Winfield Hill a écrit :Winfield a écrit :
Fred Bartoli wrote:
Jim Thompson a écrit :
Fred Bartoli >
Jim Thompson a écrit :
D from BC wrote:
[snip]
Except for the "sweet spot" bias condition, gain (small signal)
and output Q-point probably vary independently.
Are you sure that they are independent?
For a given jfet model gm is strongly linked to the drain current
(and essentially free from IDSS).
They're not "independent", but I don't think incremental gain
and bias Q-point will exactly track.
Ah, a bit of french meaning slept in there.
Now you said they'd probably vary independently, which they don't
(for ex.http://www.alldatasheet.com/view.jsp?Searchword=2SK170).
But gain and bias point sure can't exactly track. For this to be
so, you'd have to have gm proportional to Id, which you can't
ignore to be the hallmark of exponential
Well, as I understand it, and based on my measurements as well,
gm does mostly track Id, more or less independent of Idss,
especially at currents well below Idss. But I don't get the Q
discussion y'all are having here. To my mind the Id operating
point is best set independently from the particular miserable
JFET's Vgs vs. Id, etc. I mean, part-to-part, sheesh! Bummer!
I'm not arguing about "Q point". Just about gm dependency on Id.
I totally agree about the almost non dependency of gm on Idss and that
gm is mostly tied to Id for a given fet model (hence biasing is best
done by setting Id).
But gm doesn't track Id. I mean gm isn't proportionnal to Id, but rather
to sqrt(Id). Ok at very low current in the subthreshold region Id
becomes exponential, but this isn't the usual way of using jfets,
specially when we want low noise.
Woah, 500mA IDss!
Are you speaking of jfets or depletion mosfets?
I've never seen jfets that high. Any favorite part#?
On Tue, 20 Nov 2007 15:30:40 -0700, Jim Thompson
[snip]
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...
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
Winfield said:Winfield Hill a écrit :Woah, 500mA IDss!On Nov 20, 7:59 am, Fred Bartoli <" "> wrote:
Winfield a écrit :
Fred Bartoli wrote:
Jim Thompson a écrit :
Fred Bartoli >
Jim Thompson a écrit :
D from BC wrote:
[snip]
Except for the "sweet spot" bias condition, gain (small signal)
and output Q-point probably vary independently.
Are you sure that they are independent?
For a given jfet model gm is strongly linked to the drain current
(and essentially free from IDSS).
They're not "independent", but I don't think incremental gain
and bias Q-point will exactly track.
Ah, a bit of french meaning slept in there.
Now you said they'd probably vary independently, which they don't
(for ex.http://www.alldatasheet.com/view.jsp?Searchword=2SK170).
But gain and bias point sure can't exactly track. For this to be
so, you'd have to have gm proportional to Id, which you can't
ignore to be the hallmark of exponential
Well, as I understand it, and based on my measurements as well,
gm does mostly track Id, more or less independent of Idss,
especially at currents well below Idss. But I don't get the Q
discussion y'all are having here. To my mind the Id operating
point is best set independently from the particular miserable
JFET's Vgs vs. Id, etc. I mean, part-to-part, sheesh! Bummer!
I'm not arguing about "Q point". Just about gm dependency on Id.
I totally agree about the almost non dependency of gm on Idss and that
gm is mostly tied to Id for a given fet model (hence biasing is best
done by setting Id).
But gm doesn't track Id. I mean gm isn't proportionnal to Id, but rather
to sqrt(Id). Ok at very low current in the subthreshold region Id
becomes exponential, but this isn't the usual way of using jfets,
specially when we want low noise.
Yes, Fred, thanks, when I say track Id, I am thinking
of low currents, because I'm usually using large-die
parts, with 500mA Idss, etc., at "low" currents like
5mA, etc., where they act more like BJT transistors,
with Id vs Vgs exponential, as you say, and gm ~ Id.
I'd better not go further out on the proverbial limb,
not having my measured data in front of me.
Are you speaking of jfets or depletion mosfets?
I've never seen jfets that high. Any favorite part#?
Yes, I'm sure you have, Fred.
The familiar J105 "switching" JFET can make
a great low-noise amplifier. Check it out,
Idss = 500mA. Awesome, but of course we'd
never think of using it in analog mode
anywhere near such high drain currents.
Jim said:
Yeah...it keeps the bias level steady, but it cannot distinguish
between thermal drift and plain DC.
So..it's not a DC amp and the circuit acts like the electronic
equivalent of a coupling capacitor.
Here we go:
http://www.abvolt.com/misc/jfet2.gif
Paycheck time
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
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
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
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
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
Winfield said:
Fred said:
Jim said: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
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
I replaced the LM339 with a 10ns comparator... overall delay is 60ns.
Does that fit with your needs?
Jim Thompson wrote: [snip]
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
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
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
The tracking is only exact for Vgs1 == Vgs2 and Vds1 == Vds2
...Jim Thompson
