2SK117 replacement for a Curiously Low Noise Amplifier - Page 1
Forum section: electronics / beginners

author: mycroft
author_rank: Regular Contributor
timestamp: January 22, 2020, 03:55:05 pm
content: Folks,I want to build a couple of these Curiously Low Noise Amplifier from Charles Wenzel site (http://techlib.com/electronics/audioamps.html#curiously%20low%20noise) but the input JFET is no longer available. I intend to use them to measure power supply ripple and noise and some low noise sensors. Any suggestions on a suitable replacement?Thanks.

author: andy3055
author_rank: Super Contributor
timestamp: January 22, 2020, 03:59:48 pm
content: Here is a source:https://www.digipart.com/part/2SK117

author: Kleinstein
author_rank: Super Contributor
timestamp: January 22, 2020, 04:08:39 pm
content: The SK209 is relatively similar, just in SOT23 case. To92 or similar case JFETs are becoming more and more difficult to get.Anyway also keep an eye on the sub-group (e.g. color code) for Ids.

author: David Hess
author_rank: Super Contributor
timestamp: January 22, 2020, 04:30:08 pm
content: If the LSK189 from Linear Systems is not suitable, then I am sure they have a part which is. Or a dual part like the LSK389 could be used in parallel. Interfet should have something suitable.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 22, 2020, 04:41:20 pm
content: Usually the choice of JFETs from a more local supplier is relatively limited and especially TO92 parts are often NOS or similar - so it really depends on the source where you get the parts. One may still get the 2SK117 (at least in Germany). Otherwise check the usually limited list that is available.Modern JFETs usually come in SOT23 or similar.

author: MasterT
author_rank: Frequent Contributor
timestamp: January 22, 2020, 05:53:37 pm
content: My thoughts:1. input cap 0.01u has impedance 15.9k at 1kHz.2. input bias res. 47 meg, certainly is not noise free part.Q: where noise generated by res. would flow?This is definitely not "low noise" design, but rather exactly opposite.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 22, 2020, 06:05:07 pm
content: The 47 M resistor only contributes significant to noise at frequencies below the lower cross over, which is at some 0.3 Hz.At higher frequencies the noise from the resistor is shorted by the source (unless that is higher than 47 M). One would hear the noise with an open input.The capacitor impedance does not mean it would produce the corresponding amount of noise. The capacitor does produce a little noise but this is only from the loss part, which is something like 0.1 - 1 % of the impedance.So the design can be low noise, but I would not expect it to be low distortion.

author: MasterT
author_rank: Frequent Contributor
timestamp: January 22, 2020, 06:23:43 pm
content: Quote from: Kleinstein on January 22, 2020, 06:05:07 pmHow come it would be shorted, capacitor is essentially 15k resistor in between, so doesn't matter how low output impedance of the signal source, amplifier sees noise out of 47 meg loaded by 15k (at 1 kHz, input shorted to ground for simplicity).

author: Kleinstein
author_rank: Super Contributor
timestamp: January 22, 2020, 06:42:14 pm
content: 47 M loaded with the 15 K impedance is only 1/3000 the noise of the 47 M resistor. This is considerably less than the FET noise. As the FET has 1/f noise this is also true for the lower frequencies.

author: MasterT
author_rank: Frequent Contributor
timestamp: January 22, 2020, 06:54:04 pm
content: According to this pagehttps://www.daycounter.com/Calculators/Thermal-Noise-Calculator.phtmlat 25C 47meg res. has 27.8118 uV just for 1k bandwidth. Even it goes down by 3000, 9nV is still pretty high value