramp follies

cascode

The OpAmp FB pretty much takes out Early effect. So your left with making it transient-capable. ...Jim Thompson

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But only for d.c., right?--the op amp compensation doesn't work at 1V/ nS.

-- Cheers, James Arthur

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Yep. That's why I misspelled and said, "So you're left with making it transient-capable." ;-) ...Jim Thompson

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Magically corrected. :-)

Oh, but there are two cases: the op amp servo, and the transistor CCS step response.

The former matters long-term, but doesn't fix the CCS' Early non- linearity as it ramps 1V/nS. Phil made that point a ways back.

-- Cheers, James Arthur

Here's a "shorted cascode" with Ib correction.

It would need some work on the slow loop dynamics.

I used to be a big fan of the Intersil ICL7109 back in the day. Dual slope was magic.

Cheers

Phil Hobbs

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Quad slope was a trifle more practical - I can't actually remember the details, but I could probably work it out if I had to. As usual engineers dumped elegance in favour of something messier which works better, but is harder to explain to undergraduates.

-- Bill Sloman, Nijmegen

Very cute! This configuration may not be the best if you've got ripple on the positive supply, but other than that (and the whole Ccb nonlinearity problem, which none of the configurations address) it looks promising! In fact, if you reconnect the ground lead of the lowest capacitor to the positive supply, the whole thing is referenced to the positive supply which should reduce any ripple problems. Oscillation - that's another problem, you'll want to have a low HF impedance on the V+ supply anyway.

-F

The killer is probably the Early slope of the PNP, which will make the current source look resistive. Since RF parts are not specified well for DC behavior, one would have to measure some transistors to see how bad that may be. I think the LPTM said that SiGe transistors have huge Early voltages, but they probably don't come in PNP.

Still, the Ib correction is cute, and its loop compensation turns out to be interesting. Perhaps some Master Circuit Designer will address that issue.

We could dump the b-e cap to fix the Early problem and add a base resistor to kill RF oscillation in the PNP. Something like this:

As you note, C1 might better return to V+. V+ would be a bypassed copper pour, pretty stiff from DC to daylight.

Pity, I really liked the Shorted Cascode.

A BFP640 in that floating simulated-inductor circuit would fix that--its Early voltage is like a kilovolt.

Cheers

Phil Hobbs

OK, I'll think about that soon.

In some cases, I could use a negative ramp, so the Shorted Cascode with the SiGe might be interesting.

I invented a pseudo-inductor once, for one of the C5A projects of my youth. To get the impedance up, I bootstrapped the base of a darlington from its own emitter with a double RC. It oscillated at some low frequency, 10 Hz or something. I didn't realize it at the time, but I'd stumbled onto that RCRC circuit that has voltage gain. I fixed it without really understanding it. This was back in pre-Spice days, when simulation was a chore.

... and educated a lot of people about dielectric absorption.

S&Hs were my instructor. ;-)

Yup. The 7109 is pretty dead, but the related 7106/7 are still kicking vigorously after all the years- pretty amazing for a part introduced in the mid-seventies. Do we know who designed those?

It looks like the FET is expected to function without any Vds bias?

And, the base-emitter capacitor was intended to combat Miller effect; so I'm thinking Rbb is the only resistance you want in its base connection. And you don't even WANT that.

Yup. Using some gnarly disc ceramic made for some pretty amusing INL.

Cheers

Phil Hobbs

IIRC BITD I usually used polystyrene, and it seemed to work fine.

Cheers

Phil Hobbs

Even polyester was none too good in that application.

I would have thought PS caps would be too bulky in that size range. We used PC (polycarbonate) where it mattered.

I did a design that used a synthetic high-DA cap made from resistors and relatively good caps in order to linearize a sensor. Nowadays we tend to use micros for that, of course.