SORRY!
Wrong list (will appear on sdiy shortly).
Sorry again.
Richard
> -----Original Message-----
> From: motm@yahoogroups.com [mailto:motm@yahoogroups.com] On
> Behalf Of Richard Arntzen
> Sent: Friday, August 05, 2005 21:19
> To: motm@yahoogroups.com
> Subject: [motm] Newbie JFET-switch problem
>
> 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
>
>
>
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