small sihnal MOSFET or JFET - Page 1
Forum section: electronics / projects

author: sdouble
author_rank: Frequent Contributor
timestamp: March 22, 2025, 01:47:29 pm
content: Hi Everybody,Part of my job is to design charge sensitive preamplifier for nuclear physics detectors, like silicon detectors or ion chambers.In some specific cases, I'm dealing with very high capacitance devices like large and thin silicon detectors.To optimize the signal over noise ratio, i shoot for high gm jFET as an inpt transistor. Sometimes, I even parallel 2 or 3 units. Going for 4 or more often lead to instabilities.I wonder wether someone had experience with using beefy small signal MOSFETs instead of jFET for such an application. With current of about 50 mA, I could certainly get about 250mS with a modest capacitance MOSFET (about 50 pF input cap).MOSFET are supposed to be much noisier, I recon, but did someone try in such a specific (very high capacitance sensors) context ?TIAJ

author: PartialDischarge
author_rank: Super Contributor
timestamp: March 22, 2025, 01:58:00 pm
content: I guess that the higher input current noise AND voltage noise of mosfets, especially when the high input capacitance is amplifying this noise, makes mosfet unsuitable for the task and probably few have spent the time even testing

author: TimFox
author_rank: Super Contributor
timestamp: March 22, 2025, 02:35:28 pm
content: There is a general trend (read each data sheet) from BJT to jfet to mosfet:BJT has lowest voltage noise, highest current noise, lowest corner frequency.JFET has medium voltage noise, medium current noise, medium corner frequency.MOSFET has highest voltage noise, lowest current noise, highest corner frequency.(This ranking is for “audio” and “video” frequencies.)Here, “corner frequency” is where 1//fnoise starts to dominate over white noise as you go down in frequency.In actual design, FET capacitance has a large effect on current noise of the circuit.I lament the loss of the Sony 2SK152 JFET, which gave me very low noise in circuits up to roughly 10 MHz.I was surprised at the high corner frequency for good dual-gate MOSFETs, so 1/fnoise was important over the entire audio range. JFET (iirc) corners could be around 100 Hz, and BJTs could be around 10 Hz.(I’ll look up real values after breakfast.)For circuits following diode sensors, the amplifier’s voltage noise and the source capacitance can look like a current noise that rises with frequency (blue) when referring the actual noise to the usual equivalent noise at the input.

author: sdouble
author_rank: Frequent Contributor
timestamp: March 22, 2025, 04:11:10 pm
content: I usually go for 2SK932 from Onsemi and I'm more in the MHz range, like 0.5 to 5 MHz.BJT is a no go in my set up. High input impedance transistor is a must.

author: TimFox
author_rank: Super Contributor
timestamp: March 22, 2025, 04:27:21 pm
content: Yes, one could “bootstrap” a BJT to get high input impedance, but the noise current is not affected.When you get above, say, 10 MHz, you must pay attention to the gate-drain capacitance of JFETs in low-noise design, which was a good feature of the 2SK152.

author: sdouble
author_rank: Frequent Contributor
timestamp: March 22, 2025, 04:42:49 pm
content: I did measure a voltage noise of 0.7nV/sqrt(Hz) at 1MHz in some 2SK932-23.do you really get something better than that bootstrapping BJTs ?

author: TimFox
author_rank: Super Contributor
timestamp: March 22, 2025, 06:01:38 pm
content: Bootstrapping BJTs achieves high input impedance by controlled positive feedback, but does not affect the noise performance.BJTs get good noise performance with low source impedance: the optimal source impedance (if you use a transformer to vary the impedance seen by the active device) is given byRopt=en/ at frequencies where the input noise voltage densityenand current densityinare uncorrelated.Therefore, BJTs with low noise voltage work well at low source resistance, while JFETs with low noise current work well from high source resistance. It gets interesting when the source impedance is mainly capacitive.A "figure of merit" for JFET noise performance is |Yfs| /Ciss; Sony achieved a good result, but their intended market seemed to be vidicon preamplifiers, no longer important.They were also used in short-wave receivers for good noise performance.Sony quoted typical 1 nV/Rt-Hz at 10 mA, decreasing at higher current. Your 0.7 nV/Rt-Hz is very good, approximately the same voltage as the thermal noise of a 30\$\Omega\$ resistor.Their datasheet:https://www.silicon-ark.co.uk/datasheets/2sk152-datasheet-english.pdfA possible replacement for the discontinued 2SK152 is InterFet's IFN152https://www.mouser.com/datasheet/2/676/jfet_ifn152_interfet-2888029.pdf?srsltid=AfmBOoo2v05jiZJ4QKdP9YGck3uMRPfCdEIlV6ZEDRKwH-uiRMFCipaQ

author: TimFox
author_rank: Super Contributor
timestamp: March 22, 2025, 09:27:05 pm
content: That's what I wrote.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 31, 2025, 04:08:53 pm
content: CERN have a ton of papers on designing CSAs.

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2025, 05:16:40 pm
content: Quote from: sdouble on March 22, 2025, 01:47:29 pmSome low voltage noise high impedance input amplifiers have gone that route, like for audio and instrumentation. MOSFET voltage noise is greater than JFET voltage noise at a given current, however much larger MOSFETs are available allowing operation at higher currents. The compromise of course is input capacitance which is greater for larger area transistors.It used to be that large area high voltage high current "power" JFETs were available, but that was decades ago.Incidentally, current noise of MOSFETs is flat at lower frequencies whereas JFET current noise increases at low frequencies from flicker noise. This is why MOSFET input transimpedance amplifiers can deliver attoamp noise free resolution so easily.

author: tooki
author_rank: Super Contributor
timestamp: March 31, 2025, 06:29:25 pm
content: Quote from: sdouble on March 22, 2025, 01:47:29 pmOut of curiosity, why not use an op-amp instead of rolling your own amp?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 31, 2025, 06:35:57 pm
content: At the very low noise level the choice of OP-amps is very limited. With discrete JFET it is reasonable easy to get well below 1 nV/sqrt(Hz) even with low current noise. With OP's it ends at some 0.9 nV/sqrt(Hz) and this is with BJT based ones with rather high current noise.There are a few OP-amps with MOSFET inputs that have surprisingly low noise (e.g. LTC6240), but still no match for discrete transistors.

author: David Hess
author_rank: Super Contributor
timestamp: April 01, 2025, 03:33:47 am
content: I have been happy just to get precision JFET parts like the OPA140 which is 5.1 nV/SqrtHz and 0.23 uVpp from 0.1 to 10 Hz noise, and better precision than an LT1012 but with lower input bias current.Linear Systems has a p-channel JFET with 1 nV/SqrtHz noise.

author: Andree Henkel
author_rank: Regular Contributor
timestamp: April 01, 2025, 12:42:04 pm
content: sdouble,you have large cap sensors at high output impedance,you want low noise, high input impedance and also want to deal with the large capacityI guess: solution is trying to combine JFET for low noise + high input impedance with bootstrapping to deal with the large capacity