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Old 23rd December 2012, 04:26 PM   #21
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I don't know about other brands, but Philips/NXP BC550's can only reach such low noise figures when the source resistance is in the kohm range or above. That is because they have a nice high current gain (little base current shot noise), but not a very low base resistance (which sets a lower limit to the obtainable equivalent input noise voltage).

One way around this is to buy a lot of BC550's (they are cheap anyway) and connect them in parallel for AC signals. For example, you can give each its own decoupled emitter resistor to ensure an equal current division. If you select them for equal VBE maybe you can even connect them directly in parallel.
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Old 31st December 2012, 10:34 AM   #22
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I was just checking what transistor arrays are available at Farnell, and the THAT 300 series is also interesting, especially the THAT 320 PNPs: 25 ohm base spreading resistance. As there are four matched devices in one THAT 320, you can easily connect them in parallel and obtain 6.25 ohm base spreading resistance.
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Old 31st December 2012, 11:15 AM   #23
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They look pretty good and with reasonable capacitance, but rather low hfe
The THAT340 would be a nice way of implementing the W M Leach MC preamplifier as the pair matching would give some L-R gain matching
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Old 31st December 2012, 02:50 PM   #24
teleman is offline teleman  Norway
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Here are few you may want to check out.

SOT23 :MMBT5089,PMBT6429,2N4401/03,2N2222

I lifted the list below from who knows where...(don't remember now!) anyway, see for yourself if any of these can be suitable for your needs!


NPN PNP

TO-92 SOT-23 TO-92 SOT-23

2N3904 MMBT3904 2N3906 MMBT3906
2N4401 MMBT4401 2N4403 MMBT4403
BC337 BC817 BC327 BC807
2N5089 MMBT5089 2N5087 MMBT5087
BC547C BC847C BC557C BC857C
MPSA14 MMBTA14 MPSA64 MMBTA64
ZTX618 FMMT618 ZTX718 FMMT718
PN2369 MMBT2369 2N5771 MMBT5771
2N5550 MMBT5550 2N5401 MMBT5401
MPSA42 MMBTA42 MPSA92 MMBTA92
MPS5179 BFS17
BFT92 BFT93
TIP142 TIP147
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Old 30th January 2013, 07:55 PM   #25
RCruz is offline RCruz  Switzerland
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Old 3rd March 2013, 01:41 PM   #26
upstart is offline upstart  United States
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Exclamation Excess noise, watch out for reverse vbe

Even if you choose the ideal transistor, design your circuit so that you NEVER have emitter base breakdown, not even for an instant.

You can, and should, use a diode from emitter back to base to protect your base emitter junction. Install this in your circuit FIRST, then add your low-noise device, observing ESD procedures. You'll thank me later.

The physics is that in base emitter avalanche, hot electrons create defect sites in the base emitter junction, and these can lead to excess (1/f) noise or even popcorn noise . The effect is fast, and it is cumulative. Excess noise can be a problem in BJTs as high as a 1 kHz, well into the audio region.

The cardinal rule is: never reverse bias emitter base junctions into avalanche (usually at 3 to 5 volts).

Cheers.
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Old 3rd March 2013, 02:02 PM   #27
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Default MPSA18

Has worked well for me in a range of projects from single tran regenerative receivers to the N half of a CFP paired with 2SA1837 pnp driver.

http://www.redrok.com/NPN_MPSA18_45V...e400_TO-92.pdf

Good luck
Terry
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Old 3rd March 2013, 07:51 PM   #28
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Thank you.
Does it have a PNP complementary part?
Or is the 2SA1837 being a complementary to MPSA18?
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Old 3rd March 2013, 08:06 PM   #29
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Joshua,
I have searched the forum and Google, and it appears there's no complement to the MPSA18---at least as far as I know. The 2SA1837 are Toshiba devices which are somewhat more robust TO-220F package devices used as drivers/ small power output. Sorry, hope you find what you need.

Terry
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Old 4th March 2013, 01:10 PM   #30
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Quote:
Originally Posted by upstart View Post
Even if you choose the ideal transistor, design your circuit so that you NEVER have emitter base breakdown, not even for an instant.....

The physics is that in base emitter avalanche, hot electrons create defect sites in the base emitter junction, and these can lead to excess (1/f) noise or even popcorn noise . The effect is fast, and it is cumulative. Excess noise can be a problem in BJTs as high as a 1 kHz, well into the audio region.

Cheers.
I second this. Long ago it was common knowledge that bipolar RF transistors were easily damaged by BE breakdown, causing 1/f noise and gain reduction (low capacitance and small feature size makes the transistor die more vulnerable)
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