On Wednesday, December 11, 2013 7:01:06 PM UTC-5, Fred Bartoli wrote:
> Le Wed, 11 Dec 2013 10:08:52 -0500, Phil Hobbs a =E9crit:
<snip Phil's stuff>

>=20
> In fact, for the usual circuits conditions a JFET is not a MOSFET=20
> (indeed :-)
>=20
> The main difference is that, at ordinary low level currents, you use the=
=20
> JFET in its quadratic region, while you use the MOSFET in its=20
> ubthreshold region. And that makes for all the difference...
>=20
> See, for a JFET, in the triode region:
>=20
> ID =3D k ((vgs-vt) vds - 1/2 vds^2)
> when substituting vgs for vc+1/2*vds  (vc for control voltage)
> you get=20
> ID =3D k vds (vc - 2 vt)
> which indeed is the mark of a resistor.
>=20

Nice! Thanks Fred.  I tried a Jfet for agc following J.Williams.
(Not as good as a light bulb... and a much more involved circuit.) =20

<snip equally good (or better?) mosfet discussion>
>=20
> Thanks,
>=20
> Fred.

On 12/11/2013 7:01 PM, Fred Bartoli wrote:
> Le Wed, 11 Dec 2013 10:08:52 -0500, Phil Hobbs a écrit:
>
>
>>> And there's some gimmick where you add 0.5*VDS voltage to VGS to
>>> linearize.
>>> 		
>>>                                           ...Jim Thompson
>>>
>>>
>> That seems to be a lot closer with JFETs than with MOSFETs.
>>
>> I did some measurements a few years back that showed that 2N7002s kept
>> improving as I cranked the V_GS feedback up from 0.5*V_DS to 1.5*V_DS.
>> See http://electrooptical.net/www/sed/sed.html#2N7000 .  (Doesn't have
>> the 150% feedback measurements unfortunately--I'll see if I can find
>> them.)
>>
>> Cheers
>>
>> Phil Hobbs
>
>
> I recall you already saying that.
>
> In fact, for the usual circuits conditions a JFET is not a MOSFET
> (indeed :-)
>
> The main difference is that, at ordinary low level currents, you use the
> JFET in its quadratic region, while you use the MOSFET in its
> subthreshold region. And that makes for all the difference...
>
> See, for a JFET, in the triode region:
>
> ID = k ((vgs-vt) vds - 1/2 vds^2)
> when substituting vgs for vc+1/2*vds  (vc for control voltage)
> you get
> ID = k vds (vc - 2 vt)
> which indeed is the mark of a resistor.
>
> Now for a MOSFET in the subthreshold region the drain current is
> diffusion driven and you have:
> ID = is W/L Exp[vgs/(n uT)] (1 - Exp[-vds/uT] + vds/va)
>
> with va being the equivalent of BJT early voltage,
> and n an "ideality factor" above 1, ordinarily between 3 and 5 for power
> MOSFETS. (the 2N7002 is a power MOSFET)
>
> In the low level triode region vds/va << 1 and the drain current
> simplifies to:
>
> ID = is W/L Exp[vgs/(n uT)] (1 - Exp[-vds/uT])     (Eq1)
>
>  From the last term of Eq1, one can see that for vds>4*uT (roughly 100mV)
> there's no more ID dependency on vds and the MOSFET behaves like a super
> FET with constant 100mV triode to saturation "vds knee voltage".
>
> For a constant vgs, the (1 - Exp[-vds/uT]) term makes for a drain current
> barely linear up to the 25mV uT
>
> If now you set vgs=vc+k1*vds by mean of external circuitery you get:
>
> ID = is W/L Exp[(vc+k1*vds)/(n uT)] (1 - Exp[-vds/uT])  (Eq2)
>
> You factor out a beta=W/L is Exp[vc/(n uT)] term which depends only on
> physics and control voltage, then you Taylor expand ID at vds=0 to 3rd
> order (get it simple :-) and you get:
>
> ID = beta [vds/uT + (k/n-1/2)(vds/uT)^2 +
> 	 (3 k^2 - 3 k n + n^2)/(6 n^2) (vds/uT)^3)]    (Eq3)
>
> You can null the 2nd order term by setting k=n/2, (1.5 to 2.5, depending
> on the ideality factor value). The 3rd order term then simplifies to 1/24.
>
> Note that this is kind of optimal for low distortion at vanishingly low
> drain voltage.
>
> If you want a usable vds range up to say 100mV, maybe 200mV, at cost of a
> somewhat reduced linearity you just lower k1.
> For a 100mV vds range, a pretty good linearization occurs for k1=n/3
> (=1.6 for a small power MOS with n=5) which roughly tallies with your
> results.
>
> With k1=n/3 the expanded ID becomes:
> ID = beta [vds/uT - 1/6 (vds/uT)^2 + 1/18 (vds/uT)^3)]
>
> and that gives a resistance variation of only 1.5% up to 100mV vds.
>
> Note that this is totally independant from the resistor value as vc
> doesn't enter into the equation.
>
>

Thanks, Fred, that's interesting.

I've never got into the deep theoretical details of FETs, because all 
the theory I've seen has these fudge factors with huge ranges like your 
factor of 3 to 5.  Bipolars are doing badly if they're off by a few 
percent, once you include the extrinsic E, B, and C resistances.  Those 
are real resistances, are reasonably linear, drop actual voltages, and 
have Johnson noise of exactly the magnitude you'd expect.  The only real 
fudge factor in BJTs is the Early voltage.

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

On Wed, 11 Dec 2013 19:23:59 -0500, "Maynard A. Philbrook Jr."
<jamie_ka1lpa@charter.net> wrote:

>In article <49dha9lt88c56bfc3rhd3hmsgcmijtogdk@4ax.com>, 
>jlarkin@highlandtechnology.com says...
>> 
>> On Tue, 10 Dec 2013 21:07:19 -0800, Robert Baer
>> <robertbaer@localnet.com> wrote:
>> 
>> >dbr@kbrx.com wrote:
>> >> In this application, the problem with jfets is the need for a negative
>> >> control voltage. The board is small with little room for more circuitry.
>> >> Good thought otherwise.
>> >>
>> >> Hul
>> >>
>> >> asdf<asdf@nospam.com>  wrote:
>> >>> Hul Tytus wrote:
>> >>
>> >>>> Any suggestions for an enhancement mode mosfet to serve as a variable
>> >>>>   resistence element in an automatic gain control type circuit? Power
>> >>>> involved is a bare whisper; worst case voltage is probably 30 volts.
>> >>>> There may well be a better remidy than an mosfet, but it looks good at
>> >>>> this point.
>> >>
>> >>> Many audio compressor circuits use jfets as voltage dependent
>> >>> resistors. a search for "compressor fet schematic" in google images
>> >>> outputs some references.
>> >>
>> >   Variants: PMOS, NMOS, DMOS, JFET.
>> 
>> Here's an NE3509 gaasfet as a variable resistor:
>> 
>> https://dl.dropboxusercontent.com/u/53724080/Parts/NE350x/Curves_2.jpg
>> 
>> Drain capacitance is a fraction of a pF, so it's good for fast stuff.
>> More like 30 millivolts than 30 volts, though.
>
>Always had bad luck with gasfets back in the hay day.
>
>Just fine for consumer products where you can just throw them
>away.
>
> Longevity wasn't their strong point. I guess it depends
>on what you used them for.
>
> Maybe they have improved over the years.
>
>Jamie

We've had great luck with the NEC parts, and some bigger SOT-89's.


-- 

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation

In article <49dha9lt88c56bfc3rhd3hmsgcmijtogdk@4ax.com>, 
jlarkin@highlandtechnology.com says...
> 
> On Tue, 10 Dec 2013 21:07:19 -0800, Robert Baer
> <robertbaer@localnet.com> wrote:
> 
> >dbr@kbrx.com wrote:
> >> In this application, the problem with jfets is the need for a negative
> >> control voltage. The board is small with little room for more circuitry.
> >> Good thought otherwise.
> >>
> >> Hul
> >>
> >> asdf<asdf@nospam.com>  wrote:
> >>> Hul Tytus wrote:
> >>
> >>>> Any suggestions for an enhancement mode mosfet to serve as a variable
> >>>>   resistence element in an automatic gain control type circuit? Power
> >>>> involved is a bare whisper; worst case voltage is probably 30 volts.
> >>>> There may well be a better remidy than an mosfet, but it looks good at
> >>>> this point.
> >>
> >>> Many audio compressor circuits use jfets as voltage dependent
> >>> resistors. a search for "compressor fet schematic" in google images
> >>> outputs some references.
> >>
> >   Variants: PMOS, NMOS, DMOS, JFET.
> 
> Here's an NE3509 gaasfet as a variable resistor:
> 
> https://dl.dropboxusercontent.com/u/53724080/Parts/NE350x/Curves_2.jpg
> 
> Drain capacitance is a fraction of a pF, so it's good for fast stuff.
> More like 30 millivolts than 30 volts, though.

Always had bad luck with gasfets back in the hay day.

Just fine for consumer products where you can just throw them
away.

 Longevity wasn't their strong point. I guess it depends
on what you used them for.

 Maybe they have improved over the years.

Jamie

Le Wed, 11 Dec 2013 10:08:52 -0500, Phil Hobbs a écrit:


>> And there's some gimmick where you add 0.5*VDS voltage to VGS to
>> linearize.
>> 		
>>                                          ...Jim Thompson
>>
>>
> That seems to be a lot closer with JFETs than with MOSFETs.
> 
> I did some measurements a few years back that showed that 2N7002s kept
> improving as I cranked the V_GS feedback up from 0.5*V_DS to 1.5*V_DS.
> See http://electrooptical.net/www/sed/sed.html#2N7000 .  (Doesn't have
> the 150% feedback measurements unfortunately--I'll see if I can find
> them.)
> 
> Cheers
> 
> Phil Hobbs


I recall you already saying that.

In fact, for the usual circuits conditions a JFET is not a MOSFET 
(indeed :-)

The main difference is that, at ordinary low level currents, you use the 
JFET in its quadratic region, while you use the MOSFET in its 
subthreshold region. And that makes for all the difference...

See, for a JFET, in the triode region:

ID = k ((vgs-vt) vds - 1/2 vds^2)
when substituting vgs for vc+1/2*vds  (vc for control voltage)
you get
ID = k vds (vc - 2 vt)
which indeed is the mark of a resistor.

Now for a MOSFET in the subthreshold region the drain current is 
diffusion driven and you have:
ID = is W/L Exp[vgs/(n uT)] (1 - Exp[-vds/uT] + vds/va)

with va being the equivalent of BJT early voltage, 
and n an "ideality factor" above 1, ordinarily between 3 and 5 for power 
MOSFETS. (the 2N7002 is a power MOSFET)

In the low level triode region vds/va << 1 and the drain current 
simplifies to:

ID = is W/L Exp[vgs/(n uT)] (1 - Exp[-vds/uT])     (Eq1)

From the last term of Eq1, one can see that for vds>4*uT (roughly 100mV)
there's no more ID dependency on vds and the MOSFET behaves like a super 
FET with constant 100mV triode to saturation "vds knee voltage".

For a constant vgs, the (1 - Exp[-vds/uT]) term makes for a drain current 
barely linear up to the 25mV uT

If now you set vgs=vc+k1*vds by mean of external circuitery you get:

ID = is W/L Exp[(vc+k1*vds)/(n uT)] (1 - Exp[-vds/uT])  (Eq2)

You factor out a beta=W/L is Exp[vc/(n uT)] term which depends only on 
physics and control voltage, then you Taylor expand ID at vds=0 to 3rd 
order (get it simple :-) and you get:

ID = beta [vds/uT + (k/n-1/2)(vds/uT)^2 +
	 (3 k^2 - 3 k n + n^2)/(6 n^2) (vds/uT)^3)]    (Eq3)

You can null the 2nd order term by setting k=n/2, (1.5 to 2.5, depending 
on the ideality factor value). The 3rd order term then simplifies to 1/24.

Note that this is kind of optimal for low distortion at vanishingly low 
drain voltage.

If you want a usable vds range up to say 100mV, maybe 200mV, at cost of a 
somewhat reduced linearity you just lower k1.
For a 100mV vds range, a pretty good linearization occurs for k1=n/3 
(=1.6 for a small power MOS with n=5) which roughly tallies with your 
results.

With k1=n/3 the expanded ID becomes:
ID = beta [vds/uT - 1/6 (vds/uT)^2 + 1/18 (vds/uT)^3)]

and that gives a resistance variation of only 1.5% up to 100mV vds.

Note that this is totally independant from the resistor value as vc 
doesn't enter into the equation.


-- 
Thanks,
Fred.

On Tue, 10 Dec 2013 21:07:19 -0800, Robert Baer
<robertbaer@localnet.com> wrote:

>dbr@kbrx.com wrote:
>> In this application, the problem with jfets is the need for a negative
>> control voltage. The board is small with little room for more circuitry.
>> Good thought otherwise.
>>
>> Hul
>>
>> asdf<asdf@nospam.com>  wrote:
>>> Hul Tytus wrote:
>>
>>>> Any suggestions for an enhancement mode mosfet to serve as a variable
>>>>   resistence element in an automatic gain control type circuit? Power
>>>> involved is a bare whisper; worst case voltage is probably 30 volts.
>>>> There may well be a better remidy than an mosfet, but it looks good at
>>>> this point.
>>
>>> Many audio compressor circuits use jfets as voltage dependent
>>> resistors. a search for "compressor fet schematic" in google images
>>> outputs some references.
>>
>   Variants: PMOS, NMOS, DMOS, JFET.

Here's an NE3509 gaasfet as a variable resistor:

https://dl.dropboxusercontent.com/u/53724080/Parts/NE350x/Curves_2.jpg

Drain capacitance is a fraction of a pF, so it's good for fast stuff.
More like 30 millivolts than 30 volts, though.






-- 

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation

On Wed, 11 Dec 2013 17:22:06 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:

>
>An anecdote about customs:
>A colleague of mine tried to ship a "Base-Band Q" measurement
>system to Fermilab. He referred to it as "BBQ" in the paperwork,
>which made US customs think it contained meat. Oops. It took a
>month to sort that one out.

I mistakedly referred to a graphic printed item as an "overlay", which
apparently has some meaning in the textile trade (a highly non-free
segment of trade with all kinds of ugly tariffs and quotas on a
country-by-country basis). Ended up costing some extra money. 

Joerg <invalid@invalid.invalid> wrote:

> John Larkin wrote:
> > On Tue, 10 Dec 2013 17:35:19 -0800, Joerg <invalid@invalid.invalid>
> > wrote:
> > 
> >> miso wrote:
> >>> On 12/10/2013 4:10 PM, dbr@kbrx.com wrote:
> >>>> In this application, the problem with jfets is the need for a negative
> >>>> control voltage. The board is small with little room for more circuitry.
> >>>> Good thought otherwise.
> >>>>
> >>>> Hul
> >>> Would a photofet based optocoupler do the trick. Not cheap, but simple
> >>> otherwise. Fairchild makes a few.
> >>>
> >> Unfortunately not for a 30V swing. One option Hul would have is LDRs but
> >> his name sounds Scandinavian and Europe has outlawed those for most
> >> applications.
> > 
> > Cadmium?
> > 
> > What do they do about GaAs? PbSe? PLZT?
> > 
> 
> Pretty soon they'll even outlaw dihydrogen monoxide because of all the
> environmental hazards and because it contributes to "global warming":
> 
> http://www.dhmo.org/facts.html
> 
> :-)
> 
> But seriously, if one of your products contains a GaAs FET, you spell
> that out in writing as "gallium arsenide" and customs in Europe sees
> that, be prepared for a major imbroglio.

Years ago I heard a story of someone trying to import some electric
motors into a 'developing' country (they called them 'backwards' in
those days).  The customs declaration read: " 10 x electric motors, 5
horsepower, 415 volts, 50 cycles, 3-phase".

The reply from the customs office was as follows:

"The electric motors may be imported upon payment of the appropriate
duty.  No horses may be brought into the country unless accompanied by
the appropriate veterinary certificates.  The volts and phases are not
listed as permitted items and will be impounded awaiting further
investigation.    The cycles may not be imported as these are now being
manufactured in this country."


-- 
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

On Tue, 10 Dec 2013 22:49:39 +0000, Hul Tytus wrote:

> sci.electronics.design fet for automatic gain control?
> 
> Any suggestions for an enhancement mode mosfet to serve as a variable
> resistence element in an automatic gain control type circuit? Power
> involved is a bare whisper; worst case voltage is probably 30 volts.
> There may well be a better remidy than an mosfet, but it looks good at
> this point.

What frequency is the signal?

Some alternatives that I can think of to a FET (none of which are 
necessarily best for you) are:

* PIN diodes (if your frequency is high enough)
* variable-gain amplifiers
* Analog multipliers (ADC633, LM1596(?))
* variable-bias BJT stages

Dual-gate FETs have, I think, already been mentioned.

This is all assuming that you're not driving this from a microprocessor 
-- in that case, and if your frequency is not too high, consider using a 
multiplying DAC.

-- 
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

On Wed, 11 Dec 2013 10:00:06 -0600, "Tim Williams"
<tmoranwms@charter.net> wrote:

>"Jim Thompson" <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote 
>in message news:esvga9poppf2rdkc48si70a1s2pr97u87g@4ax.com...
>>>Use a single balanced mixer instead (three BJTs).
>>
>> At 30V?
>
>Sure.  It'll need unconventional supply voltages, but why not?
>
>If the source can supply a little current (or the bandwidth demand is 
>small), only two transistors would even be needed.
>
>Anyway, FETs are utterly useless here.  Curiously no references mention 
>it, I dervied that myself.
>
>Tim

Turn the input into a current, then multiply.  Then restore to
voltage.  Maybe I can spin one up using a CA30xx ;-)
		
                                        ...Jim Thompson
-- 
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