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Old 3rd November 2012, 11:20 PM   #1
lineup is offline lineup  Sweden
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Default New Lineup IDEA - Power Follower/Output stage

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.
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Last edited by lineup; 3rd November 2012 at 11:22 PM.
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Old 4th November 2012, 02:03 AM   #2
Bonsai is offline Bonsai  Taiwan
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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 ;-)
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Old 4th November 2012, 03:08 AM   #3
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Hi, Lineup! I have a similar circuit.
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Old 4th November 2012, 04:31 AM   #4
AKSA is offline AKSA  Australia
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Lineup,

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......

Cheers,

Hugh
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Old 4th November 2012, 10:17 AM   #5
lineup is offline lineup  Sweden
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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
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Old 4th November 2012, 10:22 AM   #6
lineup is offline lineup  Sweden
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Thanks thinklin, I see a diamond buffer alright. But my is not quite the same.
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Old 4th November 2012, 10:41 AM   #7
lineup is offline lineup  Sweden
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Quote:
Originally Posted by Bonsai View Post
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.
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File Type: jpg powerfollower3.jpg (108.4 KB, 1390 views)
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Old 4th November 2012, 12:17 PM   #8
lineup is offline lineup  Sweden
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I repost my circuit.
The previous was not fully stable.
This one is.
With 0.00004% THD
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Old 4th November 2012, 03:08 PM   #9
lineup is offline lineup  Sweden
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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!
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Old 4th November 2012, 03:20 PM   #10
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Quote:
Originally Posted by lineup View Post
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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