Fantastic P.I front end building block

[Woody.au]

This is an audio preamp circuit, but with slight mods to the gain path makes a cutting edge P.I front end. Minelab use AD797 chips in the front end but these become unstable at high input impedance. Using a J-fet input negates the negative effect on stabillity that is caused by off time of the front end.

Noise is still 0.9nv/hz so it makes for a fantastic building block.

http://www.audioxpress.com/magsdirx/ax/addenda/media/colin2764.pdf

[TheWizard]

http://www.analog.com/UploadedFiles/Data_Sheets/OP470.pdf

[JC1]

Low Noise Fet I Use

Hi Woody,

These are the low noise fets I use

in my home built PI and VLF detectors.

Have used the Fet/opamp deal for decades.

Works well, anyway Interfet likes to sell

quantity so have to give a story and get

samples.

Use two and get 0.25 nv/rootHz.

Hi Wizard,

Yes, that is a configuration that was

actually used on an old VLF detector,

I don't remember which detector but

it was older quad opamp, not OP470s.

http://www.interfet.com/#

http://www.interfet.com/pdf/DS_IF9030.pdf

[Elie]

Why do you always use such short lines when you type?

[Esteban]

Maybe is a poet.

[Max]

Maybe is a poet.

Or use a text editor with word-wrap at 40... some old email programs have that...or send messages just from the shell... on the fly!

I know of people that use unix-VI to send emails

(ehm... well... I've done too in some "happy hours"... but that's another story )

[JC1]

Font too large

My font is set too large

so not much fits in the

reply box they give you.

and my keyboard is old

and the carriage return

always does two.

[Carl-NC]

You don't need to hit the carriage return when you get to the right side of the reply box... just keep typing, and it will wrap itself. Hit return at the end-of-paragraph.

Oh yeah, little "up" and "down" arrows in the upper-right corner of the reply box let you enlarge the reply box.

[JC1]

It's a Joke Carl

You don't need to hit the carriage return when you get to the right side of the reply box... just keep typing, and it will wrap itself. Hit return at the end-of-paragraph.

Oh yeah, little "up" and "down" arrows in the upper-right corner of the reply box let you enlarge the reply box.

It's a Joke Carl

I know how to

work a computer.

Really.

And my keyboard

ain't broke none

either.

[JC1]

Front End

And it is easier to read. Then stuff all strung out all over the place, and to a computer it is probably not much bigger in memory or anything.

[JC1]

Don't need no feedback

And when you are designing your front end Fet electronics save yourselfs some trouble and a simple grounded source fet with a drain/gain resistor based on gfs of about a gain of 20 to 30 then AC couple to the op amp more voltage gain. The voltage gain of 20 to 30 will make all the noise spec fall onto the fet and virtually none on the op amp. 1/A^2 of the first stage. No feedback to the fet. or you can feedback if you wish no biggy as long as you don't have any trouble with the oscillations.

[sharky]

common Low noise design

http://www.analog.com/UploadedFiles/...eets/OP470.pdf

The Wizards design is common one in parallel, one can use LT1028 too. Or use low noise opamps cooled by liwuid nitrogen or helium....

[sharky]

some additions

The Wizards design is common one in parallel, one can use LT1028 too. Or use low noise opamps cooled by liwuid nitrogen or helium....

The Wizards design is common similar circuit from elecktor old edition 1997 or older in parallel using LT1028 too. Or use low noise opamps cooled by liwuid nitrogen or helium....

[sharky]

costly

http://www.analog.com/UploadedFiles/...eets/OP470.pdf

Whats the noise now cube/sqrt of the three!!! also the opamps are costly cant we use a low noise japanese Transistors in parallel !!!!

[Mr. Bean]

yes you can

Whats the noise now cube/sqrt of the three!!! also the opamps are costly cant we use a low noise japanese Transistors in parallel !!!!

Hey Sharky,

The old Corbon {misspell} does use transistor amplifier for

front end. He also uses analog switches to turn off input

during xmit. Good move. now you can have low noise fet

and then follow with transistors amplifier. Or if you are real

good you can make fet/transistor amp that recovers from

xmit flyback fast {hint don't saturate transistors or fet}

and get rid of the switches tooo.