The JFET and the VOM - John Dunn, Consultant, Ambertec, P.E., P.C.

I once had this junction field effect transistor (JFET), a 2N4391 I think it was, to which I attached a Simpson 260 VOM (volt-ohm-milliammeter), positive lead to the drain and negative lead to the source while in the ohms mode on the R x 1 scale. There was some mid-scale meter reading which meant that with the gate floating as it was, the device was carrying some kind of drain-to-source current.

This was nothing to get excited about, but then I noticed something. The meter deflection would vary in repeatable ways as I moved my hand to various positions in the vicinity of the JFET as it lay there on the table top, connected up as it was.  

Incident light, or the absence thereof, didn't seem to have any effect on this so I think that the device was oscillating at some RF frequency although I have no idea what that frequency might have been. It turns out however that this JFET is a speedy little thing. Per the data sheet, it's capable of switching speeds in the tens of nanoseconds and it has admittance definitions out to a frequency of 1 GHz.

This wasn't exactly Werner Heisenberg's "Uncertainty Principle" at work, but clearly my VOM and its wires were affecting the device whose properties I was just idly trying to examine.

In fact, in looking back on a whole variety of odd things I've seen happen over the course of time, many of them were finally explained as the results of unanticipated oscillations of one sort or another, one example being the story of the inadvertent waveguide at:

http://licn.typepad.com/my_weblog/2011/01/inadvertent-waveguide-john-dunn-consultant-ambertec-pe-pc.html#more  

Let us therefore pledge: Eternal vigilance against parasitic oscillations.  

Comments

[Mark Walter]

John,
Would not a simplier explaination be some electrostatic field that was influencing the gat?
M Walter

[John Dunn]

This was going on at my coffee table in my living room at home, then in Baldwin, NY, on a quiet and idle Sunday afternoon. The only variable was in how I dressed the VOM's meter leads.

[Aakash]

John

What type of unanticipated oscillation can it be?

[John Dunn]

The VOM's test leads were just draped helter-skelter over the table's surface and made some kind of LC-resonant structure that set the JFET up for oscillation in the HF or VHF frequency range.