Output buffers

[MiiB]

Hello....
would like to explore the differnt topologies for output buffers that per example can be driven with a tube voltage stage...

I know of the types consisting of simple mosfet follower driven in Class a or a/b....but the types are typically without feecback and based on mosfets....

The one i have in mind.....must have...

gain of 1
high damping factor
scalable
high input z (easy load)
Btj output devices

anyone with a suitable concept...???

[kristleifur]

Have you seen the JISBOS (unity gain), or the later version α20 / Alpha20 that has gain?

JISBOS - overview
The α20 Line Stage Amplifier

These have been used as buffers for tubed voltage-gain circuits, e.g. in the SOHA: The SOHA Headphone Amplifier

[StevenOH]

You can explore several ideas here:
New Class A, Super-A, Non-Switching : need a revival ?
Still another way to do SRPP power buffer is attached.

[MiiB]

JISBOS - overview...

looks really interesting....and can be scaled to high power by inserting a cascode to shield the j-fets...maybe second stage with lateral mosfets driving the BTJ's.....
But...for higher power there must be some gain...else what happens when a large signal of maybe 30V is fed to the input J-fets...??

[kristleifur]

Originally Posted by MiiB

JISBOS - overview...

looks really interesting....and can be scaled to high power by inserting a cascode to shield the j-fets...maybe second stage with lateral mosfets driving the BTJ's.....
But...for higher power there must be some gain...else what happens when a large signal of maybe 30V is fed to the input J-fets...??

I'm glad it might be useful to you! (Did you check the Alpha20? It's basically the same, except it has added gain - perhaps AMB & co. have implemented the same ideas.) At any rate, I can recommend talking a bit to AMB and Steinchen (owner of the DiamondStar JISBOS page). Very good people.

[MiiB]

It has added gain due to a resistor network in the feedback....

But as far as i see it.. for unity gain it has limited voltagesving due to the voltage capability of the input fets...but as the whole center of the fets is pulled up/down.. I may be wrong. Guess some simulation will show...

[nigelwright7557]

Originally Posted by MiiB

Hello....
anyone with a suitable concept...???

I designed a similar amplifier with an SRPP front end driving into a class AB MOSFET amplifier. The only change I made to the AB amp was to reduce its gain to counteract the gain in the valve stage.

[ontoaba]

Originally Posted by MiiB

Hello....
would like to explore the differnt topologies for output buffers that per example can be driven with a tube voltage stage...

I know of the types consisting of simple mosfet follower driven in Class a or a/b....but the types are typically without feecback and based on mosfets....

The one i have in mind.....must have...

gain of 1
high damping factor
scalable
high input z (easy load)
Btj output devices

anyone with a suitable concept...???

That must be voltage follower, I have many version, but all mosfets outputs with floating drivers. I ever trying with bjt and not success. I don't have any no feedback, but all with local and global feedback.
Why not just using multi stage emitter follower? or cascoded one?

[Arnulf]

Originally Posted by MiiB

gain of 1
high damping factor
scalable
high input z (easy load)
Btj output devices

anyone with a suitable concept...???

I posted a schematic of PP buffer here, asking for further pointers and/or alternatives. I went with a version very similar to what I posted in the end and it works perfectly. I have updated schematic somewhere. I used tube front end (6N3P) to drive the 0 dB gain output stage.

As for your requirements: first two fall under same category (add more output devices and adjust drive accordingly), third is a matter of adding another follower to the input if you dislike the existing solution and the latter is a matter of choice (I used BJT integrated darlingtons that many denizens of this forum will shun even though they work just fine). Zero GNFB in the amplifier. Some call it "Zen", others bash it

[MiiB]

Arnulf ...

Would you be so kind and repost the schematics..... can't seem to find it....

I have tried to simulate the alpha20 schematics...both with and gain and unity....It simulates very very well.. but is off course limited to the voltage sving of the Jfets.. app +-25 V....

I have tried to cascode the input with floating bjt cascodes lifted by zeners...It works but...does not simulate very well...and comes to a simulated distortion off 0.02%.. with 40 V sving and 60 V rails...