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Old 3rd July 2003, 10:26 PM   #1
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Default ultra-low noise transistors.

Does anybody know any ultra low-noise BJT transistor in SOT-23?

Not BC850 or so.
thanks
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Old 4th July 2003, 07:07 AM   #2
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If you are looking for ultra low voltage noise (2 ohm base resistance), BFG16A (SOT223A) or BFQ17 (SOT89) might be interesting. Mind you, hFE is not very high and as far as I remember the 1/f corner is not particularly low, so these are not the right transistors if you want low current noise. See www.semiconductors.philips.com for more info.

You could also try to find out if there is an SMD-version of the 2SC2547. It has a base resistance of about 14 ohm, hFE~=600 and a low 1/f-corner.
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Old 4th July 2003, 07:54 AM   #3
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May I ask what you are going to do with this low noise transistors?

Have you checked
http://www.thatcorp.com/

BC850 isn't so bad. I get 600 nV noise in my monster headphone amp.
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Old 5th July 2003, 03:00 PM   #4
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Default lower noise

Hello peranders & Marcel

I'd use it for a MC RIAA preamp. So I look for very low voltage noise and also a low 1/f corner transistor.


Those wideband transistors use to have a high 1/f corner. It happend to me somethign simmilar before with an op-amp, the AD8051. Everything is indeed good in it, (Low cost, 150mHz, 150V/us, 5nV...), but its 1/f corner is at 1000Hz, not good for audio. Perhaps good for GICs active filters.


I believe that a lower voltaje noise can be obtained with BJTs (so happens in OP-AMPs), at a cost of more current noise. But... by now, the lowest noise single transistor I found is the 2SK170 and 2SJ74, reaching the 0,95nV/sqrt(Hz), the same as dual ultra low noise LM394, SSM2210... BJTs. With one LM394 I could obtain 0,65nV/sqrt(hZ), but there must be any single transistor doing the same as two. Also avaibility of SSM220 and LM394 is bad (are they discontinued? MAT03 is...)

Also there's no PNP "friend" for LM394, and I'd like to make a current feedback first stage.

2SC2547 are quite more noisy than LM394.

I also use BC850C for my power amps, with low NFB resistors you can get low noise at the output, but it's not enough for a MC preamp (I've already tried it in a second stage and I had to substitute them for 2SK107 and 2SJ74)
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Old 5th July 2003, 03:21 PM   #5
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Default It depends on the application, man........

Low-noise BJTs are quiet, but only if they have a low driving impedance. JFETs don't really care about impedance, because they do not have the noise current contribution. Noise figures for JFETs with low source impedance aren't really anything to go by.

Hmmm.......current feedback front end. Guess he didn't read P-As thread on current feedback.

Rohm used to make some BJTs for these applications. I forget the number......maybe I have it somewhere. But I have no idea if they still make them, and if they come in a SOT package.

Jocko
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Old 5th July 2003, 04:07 PM   #6
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Default Ultra-low noise transistors

Hi PCP,
See f.a.:
http://db.audioasylum.com/cgi/m.pl?f...89&r=&session=
and
http://db.audioasylum.com/cgi/m.pl?f...89&r=&session=

Edit: the circuit I refered to in the MAT04 datasheet is current feedback!
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Old 5th July 2003, 04:13 PM   #7
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Default what's wrong with CFB front end?

I don't know what's worng with CFB front end (is not really a front end, its an small signal amplifier itself, to give variable gain without bandwith variations).

Voltaje noise can be as low as in VFB amplifiers, current noise is high, but with 100R source it wouldn't be a problem. I've already made it for MM with FETs, 2SK389 ,and the result was ok.



Quote:
"Guess he didn't read P-As thread on current feedback."
where is it?
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Old 5th July 2003, 04:38 PM   #8
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Default Re: what's wrong with CFB front end?

Quote:
Originally posted by PCP
where is it?
The link about CFB is here
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Old 5th July 2003, 05:20 PM   #9
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Hi PCP

You can try 2SA1312 & 2SC3324 (TOSHIBA)
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Old 7th July 2003, 05:20 AM   #10
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Quote from PCP:
"2SC2547 are quite more noisy than LM394."

This surprises me, as their datasheet specifies a 0.5nV/root(Hz) voltage noise at 10mA collector current. I've never checked that by measurement, I've only used them in microphone amplifiers with bias currents in the order of 1mA and got noise figures quite close to the calculated values.

Anyway, the other transistor types I recommended have a highish 1/f-corner in the CURRENT noise, but for extremely low source impedances, especially if they are inductive, that doesn't matter much. I know their non-SMD predecessor BFW16 has been used successfully in an ultra low noise MC amplifier.
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