Simpler PI...

[Aziz]

http://mujweb.cz/janascard2/PDF/Desi...amplifiers.pdf

[Waikiki_Sweep]

http://mujweb.cz/janascard2/PDF/Desi...amplifiers.pdf

There are really amazing solutions to build low noise low cost preamplifiers. Thanks for digging in it!

[Aziz]

Ok guys,

here is a reasonable competition. The winner will know, that I will use his genius design in a world beating detector.

Here are the conditions for an ultra low noise amplifier:
- Single supply (+8.5 V)
- AC amplifier
- stable gain (100 - 200 x) by resistor divider via feedback loop
- < 1 nV/sqrt(Hz) noise density @1 kHz (ideally < 0.7 nV/sqrt(Hz) )
- wide band (1 kHz - 70 kHz)
- cheap and simple solution

Is there anybody who is willing to win the competition?

Aziz

[Aziz]

Hi guys,

ok, I have won the competition. *LOL*

I have modified the improved version of OP27 circuit in the pdf paper mentioned above ("Design of ultra low noise amplifiers" by Vojtěch Janásek).
You can make it work to the conditions listed above. It's really a nice and simple circuit and I'm going to test it soon.

Except you beat me with a much better circuit. I'm waiting...
The competition run hasn't been finished yet.
Good luck.

Cheers,
Aziz

[ppodg]

Aziz, try bcm847bs+lm318, see fig.3 in an222 by National, but some simple tricks are a must...

[Aziz]

Aziz, try bcm847bs+lm318, see fig.3 in an222 by National, but some simple tricks are a must...

Hi ppodg,

no, I can beat that easily.
We need easily available & cheap BJTs in a simple & flexible KISS design.

I'm already below 0.5 nV/sqrt(Hz).
Hard to beat me. I'm willing to wait for the super-duper genius design before I fire up my soldering iron however.
Comeon guys, give your ultimate best.

Cheers,
Aziz

[ppodg]

'We need easily available & cheap BJTs in a simple & flexible KISS design'

well, bcm847bs - bjt, of course - is 0,5euro at Farnell; old lm318 well below 1euro
3-4 resistors, one cap is advisable
~1nv/hzsqrt, but you don't need it - see em noise in Europe, should be enough even in oz, we are talking about a metal detector...

[Aziz]

'We need easily available & cheap BJTs in a simple & flexible KISS design'

well, bcm847bs - bjt, of course - is 0,5euro at Farnell; old lm318 well below 1euro
3-4 resistors, one cap is advisable
~1nv/hzsqrt, but you don't need it - see em noise in Europe, should be enough even in oz, we are talking about a metal detector...

Ok, BCM847bs would make up an ultra low noise application (both transistors parallel at 0.5 nV/sqrt(Hz) ).
But I can't get it at www.reichelt.de . And you can't get it easily in most countries as well.
(I could get it from German Farnell of course.)

Aziz

[Aziz]

The BF459 (0,27 € at www.reichelt.de ) seems to be very very attractive. Two (rough) matched pair of them would be enough to go below 0.5 nV/sqrt(Hz).

Aziz

[Aziz]

The BF459 (0,27 € at www.reichelt.de ) seems to be very very attractive. Two (rough) matched pair of them would be enough to go below 0.5 nV/sqrt(Hz).

Aziz

Nice!, very very nice. I'm below 0.4 nV/sqrt(Hz) with two rough matched BF459 pair and load balancing circuit as shown in the PDF paper (emitter load balancing). Seems to be an ultimate amplifier circuit destined for my anti-interference "Top-Hat"(c)(r)(tm) IB coil.
I'm going to release the ultimate Laptop/Netbook/Tablet PC detector controller for the world best beating metal detector application. *LOL*
Oh sh1t!, I'm totally bankrupt! I have to wait several weeks to afford a bunch of BF459 transistors (50 - 100).
(What a great luck for Greedlab that I'm so poor! )

Cheers,
Aziz

[ppodg]

Aziz, don't use single transistors with 318, their thermal drifts will kill you!
You need a pair in ONE case, on ONE substrate! Audio is different from pi!
And, if I remember, your bfs need large current in noiseless applications.

[Aziz]

Aziz, don't use single transistors, their thermal drifts will kill you!
You need a pair in ONE case, on ONE substrate! Audio is different from md!

No, I don't think, that the temperature gradient won't be an issue. The BF459 can be mounted well for good thermal coupling using a good thermal heat sink paste.
BTW, the temp gradient is in the low frequency region and I'm going to use 1.5 kHz to 3 kHz pulse frequency. All not critical. The DC drift won't be an issue as well due to AC amplifier configuration! The gain is adjusted by a resistor divider feed back loop. It is very stable.

Another good matched transistors in the same chip die: THAT300 (four matched NPN transistor!!!!). But this is expensive and difficult to get. I'm using the poor man's solution and not my expensive LM394 chip at home (yes, I have one at hand). *LOL*

Cheers,
Aziz,
poor but not dumb! *LOL*

[ppodg]

ok, I simply hate ac amps in pi

[Davor]

If carefully designed ... why not. They do get you well below 1nV/sqrt(Hz) on a budget.

[Aziz]

Very Ultra Low Noise AC Amplifier

Hi all,

I have forgotton to mention, why the perfect matching of the transistors isn't much important here:
We don't have a differential pair amplifier topology here.
Just parallel the (input) transistors and take care, that they have enough collector current for each transistor. I have chosen a collector current of 1 - 2 mA each (actually 1.5 mA).
And we can load balance the emitters with one elco and one 100 Ohm resistor for each transistor.

Ok, I have taken the OP27 for the NJM2068 for the noise performance simulation. Both have around 3 nV/sqrt(Hz) noise density. So a dual op-amp can be used to minimize further noise and has more current drive capability in the gain resistor divider network. You can use the OP27 for the single op-amp version too. Just take care, that the resistive load does not go below 600 Ohm. You can use a series resistor at the op-amp output (just like in the dual-opamp version) if you have a very small gain resistor divider network (< 600 Ohm series).

You can use the NE5532 too. The noise density spec just increases marginally (totally neglectable IMHO). But the OP27 is a better choice for the single op-amp version (has more output voltage swing).

And you can use a single transistor as well. Then leave the emitter load balancing part (elco + 100 Ohm resistor). Noise density still way well below 1 nV/sqrt(Hz).

This is just to give you some ideas:


The "Ultimate Thermo Nuclear Melt-Down"(c)(r)(tm) is coming.... *LOL* "Rette, wer sich kann!"

Cheers,
Aziz