four transistor emitter follower (diamond buffer) as power output stage?

[capslock]

The good old four transistor unity-gain buffer which consists of a complementary emitter follower, another emitter follower pair that replaces the bias diodes and two current sources is very popular in complementary integrated op amp designs as well as some discrete designs. For a schemetic, take a look at Sonny's page:

http://www.geocities.com/sonnya5000/basicb1.jpg

He did some nice simulations about distortion and how to improve it for low impedance loads.

What I wonder about: why is this nice topology apparently not used for power output stages? Is it because it is too difficult to keep the bias constant?

Puzzled,

Eric

[HarryHaller]

http://www.geocities.com/sonnya5000/basicbuffer.html

Good article. Check it out.

H.H.

[djk]

http://www.geocities.com/~ciuffoli/my/Hot_Follower1.GIF add capacitor to switch to class AB a high power http://www.geocities.com/~ciuffoli/m..._Follower1.GIF add DC servo to run without feedback http://66.34.95.145/gif/a18a.gif
--- Andrea Ciuffoli <ciuffoly@tin.it> wrote:
> Andrea Ciuffoli
> via degli Zingari 50
> 00184 Rome
> ITALY
>
> thank you
>
> dennis kleitsch wrote:
>
> > djk:Sorry,I don't have access to a scanner at
> work.If
> > your library doesn't have Wireless World you can
> send
> > me your postal address and I can mail you a
> > photocopy.It was in October 1976 Wireless World
> page
> > 74
> >
> > --- Andrea Ciuffoli <ciuffoly@tin.it> wrote:
> > > please, send me a san image
> > >
> > > dennis kleitsch wrote:
> > >
> > > > djk: It was published in Wireless World
> magazine
> > > many
> > > > years ago in the Circuit Ideas section.It had
> a
> > > single
> > > > transistor running class A for voltage gain as
> a
> > > front
> > > > end and had no feedback.I'll see if I can find
> and
> > > > copy it for you if you are interested.It would
> be
> > > easy
> > > > to add a DC servo to the current source
> setting
> > > > resistors.

[capslock]

djk:

Thanks for posting those. I was aware of the first two, but I thought those were just paper designs, i.e. never really used and tested.

I did not know the third one. Where does it come from, what is it?

Eric

[djk]

http://www.audio.nl/designeng.html I linked to this as I thought it was a good non-invasive way to implement a DC servo.If you examine this closely you will see that it is basically a Hafler DH-200 topology with the feedback removed, reduced B+,and biased into class A.

[Jocko Homo]

Can't believe I missed this one.......

I've been using a version of this for years. Only I use many devices in parallel for the output followers. I get distortion numbers down near -60 dB under load.

I first saw it used in the old Audio Research D-100.......from back in the 70's. Ran hot, sounded rotten, blew up all the time.

Jocko

[lumanauw]

Asking the same question :

Why diamond buffer topology, like Walt Jung's never come to power amplifier output stage usage? I only see it on preamp or headphone output stage, but never in power amplifier output stage.

Why is it? Bias is not stable enough?

[wrenchone]

I've been using a mosfet version of the diamond buffer as a power output stage for a while. I bias the first two followers with current sources and put all the devices on the same heat sink. The bias current is pretty stable.

[lumanauw]

Hi, Wrenchone,

Do you have any idea why this topology never used in commercial power amp output stage? (I never saw one)

From your experience, what is the pro/cons of using this diamond buffer topology compared with EF+VBE multiplier biasing scheme?

How much bias do you use in final stage? ClassA? low biased ClassAB?

And how is the sound?

Is this topology has a "Non-turnoff" properties (like Blomley)?

[anatech]

Marantz 300DC. I've been using a variation on this. Bias stability is not a problem
-Chris