[sdiy] Newbie JFET-switch problem

[Tony Clark]

   Hi Richard,

> 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.

   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

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