Dumb newby has a question...

[Dave]

I am trying to troubleshoot something I built (or tried to build) an am in
need of a piece of information I can't find anywhere. I downloaded the
datasheets (with a dozen or more graphs and two pages of other info) for the
2N5486, but can't find anything that looks like typical forward
common-source voltage gain (the 2N5486 is an N-channel JFET RF amplifier.)
The charts/graphs mostly concern themselves with one type of
transconductance/transadmittance or another. What am I missing ?

I did find an equation in one of my design texts that looks like it woud do,
but would really like a nice friendly chart/graph. I don't mind working
with arcane formulas and my handly graphing calculator, but was hoping for
something a little faster (instant gratification and all that.)

I know I am in foreign territory here, but thought I was smart enough to
figure it out. Apparently not.

Anyone know how to find what I am looking for? Care to enlighten a dumb
newby? (It would really be appreciated...)

Many thanks to anyone who responds.

Dave

 

[John Larkin]

I am trying to troubleshoot something I built (or tried to build) an am in
need of a piece of information I can't find anywhere. I downloaded the
datasheets (with a dozen or more graphs and two pages of other info) for the
2N5486, but can't find anything that looks like typical forward
common-source voltage gain (the 2N5486 is an N-channel JFET RF amplifier.)
The charts/graphs mostly concern themselves with one type of
transconductance/transadmittance or another. What am I missing ?

I did find an equation in one of my design texts that looks like it woud do,
but would really like a nice friendly chart/graph. I don't mind working
with arcane formulas and my handly graphing calculator, but was hoping for
something a little faster (instant gratification and all that.)

I know I am in foreign territory here, but thought I was smart enough to
figure it out. Apparently not.

Anyone know how to find what I am looking for? Care to enlighten a dumb
newby? (It would really be appreciated...)

Many thanks to anyone who responds.

Dave

First approximation, low frequencies,

Vgain = Gm * Rl

where Gm is transconductance in mhos (or Siemens, if you prefer) and
Rl is the drain load resistance (or impedance.)

A bit more precise would be to look at the slope of the drain curve,
convert that to resistance, and put that in parallel with Rl to get
effective drain resistance Rd, then

Vgain = Gm * Rd

which will be somewhat lower than the first equation. If Rl is small,
1 k or thereabouts, you can ignore the drain resistance thing.

At RF, the drain load gets harder to compute and the capacitances
start to matter, so it's not as simple.

John

 

[Tim Wescott]

I am trying to troubleshoot something I built (or tried to build) an am in
need of a piece of information I can't find anywhere. I downloaded the
datasheets (with a dozen or more graphs and two pages of other info) for the
2N5486, but can't find anything that looks like typical forward
common-source voltage gain (the 2N5486 is an N-channel JFET RF amplifier.)
The charts/graphs mostly concern themselves with one type of
transconductance/transadmittance or another. What am I missing ?

I did find an equation in one of my design texts that looks like it woud do,
but would really like a nice friendly chart/graph. I don't mind working
with arcane formulas and my handly graphing calculator, but was hoping for
something a little faster (instant gratification and all that.)

I know I am in foreign territory here, but thought I was smart enough to
figure it out. Apparently not.

Anyone know how to find what I am looking for? Care to enlighten a dumb
newby? (It would really be appreciated...)

Many thanks to anyone who responds.

Dave

They didn't put that in because it depends on the load resistance and
frequency (see John Larkin's answer). At high frequencies it also
depends heavily on board layout.

So it's traditional to just include what you got, and leave it to you to
design your circuit.

See if you can get a copy of the ARRL Handbook -- last time I looked
they still had a pretty good basic theory section for transistors.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Dave]

First approximation, low frequencies,

Vgain = Gm * Rl

where Gm is transconductance in mhos (or Siemens, if you prefer) and
Rl is the drain load resistance (or impedance.)

A bit more precise would be to look at the slope of the drain curve,
convert that to resistance, and put that in parallel with Rl to get
effective drain resistance Rd, then

Vgain = Gm * Rd

which will be somewhat lower than the first equation. If Rl is small,
1 k or thereabouts, you can ignore the drain resistance thing.

At RF, the drain load gets harder to compute and the capacitances
start to matter, so it's not as simple.

John

Hey John,

Yeah, that is basically what I found in Practical RF Design Manual by Doug
DeMaw, although it was a little more complex. So, basically there aren't
any charts/graphs that give even approximations of common-source voltage
gain at RF, like even 10 MHz? (Guess that's why I can't find it.) I was
wondering if I was missing something in the transconductance/transadmittance
thing. So have I found my answer, it's just not as easy as I wanted?

Thanks much,

Dave

 

[Dave]

They didn't put that in because it depends on the load resistance and
frequency (see John Larkin's answer). At high frequencies it also depends
heavily on board layout.

So it's traditional to just include what you got, and leave it to you to
design your circuit.

See if you can get a copy of the ARRL Handbook -- last time I looked they
still had a pretty good basic theory section for transistors.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Hey Tim,

Hmmm. The ARRL Handbook is the one place I don't think I have looked for
this question. Have my copy in the other room, will go check it out. Thank
you. Told you I was a dumb newby... (shaking head.)

Thanks much.

Dave

 

[John Larkin]

Hey John,

Yeah, that is basically what I found in Practical RF Design Manual by Doug
DeMaw, although it was a little more complex. So, basically there aren't
any charts/graphs that give even approximations of common-source voltage
gain at RF, like even 10 MHz? (Guess that's why I can't find it.) I was
wondering if I was missing something in the transconductance/transadmittance
thing. So have I found my answer, it's just not as easy as I wanted?

Thanks much,

Dave

RF part datasheets will sometimes have S-parameters, which give you
power gain versus frequency for 50 ohms in and out, and corresponding
input and output complex impedances. That's enough to figure out
actual RF gain in most situations, but it's not real simple. Bowick's
paperback, RF Circuit Design, is a good place to start if you want to
get serious about this.

10 MHz isn't super-fast, so Gm * Zl will sorta work.

If this stuff was easy, anybody could do it, and we wouldn't want
that, would we?

John

 

[Winfield Hill]

John Larkin wrote...

RF part datasheets will sometimes have S-parameters, which give you
power gain versus frequency for 50 ohms in and out, and corresponding
input and output complex impedances. That's enough to figure out
actual RF gain in most situations, but it's not real simple. Bowick's
paperback, RF Circuit Design, is a good place to start if you want to
get serious about this.

10 MHz isn't super-fast, so Gm * Zl will sorta work.

If this stuff was easy, anybody could do it, and we wouldn't want
that, would we?

It's not quite that easy. Yes, G = Gm * Z_load, but the trick is
figuring out the value of the transconductance, Gm. Dave, we should
mention that Gm depends on the JFET's operating current, Id. We can
write Gm = 0.025 Id/n, where n is a gain-reducing parameter that we
hope is nearly constant for a given part, it least at modest currents.
I've seen n range from 4 to 200. Let's examine the 2n5486 datasheet.

For the 2n5486 we see Gm ranges from 4 to 8 mS at Idss (we know it's
Idss because Vgs = 0V). Searching further we see that Idss ranges
from 8 to 20mA. It seems they're hiding the critical pieces of
information from us. But we can try associating the 4mS value with
8mA, and 8mS with 20mA (Siemens, abbreviated S, replaces the old mho
unit for conductance). Comparing with my formula, this gives us two
values of n, 80 and 100. (BTW, these values tell us we're not going
to get a very high gain, e.g., compared to a BJT, which would have
Gm = 0.025 Id, or 320 and 800 mS at 8mA and 20mA collector current.)

These Gm values seem rather low to me, so I poked around, and found
a plot of Gm vs Id in a Vishay/Siliconix datasheet. With their data,
n = 16 at 1mA, and degrades to 66 at 8mA. The Fairchild datasheet
has a plot covering operation at lower currents, with n = 5 at 0.1mA,
16 at 1mA, and 80 at 10mA. So we get a picture of an increase in n
and the degradation of 2n5486 transconductance at higher currents.

But, oops! now you still need to know the Id current in your circuit.
(This is a painful thing about using JFETs, what's Vgs and what's Id,
given that they have such a wide range of allowed variation? Well,
for a one-off design, you could simply measure the relationship for
your specific part.) If you are using zero gate bias, then Id = Idss,
which the datasheet says will be between 8 and 20mA. If you measure
Id, or use a circuit that sets Id, you'll be closer to being able to
accurately calculate your JFET amplifier's gain from John's formula.

 

[Tony Williams]

Let's examine the 2n5486 datasheet.

For the 2n5486 we see Gm ranges from 4 to 8 mS at Idss (we know
it's Idss because Vgs = 0V). Searching further we see that Idss
ranges from 8 to 20mA. It seems they're hiding the critical
pieces of information from us. But we can try associating the
4mS value with 8mA, and 8mS with 20mA (Siemens, abbreviated S,
replaces the old mho unit for conductance).

[snip]

You are far too trusting about the windows on JFET
data sheets Win. There's the device data sheet, and
then there's the data sheet for the raw source chip.

Old Mr Cynical here always reckoned that Siliconix
did the quickest measurement they could on a chip,
(which is Idss), to point raw chips into various device
bins and then used each selected Idss range to construct
the secondary windows on the device data sheets.

Take that 2N5486, drawn from Siliconix source chip NH.

Selected Idss range is 8-20mA.

The NH chip data says that Vgs(off) will range from
-3 to -6.x volts and forward transconductance from
about 5800 to 6500 umhos.

The data sheet uses -2 to -6 V and 4000 to 8000 umhos.

Device window MIN = Source chip MIN, minus a bit.
Device window MAX = Source chip MAX, plus a bit.

In that way all devices selected for an 8-20mA Idss
will automatically be in spec for all the other windows
on the data sheet, without having to be measured.

But, and this is the big but, anyone doing the classic
JFET sums off the data sheet windows may not get
accurate answers.

That NH source chip was used in over a dozen devices,
and you can do the same comparison sums on all of them.

But, oops! now you still need to know the Id current in your
circuit. (This is a painful thing about using JFETs, what's Vgs
and what's Id, given that they have such a wide range of allowed
variation? Well, for a one-off design, you could simply measure
the relationship for your specific part.) If you are using zero
gate bias, then Id = Idss, which the datasheet says will be
between 8 and 20mA. If you measure Id, or use a circuit that
sets Id, you'll be closer to being able to accurately calculate
your JFET amplifier's gain from John's formula.

I think we've discussed here before: Siliconix Technical
Article, TA70-2, "FET Biasing" by James Sherwin. AFAIR
now available on the web, but with a different name.

This article gives a good insight into coping with
JFET manufacturing spreads.

 

[Winfield Hill]

Tony Williams wrote...

Let's examine the 2n5486 datasheet.

For the 2n5486 we see Gm ranges from 4 to 8 mS at Idss (we know
it's Idss because Vgs = 0V). Searching further we see that Idss
ranges from 8 to 20mA. It seems they're hiding the critical
pieces of information from us. But we can try associating the
4mS value with 8mA, and 8mS with 20mA (Siemens, abbreviated S,
replaces the old mho unit for conductance).

[snip]

You are far too trusting about the windows on JFET
data sheets Win. There's the device data sheet, and
then there's the data sheet for the raw source chip.

I agree completely, and was oversimplifying for purposes
of making an easier post.

Old Mr Cynical here always reckoned that Siliconix
did the quickest measurement they could on a chip,
(which is Idss), to point raw chips into various device
bins and then used each selected Idss range to construct
the secondary windows on the device data sheets.

Take that 2N5486, drawn from Siliconix source chip NH.

Selected Idss range is 8-20mA.

The NH chip data says that Vgs(off) will range from
-3 to -6.x volts and forward transconductance from
about 5800 to 6500 umhos.

The data sheet uses -2 to -6 V and 4000 to 8000 umhos.

Device window MIN = Source chip MIN, minus a bit.
Device window MAX = Source chip MAX, plus a bit.

In that way all devices selected for an 8-20mA Idss
will automatically be in spec for all the other windows
on the data sheet, without having to be measured.

But, and this is the big but, anyone doing the classic
JFET sums off the data sheet windows may not get
accurate answers.

That NH source chip was used in over a dozen devices,
and you can do the same comparison sums on all of them.


I think we've discussed here before: Siliconix Technical
Article, TA70-2, "FET Biasing" by James Sherwin. AFAIR
now available on the web, but with a different name.

This article gives a good insight into coping with
JFET manufacturing spreads.

Bringing back some olde info, here's my post of last year,
with your reference from 2004.

TA70-2 ==> Siliconix / Vishay AN102 From an old post by Tony,
"AN102 is obviously drawn from the original TA70-2 and seems
to be a 1997 update+rewrite in electronic form. The guts of
the information given in TA70-2 is still there, relatively
unchanged." http://www.vishay.com/docs/70595/70595.pdf

You wrote, "first published in Electronics Design in May 1970,
then included in the App Notes at the rear of most Siliconix
FET data books for the next 15 or 20 years thereafter."

The ED article author was James Sherwin, and you wrote,
"a J.S Sherwin is referenced as publishing many other
technical articles or tech notes. In no particular
order........

"Liberate your FET amplifier". EDN May 1970.

"Distortion in FET amplifiers". Electronics Dec 1966.

"Voltage Controlled Resistors (FET)". Solid State Design Aug 1965.

"How, Why and Where to use FETs". Electronic Design May 1966.

"Knowing the Cause helps cure distortion in FET Amplifiers".
Electronics Dec 1966."

 

[John Jardine.]

[...]
Vishay supplies the voltage gain as a graph in their JFET data sheets. Gain
runs from x60 to x2. Obviously nothing to be proud of, it's nice that it's
there though and the same graph can be used with most other JFETS.
Indeed, JFET spec' limits are so wide that it seems most JFET circuits can
be designed (ha!) using any old data sheet coupled with any old JFET.
http://www.vishay.com/fets-small-signal/SSFsgnchjampP/
john

 

[John Larkin]

Take that 2N5486, drawn from Siliconix source chip NH.

Selected Idss range is 8-20mA.

The NH chip data says that Vgs(off) will range from
-3 to -6.x volts and forward transconductance from
about 5800 to 6500 umhos.

The data sheet uses -2 to -6 V and 4000 to 8000 umhos.

Interesting that jfets have such astounding spreads. I've seen Idss
specs over 10:1. I wonder what the physics is here.

Some phemts, compound-semi jfet equivalents, are remarkable
consistant. If you do the curves on two parts, the lines are just
about on top each other. Precision ion implanting maybe, as opposed to
chancy diffusion?

John