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lineup 3rd November 2012 10:20 PM

New Lineup IDEA - Power Follower/Output stage
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This is an idea I have had in my head for a long long time. Now I have finally got hold of it and made some excersises in my SPICE. It turns out that my idea actually works.

As you can see from the simplified diagram below there are 2 MOSFETs arranged in push-pull output. Betweeen the gates are 4 transistors in diamond style giving feedback from the output to the input. We can say the circuit tries to keep output at same voltage level as the input.

BJTs can be used for output devices. But then you better use darlington EF pair. Otherwise the input impedance is very low.

I will post some working circuits. Just want to make them better first. But why dont you try my idea in SPICE. There is room for improvement when there are more than one head in use.

Bonsai 4th November 2012 01:03 AM

Nice concept.

You need to add some simple bias adjustment for the output pair.

I'm also thinking if you turned the diamond buffer types around, you may be able to figure in some error correction. Just a thought ;-)

thinkhlin 4th November 2012 02:08 AM

Hi, Lineup! I have a similar circuit.
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AKSA 4th November 2012 03:31 AM


Nice idea, but matter this is not a diamond buffer since signal is passed from npn to nmos, and pnp to pmos. Reverse is usual, of course.

Your FetZilla ideas were really good, and I think you for your thought, a lot of people here built that amp and really liked it!

However, Bonsai has a great suggestions......



lineup 4th November 2012 09:17 AM

AKSA I did not say this is a diamondbuffer, only that those four smaller transistors in the feedback is a diamond buffer.
Thanks for the nice reminder of Fetzilla. I have almost forgotten it :)

lineup 4th November 2012 09:22 AM

Thanks thinklin, I see a diamond buffer alright. But my is not quite the same.

lineup 4th November 2012 09:41 AM

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Originally Posted by Bonsai (Post 3227228)
Nice concept.

You need to add some simple bias adjustment for the output pair.

I'm also thinking if you turned the diamond buffer types around, you may be able to figure in some error correction. Just a thought ;-)

Bonsai. First I tried the circuit with Class A current in output and instead of making bias adjust I simply throw a big output cap. 4700uF.
This solves any bias point.

As I mention in topic "Power Buffer/Output stage" this buffer can very suitable be inside the feedback loop and act as a buffer within. In fact this is how I use it. Now in Class AB.

See my diagram.
It has also got a TMC error correction. It is the first time I got TMC to work. I finetune TMC with trial and error. Changing those RC components.
Very low distortion. It is a precision discrete Opamp that controls it all.
40Vp-p THD 0.00004%
That is 25 Watt output into 8 Ohm.
See the numbers in the second image.

lineup 4th November 2012 11:17 AM

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I repost my circuit.
The previous was not fully stable.
This one is.
With 0.00004% THD

lineup 4th November 2012 02:08 PM

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Here is another application where the circuit is used. It acts as the output stage. OPA134 is used for the frontend and receiver of feedback.

It can be of interest to see the INPUT voltage versus the OUTPUT voltage from the buffer.
INPUT = 32.01 Vp-p
OUTPUT 32.01 Vp-p
Which tells us that what is on the input comes out. This is a very precise output buffer!

Fast Eddie D 4th November 2012 02:20 PM


Originally Posted by lineup (Post 3227726)
This is a very precise output buffer!

Kudos. I like this implementation with the local feedback in the output stage.

Is this buffer linear enough to operate out of the global feedback loop? It does have local feedback. Because if you build it as a two stage amplifier, you will have less distortion fed back to the op amp. Do you think this is worth a try?

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