The process is complicated by the wide parameter spread that is unique to JFETs. The information in this page is intended to show both the advantages and disadvantages of simple JFET stages.

'General Purpose' JFETs such as the once-ubiquitous 2N5459 might show up in a search, but be designated 'non-stocked' or similar, with orders accepted only for large quantities with a significant lead-time. Linear Systems makes the LSK170, which is pretty much a direct equivalent, but they aren't available from most major distributors. other than on eBay, where you might get a JFET of one type or another, but it's unlikely to be genuine. The really low noise devices such as the 2SK170 are gone . While you can use a small adapter board (available from a few suppliers), this is still a nuisance, as each device you wish to test or experiment with needs its own adapter. The few remaining devices from the major suppliers are often only available in a surface-mount device (SMD) package, making it next to impossible to use traditional prototyping systems such as a breadboard or Veroboard. They do work as amplifiers, but some have so much input capacitance that they are unusable with high-impedance signal sources. This is not because I dislike then (quite the opposite), but because the range from most of the larger suppliers has been reduced to a few devices intended for switching, rather than linear operation. The challenges are covered below, and they are not insignificant.Īs noted within this article, I have very few designs that use JFETs. One of the advantages is that because the gate is a reverse-biased diode, there is far less likelihood that any stray radio-frequency signals will be detected and amplified, as can happen easily with BJTs (bipolar junction transistors) and many opamps (operational amplifiers). While JFETs have many desirable features, they also come with many challenges. I have avoided the more complex designs, simply because they are complex, and because you need to go to considerable trouble to match the JFETs closely enough to get a working circuit. This is important when experimenting, as it makes it more likely that you'll have a successful outcome. It doesn't excel at anything in particular (although it does have fairly low noise of around 4nV√Hz), but it also has few 'bad habits'. I used this because it's one of the few low-cost JFETs that you can still get from (some) major suppliers, and it has basic specifications that make it ideal for general-purpose low current applications. JFETs provided by Linear Systems notwithstanding, most of the designs shown use a rather pedestrian 2N5484. Having received these, I decided that it was a worthwhile exercise to look at the basic design processes for JFET stages in general.

One of these is the LSK170B (equivalent to the revered 2SK170, but with graded maximum drain current). I mention this because they kindly sent me some samples (full disclosure here) of two different types. This has made it very difficult to build some of the more esoteric designs from readily available types, but Linear Systems produces a range that's ideal for many designs. JFETs (junction field-effect transistors) are beloved by many, but unfortunately the range has shrunk dramatically in the past few years.

5 - Active (Current Source) Load JFET Amplifier.

Jfet transistor diagram