{"id":665,"date":"2015-01-25T19:44:50","date_gmt":"2015-01-25T19:44:50","guid":{"rendered":"http:\/\/stompville.co.uk\/?p=665"},"modified":"2020-10-11T14:19:14","modified_gmt":"2020-10-11T14:19:14","slug":"jfet-idss-matching","status":"publish","type":"post","link":"https:\/\/stompville.co.uk\/?p=665","title":{"rendered":"JFET Idss Matching"},"content":{"rendered":"<p><strong>Idss<\/strong> for the <strong>2SK596s<\/strong> JFET &#8211; as introduced in my <a title=\"OnSemi 2SK596S Low Noise JFETs\" href=\"https:\/\/stompville.co.uk\/?p=658\">previous blog<\/a> &#8211; is an order of magnitude lower than (say) a <strong>2N5457<\/strong>, so I have been revisiting the design of my Arduino-based automatic JFET matcher to be sure that I can use it to characterise the <strong>2SK596s<\/strong> with good resolution. Along the way, I was idly comparing readings between my automated tester and the new <a title=\"Matching JFETs \u2013 Revisited\" href=\"https:\/\/stompville.co.uk\/?p=601\">JFET Matcher project<\/a> (kits available <a title=\"http:\/\/stompville.co.uk\/shop\/23-jfet-matcher-kit.html\" href=\"https:\/\/stompville.co.uk\/cart\/kits-modules\">here<\/a>) and noticed that whilst the <strong>Vgs<\/strong> readings between the two testers were comparable, the automatic tester\u00a0 consistently showed a lower <strong>Idss<\/strong> than the new manual tester (by about 3%). I went right through the hardware design of both testers (and\u00a0and the firmware for the auto tester) looking for a bug or any clue to the discrepancy. Eventually (doh!), it dawned on me that the reason the two testers give different results is that <strong>Idss<\/strong> is dependant on the drain-source Voltage <strong>Vds<\/strong>.<\/p>\n<p>In this case, the 3% difference is due to the fact that the automated matcher operates at a drain-source voltage of nominally <strong>5V<\/strong>, whereas the manual matcher is operating off a <strong>9V<\/strong> battery.<\/p>\n<p>So, I made the world&#8217;s simplest <strong>Idss<\/strong> tester&#8230;<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter\" src=\"http:\/\/stompville.co.uk\/wp-content\/uploads\/2015\/01\/167SV.png\" alt=\"167SV\" width=\"350\" height=\"250\" \/><\/p>\n<p>&#8230;and measured the Idss for a single sample <strong>PF5102<\/strong> JFET over a <strong>Vds<\/strong> range of 2-25V. The results:<\/p>\n<p><a href=\"http:\/\/stompville.co.uk\/wp-content\/uploads\/2015\/01\/166SV.png\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter\" src=\"http:\/\/stompville.co.uk\/wp-content\/uploads\/2015\/01\/166SV.png\" alt=\"166SV\" width=\"620\" height=\"457\" \/><\/a>The low value (2V) is 3.84 mA and the high value (25V) is 4.51 mA. This is a 15% change and entirely explains the discrepancy between the automated and manual matchers.<\/p>\n<p>So the moral is:\u00a0 if you want an Idss of a particular value (for example if you&#8217;re using your JFET as a constant current source or sink), then you need to test for <strong>Idss<\/strong> at the same <strong>Vds<\/strong> as your application circuit.<\/p>\n<p>Incidentally, I also noticed that <strong>Vgs(10M)<\/strong> measurements are susceptible to RF interference. If you are getting inconsistent results, you should go and do your JFET-testing\/matching away from potential sources of interference.\u00a0 If you set your tester for <strong>Vgs(M)<\/strong> and the reading on your multimeter changes as you move your hand close to the JFET, you have a problem with interference.\u00a0 <img loading=\"lazy\" decoding=\"async\" src=\"http:\/\/stompville.co.uk\/wp-content\/uploads\/2012\/02\/svfavicon.png\" alt=\"svfavicon.png\" width=\"16\" height=\"16\" \/><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Idss for the 2SK596s JFET &#8211; as introduced in my previous blog &#8211; is an order of magnitude lower than (say) a 2N5457, so I have been revisiting the design of my Arduino-based automatic JFET matcher to be sure that I can use it to characterise the 2SK596s with good resolution. Along the way, I\u2026 <span class=\"read-more\"><a href=\"https:\/\/stompville.co.uk\/?p=665\">Read More &raquo;<\/a><\/span><\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5],"tags":[],"_links":{"self":[{"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/665"}],"collection":[{"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=665"}],"version-history":[{"count":3,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/665\/revisions"}],"predecessor-version":[{"id":1414,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/665\/revisions\/1414"}],"wp:attachment":[{"href":"https:\/\/stompville.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=665"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=665"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=665"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}