Will a true class B have thermal runaway? - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

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
Reply
 
Thread Tools Search this Thread
Old 11th January 2004, 04:32 AM   #1
azira is offline azira  United States
diyAudio Member
 
Join Date: Dec 2003
Location: Near Seattle
Default Will a true class B have thermal runaway?

Lets define for this post that a class B is a a NPN back-to-back with a PNP in push-pull with no diodes or VBE multiplers so that it will have the 1.2V of deadzone and xover distortion.
Since the transistors aren't biased on with a fixed voltage, and say I use feedback instead to correct the xover, will it have thermal runaway?
--
Danny
  Reply With Quote
Old 11th January 2004, 10:49 AM   #2
diyAudio Member
 
Tube_Dude's Avatar
 
Join Date: Mar 2002
Location: Aveiro-Portugal
Default Re: Will a true class B have thermal runaway?

Quote:
Originally posted by azira
Lets define for this post that a class B is a a NPN back-to-back with a PNP in push-pull with no diodes or VBE multiplers so that it will have the 1.2V of deadzone and xover distortion.
Since the transistors aren't biased on with a fixed voltage, and say I use feedback instead to correct the xover, will it have thermal runaway?

No... in this case you don't have thermal runaway.


Regards
__________________
Jorge
  Reply With Quote
Old 11th January 2004, 10:56 AM   #3
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Default Re: Will a true class B have thermal runaway?

Quote:
Originally posted by azira
and say I use feedback instead to correct the xover
Danny
Just note the xover cannot be "corrected" by feedback,
feedback will only reduce the crossover distortion, and
due to compensation as the amount of feedback reduces
the distortion will increase with frequency.

It will sound extremely unpleasant.

sreten.
  Reply With Quote
Old 11th January 2004, 11:16 AM   #4
Account Disabled
 
Join Date: Apr 2003
Location: US
thermal run-away is a characterisics of biplor transistors and have no regard for topology. so the answer in this case is a "yes".

It is that with the cross-over section unbiased, you reduce the risk of thermal run away.

But if you run the transistors too hot (driving heavy load for example), you will get thermal run-away.
  Reply With Quote
Old 11th January 2004, 11:20 AM   #5
Account Disabled
 
Join Date: Apr 2003
Location: US
Default Re: Re: Will a true class B have thermal runaway?

Quote:
Originally posted by sreten
Just note the xover cannot be "corrected" by feedback,
sreten.
what will the front stages do if they are driving an unbiased ops (class C) within a feedback loop? assume of course that all stages are perfect other than the cross-over region for the OPS.

if you apply a small voltage to the input that's within the cross-over at the OPS, there will be zero output. so the front stages will output more until and unless the ops is generating the right output.

This would suggest that feedback indeed corrected the cross-over distortion by working in a non-linear fashion.

Of course, how that works in real life with less-than-perfect front stages will be much more complicated.
  Reply With Quote
Old 11th January 2004, 11:27 AM   #6
dhaen is offline dhaen  Europe
diyAudio Moderator Emeritus
 
dhaen's Avatar
 
Join Date: Jun 2002
Location: U.K.
Default Fixing xover dist..

Yup, you'd need an infinitely fast amp working in a less than infininite passband.

Millwood, you surely don't mean class C..
By definition it only conducts for 120 deg.
  Reply With Quote
Old 11th January 2004, 11:33 AM   #7
diyAudio Member
 
Tube_Dude's Avatar
 
Join Date: Mar 2002
Location: Aveiro-Portugal
Quote:
Originally posted by millwood


But if you run the transistors too hot (driving heavy load for example), you will get thermal run-away.

Wrong!...in this case you will have high dissipation !And not thermal run away.

If you stop to drive the heavy load the transistors will return
to near zero standing current...and will cool down!
Thermal run away is when the current in the transistor increase...the transistor become hoter ...as the transistor become hoter the current increase...and the process ends with the self destruction of the device.
__________________
Jorge
  Reply With Quote
Old 11th January 2004, 11:49 AM   #8
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Quote:
Of course, how that works in real life with less-than-perfect front stages will be much more complicated.
Its not complicated at all. Open loop gain is easily calculated
giving you the amount of closed loop feedback. This reduces
the distortion but does not eliminate it at high frequencies.

sreten.
  Reply With Quote
Old 11th January 2004, 11:57 AM   #9
diyAudio Member
 
nemestra's Avatar
 
Join Date: Jul 2002
Location: UK
Default Class C?

Quote:
Originally posted by dhaen
Millwood, you surely don't mean class C..
By definition it only conducts for 120 deg.
John, can you give a reference for the 120 deg. figure you mentioned? I have read an article in which an audio designer who was classifying all the different types of audio amplifiers (Class A, AB, B, D, G etc.) has referred to a an output stage with no bias, as Danny mentioned in the first post, as class C. They defined Class B as an output stage in which each device conducts for 180 deg, i.e. optimally biased. Using that definition, any output stage in which each device conducts for less than 180 deg. would be considered as Class C. From a quick glance at my bookself Duncan's, Self's and Slone's books would not contradict this definition, though none of them state it outright in these terms.

James
  Reply With Quote
Old 11th January 2004, 12:12 PM   #10
dhaen is offline dhaen  Europe
diyAudio Moderator Emeritus
 
dhaen's Avatar
 
Join Date: Jun 2002
Location: U.K.
Default oops...

I was getting mixed up between the optimum conduction angle for some RF class C circuitry and a real definition.

You (and Millwood) are of course right that class C is defined as "Less than 180 degrees conduction angle".

Aplogies to all concerned

Maybe I should have bid harder on 7V's auction
Attached Images
File Type: jpg 58_1.jpg (23.6 KB, 318 views)
  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
Thermal runaway question ricsmuts Car Audio 39 22nd May 2008 03:03 PM
thermal runaway problem Dan2 Solid State 6 25th March 2008 07:11 PM
Krell KSA-250 not true class-A? K-amps Solid State 11 27th May 2004 08:54 PM
thermal runaway diodes Wagener Solid State 19 22nd May 2004 10:44 AM
Transistor Thoery Help. Thermal Runaway hugeli60 Pass Labs 1 1st June 2003 09:29 AM


New To Site? Need Help?

All times are GMT. The time now is 10:07 AM.


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