FET in the high end FET probes - Page 1
Forum section: electronics / projects

author: ezalys
author_rank: Frequent Contributor
timestamp: August 12, 2021, 01:27:53 am
content: Several high end FET probes seem to have < 1 pF of input capacitance. Are there any known publicly available parts that will do this? I'm trying to do a charge measurement on a very tiny capacitance, and input capacitance more than the capacitor under test will act like a voltage divider. When I poked around on digikey, there was a part around 0.8 pF, but upon calling the supplier, they said this was a typo.Anyone know any exotics that will do this?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 12, 2021, 01:30:43 am
content: At what bandwidth?And physical size, for that matter? Down in that range you'd better not have any trace or pin length between DUT and probe!In that range I think you'll mostly be finding circuit solutions i.e. bootstrapping the drain and fixture capacitance. Phil Hobbs has done a few this way, I think that went into the 10fF range or something like that pretty crazy.Tim

author: ezalys
author_rank: Frequent Contributor
timestamp: August 12, 2021, 01:55:57 am
content: Physical size -- should fit inside a cerdip package, since that's what we package all our samples in.Any references to this technique by Phil Hobbs? I'm super curious.

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 12, 2021, 03:01:34 am
content: FET probes don’t give you direct access to the gate, but to an input divider, so there is a trade off for the input capacitance. You can play with this, although in the end your construction is going to limit this capacitance, don’t use coaxial cables, just thin wires with wide spacings. I don’t think the interpin capacitance of a cerdip can get you that low

author: ezalys
author_rank: Frequent Contributor
timestamp: August 12, 2021, 04:10:22 am
content: The node being sensed never appears on a pin. It's a small metal island that's wire-bonded to.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 12, 2021, 09:17:19 am
content: This one maybe?https://electrooptical.net/static/oldsite/www/sed/pHEMT_probe/pHEMT_probe.htmlNot sure if he has any schematics public or what.. I recall something about a bootstrap probe for wafer test, with extremely low capacitance. He's happy to email I think, worth a try.Tim

author: David Hess
author_rank: Super Contributor
timestamp: August 12, 2021, 04:07:53 pm
content: Quote from: ezalys on August 12, 2021, 01:27:53 amEarly designs used air wired construction with leaded and beam lead parts to reduce input capacitance. The same thing can be done with surface mount parts to some extent to minimize added capacitance. Later chip and wire hybrids with thin or thick film elements were used which suggests that the FET would need to be provided and assembled in chip form for your use.Measurements of tiny changes in capacitance rely on circuit design, like bootstrapping, rather than low absolute input capacitance.

author: ezalys
author_rank: Frequent Contributor
timestamp: August 12, 2021, 07:32:59 pm
content: Cgs can't be bootstrapped away, can it? I get that you can bootstrap away miller capacitance.

author: David Hess
author_rank: Super Contributor
timestamp: August 12, 2021, 08:35:40 pm
content: Quote from: ezalys on August 12, 2021, 07:32:59 pmIf the input transistor is used as source follower which is the common case for an active probe, Cgs is already bootstrapped by the transistor's alpha.High performance designs bootstrap the drain with a series RC network from source to a cascode which drives the drain. Dual-gate MOSFETs are convenient for this but their availability is limited to below 1 GHz for VHF and UHF applications; their reverse transfer capacitance is measured in 10s of femtofarads.

author: ezalys
author_rank: Frequent Contributor
timestamp: August 12, 2021, 10:38:25 pm
content: Sorry do I expect Cgs to be much larger than Cgd?

author: David Hess
author_rank: Super Contributor
timestamp: August 13, 2021, 12:20:06 pm
content: Quote from: ezalys on August 12, 2021, 10:38:25 pmCgs is somewhat larger than Cgd, but Cgs is already bootstrapped in the source follower configuration.

author: ezalys
author_rank: Frequent Contributor
timestamp: August 13, 2021, 04:42:05 pm
content: That sounds like a little over a factor of 2. Is the RC filter to try and cancel Cgd too?

author: TimFox
author_rank: Super Contributor
timestamp: August 13, 2021, 04:46:10 pm
content: Some of my -hp- equipment used tiny PTFE-insulated solder cups for gate leads of TH FETs for high resistance nodes in voltmeters, etc. I wonder if they could be guarded (actively) to reduce capacitance?

author: David Hess
author_rank: Super Contributor
timestamp: August 13, 2021, 06:17:30 pm
content: Quote from: ezalys on August 13, 2021, 04:42:05 pmIn the examples I have seen the series RC network is there to do exactly that.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 13, 2021, 08:04:47 pm
content: Quote from: TimFox on August 13, 2021, 04:46:10 pmIn the 3456 they use the capacitance of the PTFE cap as part of a capacitor that is wanted. So they make the parasitic capacitance part of a wanted one.With a fet follower the drain side may be actively driven, but this can be tricky as the extra diver adds some delay and this can in consequence leed to extra phase shifts and possibly high frequency osciallation . Usually the boostrapping is thus with a reduced gain (which kind of comes naturally as a source follower / emitterfollower has a gain a little below 1).