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fet for automatic gain control?

H

Hul Tytus

Jan 1, 1970
0
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.

Hul
 
A

asdf

Jan 1, 1970
0
Hul said:
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.
 
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
 
S

Spehro Pefhany

Jan 1, 1970
0
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.

Hul

A MOSFET stops behaving like a resistor once the voltage across it
exceeds Vgs - Vth. For low distortion, maybe you want to keep it to
tens of mV or something like that, not 30V.



Best regards,
Spehro Pefhany
 
B

Bill Bowden

Jan 1, 1970
0
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

Maybe you can ground the gate of the jfet and use the source to control it. That way, as the source moves positive, the jfet turns off since the gate is at ground.

-Bill
 
M

miso

Jan 1, 1970
0
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.
 
J

Joerg

Jan 1, 1970
0
miso said:
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.
 
S

Spehro Pefhany

Jan 1, 1970
0
Cadmium?

What do they do about GaAs? PbSe? PLZT?

I see devices with LCDs being advertised as "mercury-free backlight"
and "arsenic-free glass".

Of course the backlights may not be completely benign:-

".. according to California regulations, excessive levels of copper
(up to 3892 mg/kg; limit: 2500), Pb (up to 8103 mg/kg; limit: 1000),
nickel (up to 4797 mg/kg; limit: 2000), or silver (up to 721 mg/kg;
limit: 500) render all except low-intensity yellow LEDs hazardous".

http://pubs.acs.org/doi/abs/10.1021/es101052q


Best regards,
Spehro Pefhany
 
R

Robert Baer

Jan 1, 1970
0
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
Variants: PMOS, NMOS, DMOS, JFET.
 
T

Tim Williams

Jan 1, 1970
0
Jim Thompson said:
And there's some gimmick where you add 0.5*VDS voltage to VGS to
linearize.

Too bad it still only works to fractions of a volt, at least if you want a
wide range in R(on)/R(off).

Use a single balanced mixer instead (three BJTs).

Tim
 
P

Phil Hobbs

Jan 1, 1970
0
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.

Hul

Can you use range switching? Failing that, current-mode is your friend.
A BJT diff pair can have an enormous range of gain depending on bias,
so you can use one pair as a variable current divider to get the maximum
input signal down to something manageable, then maybe another pair to
get some gain. If you run the two in cascade, you can get probably 40
dB of gain range easily, and 60 dB if you work at it a bit.

You can improve the linearity by ~20 dB or a bit more by using matched
diode-connected transistors on the second diff pair's inputs. (See the
LM13700 data sheet.)

Any simple AGC design using elements that respond fast enough to follow
your signal (as opposed to light bulbs, thermistors, or CdS
photoconductors) is going to cause some distortion.

One low-distortion method that may work in a restricted ambient
temperature range is a few cascaded voltage dividers whose shunt legs
are NTC thermistors, with external heating applied for control.

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
 
J

Joerg

Jan 1, 1970
0
John said:
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.
 
T

Tim Williams

Jan 1, 1970
0
Jim Thompson said:

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
 
G

George Herold

Jan 1, 1970
0
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.

Hul

Lotsa nice ideas already posted. How much gain control do you need?
and how much distortion can you tolerate?
I did these Wein bridge oscillators a few years ago, and clipped off the tippy tops of the sine wave with just some diodes. I'd have to check but IIRC the third harmonic was down 50-60dB.

George H.
 
J

Jeroen Belleman

Jan 1, 1970
0
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.

At one time, I took care of some variable gain amplifiers that
used a MOSFET as a variable resistance in the emitter of a CE
amplifier stage. Not only was the gain vs. Vg curve very non-
linear and unpredictable, but at some settings, the gain from
Vg to output was so high that the FET's noise was the greatest
noise contributor. (From the point of view of the FET, this
was a cascode circuit after all.) I'm glad to be rid of them.
I re-designed the amplifiers using an AD630 as the VGA stage.

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.

(In the particle accelerator world, the Q of a synchrotron is
the number of oscillations the beam makes around its central
orbit in one revolution. It's an important parameter for beam
stability.)

Jeroen Belleman
 
A

Adrian Tuddenham

Jan 1, 1970
0
Joerg said:
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."
 
S

Spehro Pefhany

Jan 1, 1970
0
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.
 
M

Maynard A. Philbrook Jr.

Jan 1, 1970
0
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
 
P

Phil Hobbs

Jan 1, 1970
0
Le Wed, 11 Dec 2013 10:08:52 -0500, Phil Hobbs a écrit:

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
 
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