[sdiy] JFET switch question

[Tim Stinchcombe]

> Now I wonder: We rarely see this kind of switch without this 
> capacitor and 
> without the resistor.
> Why is this? For a very simple switch, wouldn't just the FET 
> and the diode 
> be enough?
> (Obviously no smooth transition without a capacitor - but what's the 
> disadvantage compared to the FET + Resistor + Diode solution?)

I have a couple of old JFET books, which have a little about this circuit in
them. In the first, without a G-D or G-S resistor, the development seems to
be along these lines:

- if you apply the switch control voltage directly to the gate, then due to
internal G-D and G-S capacitances, the switching voltage couples to the
switch output, so you get transients in the output when the control voltage
changes.

- the diode helps with this, as it better isolates the control voltage from
the gate

- however, with the diode reverse-biased for the switch to be ON, there is
now no path for a current to remove the charge in the reverse-biased channel
junction, which is needed to allow the JFET to actually turn on. The cap is
necessary to supply this path, the implication being that the JFET might not
actually switch on without it.

The second book does have a G-S resistor, which it says does supply this
path for the channel junction to discharge, but says that adding a capacitor
to shunt the diode will greatly reduce the turn-on time.

Thus it looks like if you have the resistor you could ignore the cap (and
suffer slower switching times), but without the resistor, you really do need
the cap!

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk