Lowest noise BJT transistor? - Page 2 - diyAudio
Go Back   Home > Forums > Design & Build > Parts

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 11th December 2012, 11:45 PM   #11
diyAudio Member
 
dchisholm's Avatar
 
Join Date: Mar 2011
Location: St Louis, Mo
Quote:
Originally Posted by gentlevoice View Post
. . . I searched for the 2SC2545 on Mouser's pages but it doesn't appear to be in their catalog ...
Look for Fairchild's KSC1845/KSA992 devices.

If you are not scared by SMT devices, there is the Toshiba 2SC3324/2SA1312.

Dale
  Reply With Quote
Old 12th December 2012, 10:34 PM   #12
diyAudio Member
 
Join Date: Mar 2003
Location: Haarlem, the Netherlands
Quote:
Originally Posted by gentlevoice View Post
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?
Hi Jesper,

Maybe there is an SMD version of the good old 2SC2545 in production somewhere? I don't know. They were always a bit hard to get.

About the BFU760F: yes, definitely if the circuit cannot handle the negative base current you get at larger collector-emitter voltages. If it can, you may be able to go higher, between 2.8 V and 10 V, although the datasheet is not very clear about that. I would try to stay below 2.8 V, just to be safe. The low voltage handling is a consequence of the small device sizes and thin base regions they use to make the fT very high.

Best regards,
Marcel
  Reply With Quote
Old 12th December 2012, 10:41 PM   #13
diyAudio Member
 
Join Date: Mar 2003
Location: Haarlem, the Netherlands
Quote:
Originally Posted by dchisholm View Post
Look for Fairchild's KSC1845/KSA992 devices.

If you are not scared by SMT devices, there is the Toshiba 2SC3324/2SA1312.

Dale
3 dB noise figure in 20 ohm at 10 mA for the 2SC3324? That is almost as good as a 2SC2545!
  Reply With Quote
Old 13th December 2012, 06:38 AM   #14
diyAudio Member
 
Join Date: Oct 2007
Location: Aarhus, Denmark
Hi - thanks Marcel & Dale for replying.

@Dale: Hmmm... Thanks for the tip on the KSC1845 & 3324. I actually have some of the KSC1845s so maybe I should give them a listen ...

@Marcel: Can I ask you if there are there any "side-effects" - audio-wise - with these transistors? I mean: Do the design parameters in a transistor like the BFU760F mean that they may not be the most suitable for audio design? Here I'm not thinking about their potential for oscillating but would there be something else that is less feasible - besides the size :-(

Winter greetings from Denmark - cold it is ...

Jesper

P.S.: Oups - forgot this: Do you know of any PNP transistor that compares with the BFU760F? Maybe a silicon-germanium type?
__________________
... Being mindful about sustainability also in my audio endeavors matters to me ... paying CO2 - and a quite fair pollution compensation helps create a good listening "energy" ...

Last edited by gentlevoice; 13th December 2012 at 06:41 AM.
  Reply With Quote
Old 14th December 2012, 10:15 AM   #15
diyAudio Member
 
Join Date: Dec 2005
Location: Kuala Lumpur
High frequency devices have low current gain and breakdown voltage
  Reply With Quote
Old 15th December 2012, 07:04 AM   #16
diyAudio Member
 
Join Date: Mar 2003
Location: Haarlem, the Netherlands
Quote:
Originally Posted by gentlevoice View Post

@Marcel: Can I ask you if there are there any "side-effects" - audio-wise - with these transistors? I mean: Do the design parameters in a transistor like the BFU760F mean that they may not be the most suitable for audio design? Here I'm not thinking about their potential for oscillating but would there be something else that is less feasible - besides the size :-(

P.S.: Oups - forgot this: Do you know of any PNP transistor that compares with the BFU760F? Maybe a silicon-germanium type?
No, I don't know a complementary version.

About disadvantages of an RF transistor like the BFU760F:


1. Parasitic oscillations. You will very probably need a base stopper right next to the transistor, and for absolute minimum audio frequency noise that will need to be a lossy ferrite bead rather than a resistor. For SMD boards you have special SMD ferrites, like the Murata BLM18EG series. A small resistor in the collector connection may also help. If the transistor oscillates, you will only be able to measure that directly with a very expensive RF spectrum analyser. If you haven't got one, check for excessive distortion and for bias points that change when you hold your finger or a small screwdriver close to the transistor.

2. Vulnerability to ESD and electrical overstress. Bipolar transistors are sensitive to emitter-base junction breakdown, it can cause beta degradation at low collector currents and increased 1/f noise. RF devices tend to be more sensitive than low-frequency devices; the BFU760F has an absolute maximum emitter-base voltage of only 1 V. Make sure you include an antiparallel diode to protect the emitter-base junction against reverse breakdown.

3. Uncharacterised 1/f noise. There is nothing in the datasheet or in the Spice model about 1/f noise. Chances are that 1/f noise is quite low in such a modern transistor, but it would have been nice if they had included measured data somewhere, now you will have to measure it yourself. In bipolar transistors, 1/f noise is mainly current noise, so it doesn't have much effect if you have an extremely low source impedance (like an MC cartridge).


Besides, some RF transistors have a low hFE, but this does not apply to the BFU760F. Low hFE means large base currents and, hence, large base current shot noise at a given collector current. This again doesn't matter much if you have an extremely low source impedance. Anyway, for the BFU760F, hFE is typically 340 at 10 mA collector current, which is not bad at all.

Last edited by MarcelvdG; 15th December 2012 at 07:09 AM.
  Reply With Quote
Old 15th December 2012, 02:28 PM   #17
diyAudio Member
 
Join Date: Dec 2005
Location: Kuala Lumpur
1/f noise is undesirable in rf oscillators, causing phase noise, so it is a parameter the manufacturer worries about
  Reply With Quote
Old 16th December 2012, 06:39 AM   #18
diyAudio Member
 
Join Date: Mar 2003
Location: Haarlem, the Netherlands
Yep, that's what I would also think, but still the manufacturer doesn't specify anything, neither in the datasheet nor as a KF parameter in the Spice model.
  Reply With Quote
Old 18th December 2012, 02:21 PM   #19
diyAudio Member
 
Join Date: Oct 2007
Location: Aarhus, Denmark
Hi Marcel ... thanks again for replying to my questions. What I'm considering using these transistors for is a wide-band analog stage prior to an A/D converter. I have no experience with how such devices sound but if they sound well their low voltage capacity may be suitable for this application - I guess.

I've experimented with listening to the 2SC2240 and to my ears it sounds at its best with a Vce of about 15 - 20 volts which would mean that to avoid destroying the A/D converter I'd have to add some kind of protection circuitry - in case there's an unforeseen high level signal on the input. That may not be necessary if the transistor itself sounds well at low voltages and the circuitry as such can be designed around low voltage rails.

So I've ordered a couple of the BUF760s and PNP versions from NXP and will try them out. Thanks also for the tip on ferrite beads & a resistor to - hopefully - avoid oscillation. I will keep this in mind.

Merry Christmas to you all ;-)

Jesper
__________________
... Being mindful about sustainability also in my audio endeavors matters to me ... paying CO2 - and a quite fair pollution compensation helps create a good listening "energy" ...
  Reply With Quote
Old 21st December 2012, 04:36 PM   #20
diyAudio Member
 
Join Date: Jan 2011
Location: Fosser, Aurskog-Holand, Akershus, Norway.
BC550 is also a quite good transistor regarding to noise. NF1 is typically around 0.6dB, but then one should remember: There is a lot of mistakes to do wich has greater influence to the noise a transistor emits, wich makes the total design around it a lot more to take in consideration than the actual Noisefigure for the transistor. Just try to drive a LowNoise transistor just a tad outside the "currentwindow" where the transistor is at its most optimum regarding to low noise.

But have you considered J-FETs in this stage of the A/D-converter?
__________________
Sooner or later you end up with TANDBERG
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Can I build up these Darlington transistor if I use two BJT transistor ? gaborbela Parts 9 15th July 2010 06:30 PM
Which tubes are lowest noise? Joshua_G Tubes / Valves 49 30th January 2010 02:44 PM
Which is the lowest noise FET? Young Solid State 10 11th February 2006 04:20 AM
which is lowest noise bipolar? bane_982 Parts 1 4th March 2005 11:45 AM
Lowest noise levels difi Chip Amps 3 3rd March 2004 10:42 PM


New To Site? Need Help?

All times are GMT. The time now is 11:12 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2