why is the J201 transistor so "unstable"?

Thread Starter


Joined Feb 2, 2024
is it a problem with my transistor, or did I miswire it?

the voltage source is 5v.
Drain is connected to a LED which is connected to 5v-
Source is connected to 5v+
Gate is connected to a pushbutton, which is connected to 5v+

so, I'm testing out this J201, and this happens:


Now, I'm not any kind of expert but... I've heard that J201 is "Unstable," but it shouldn't be THIS unstable... right?
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Joined Mar 30, 2015
Welcome to AAC!
I've heard that J201 is "Unstable," but it shouldn't be THIS unstable... right?
Why would anyone say it was "unstable".

The gate is a reverse biased diode (e.g. high impedance). You're coupling line voltage to the gate with your hand.

A schematic is always preferred to a textual description:

schmitt trigger

Joined Jul 12, 2010
Leaving a gate unterminated is a recipe for malfunctioning, as you have discovered already.
Additionally, an N-channel JFET requires a *negative voltage* to turn off. With zero VGS voltage, the channel current is maximum, what the datasheet calls IDSS.


Joined Feb 24, 2006
One might ask why you rushed through the description of how the device works before hooking one up. That or you didn't bother to read the description in the firstplace. Either one is a problem you should attempt to fix. This sit has a wealth of introductory information. You should at least read it once before the next "experiment".

sparky 1

Joined Nov 3, 2018
In these examples below the J201 improves different types of stability.
The sensitive nature of FET low current switching and liquid crystal displays forced us to learn to work lower current.

Could be used as a current limiting diode or low current source where very small current is needed.
J201 low current source can improve performance where a resistor alone may tend to heat up, the J201 takes the heat allows the resistor to perform within a narrower tolerance range. Using a heat sink will further improve the current regulation. TO92 package are ok but less helpful for mounting.

In a resonant circuit that uses a varactor diode the frequency stability is improved by low current and voltage regulation.
It was the wide voltage range that gave voltage-controlled oscillators adjustable frequency range over simple crystal oscillators.
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