another FETish question

[René Schmitz]

Hi all!

I think that Vp and Idss are directly related to each other. The ones with
the small Idss have the lowest Vp. To me they're two different ways of
looking at the FET. In your matching circuit Vp usually will depend on the
amount of channel current. I.e. you run some current thru the D-S ground
the gate and look at the difference of Vg and Vs that is actually Ids(Vgs)
over the source resistor, which has to be large to get into the region
where the current cuts off.
So Idss the other extreme, Vgs is zero, and the load resistance is zero
either (and in a follower your signal!). This is just another way of saying
Idss is the current which must flow thru the channel so that there is zero
offset from gate to source or Vgs=0. (Which is what that SEM-1a buffer in
the VCO does.) 

In the saturation region of the FET this boils down to solving 
Id=k(Vgs-Vp)^2 for either Id=0 or Vgs=0.
One sees that Idss=k(-Vp)^2 in the latter case.
Voila!

I could be totally off here, I have to admit.

Bye,
 René


At 13:25 12.04.00 +0200, you wrote:
>	>hi all, in the synthi AKS they use a fet , type 2n5163. the service
>manual
>	>calls for a Vp of between 2-3v.
>	>since this part is as rare as hens teeth, is it possible to replace
>the
>	>2n5163 with say a 2sk30, if we select one with the right Vp?
>	>are there any other parameters we need worry about?
>	>regards
>	>geoff james
>
>Vp is the important parameter, as far as I can tell. I've used BF245
>(selected for
>Vp) throughout my Clone (apart from the one p-channel type in the trapezoid
>of course.)
>
>JH.

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