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Old 29th October 2012, 09:34 PM   #1
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Default Lowest noise BJT transistor?

Which are the lowest noise BJT transistors, both NPN and PNP, which are in current production, regardless of price?
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Old 29th October 2012, 10:41 PM   #2
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Hi Joshua,

Are you interested in voltage noise, current noise or both? If the source impedance is extremely low you would normally be interested in voltage noise, if it is not, you would normally be interested in both.

Best regards,
Marcel
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Old 29th October 2012, 11:13 PM   #3
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I'm interested in both, but especially in voltage noise.
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Old 30th October 2012, 08:49 PM   #4
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My favourites are 2SC2545/2SC2546/2SC2547 and their PNP complements (2SA1085 or something, I don't remember the exact type number). SSM2210/SSM2220 (cheap version of MAT02 and its complement) are also pretty good. I don't know if any of them are still in production, or maybe in production with a different package and a different type number.

For extremely low voltage noise, the Philips/NXP BFW16A used to be a good choice because of its low base resistance, but it is out of production. You could try a more modern low-noise wideband transistor, such as the BFU760F (RB=3.32 ohm according to its Spice model). It will be difficult to keep it stable due to its high transition frequency.
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Old 30th October 2012, 08:58 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

Again as in the other zener thread, for low numbers parallel devices.

rgds, sreten.
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Old 30th October 2012, 09:03 PM   #6
MiiB is offline MiiB  Denmark
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Does the BFU760F have a complement...??
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Old 30th October 2012, 11:07 PM   #7
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Quote:
Originally Posted by MiiB View Post
Does the BFU760F have a complement...??
None that I know about.
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Old 6th December 2012, 12:24 AM   #8
teleman is offline teleman  Norway
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2SA 872A (Hitachi)!.....this is the pnp version. Noise is specified between 2-5 dB maximum! I'm no expert, but perhaps someone else could through some light on this.
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Old 10th December 2012, 07:08 PM   #9
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Quote:
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2SA 872A (Hitachi)!.....this is the pnp version. Noise is specified between 2-5 dB maximum! I'm no expert, but perhaps someone else could through some light on this.
According to the last graph in the datasheet, the optimal noise figure is 2 dB related to 190 ohm at 1 mA collector current, 1 kHz. The noise contribution of the transistor is then equivalent to (10^(2/10)-1)*190 ohm ~= 111.1297 ohm. About 13 ohm of this is collector shot noise translated back to the input (half of kT/(qIc)). As 1 mA is the optimum according to the graph, the contribution of base shot noise must be roughly the same. That leaves some 85 ohm for base resistance thermal noise.

85 ohm is not bad at all for a discrete low-frequency transistor, but not exceptionally good either.
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Old 11th December 2012, 04:04 PM   #10
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Hi Marcel,

Interesting thread ... would you happen to have advices on current production low-noise complementary BJT's? I searched for the 2SC2545 on Mouser's pages but it doesn't appear to be in their catalog ... In my context can be low voltage/current/high fT as well.

Best regards,

Jesper

P.S.: is it correctly understood that the BFU760F only accepts 2.8 VDC between collector and emitter?
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Last edited by gentlevoice; 11th December 2012 at 04:25 PM.
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