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peskywinnets 27th April 2012 02:03 PM

jfet preamp - does a jfet exist that'll work at just 5V supply?
Subject says it all really...I'm constrained to only using a jfet, but I really want to use the single 5V rail I intend using in this circuit ...else I'm faced with having to add in an additional supply just to feed the jfet!

The other constraint is that the jfet must be self biased, with it's gate grounded via a high resistor (which rules mosfets and opamps out)

Top tips on any jfets that'll operate ok at 5V supply welcomed!

jcx 27th April 2012 02:15 PM

actually several CMOS op amps work with input CM V at the rail or even ~100 mV beyond - in fact its a easily searchable class: "rail-to-rail" input (and often output)

sw-C V inverter chips let you invert supplies cheaply, there is even op amp incorporating the sw-C inverter internally

peskywinnets 27th April 2012 02:16 PM

Thanks but like I say ....I *must* use a jfet ...opamps are out (I tried a dual rail opamp & therefore mid point biasing to ground, but it wasn't any good).....I want to keep this with the lowest jfet count possible...hence seeking a single jfet to perform the task.

Now I'm fairly new to jfets...I understand how they work etc, but not experienced enough to know of a particular jfet that will operate as low as 5v rail...and wondered if anyone on here did?

franzm 27th April 2012 02:33 PM

You'll need a jfet with low Vp and high gm0 like the BF246A or 247A or some japanese types specially designed for electret microphone amplifiers. Select them for low Vp. Then even a SRPP is possible at 5V.
2SK170 (Vp= -0,2...-1,5V), 2SK190 (-0,15..-1,5V)

peskywinnets 27th April 2012 02:49 PM

I guess this is a pretty niche area, becuase none of your suggested jfets I can source. I also stumbled across this one...TF202THC ...which might be a contender, but again, unless i buy 8,000, then I can't source!

Tekko 27th April 2012 03:00 PM

Why are opamps out of the question ?

peskywinnets 27th April 2012 03:02 PM

Just has a rummage in my components tray and found a PMBFJ110....a quick check of the datasheet, reveals it might be a contender, but I can't see the gm0 figure you mention?


Originally Posted by Tekko (
Why are opamps out of the question ?

Opamps are out the question as I want the input to be biased at 0V with no DC blocking cap, which means a dual rail supply...I don't want to go there (& the testing I did with opamps wasn't encouraging vs a single jfet & two resistors)

jcx 27th April 2012 03:15 PM

which means you don't understand any of the terms in my post?

peskywinnets 27th April 2012 03:17 PM


Originally Posted by jcx (
which means you don't understand any of the terms in my post?

I did, but equally, you clearly haven't locaked in to my comment about NOT using opamps!

Tekko 27th April 2012 03:26 PM

What is the application ? Is it an amplifier or just a inverter ? Even the single jfet amplifier needs its gate biased somewhere between GND and Vcc = coupling cap needed.

If its just a inverter, theres ttl and cmos logic which is designed for 3.3 and 5 volts.

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