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.
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.
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"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
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.
And there's some gimmick where you add 0.5*VDS voltage to VGS to linearize. ...Jim Thompson
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| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
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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.
--
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
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".
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Best regards, Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
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
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
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.
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
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
Pretty soon they'll even outlaw dihydrogen monoxide because of all the environmental hazards and because it contributes to "global warming":
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:-)
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.
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