[sdiy] Newbie JFET-switch problem

[Richard Arntzen]

> > 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.
> 
>    FETs don't necessarily want to work as a switch with a 
> small voltage applied to the base/gate.  You should ideally 
> feed a FET a few volts in order for them to conduct full on.  
> Otherwise, you may be running it in the small linear region.
> 
>    Also, other considerations need to be made if you are 
> trying to switch power through the FET instead of switching 
> to ground.  IE, if you have drain tied to power and you are 
> trying to switch that through to the source side, your gate 
> drive will have to be a few volts _higher_ than your supply 
> line!  Obviously if source is connected to ground and you 
> switch the drain side to ground, then your gate drive can be 
> anything from a few volts up to supply.  
> 
>    Hope that helps some,
> 
>    Tony

Thanks for the tips - I can boost the gate voltage. But I still don't
understand how a negative voltage can appear at the base when feeding a
small positive VS and a small positive VG (well I am obviously not feeding a
positive VG since it is indeed measured negative and smaller than the
required voltage for opening the diode)...that would affect my options in
how to boost the gate signal.

I have uploaded the schematic at
http://home.chello.no/~richarnt/ewicv-v1.pdf and the measurements at
http://home.chello.no/~richarnt/JFET-meas.jpg - now if I add let's say an
opamp with strong positive amplification between R24/D3/C17 and the base of
Q2, will that move the JFET operation to the extremes of the transfer
function?

I will have to test that.

Richard