# [sdiy] variable width pulse wave for JEFT sample and hold

Dan Snazelle subjectivity at hotmail.com
Wed Jan 7 09:58:58 CET 2009

thanks david

your help has been greatly appreciated today!

i got out my copy of Hor and HIll and wow...great chapter!!!

i now have a list of things to try.

thanks to all who helped!

--------------------------------------------
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http://www.myspace.com/lossnyc

http://www.soundclick.com/lossnyc.htm

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> Date: Wed, 7 Jan 2009 00:33:21 -0800
> From: dixon at interchange.ubc.ca
> Subject: RE: [sdiy] variable width pulse wave for JEFT sample and hold
> To: lanterma at ece.gatech.edu; synth-diy at dropmix.xs4all.nl
> CC: subjectivity at hotmail.com
>
> A JFET has a gate (G), a source (S) and a drain (D). The gate is like the
> base on a BJT, the source is like the emitter, and the drain is like the
> collector. Just like the flow through a BJT is controlled by the voltage
> difference between the base and the emitter (V_BE), so the JFET is
> controlled by the voltage difference between the gate and the source (V_GS).
>
> However, JFETs are OFF when V_GS is negative (gate voltage less than source
> voltage) by about 4 V. Hence, if the source is at ground (common source),
> the gate must be held negative to prevent flow through the JFET. If the
> source is at ground, then putting the gate at ground will turn the JFET
> fully on. However, it's the difference which drives the device. Hence, if
> the source is at 5 V, the JFET will be fully off when the gate is below
> about 1.5 V and fully on when the gate is at about 5 V.
>
> Clear as mud? Look at Figure 3.8 in H&H. Using JFETs as switches can be a
> little bit tricky if the source voltage is moving around, but it's not that
> different from BJTs, except that virtually no gate current flows (unlike the
> small base current in a BJT).
>
>
> David G. Dixon
> Professor
> Department of Materials Engineering
> University of British Columbia
> Vancouver, B.C. V6T 1Z4
>
> Tel 1-604-822-3679
> Fax 1-604-822-3619
>
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> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Aaron Lanterman
> Sent: Tuesday, January 06, 2009 11:58 PM
> To: SDIY DIY
> Subject: Re: [sdiy] variable width pulse wave for JEFT sample and hold
>
> On Jan 7, 2009, at 2:54 AM, Dan Snazelle wrote:
>
>> what does positive with respect to the source mean?
>
> Uhm, I haven't been following that thread, but I suspect that it means
> the voltage on whatever they were referring to is greater than the
> voltage on the source...
>
> - Aaron
>
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