Differential probe - Page 1
Forum section: electronics / projects

author: GGMM
author_rank: Frequent Contributor
timestamp: May 13, 2023, 07:05:54 am
content: Hello,Good morning,I’m thinking about making a differential probe.Since I have BF988S in advance, I’d like to use them.However, I would like to know the function of the 5 transistors on this diagram. It's similar and does not exist on the D.Kramniks probe.It is not to light a led. LOLAnother modification, I want to find another op amp for the output, to gain bandwidth.your opinionscdt

author: GGMM
author_rank: Frequent Contributor
timestamp: May 13, 2023, 07:07:52 am
content: other pictures

author: mk_
author_rank: Regular Contributor
timestamp: May 13, 2023, 07:12:49 am
content: Quote from: GGMM on May 13, 2023, 07:07:52 amSearch TI`s collection of opamps and related partsQuoteKramniks Design is "only" AC

author: tggzzz
author_rank: Super Contributor
timestamp: May 13, 2023, 08:39:17 am
content: Quote from: GGMM on May 13, 2023, 07:05:54 amThey are a current source for the differential amp above your red outline.The LED and Q2 form a temperature compensated current source, with the current (~150µA) coming out of Q2 collector.U2A and Q20 form a basic 2:1 current mirror, and Q1 "turns" that into a Wilson current mirror with the current (~300µA) coming out of Q1 collector. Both are described in wackypedia.

author: moffy
author_rank: Super Contributor
timestamp: May 13, 2023, 08:47:37 am
content: The red section is a PNP current source using Q2 and the LED as a temperature tracking reference. The three transistors from the LM3046 and Q1 form a current mirror with I think twice the current going out as is coming in. The current mirror requires very little voltage drop, basically 2Vbes to operate, so it allows the input stage to operate over a wider voltage range.

author: GGMM
author_rank: Frequent Contributor
timestamp: May 13, 2023, 08:53:45 am
content: Hello,Thank you for your answers.So actually the diode is not a gadget. LolI put the link for the explainationcdthttps://en.wikipedia.org/wiki/Wilson_current_mirror

author: GGMM
author_rank: Frequent Contributor
timestamp: May 13, 2023, 09:09:15 am
content: Hello MK_I’m more inspired by that.https://www.diyaudio.com/community/threads/differential-probe-reverese-engineered.248505/Can you tell me if the DC passes, or possibly what changes need to be made.Also the fets will be replaced by BF998 (As explained, for a few euros you have more than 10)So surely modify some resistorSchematic will be a lot of inspiration of the 2 designcdt

author: tggzzz
author_rank: Super Contributor
timestamp: May 13, 2023, 09:42:35 am
content: Quote from: GGMM on May 13, 2023, 07:05:54 amYou are considering how it works.As it is ahigh voltagedifferential probe you will also need to consider how itfails. That should include:appropriate PCB layout, material, and shape (possibly including slots/cutouts)appropriate resistor types; some are more suitable for HV operation and othersditto capacitorshttps://www.diyaudio.com/community/threads/differential-probe-reverese-engineered.248505/post-3765125

author: daisizhou
author_rank: Super Contributor
timestamp: May 13, 2023, 09:52:49 am
content: https://www.eevblog.com/forum/projects/high-voltage-differential-probe-design-for-review/?nowapThe bandwidth here is 50M, good plan

author: mk_
author_rank: Regular Contributor
timestamp: May 13, 2023, 09:54:03 am
content: Quote from: GGMM on May 13, 2023, 09:09:15 amwhat kind of diffprobe you need/want to build? a fast one, low voltage or a slower one with hV?

author: GGMM
author_rank: Frequent Contributor
timestamp: May 13, 2023, 05:16:17 pm
content: Re,Do you have an explanation that on the Kramniks version 2 probe, we went from +/-5V to +/-3.3V to power the op amp?This implies a circuitry with converter and 2 additional regulator.why?cdt

author: mk_
author_rank: Regular Contributor
timestamp: May 13, 2023, 06:24:37 pm
content: Quote from: GGMM on May 13, 2023, 05:16:17 pmYes, but if it is needed... then it has to be done, whats the problem?QuoteI have no idea why Kramnik he decided to go that way.I design my own probes for max. outputvoltage and therefore I generate the supplyvoltages as needed by the design.

author: GGMM
author_rank: Frequent Contributor
timestamp: May 13, 2023, 07:03:10 pm
content: Re,it's good.You have to read the article to the end.He had too much noise on the power supplieshttps://xellers.wordpress.com/electronics/1ghz-active-differential-probe/cdt

author: daisizhou
author_rank: Super Contributor
timestamp: May 15, 2023, 03:47:02 am
content: Quote from: GGMM on May 13, 2023, 07:05:54 amLooking forward to sharing it after your success