Bob Cordell Interview: BJT vs. MOSFET - Page 112 - 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 20th April 2007, 08:42 PM   #1111
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Quote:
Originally posted by Bob Cordell

........................
With regard to feedback loop stability, I believe that the substitution of a faster transistor in a stage should never cause instability. If it does, it is a sign that you don't have enough stability margin in your compensation, and that to some extent your compensation is depending on slowness of transistors, which can vary a lot, both from transistor to transistor and with current and voltage on the transistor.
Cheers,
Bob
Hi Bob,

I suppose you are talking about the global feedback stability. If that's the case, it has nothing to do with the kind instabilty that I have reported to Glen. They were oscillations around 40MHz, and they disapeared when base stopper resistors in the O/P stage were inserted (or when the series inductunces of the leads to these devices was reduced to zero, which is impossible in practice).
BTW the unity gain frequency of the global FB loop was 1.2MHz.
Anyhow, the instabillty occured inside the Miller loop or inside the output stage and, probably, this cannot be cured by chosing just an other Miller cap.
If you had an other mechanism in mind, I'm eager to learn about it.

Regards, Edmond.

To All,
To avoid unnecesarry discussions, allways specify exactly which feedback loop.
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
  Reply With Quote
Old 20th April 2007, 08:46 PM   #1112
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Quote:
Originally posted by G.Kleinschmidt
Fantastic! Edmond, you've earned yourself a Heineken
Cheers!
Glen
NO, it's your design, not mine!
Cheers,

PS: In a couple of days, I'll try slower devices.
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
  Reply With Quote
Old 20th April 2007, 10:19 PM   #1113
ingrast is offline ingrast  Uruguay
diyAudio Member
 
Join Date: Sep 2004
Location: Montevideo
Quote:
Originally posted by Bob Cordell

......
With regard to feedback loop stability, I believe that the substitution of a faster transistor in a stage should never cause instability......
Completely agree, and as Mr. Curl pointed earlier, faster y better in a general sense.

Just to add a dime, note that faster transistors push the output stage pole forward, in turn allowing for a higher frequency dominant pole (or higher low frequency OL gain for the same 6 dB/oct. rolloff), meaning there is a larger high frequency feedback correction factor available. Of course this makes sense only for global negative feedback topologies.

Rodolfo
  Reply With Quote
Old 21st April 2007, 01:10 AM   #1114
diyAudio Member
 
Join Date: Sep 2006
Quote:
Originally posted by estuart


Hi Bob,

I suppose you are talking about the global feedback stability. If that's the case, it has nothing to do with the kind instabilty that I have reported to Glen. They were oscillations around 40MHz, and they disapeared when base stopper resistors in the O/P stage were inserted (or when the series inductunces of the leads to these devices was reduced to zero, which is impossible in practice).
BTW the unity gain frequency of the global FB loop was 1.2MHz.
Anyhow, the instabillty occured inside the Miller loop or inside the output stage and, probably, this cannot be cured by chosing just an other Miller cap.
If you had an other mechanism in mind, I'm eager to learn about it.

Regards, Edmond.

To All,
To avoid unnecesarry discussions, allways specify exactly which feedback loop.

Yes, global is largely what I had in mind, and I agree with your point about very high frequency local oscillations.

Cheers,
Bob
  Reply With Quote
Old 21st April 2007, 02:37 AM   #1115
diyAudio Member
 
Workhorse's Avatar
 
Join Date: Dec 2004
Hi Bob,

Did someone ever mentioned the Ft of Vertical Mosfets in comparision to newer 30MHz Ft Bipolars[2SC5200/2SA1943]....
I think mosfets exhibit greater Ft in range of 100MHz or greater ...

What are your views regarding this..
  Reply With Quote
Old 21st April 2007, 03:45 AM   #1116
GK is offline GK  Australia
Account disabled at member's request
 
Join Date: Jan 2006
Quote:
Originally posted by ingrast


Completely agree, and as Mr. Curl pointed earlier, faster y better in a general sense.

Just to add a dime, note that faster transistors push the output stage pole forward, in turn allowing for a higher frequency dominant pole (or higher low frequency OL gain for the same 6 dB/oct. rolloff), meaning there is a larger high frequency feedback correction factor available. Of course this makes sense only for global negative feedback topologies.

Rodolfo

Quote:
Originally posted by Bob Cordell



Yes, global is largely what I had in mind, and I agree with your point about very high frequency local oscillations.

Cheers,
Bob

Hi all. As Edmond has already pointed out, the 40MHz was parasitic oscillation of the output devices cured with base stopper resistors, not global loop instabliity.
Since I am publishing this design on the net, I will optimise the miller compensation of this design for the 4MHz devices and include base stopper resistors. That way, if anyone want's to copy the design, they can bung in 30MHz transistors without a problem for a modest improvment in high frequency lineaity. And if they are feeling adenturous, and want to get more out of the design with high fT transistors, they can then tweak the compensation themselves.

With regards to 30MHz transistors being better than 4MHz ones, I agree completely. But the performance differences when applied in typical audio circuits are not as great as some expect. D. Self has demonsatrated what can be achieved with lowly low-fT transistors with his 'Blameless' - A very basic topology that achieves in the vicinity of 0.01% THD+N at 20kHz and ~0.001% THD+N < 1kHz - with 2MHz fT output devices - definately "Hi-Fi", IMO.


Cheers,
Glen
  Reply With Quote
Old 21st April 2007, 04:34 AM   #1117
GK is offline GK  Australia
Account disabled at member's request
 
Join Date: Jan 2006
Quote:
Originally posted by estuart

NO, it's your design, not mine!
Cheers,

PS: In a couple of days, I'll try slower devices.

Thanks, Edmond.

Cheers,
Glen
  Reply With Quote
Old 21st April 2007, 04:58 AM   #1118
GK is offline GK  Australia
Account disabled at member's request
 
Join Date: Jan 2006
Quote:
Originally posted by lumanauw


Hi, Glen,

This is very interesting.
A question : what is the "better" result if we use 20-30mhz ft output transistor compared if we use 4-5mhz ft output transistors? In what POV the faster one is better? Xover distortion? Or what? Many well regarded (and robust) commercial amps are indeed using MJL21193/4 until this day in 2007 (while output transistors with 30mhz are easily available).
Are they really differ in sonics result?

Hi lumanauw.

30MHz is better than 4MHz for reasons others have already mentioned here, but the sonic performance of an amplifier ultimately depends on much more than the devices selected for the output stage.
I contend that one can make a perfectly adequate HiFi or PA amplifier with 4MHz devices and a horribly inadequate one with 30MHz devices, and vice versa.

Cheers,
Glen
  Reply With Quote
Old 21st April 2007, 07:02 AM   #1119
diyAudio Member
 
Join Date: Apr 2007
While BJT's have non linear gain if you have a good class A driver for them it will work just fine. My drivers can supply 100ma of current to the darlington pair on the output. Even at full current output this is more that enough. MOSFETs at low currents are nonlinear and in my experiences are not to be used in audio because of the large current required to charge the gate at sufficient speeds. My current amp can drive a 2 ohm load to 240W without a problem. It uses BJT's and with the heatsink i have they don't even get hot.
This is my amp design. Let me know what ya think. I know it can use improvements.
http://byrne.isa-geek.com/brendan/amp.bmp
  Reply With Quote
Old 21st April 2007, 10:44 AM   #1120
diyAudio Member
 
Workhorse's Avatar
 
Join Date: Dec 2004
Quote:
Originally posted by IrishboyM4
MOSFETs at low currents are nonlinear and in my experiences are not to be used in audio because of the large current required to charge the gate at sufficient speeds.
Its your own specific view.....The non-linearity could be solved by implementing/adopting some good techniques such as Transconductance Gain cell, Error-Correction..
For driving the mosfet you need sufficient gate current which is given by Ig=F X Qg, where F is operating frequency and Qg is total gate charge... and this value of Ig is much less when compared to the base current required for driving Bipolars at same frequency, which require much larger magnitude of base current in proportion to mosfets...
  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



New To Site? Need Help?

All times are GMT. The time now is 05:10 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