Variable operating biass output ?

[Bigun]

This isn't quite a coherent idea yet, but would be interested to hear if there are already some others who have thought along these lines. My apologies if this is 'old news' as my Search skills are not yet great.

The problem is how to avoid cross-over distortion in AB but still have good power efficiency.

I was thinking if we start with Class A but adjust the operating point (dc bias point ?) of the output device to suit the signal. A big signal means moving the operating point up so that negative swings don't clip. Lower signal means the operating point moves down to conserve power.

How I think that this can be done - the bias circuit needs to know in advance what the signal is going to look like so it can adjust the biass. Well, how about the biass circuit 'sees' the input signal before the power devices. You introduce a 'delay' to the signal between the input and the power stage. A 'delay line' so to speak. The non-delayed signal is fed through LP filter (below amplifier LF cut-off) and the output used to drive the biass. Perhaps this can be done symmetrically for a Class A push-pull output.

Now the output biass tracks the signal level in Class A but with greater power efficiency.

Am I crazy ???

[Williams Audio]

hi ,
i think the idea as idea is fine but:
1- you will have to deal with feedback that is delayed so you add some distortions
2- there are some big names are trying to work so they call it in different names, please check up with KRELL, CARVER and so
3- the idea is good but not easy to make it work, if you success let me know too
4- NEVER GIVE UP with innovating ideas
best regards
Williams

[PaulBysouth]

Gareth,
You idea is a good one, but old (most ideas usually are). While there are many variations of this theme, the most interesting group is for `Non Switching Class B Amplifiers’. The most useful references on these are:

P. Blomley `New Approach to Class B Amplifier design’
Wireless World, Feb. 1971

S. Tanaka `New Biasing Circuit for Class B Operation’
AES 65th Convention, paper 1615, Feb. 1980

E. Margan `Crossover Distortion in Class B Amplifiers’
Electronics & Wireless World, July 1981

Marcel van de Gevel `Audio Power with a New Loop’
Electronics World, Feb. 1986

Huon, Dower US Patent 6-630-865
Oct. 2003

I think the most interesting of these is the Gevel and Huon ideas (Huon and Dower’s patent is an extension to van de Gevel’s work), in which the bias is constantly adjusted during each cycle so that the `Off’ output transistor never actually turns off, but settles to a minimum current. I’ve spend a lot of the last 18 months carefully looking at the Huon / Dower scheme, and while it fixes a number of problems (bias stability in particular) it also introduces some more (higher order distortion principally). Incidentally I went to University with Graeme Huon and we are currently building loudspeakers together, and Wal Dower and I spend 11 years working together designing, building and installing radar equipment all over Australia.

I think there is potential to monitor the output bias current, and continuously adjust the bias circuit using a small microprocessor. Now that most inputs are digital, this gives the opportunity of implementing a small digital delay, so that a look ahead bias scheme could be implemented. But this is far to complex for me!

I haven’t given up on the Huon / Dower scheme as I think it has potential. I also have an extension to the patent that makes it simpler to implement the bias scheme, but stability is an issue. In effect the bias scheme has to operate at about 5 to 10 times the frequency response on the actual amplifier, so 250 KHz plus – not easy!

There is also an interesting variation of the van de Gevel scheme by Edmund Stuart (Electronics World, Dec. 2003) for a Class B amplifier, in which the bias scheme operates at very low frequencies to set the bias current only, and hence eliminates the need for thermal feedback.

I hope all this is of interest.
Paul Bysouth, May 2009.

[Bigun]

Originally posted by PaulBysouth
I think the most interesting of these is the Gevel and Huon ideas (Huon and Dower’s patent is an extension to van de Gevel’s work), in which the bias is constantly adjusted during each cycle so that the `Off’ output transistor never actually turns off, but settles to a minimum current.

Flipping Heck!, this was the 2nd idea I had, I woke up very early this morning because it was running through my head and I was thinking of how to implement this. Darn frustrating to reinvent something old


- but seriously, thank you for responding to my post, I'd never have found those references myself and it saves bigger heartache down the road

Do you have any prototypes on the go, anything you can share ?

[Bigun]

Originally posted by PaulBysouth
I think there is potential to monitor the output bias current, and continuously adjust the bias circuit using a small microprocessor. Now that most inputs are digital, this gives the opportunity of implementing a small digital delay, so that a look ahead bias scheme could be implemented. But this is far to complex for me!

Paul,

I was thinking of analogue delays, but I believe this route could produce lots of distortions. The digital option seems much better.

Mr. Williams - I don't see feedback as an issue, the 'advanced notice' of what the signal looks like can come from the pre-amp or dedicated buffer at the front, the power amplifier can include a feedback loop that stays within the scope of the 'delayed' signal.

I fear I need a partner in crime to try out these things or they will likely never get off the drawing board.

[Bigun]

Originally Posted by PaulBysouth

There is also an interesting variation of the van de Gevel scheme by Edmund Stuart (Electronics World, Dec. 2003) for a Class B amplifier, in which the bias scheme operates at very low frequencies to set the bias current only, and hence eliminates the need for thermal feedback.

I hope all this is of interest.
Paul Bysouth, May 2009.

Anybody have a copy of the Edmund Stuart article ??

[latala]

To be honest you are flogging a dead horse !
its been tried since the early sixties with sliding bias etc but has never taken off!
If you google improved "class b amplifier" by Renardson you will see that he has done this very successfully He uses a method of feedforward over the output pair to prevent Gm doubling on a class AB output stage thus avoiding any trace of crossover distortion,All it really means in practical terms is a pair of extra resistors over a normal output stage!
Most annoying that he has done this!!! 40 years I have beeen looking into this problem without success
Regards Trevor

[Ian Finch]

It may have slipped the memory somewhere but an obvious and proven alternative, "class XD", substantially avoids the crossover problem. If Self's documentation doesn't prove it, perhaps we may only be considering earlier attempts, such (as you say) the sliding bias concept.

As I understand Blomley's method, it didn't....not in an OP stage anyway!

[Ian Finch]

Thanks for ref. Iatala, Renardson (easiest to just Google his Surname) has considereable documentation on his project site at Angelfire. A lot of reading there. Bon apetit.

[Edmond Stuart]

Originally Posted by Bigun

Anybody have a copy of the Edmund Stuart article ??

Of course I have one! See: Auto Bias
BTW, this design is a bit obsolete. In a few month or so I will update my website with a new version: fully symmetrical and high speed (>1MHz).

Regards,
Edmond.