Author Topic: What's the real story on SOURCE v/s DRAIN trimmers in JFET circuits?  (Read 48286 times)

Renegadrian

QUOTEEEEEEEEEE
« Reply #100 on: August 23, 2008, 06:14:30 PM »
...my approach is a bit more amateur-ish, all this "Ids, Igs, Zdgs, Abc, tick tack toe" still hits me in the face, and makes me think "what the heck..im going for the parlor trick!!"

QUOTE OF THE YEAR!!!  :icon_lol:
Done an' workin'=Too many to mention - Tube addict!

Uma Floresta

Re: What's the real story on SOURCE v/s DRAIN trimmers in JFET circuits?
« Reply #101 on: September 03, 2008, 03:35:49 PM »
...And it can be done. Rearranging a couple of wires around the trimpot you can have CONSTANT output impedance equal to the total trimpot value regardless of the trimpot's rotation.  The change is pretty obvious, yet I have never seen it used or mentioned before, so I decided to call it "stupidly wonderful drain biasing" or SWDB, following Mark Hammer's philosophy when naming his "stupidly wonderful tone control" or SWTC.

So, this is my newest "parlor trick": http://www.aronnelson.com/gallery/main.php/v/STMs-Circuit-Ideas/SWDB.png.html

After these arguments I see both biasing methods are tied, unless there is something else I am missing.

So, if I understand correctly, if you use a 100k trim pot, you get an output impedance of 100,000 ohms? Would it be possible to make the output impedance extremely low (say, 8-10 ohms) while still biasing the JFET correctly?

Dragonfly

Re: What's the real story on SOURCE v/s DRAIN trimmers in JFET circuits?
« Reply #102 on: September 03, 2008, 04:24:20 PM »
« Last Edit: September 03, 2008, 04:26:52 PM by Dragonfly »

fpaul

Re: What's the real story on SOURCE v/s DRAIN trimmers in JFET circuits?
« Reply #103 on: September 03, 2008, 07:01:02 PM »
Could someone explain in 10 pages or less how would I increase the output on the trimmerless jfet preamp?
Frank

Dragonfly

Re: What's the real story on SOURCE v/s DRAIN trimmers in JFET circuits?
« Reply #104 on: September 03, 2008, 07:41:53 PM »
Could someone explain in 10 pages or less how would I increase the output on the trimmerless jfet preamp?

in the text it mentions that, as drawn, its about 6dB gain...you can increase gain by decreasing the size of the resistor (5k6) between the 1u cap and ground...if you omit the resistor completely, you'll get about 20dBs of gain.  The layout I posted uses a pot to make the gain variable. :)

snap

Re: What's the real story on SOURCE v/s DRAIN trimmers in JFET circuits?
« Reply #105 on: September 04, 2008, 01:30:12 AM »
Could someone explain in 10 pages or less how would I increase the output on the trimmerless jfet preamp?

in the text it mentions that, as drawn, its about 6dB gain...you can increase gain by decreasing the size of the resistor (5k6) between the 1u cap and ground...if you omit the resistor completely, you'll get about 20dBs of gain.  The layout I posted uses a pot to make the gain variable. :)

But if you omit R6, there`s no DC return from source to ground  :icon_question:

Gus

Re: What's the real story on SOURCE v/s DRAIN trimmers in JFET circuits?
« Reply #106 on: September 04, 2008, 07:56:40 AM »
go back a page and read the notes on the schematic. 

slacker

Re: What's the real story on SOURCE v/s DRAIN trimmers in JFET circuits?
« Reply #107 on: September 04, 2008, 07:58:17 AM »
Andy was referring to R7 on the original schematic http://www.aronnelson.com/gallery/main.php/v/STMs-Circuit-Ideas/Trimless+JFET+Preamp-STM-rev1_0.png.html not R6. If you omit that and simply connect the free end of C3 to ground that gives you maximum gain.

Or what Gus just said :)

snap

Re: What's the real story on SOURCE v/s DRAIN trimmers in JFET circuits?
« Reply #108 on: September 04, 2008, 09:14:09 AM »
I misunderstood from Dragonflys layout the only 5.6K I could see.
I saw it physically between  the 1uF (C2 in this case) and ground.
I misunderstood omit, which in case of resistors means infinite resistance to me.

Thank you Slacker, for pointing out to me that omitting means replacing with a dead short in this case.

Ben N

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Re: What's the real story on SOURCE v/s DRAIN trimmers in JFET circuits?
« Reply #109 on: September 04, 2008, 09:17:33 AM »
I misunderstood omit, which in case of resistors means infinite resistance to me.
I'd have understood the same thing, as would be the case, say, with a pcb, where omitting the component means an open circuit. Probably not a bad idea to avoid words like "omit" in such circumstances and explicitly state whether the circuit is to be open or short.

Dragonfly

Re: What's the real story on SOURCE v/s DRAIN trimmers in JFET circuits?
« Reply #110 on: September 04, 2008, 11:22:45 AM »
I misunderstood from Dragonflys layout the only 5.6K I could see.
I saw it physically between  the 1uF (C2 in this case) and ground.
I misunderstood omit, which in case of resistors means infinite resistance to me.

Thank you Slacker, for pointing out to me that omitting means replacing with a dead short in this case.


Thats why I said ....

Quote
in the text it mentions that, as drawn, its about 6dB gain...

There's no "text" on the layout, so I thought you'd know that I was referring to the text on the schematic....my bad for assuming. :)



Ben N

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Re: What's the real story on SOURCE v/s DRAIN trimmers in JFET circuits?
« Reply #111 on: September 04, 2008, 01:44:41 PM »
Sorry, Andrew, my point was not so much about this particular instance, nor that you were not clear, as about using clear terminology in general--and this is one that I come across fairly often.