Testing a femtocoulomb preamp  - Page 1
Forum section: electronics / projects

author: BrunoGelli
author_rank: Contributor
timestamp: December 21, 2022, 03:31:04 am
content: Good evening my friends,I'm trying to test (and understand) an old preamplifier. It was designed in CERN and was intended to be used with really really small charge and fast pulses. (Perhaps 10 fC and 1 ns or so...)I've attatched a picture of the circuit alongside this post.The thing is, I'm applying a pulse to its input and it seams to be working. But I'm measuring a gain which is way smaller than it should be (measuring about 40x instead of 10000x or so). I'm afraid I'm applying too much charge into the preamp and it simply can not keep up to the "large" 10mV signal I'm applying. (Overloading the preamp perhaps)I thought I could use a simple resistive divider to atenuate the pulses. But I'm not sure this is a good idea to be honest! Perhapse something a little more sophisticated? I was wondering if there is circuit to generate controlled and repetable small-charge pulses.Do any of you have a good idea or tips I could try?Thank you and have a wonderfull end of the year,Bruno Gelli

author: Swainster
author_rank: Frequent Contributor
timestamp: December 21, 2022, 04:11:29 am
content: This appears to be a differentiator circuit - the output should be proportional to the rise/fall time of the input signal (within a certain bandwidth), not the amplitude. Perhaps testing it with a triangle wave will give more interesting results. If you want to reduce the input signal then a simple resistor divider should be fine (maybe matching the impedance of your signal source).

author: matthuszagh
author_rank: Frequent Contributor
timestamp: December 21, 2022, 04:17:12 am
content: Quote from: BrunoGelli on December 21, 2022, 03:31:04 amHm, are you sure this is meant for 1ns pulses? The NJ903 JFET (https://www.digchip.com/datasheets/parts/datasheet/230/NJ903-pdf.php) input stage reports rise time 1ns, fall time 2ns, turn on delay 7ns, and fall time delay 12ns. And, that input capacitance (45pF) is pretty high, which will limit the bandwidth. Cool part though.

author: wasedadoc
author_rank: Super Contributor
timestamp: December 21, 2022, 11:06:22 am
content: The 2N3904 has a current gain bandwidth product of 300 MHz. For the 2N3906 it is 250 MHz. Not the transistors I would expect to see in something expected to handle 1 ns pulses.

author: BrunoGelli
author_rank: Contributor
timestamp: December 21, 2022, 01:20:34 pm
content: Thank you for the replies! I'll take a look in to the sugestions.To be honest, I'm not sure if it was intended to be used with 1 ns pulses.As it is an old design, the only references I have are anecdotal. That's why I want to characterize it!We are interested in using it to measure discharges in GEM detectors, which would produce a charge pulse of about 100 fC lasting about 10 ns or so. (It was originally designed to be used in gas detectors, which would have a similar [if not smaller] response)

author: matthuszagh
author_rank: Frequent Contributor
timestamp: December 21, 2022, 02:20:46 pm
content: You might try firing this up in spice simulator: I like ngspice personally, but I know ltspice is a common favorite. That would make testing with low-current, high-speed pulses simple, including to see how it would behave in the GEM detector where you want to use it. There's a spice model available for the ha5002 (https://www.renesas.com/us/en/products/space-harsh-environment/harsh-environment-products/harsh-environment-amplifiers/ha-5002-110mhz-high-slew-rate-high-output-current-buffer#overview). I didn't immediately find one for the NJ903 but you should have enough to go by the datasheet to at least get a decent approximation. Or, you might try contacting interfet directly and ask for one.Otherwise, there's the traditional approach of reasoning about the circuit. I'd approach this like any other analog circuit: first find the bias points and then think about the small signal operation about those bias points.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 21, 2022, 04:48:06 pm
content: A simple reistive divider will have lots of stray capacitance and leakage- you would need specialised components and construction.For some hot tips search for "what's all this femtoampere stuff anyhow" See the video of the same title on testing ultralow bais opamps.quote "This appears to be a differentiator circuit - the output should be proportional to the rise/fall time of the input signal (within a certain bandwidth), not the amplitude. Perhaps testing it with a triangle wave will give more interesting results. If you want to reduce the input signal then a simple resistor divider should be fine (maybe matching the impedance of your signal source)"Where does it appear in this thread?? Hmmm...

author: Kleinstein
author_rank: Super Contributor
timestamp: December 21, 2022, 05:05:48 pm
content: There is already a test input with a small capacitor. Apply something like a 100 mV peak to peak square wave for a test. This would give 0.1 pC size pulses and should give output pulses with some 100 mV peak hight and than decaying according to the 1 pF*50 M = 50 µs time constant. Due to parasitic capacitance and a somehwat limited gain the pulse may be a bit smaller than that.The FB capacitor may vary.It would take quite some amplitude to overload the input - this could effect the rising slope part a little, but the overall pulse area after pulse forming should not change very much.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 21, 2022, 05:18:02 pm
content: Based on the description and 1kV input cap, could this be the amp to look at pulses from a PMT?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 21, 2022, 05:33:00 pm
content: A PMT could be a possible signal source, though this would normally not need that much sensitivity.I would more consider a GM tube type ion current detector with less internal gain, possibly in a multi wire configuration.The amplifier input looks like it is made for a detector with quite some capacitance and a relatively small signal.

author: David Hess
author_rank: Super Contributor
timestamp: December 21, 2022, 05:35:25 pm
content: For testing at such low currents or amounts of charge, I would use a small value capacitor with a step input for charge or triangle input for current.

author: BrunoGelli
author_rank: Contributor
timestamp: December 22, 2022, 12:05:06 am
content: QuoteThis is exactaly the description of a GEM detector, which is basically a big capacitor. (I've atached a schematic drawing if anyone is interested. There is about 300 V between the top and boton part of each GEM foil and about 500 V from one foil to another. )PMT pulses are a great idea! I have a few here and we are quite familiar with them. Thank you, guys! For this sugestion and all others as well!I'll provide a feedback once I'm able to test it!

author: Sengcid
author_rank: Regular Contributor
timestamp: December 28, 2022, 07:41:48 pm
content: First draft, awaiting test results.