[HN Gopher] The marvelous disappearing capacitor
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The marvelous disappearing capacitor
Author : peter_retief
Score : 86 points
Date : 2024-12-08 07:30 UTC (4 days ago)
(HTM) web link (lcamtuf.substack.com)
(TXT) w3m dump (lcamtuf.substack.com)
| IndrekR wrote:
| Have used that JFET trick on large area photodiode amplifiers in
| a satellite based radiation detector. Works quite well.
|
| Also quite similar is actively driving the shield used in many
| high impedance sensor circuits (most notably capacitive touch
| sensing) to reduce the parasitic capacitance you would get when
| ground is used for the E-field shield:
|
| https://www.ti.com/lit/an/snoa926a/snoa926a.pdf
| milleramp wrote:
| Can a similar technique be used to increase the bandwidth of an
| amplifier driving an LED?
| frabert wrote:
| I think that if what you're looking for is to increase the
| speed at which you can drive something which has some
| parasitic capacitance, you would do that by using the
| techniques that are employed by e.g. gate drivers.
| frabert wrote:
| Very cool! Can this trick be used for piezo sensors as well? My
| understanding is that those too are essentially magic capacitors
| that sometimes move charges on their own
| relaxing wrote:
| Yes, you can. In fact bootstrapping is a useful technique for a
| variety of circuits involving some small input signal, and even
| large ones, like power supply design.
|
| This article is a little weird in framing the concept solely
| around the author's journey in photodiode circuits, but that's
| blogging for you I guess.
| formerly_proven wrote:
| AoE and AoE X has a bit on TIAs as well, their "final form"
| circuit idea enhances the basic JFET follower idea with an
| additional emitter follower to reduce source impedance and by AC-
| coupling the compound follower's output they get to set favorable
| DC working points.
|
| The tricky bit is going to find a suitable replacement for the
| BF862.
| jeffbee wrote:
| Depends on the critical parameters, but 2SK2394?
|
| You can try to source old JFETs but all of the ones advertised
| online are fake.
| formerly_proven wrote:
| It has been a couple years, but iirc the BF862 back then had
| a pretty unique combination of low capacitance, low e_n and a
| low 1/f knee. The 2SK2394 is also earmarked for use as an AM
| radio amplifier, same as the BF862, which probably also
| explains why these went from jellybean to rarity some time
| ago...
| CamperBob2 wrote:
| Phil Hobbs has also written a lot about photodiode
| bootstrapping, and he was also a very unhappy camper when the
| BF862 went EOL. Anyone working in this area should check out
| his book (see https://electrooptical.net/), which is very much
| in the same spirit as AoE. I used to read a lot of posts by
| Phil on Usenet via Google Groups, before Google decided they
| had better things to do.
|
| For that matter, I have a feeling Michal (lcamtuf) has a book
| or two in him that could stand up next to Hobbs and
| Horowitz/Hill on the shelf. Super impressed with his work over
| the years. (Many may recognize him as the 'Guerilla Guide to
| CNC' guy, which has been linked on HN several times.)
|
| CPH3910 may be worth considering as a BF862 replacement. I
| don't recall the part number but there's also a dual CPH3910 in
| a 5- or 6-pin SOT package that could be used with the
| transistors in parallel.
| zh3 wrote:
| Digikey has a useful article as well. Note how in Figure 5 the PD
| is reverse-biased to reduce the capacitance without any extra
| components.
|
| https://www.digikey.co.uk/en/articles/how-to-use-photodiodes...
| amelius wrote:
| I'd like to see an I-V plot for various levels of light, for
| the photo diode.
| jonjojojon wrote:
| This is called a bootstrap circuit. It is basically positive
| feedback that changes the input impedance. You can see this sort
| of jfet circuit not only in trans-impedance amps, but also in a
| lot of audio circuits.
| rapjr9 wrote:
| This reminds me of a somewhat similar circuit, the capacitance
| amplifier:
|
| https://en.wikipedia.org/wiki/Capacitance_multiplier
|
| Which leads me to think about planned obsolescence in electronic
| circuits. In many electronic circuits the electrolytic capacitors
| are the first to degrade, limiting electronics lifetime to 10+
| years. What if those capacitors were replaced with amplified
| capacitors where the capacitor was not electrolytic, say a film
| capacitor. Maybe the circuit would work the same and last 20-30
| years, eliminating a lot of electronic waste and reducing
| environmental carbon due to manufacturing? Part of the solution
| to climate change has to be manufacturing products that last
| longer. Electrolytic capacitors, built in batteries, fans, and
| parts made of rubber instead of silicone are some of the prime
| components that cause failures. I have a suspicion car companies
| have tuned steel formulations and paint to cause rust after a
| specific number of years as well.
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