John Curl's Blowtorch preamplifier part II

[gerhard]

Originally Posted by bear

Hold on Scott, if you physically reverse the DUT, does the cgs and cgd stay the same or not??
_-_-

There is only one junction in the JFET. It is between the gate and the channel.

Source and drain are only the ends of the channel. Since there is a voltage drop
across the channel, the depletion zone at the drain is wider than at the source.

This results in less capacitance between gate and drain because the capacitor
plates are further apart.

In Massobrio/Antognettit "Semiconductor Device Modeling with SPICE"
is a nice picture of this on the second page of the JFET chapter, and it is probably
in every beginner textbook.

Search for pinch-off point of the channel etc.

So, if you reverse the FET, the pinch-off point will be mechanically on the
other side, too.

Gerhard

OK, graphically there is a second junction on the substrate side, but this is
exactly in parallel to the top one. The reason why in metal case JFETs the
gate is usually on the case.

[scott wurcer]

Originally Posted by bear

Hold on Scott, if you physically reverse the DUT, does the cgs and cgd stay the same or not??

_-_-

Have not had the chance, my gear is in a state of flux right now. From the text book model there is symmetry as mentioned above, if the channel is doped uniformly there is only one type junction. For RF FETs sometimes there are parasitics that make small differences.

On second thought this is making a mountain out of a mole hill, a small error in reading the data could give a large error in curve fitting and maybe the difference is trivial and not that large. In any case the models on the web are all over the place. Back to regular programming

[john curl]

Scott, wake up, and look at the die pictures or photographs.

[scott wurcer]

Sorry John for only engaging 1/2 my brain. Of course both the LSK170 and 2SK170 look symmetric and they measure roughly that way. They both (at least my samples of one) look to be a little better than the DS. The LSK part looks much better than that model they provide.

It only takes a small slip of the ruler to misread one of those tiny log log plots.

[simon7000]

John,

You mentioned you prefer the Resista resistors if I recall correctly. I have come up with a method I think to measure the distortion in resistors. I plan to test 1000 ohm 1/4 watt samples. Is this a typical value for you? Do you have a source of these or other preferred resistors?

[john curl]

We usually use 1/2W except for servos, and we used the OLD, original Resistas. This is important.

[bear]

Suggest Simon7000 that you open a new thread, in an appropriate sub section of this forum on this topic??

Would be very interested in what you find, and your test methodology.

Probably would be a good idea to test different brands rather than a run of a single brand many examples.

Be sure to try a Caddock or Vishay (bulk film type) as a "gold standard".


_-_-bear

[simon7000]

Originally Posted by bear

Suggest Simon7000 that you open a new thread, in an appropriate sub section of this forum on this topic??

Would be very interested in what you find, and your test methodology.

Probably would be a good idea to test different brands rather than a run of a single brand many examples.

Be sure to try a Caddock or Vishay (bulk film type) as a "gold standard".


_-_-bear

I am acquiring every kind I can get. All 1K preferably 1/4W because at the levels I can source they will have a bit more thermal problems than 1/2W.

Anybody have 2 old 1K 1/2 Resistas?

A thread is not appropriate as most I am not really interested in idle chatter. Publishing is more appropriate.

[john curl]

Understood, Simon.

[bear]

Simon7000,

Are you testing the thermal stressing of resistors WRT signal?

If so, can we not merely design with higher wattage resistors and avoid the problem entirely by not designing "on the edge"??

Is this question "idle chatter"?

dunno... (rhetorical?)

I guess if John Curl is ok with this topic here, I am... but it seems like it is OT to some extent.

I'll be looking for your published paper in JAES or another journal? C'mon... just joking here.

_-_-bear