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
fet for automatic gain control?
Started by ●December 10, 2013
Reply by ●December 10, 20132013-12-10
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.
Reply by ●December 10, 20132013-12-10
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.
Reply by ●December 10, 20132013-12-10
On Tue, 10 Dec 2013 22:49:39 +0000 (UTC), the renowned Hul Tytus <ht@panix.com> 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. > >HulA 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
Reply by ●December 10, 20132013-12-10
On Tuesday, December 10, 2013 4:10:10 PM UTC-8, d...@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. > > HulMaybe 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> > asdf <> 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.
Reply by ●December 10, 20132013-12-10
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. > > HulWould a photofet based optocoupler do the trick. Not cheap, but simple otherwise. Fairchild makes a few.
Reply by ●December 10, 20132013-12-10
On Tue, 10 Dec 2013 19:24:34 -0500, Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:>On Tue, 10 Dec 2013 22:49:39 +0000 (UTC), the renowned Hul Tytus ><ht@panix.com> 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. >> >>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 PefhanyAnd there's some gimmick where you add 0.5*VDS voltage to VGS to linearize. ...Jim Thompson -- | 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.
Reply by ●December 10, 20132013-12-10
On Tue, 10 Dec 2013 19:24:34 -0500, Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:>On Tue, 10 Dec 2013 22:49:39 +0000 (UTC), the renowned Hul Tytus ><ht@panix.com> 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. >> >>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. > >Not to mention the substrate diode! -- 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
Reply by ●December 10, 20132013-12-10
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. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●December 10, 20132013-12-10
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? -- 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
