On a sunny day (Wed, 21 Nov 2007 00:12:17 +0100) it happened Fred Bartoli wrote in :
OK, my mistake, I thought he took the output from the drain of the lower FET. Yes the opamp output will be Vin + 2.5V
But then, if you have an opamp, why use the JFET end the extra 2.5 V? OK, maybe it was not a FET opamp. My apologies anyways to Vladimir, I just wanted to go to sleep and realised that you ment the opamp output. So I switched on the PC to correct it and found Fred's posting.
Yup...I think that's where I'm headed..
Maybe call J2 a 'reference amp'. Where J2 is a reference circuit. Relative to J2 drift, compensate to maintain J1's bias point. Something like that... :P
Nice.. You did a temp sweep.
Heck...I don't even know if I can trust LT's JFET models and I haven't spotted yet if I can do a temp sweep in LTSpice.
I'd say 90% of circuits I've seen use Rs in JFET linear circuits. Without Rs...it's interesting to see the performance of the alternatives. :)
D from BC
r
Yes, I'm sure you have, Fred.
The familiar J105 "switching" JFET can make a great low-noise amplifier. Check it out, Idss =3D 500mA. Awesome, but of course we'd never think of using it in analog mode anywhere near such high drain currents.
[snip]
Sure you can. Mikey did a good job with LTspice.
If you're really trying to make a comparator rather than a linear amplifier, you might experiment with current source loads (hint, hint ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
Load this file again...
It now contains another version... of course use a better OpAmp or Comparator to get the speed ;-)
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |
Load this file yet again...
It now contains still another version... Comparator as Output.
I can now walk, swaying a bit, without aid of a walker or cane... guess it's time to try the water ;-)
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |
I don't remember whether the P8000/8002 were classified as switching JFETs but we used them up to where the heat sink had that new wood stove smell.
-- Regards, Joerg http://www.analogconsultants.com/
Great!
Try Shiraz, tastes better. But it won't exactly help with that swaying.
-- Regards, Joerg http://www.analogconsultants.com/
Neato!
DC amp: check Thermal compensation: check BW to 2Mhz: check (Looks fast! (1) (2)) Amplifies below rail voltage: check Inverting output: n/a check Uses matched dual JFET pack: check Low noise: op amp noise introduced (eh...might be ok).. check Parts count: great Prone to oscillation: dunno yet
(1) No gain! (2) I suspect thermal compensation can be traded off for more gain by including negative feedback about the op amp.
In some ways this circuit is like the inclusion of Rs... Gain is traded off for thermal stability.
Impressive :)
D from BC
Hmmm.. :) Constant current combining with thermal compensation current. Wow .. Thermally stable for Vgs = -200mV. That's only point that really needs to be temp. stable in my app.
The comparator version might seem out of place here... But I suspect some eerie precognition. :) The circuit following the JFET amp in my app is ..a comparator :P
I guess the idea here is to have Vgs J2 = Vgs J1.. When that happens, the output shouldn't drift much. It's like a voltage divider. For R1 = R2, if R1 drifts just like R2 then it doesn't show up in the output.
D from BC
May I also add, the J105 in its original TO-92 package had many manufacturers in its day, even tho now there are only a few. 10 years ago Fairchild introduced the smt JFTJ105, in a hefty sot-223 package. NSC's NDSJ105 sot-23 package was discontinued, being far too wimpy to cage the beast!
I don't remember the P8000 and P8002 JFETs, who made them and what kind of package did they come in?
I meant WALK on the water ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
The tracking is only exact for Vgs1 == Vgs2 and Vds1 == Vds2
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
I replaced the LM339 with a 10ns comparator... overall delay is 60ns. Does that fit with your needs?
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
This is how we used them in high-IP3 RF amps back in the old days:
Of course me being in high school and then university the budget often only allowed for the BF245 where you had to glue half a dozen together to achieve that kind of performance. Later I did some projects with P8002 transistors after I landed a nice side job at our RF institute.
Search for the text string "In deze schakeling heeft mijn P8002" in this page and underneath you find a P8002 in TO220 style housing:
IIRC TI and Siemens produced them. Marvelous FETs. But all good things come to an end some day.
-- Regards, Joerg http://www.analogconsultants.com/
I too found the Atlas page, and another one leading to the 8002 datasheet:
Nice fet but idss is spec'ed at 140mA max.
-- Thanks, Fred.
Yes, but you could easily have it dissipate several watts. Try that with those SOT23 JFETs :-)
-- Regards, Joerg http://www.analogconsultants.com/
Got a fast comparator in mind that isn't a lot more expensive than the LM339? Always looking for a bargain here ...
And yes, I know your standard answer that you just roll your own :-)
-- Regards, Joerg http://www.analogconsultants.com/
Yep ;-)
Larkin can probably give you a part number... he's fond of using LVDS and PECL parts as comparators.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
Ahhh.
From another point of view... I think I see common mode and differential mode. The DC drift is set up to be common (common mode). The DC signal is from a differential.
D from BC
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.