Why collector/drain output stages are so rare???

Leolabs · 18th December 2006, 02:08 AM

Is it because of stability issue???

vynuhl.addict · 18th December 2006, 09:12 AM

If you mean FET outputs vs BJT doug self touches on this, not sure about others but he quaotes that fet outputs suffer from poor linearity and jagged crossover region compared to even the poorest BJT outputs..

Colin

richie00boy · 18th December 2006, 11:17 AM

They are not really all that rare IMO. They can be harder to get right in class-b.

lumanauw · 18th December 2006, 11:29 AM

Do you mean CFP or something like SKA or MusicalFidelity uses?

edl · 18th December 2006, 12:00 PM

Quote:

Originally posted by Leolabs
Is it because of stability issue???

QSC used this output topology.
Let's visit their site for schematics.

regards,

peranders · 18th December 2006, 02:33 PM

Quote:

Originally posted by Leolabs
Is it because of stability issue???

If you compare the properties between source out vs. drain out, I'll think you'll have the answer.

Most of the cases you want a low output impedance which you hardly will get if you have the drain as output.

Ultima Thule · 18th December 2006, 07:27 PM

Maybe as PerAnders wrote,

but on the ohter hand common emitter/source have much more gain added to the overall open loop which by the means of higher GFB rate also will add up to decrease output impedance/increase the damping factor where an all else equal common collector/drain will instead have initial lower output impedance and higher damping factor before the GFB is wrapped around, but also less open loop gain...

Gregs's (user Amplifierguru) is one example who have by now a well renoved amplifier design with his SKA proven to work well "despite" having a common soruce output stage.

I think it's a matter of implementation of both types, but I would as well like to hear especially from our heavy weighters what their views would be.

Cheers Michael

Ed Holland · 18th December 2006, 07:41 PM

Funny, I was thinking about this again recently, before this thread appeared.

If I recall correctly, the famous (in the UK at least) 50W amp kit sold by Maplin, had a common emitter topography. I'd always wondered about this, as nearly everything else is emitter-follower based. I reasoned that the designer was attempting either to increase open loop gain, or allow for local negaitive feedback in the o/p stage. However I can't find a copy of the schematic to check this out.

I built my first Hi fi amp from two of these modules, at age 17. I'm now 36 and still own and use that amplifier - it has a very clean sound and seems well behaved and "musical". However on one occasion, it did receive Swedish International Shortwave radio on 9 MHz. I was alerted (and startled) by the time signal, heard between tracks on the CD to which I was listening. The speaker leads seemmingly acted as antenae! Perhaps this pickup was an artefact of the common emitter circuit design, gain in the output stage?

Ed

Wavebourn · 18th December 2006, 07:55 PM

Quote:

Originally posted by Leolabs
Is it because of stability issue???

Fashion from one side, belief that CC is simplier from another side. Also, a lot of designers don't think of linearity of current amplification and stuck to voltage amplifications, even when call own amps POWER amps, not VOLTAGE amps. Emitter followers seem to amplify voltage more linear (however, when no current is drawn by a load).

pixpop · 18th December 2006, 07:56 PM

I just assumed it was because of Miller effect. Is that not the case?

18th December 2006, 02:08 AM	#1
Leolabs diyAudio Member Join Date: Dec 2004 Location: Bukit Mertajam	Why collector/drain output stages are so rare??? Is it because of stability issue???

18th December 2006, 11:17 AM	#3
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	They are not really all that rare IMO. They can be harder to get right in class-b. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

18th December 2006, 07:27 PM	#7
Ultima Thule diyAudio Member Join Date: Jan 2004 Location: Koskenkorva Land	Maybe as PerAnders wrote, but on the ohter hand common emitter/source have much more gain added to the overall open loop which by the means of higher GFB rate also will add up to decrease output impedance/increase the damping factor where an all else equal common collector/drain will instead have initial lower output impedance and higher damping factor before the GFB is wrapped around, but also less open loop gain... Gregs's (user Amplifierguru) is one example who have by now a well renoved amplifier design with his SKA proven to work well "despite" having a common soruce output stage. I think it's a matter of implementation of both types, but I would as well like to hear especially from our heavy weighters what their views would be. Cheers Michael __________________ "If transistors are blueberries and FETs are strawberries, then tubes must be.. pears" Michael 29th January 2010

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18th December 2006, 09:12 AM	#2
vynuhl.addict diyAudio Member Join Date: Mar 2006 Location: Vancouver, BC,Canada	If you mean FET outputs vs BJT doug self touches on this, not sure about others but he quaotes that fet outputs suffer from poor linearity and jagged crossover region compared to even the poorest BJT outputs.. Colin

18th December 2006, 11:29 AM	#4
lumanauw diyAudio Moderator Emeritus Join Date: Oct 2002 Location: Bandung	Do you mean CFP or something like SKA or MusicalFidelity uses?

18th December 2006, 07:41 PM	#8
Ed Holland diyAudio Member Join Date: Dec 2004 Location: Mountain View, California	Funny, I was thinking about this again recently, before this thread appeared. If I recall correctly, the famous (in the UK at least) 50W amp kit sold by Maplin, had a common emitter topography. I'd always wondered about this, as nearly everything else is emitter-follower based. I reasoned that the designer was attempting either to increase open loop gain, or allow for local negaitive feedback in the o/p stage. However I can't find a copy of the schematic to check this out. I built my first Hi fi amp from two of these modules, at age 17. I'm now 36 and still own and use that amplifier - it has a very clean sound and seems well behaved and "musical". However on one occasion, it did receive Swedish International Shortwave radio on 9 MHz. I was alerted (and startled) by the time signal, heard between tracks on the CD to which I was listening. The speaker leads seemmingly acted as antenae! Perhaps this pickup was an artefact of the common emitter circuit design, gain in the output stage? Ed

18th December 2006, 07:56 PM	#10
pixpop diyAudio Member Join Date: Jan 2005 Location: Los Angeles	I just assumed it was because of Miller effect. Is that not the case?

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