[sdiy] Newbie JFET-switch problem

[Ian Fritz]

Hi Richard (et al.)

Looks like perhaps the FET in my circuit has a higher pinchoff voltage than 
yours.  Also, I use the circuit only up to ~ 4V.  The trouble you are 
having is most likely due to the diode.  I don't even remember why I put it 
in -- probably to reduce the amount the gate has to swing.  Try just taking 
it out or replacing it with a 1M (or so) resistor.

   Ian



At 01:22 PM 8/5/05, Richard Arntzen wrote:
>Hello all.
>
>I have built myself a JFET-switch (2N3819) from a schematic kindly provided
>by Ian Fritz, and I have some problems. Brief description of the circuit:
>
>The JFET base is connected to the anode of a diode (cathode is grounded).
>Base is also is connected to positive rail (+15V) through a 1.5M resistor,
>and to a cap. The basic idea is that:
>
>i) Static mode is that the anode of the diode should have a small positive
>voltage, and the JFET conducts.
>
>ii) when the cap is pulled down (by another part of the circuit), the base
>will be pulled down and the JFET will close.
>
>iii) The cap will then be slowly charged through the resistor, and
>eventually the JFET will conduct again.
>
>Now; my problem is with the input range of the switch. I see three modes:
>
>A) When the source voltage VS is below 1.19V, it apparently pulls the gate
>down. I see an <almost> linear relation between VG (gate voltage) and VS, up
>to VS between 1.11 and 1.19V, after which VG flattens out at 0.37V
>regardless of VS. In this region (VG < 0.37V), the drain voltage VD is
>always 0.47V below VS.
>
>B) When 1.19V < VS < 3.22V, the JFET is open (VD=VS).
>
>C) When VS > 3.22V or thereabout, VD becomes <almost> constant at 3.26-3.3V
>regardless of VS.
>
>Having done some reading I see that C) happends because of saturation.
>Question 1: is 3.3V a normal saturation limit for the 2N3819? Can't see it
>from the datasheet (at least, I can't).
>
>A) I don't understand - is this due to the threshold voltage? Why does the
>JFET conduct anyway (with a constant resistance?), and why does it pull
>current from the base?
>
>And finally; how do I fix this? Do I have to scale the source voltage to be
>between 1.2 and 3.2V? Does these JFET parameters vary, and how can I get
>this information from a datasheet?
>
>Hoping for some help from the experts,
>
>Richard