fet for automatic gain control?

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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

Re: fet for automatic gain control?
Hul Tytus wrote:

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Many audio compressor circuits use jfets as voltage dependent
resistors. a search for "compressor fet schematic" in google images
outputs some references.


Re: fet for automatic gain control?
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

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Re: fet for automatic gain control?
On Tuesday, December 10, 2013 4:10:10 PM UTC-8, snipped-for-privacy@kbrx.com wrote:

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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

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Re: fet for automatic gain control?
On 12/10/2013 4:10 PM, snipped-for-privacy@kbrx.com wrote:
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Would a photofet based optocoupler do the trick. Not cheap, but simple  
otherwise. Fairchild makes a few.




Re: fet for automatic gain control?
miso wrote:
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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.

--  
Regards, Joerg

http://www.analogconsultants.com/

Re: fet for automatic gain control?
wrote:

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Cadmium?

What do they do about GaAs? PbSe? PLZT?






--  

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
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Re: fet for automatic gain control?
On Tue, 10 Dec 2013 18:06:01 -0800, the renowned John Larkin

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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
--  
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: fet for automatic gain control?
On Tue, 10 Dec 2013 21:13:34 -0500, Spehro Pefhany

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Copper is hazardous? Maybe we can go back to using oxen for our prime
energy source.

Silver? No more jewelry?

How come car batteries get a pass? They are huge masses of lead
*compounds*, much worse than metallic lead.

Rum is, by California law, a toxic substance. It contains *alcohol*




--  

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
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Re: fet for automatic gain control?
John Larkin wrote:
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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.

--  
Regards, Joerg

http://www.analogconsultants.com/

Re: fet for automatic gain control?
On 2013-12-11 16:36, Joerg wrote:
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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

Re: fet for automatic gain control?
On Wed, 11 Dec 2013 17:22:06 +0100, Jeroen Belleman

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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.  


Re: fet for automatic gain control?

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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)
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Re: fet for automatic gain control?
snipped-for-privacy@kbrx.com wrote:
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   Variants: PMOS, NMOS, DMOS, JFET.


Re: fet for automatic gain control?
On Tue, 10 Dec 2013 21:07:19 -0800, Robert Baer

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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
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Re: fet for automatic gain control?
snipped-for-privacy@highlandtechnology.com says...
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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


Re: fet for automatic gain control?
On Wed, 11 Dec 2013 19:23:59 -0500, "Maynard A. Philbrook Jr."

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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
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Re: fet for automatic gain control?
On Tue, 10 Dec 2013 22:49:39 +0000 (UTC), the renowned Hul Tytus

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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
--  
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: fet for automatic gain control?
On Tue, 10 Dec 2013 19:24:34 -0500, Spehro Pefhany

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And there's some gimmick where you add 0.5*VDS voltage to VGS to
linearize.
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: fet for automatic gain control?
On Wednesday, 11 December 2013 12:21:29 UTC+11, Jim Thompson  wrote:
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esistence 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.  
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 of mV or something like that, not 30V.    
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e.

More a circuit design technique than a gimmick.

http://users.ece.gatech.edu/~lanterma/sdiy/datasheets/transistors/vishay_fe
t_cvr_an.pdf

and it was around before 1997.

--  
Bill Sloman, Sydney

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