Mon Dec 6 02:47:38 EST 2010

Picking a staple transistor (BC549-BC559)

EDIT: Starting from futurlec transistor value pack and what I find
commented on http://www.diyaudio.com and http://www.diystompboxes.com

What I want: to pick a single NPN-PNP transistor pair that's cheap and
good enough for non-power audio apps.  Needs high current gain (for
input impedance), low noise for small signals/preamps.  Not so
important: bandwidth, max current/voltage/power.


Transistor selection is still a bit of a mistery to me.  What I do
(think I) understand is this:

  - Bandwidth, not so important for audio
  - Noise, important for small signals
  - Current gain, important for input impedance of emitter follower
  - Max voltages, max power only really relevant for power amps.

What I don't get:
  - Difference between switching, small signal, general purpose.
  - @Ic, collector cutoff current?  (probably only for switching)
  - price: why is there such a huge difference (factor 50).

This is what's in the value pack which contains the most popular

10 BC327  PNP  500mA 100-600

10 BC547  NPN  100mA 110-800
10 BC548  NPN  100mA 110-800
 5 BC549  NPN  100mA LN 200-800

10 BC338  NPN  500mA 100-600
10 BC337  NPN  500mA 100-600

 5 BC107  NPN  100mA 110-450

10 2N2222  NPN SW
 5 2N2907  PNP SW

 5 C1815  NPN
 5 C9013  NPN SS
 5 C9012  PNP SS

My summary of on-line search is this:  (PNP / NPN)
2N5087 / 2N5089 for low-noise, general purpose
BC560 / BC550   for low noise.
C9012 / C9013   for small signal

The latter are cheapest.

I need to look at the datasheets to check these out.  It's probably
best to stick to one staple transistor and leave component
optimization for a later stage.

Sheets (internal refs)
These are pretty thin.  Cheap, current gain: 64-250
C9012 PNP md5://2a5fbc943ff0fd700f9db3501ca54cb7
C9013 NPN md5://0dcb0103571f86d446f01d20b5589683

More current gain: 400-1200
2N5087 PNP md5://6bc41770417204ff4953c15ec2fee282
2N5089 NPN md5://41bab55c79bf466d87189edd8e920003

Low noise: 
BC560 PNP md5://a4fb0b631e6d9a9016785126a73fce07
BC550 NPN md5://0a781a7c276c53cc2c1e77cac77f26ed

Looks like BC559 / BC549 would also be ok, but 30V instead of 50V.
They can probably take the place of anything I want to do so let's
drop the the C9012/13 and 2N5087/89.