BJT EF Output stage bias distribution? - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

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
Thread Tools Search this Thread
Old 4th September 2009, 08:41 PM   #1
OliverD is offline OliverD  Germany
diyAudio Member
OliverD's Avatar
Join Date: Nov 2002
Location: Germany
Default BJT EF Output stage bias distribution?

Hi everyone,

I'm having trouble with On Semiconductor MJL1302A/MJL3281A used as output devices. I acquired a lot of these devices. They are advertised as direct replacements for the old and well-known Toshiba 2SA1302/2SC3281, of which only fakes are available today.

In my circuit, I have three in parallel on each rail. I'm aiming for 300mA of total bias for the output stage. However, with devices from the same date code, I get quite large variations in the current through individual devices. I see anything from 8mV to 14mV accros the 0,1R emitter resistors when I adjust for 300mA current drain of the output stage.

Is this normal for these kinds of devices?
Do you select output devices?
Does the difference become smaller with higher currents?
Before I start further testing, do you think it matters?

I have never experienced these problems with other devices before.
  Reply With Quote
Old 4th September 2009, 09:02 PM   #2
diyAudio Member
nigelwright7557's Avatar
Join Date: Apr 2008
Location: Carlisle, England
This is a very old topic on here.

Some people demand tightly controlled transistors and others dont think it matters much.

The emitter resistors are supposed to help balance things out a bit.
Murton-Pike Systems PCBCAD51 pcb design software.
  Reply With Quote
Old 4th September 2009, 09:15 PM   #3
diyAudio Member
ostripper's Avatar
Join Date: May 2008
Location: Smoky Mountains , Tennessee
Is this normal for these kinds of devices?
had same issue with the similar NJW0281/0302 ' s(ON-semi) . Used 2 different batches on one amp (20-30ma variation ), built the other amp from one "batch" (same date code) ... just 2-3ma variation. I did run the hell out of the "mismatched" amp , it did not sound any worse , did not go up in smoke .

I did remake it later with new devices from the same "batch" , but it showed no undue stress while mismatched.
  Reply With Quote
Old 7th September 2009, 06:50 PM   #4
diyAudio Member
megajocke's Avatar
Join Date: Jan 2003
Current sharing stability depends on power supply voltage and non-shared thermal resistance too. It gets worse towards higher voltages and higher thermal resistances.

What kind of insulators, if any, are you using? How high is the power supply voltage? Having 0.1 ohm emitter resistors sounds quite low, but if power supply voltage isn't too high it could work.
  Reply With Quote
Old 8th September 2009, 12:34 AM   #5
GK is offline GK  Australia
Account disabled at member's request
Join Date: Jan 2006
100mA bias per pair with 0.1R is about half of what you should be running for "optimal bias", and is significantly underbiased.

Better to be slightly over biased than under. If the extra heat of a 600mA Iq can't be tollerated, I'd increase the emitter resistors to at least 0.22R.
  Reply With Quote
Old 21st December 2009, 07:17 PM   #6
diyAudio Member
tiefbassuebertr's Avatar
Join Date: Jul 2005
Location: D-55629 Schwarzerden
In this case also this thread could be of interest:
bipolar (bjt) transistor families for audio power output stages
  Reply With Quote
Old 21st December 2009, 07:32 PM   #7
wahab is offline wahab  Algeria
diyAudio Member
Join Date: Nov 2009
Location: algeria/france
the differences you re measuring are quite common in
devices dispersion...these are differences in vbe, and are even
lower than the dispersion we find in diodes jonctions of the same device..
  Reply With Quote
Old 21st December 2009, 08:15 PM   #8
AndrewT is online now AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
as the others are telling you, your parallel devices have not been matched.

If you want optimal operation from parallel devices you must match them.

You can use your completed 3pair PCB as your test device. But I'd suggest that each group of DUTs are attached with short leads to save the PCB and the devices from damage when you try to remove them.

Set up your PCB Vbe to give the bias you need. Don't change it.
Attach your DUTs. Measure Vre. Remove and group the DUTs by Vre readings.

Select PNP DUTs that have very similar Vre readings and select NPN DUTs that have similar (but not necessarily the same as PNP) and attach your 3pair of these selected devices to your test PCB.
measure the Vre. You should find that the devices Vre is now very much closer together. If they are within 10% total spread you can stop. If you are <5% total spread you have an excellent match of Vbe.
If you are around 20% spread then a second set of DUT testing is looming in.

Now to an optimum ClassAB bias current.
The Vre of the external resistor and the effective internal resistor should be 26mV when the transistor is cold (Tc=25degC). This falls slightly as the Tc & Tj rises.
Assuming Re=0r22 and that the transistor has an effective re=0r04, then total Re+re=0.26ohms.
The required bias voltage across Re is 22/26 * 26mV ~=22mV
Set Vre to average 22mVre across the 3devices.

If you decide to use Re=0r1 then try setting Vre to ~18mVre. This will require a heatsink about twice as big.
regards Andrew T.
Sent from my desktop computer using a keyboard

Last edited by AndrewT; 21st December 2009 at 08:34 PM.
  Reply With Quote
Old 21st December 2009, 08:26 PM   #9
Did it Himself
diyAudio Member
richie00boy's Avatar
Join Date: Nov 2003
Location: Gloucestershire, England, UK
I would recommend 0.22 ohm emitter resistors if you have not matched the transistors. 100mA bias per pair is fine.
__________________ my website for UK diy audio people - designs, PCBs, modules and more.
  Reply With Quote
Old 21st December 2009, 08:32 PM   #10
diyAudio Member
nigelwright7557's Avatar
Join Date: Apr 2008
Location: Carlisle, England
I usually find 5mA through each MOSFET is enough to get rid of crossover distortion.
Any more is just wasted as heat.

I use a sig gen and a scope, apply a 1 volt sine wave and monitor the output into a speaker. I turn the bias up slowly until the sine wave is no longer distorted and leave it there.
Murton-Pike Systems PCBCAD51 pcb design software.
  Reply With Quote


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
Output stage optimal bias hpl Everything Else 2 4th February 2008 02:53 PM
How to set the bias of a FET output stage? SpittinLLama Solid State 20 19th March 2007 01:21 PM
bias trouble in pnp output stage Easyamp Solid State 5 17th December 2003 06:36 PM
20 output distribution amp biff584 Solid State 14 21st July 2003 03:17 PM
Lt1360 power output stage (auto bias) partyjups Solid State 0 26th February 2002 06:53 PM

New To Site? Need Help?

All times are GMT. The time now is 08:09 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2