nice fA opamp

Cute part--thanks.

Cheers

Phil Hobbs

Typ vs guaranteed, I guess.

Well, that's getting down there. A fifty-year old Cary 31 electrometer has about 1E-18 A input current, so there's only another decade or two before solid state catches up. To vacuum tubes. Gold-plated elecrodes, cast iron case, sapphire insulators... they say a lot of the cost is in the packaging.

The chopper for that old amp operated with a 440 Hz (that's A above middle C, to a musician) tuning fork oscillator.

ebay has at least one, but the shipping looks expensive.

I looked for a manual for the Cary but didn't find one. I think maybe a vibrating reed acted like a variable capacitor and modulated the input voltage into an AC amplifier. Is that how it worked? If so, a vibrating reed with a solid-state amplifier would be at least as good as the tube version.

A changing capacitance in an electric field generates an AC current.

Analog Devices had a solid state competitor to the vibrating reed with a back-biased varactor bridge which worked much the same way.

I once thought about building one of my own, but figured that I'd have needed very well matched diodes in the bridge and gave up at that point.

The Cary would have needed a very low input capacitance amplifier, which would have been easier to realise with a valve/tube back then.

Not as much as you might think: the smallest triodes (like the "acorn" style

Likewise the follower performance of a triode or pentode isn't very good, due to the lower gain.

Tim

I think there were tubes, used for tape head and mic inputs, that got close to 1 nV/rtHz noise levels.

Really? The only one of those I remember is the Philbrick P2.

Cheers

Phil Hobbs

TI briefly made an integrated opamp that had a varactor-modulator front end.

Yep, that's the principle. The cast-iron probe head, and selected tubes for low microphonics (and there were Nuvistor upgrades recommended) were necessary elements. I found one for the old lab, around thirty years ago, but the manual was a faded mimeograph.

See here:

AD310 (inverting) and AD311 (noninverting) were the part numbers. Those, Keithley 427 current amplifiers, and the Cary electrometer were all useful for accurate ion-chamber measurements of X-rays, gammas, C14 dating. The predecessors were gold-leaf electroscopes, used with stopwatches...

Cool, thanks.

Cheers

Phil Hobbs

..are you suggesting that one can BUILD one using a tuning fork for the vibrating reed?

Seems that you are spot-on!

Cary did. The Cary 401 Vibrating Reed Electrometer is firmly embedded in the scientific literature.

How to Convert Amps to Electrons Per Second (1:11)