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Old 14th November 2012, 11:59 AM   #121
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Quote:
Originally Posted by lineup View Post
Great jkuetemann.

If you go back to the beginning of this thread,
you see I use simple pair of IRFP240/IRF9240 for output.
The numbers were very good.

This circuit now shown to work good with following outputs
- pure MOSFET pair
- Lazy Cat BIGBT
- Triple darlington BJT

I must say it is a nice Power Buffer.
EVerything points this way.

Next I will go back to the MOSFET output, once again
The somewhat higher input impedance of the MOSFET Gates
is good for the results.
There is also higher voltage across the input transistors.
Like 4+4 Volt.
I spent some time playing with the lower CCS currents to get the circuit into a balanced condition (BTW, the input pair balance is very touchy and important for performance) and got some nice numbers. I'm wondering about some kind of current mirror with one leg reflected to the opposing rail for the front end to keep it well balanced. The diamond pair seems to be more important with respect to output offset and as such I think is best left adjustable. Currents were ~100mA through the BIGBT BJTs and 50mA through the MOSFET driver.
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File Type: jpg Lineup_PowerFollower_BIGBT.jpg (129.0 KB, 251 views)
File Type: jpg Lineup_PowerFollower_BIGBT_FFT.jpg (295.9 KB, 239 views)
File Type: png Parallels Picture.png (45.3 KB, 212 views)
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Old 14th November 2012, 12:03 PM   #122
lineup is offline lineup  Sweden
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This is a Combined Voltage Amp Opamp
and
The Power Buffer.

The OPamp has got voltage feedback and TMC error correction.
It is rather precise.

The LU Power Buffer is one version with Triple EF.
The buffer and voltage amplifier work separately.
There is no global feedback.

These are the simulated THD figures
Quote:
Combined VoltageAmpSC2240_1c PowerBuffer_6p
Power Supply is +/-42 Volts

THD 1kHz
04 Vp, 01 Watt THD 0.00016%
08 Vp, 04 Watt THD 0.00024%
12 Vp, 09 Watt THD 0.00034%
16 Vp, 16 Watt THD 0.00044%
20 Vp, 25 Watt THD 0.00054%
24 Vp, 36 Watt THD 0.00067%
28 Vp, 49 Watt THD 0.00080%
32 Vp, 64 Watt THD 0.00095%
36 Vp, 81 Watt THD 0.00115%
40 Vp,100 Watt THD 0.89761%

THD 20kHz
04 Vp, 01 Watt THD 0.00227%
08 Vp, 04 Watt THD 0.00219%
12 Vp, 09 Watt THD 0.00223%
16 Vp, 16 Watt THD 0.00212%
20 Vp, 25 Watt THD 0.00213%
24 Vp, 36 Watt THD 0.00260%
28 Vp, 49 Watt THD 0.00381%
32 Vp, 64 Watt THD 0.00593%
36 Vp, 81 Watt THD 0.00919%
Those a good numbers, but no doubt can be made better with other versions of the LU Power Buffer.

We now call this buffer: LU Power Buffer
where LU stands for Lineup
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File Type: jpg powerfollower25-Cmb1.jpg (141.4 KB, 229 views)
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Old 14th November 2012, 12:17 PM   #123
lineup is offline lineup  Sweden
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Quote:
Originally Posted by jkuetemann View Post
I spent some time playing with the lower CCS currents to get the circuit into a balanced condition (BTW, the input pair balance is very touchy and important for performance) and got some nice numbers. I'm wondering about some kind of current mirror with one leg reflected to the opposing rail for the front end to keep it well balanced. The diamond pair seems to be more important with respect to output offset and as such I think is best left adjustable. Currents were ~100mA through the BIGBT BJTs and 50mA through the MOSFET driver.
The mirror current source is a good idea.

The DC Offset should be solved some way.
If people see an Output cap,
they shake their head and go elssewhere.
Myself is not at all afraid of using Output capacitor. Say 10.000uF

When we use NPN + PNP MOSFET output pair
we can get offset like several 0.1 Volt.
Because one MOSFET can have 4.3 and another 4.5 VGS.

One idea is the let the frontend 10+10 Ohm be one trimpotentiometer.
A precision trimpotentiometer of 20 Ohm
and take the input through the wiper somewhere in middle.
This calls for having maybe 0.3+0.3 Volt across the input trimpot.
If this voltage is too low, then the adjustment may not be enough.
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Old 14th November 2012, 12:25 PM   #124
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Those figures are dissapointing given the low THD from the buffer. Now try a straight forward Blameless amp and use TMC with it. It will result in much better figures.
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Old 14th November 2012, 12:33 PM   #125
MiiB is online now MiiB  Denmark
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What about a Lateral MOSFET output... Could be be good also...
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Old 14th November 2012, 01:36 PM   #126
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Originally Posted by homemodder View Post
Those figures are dissapointing given the low THD from the buffer. Now try a straight forward Blameless amp and use TMC with it. It will result in much better figures.
Yes, I think the bottleneck is the driving circuit. About the buffer itself, it is a buffer, with normally low distortion. Now the question is, is the extra low distortion worth the extra complex buffer circuit. I mean, many other simpler buffers are low distortion, even if they are not super low.
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Old 14th November 2012, 01:51 PM   #127
lineup is offline lineup  Sweden
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Try to setup a diamond normal power buffer,
and you see what distortion figures you get.
That is what I call disappointing


I agree with homemodder regarding a Blameless with negative feedback and TMC.
But then you have global negative feedback over your ears.

This buffer approach try to get away from GNFB.
And so, like Struth understands we get 2 distinct parts with only local feedback.
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Old 14th November 2012, 01:55 PM   #128
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Originally Posted by MiiB View Post
What about a Lateral MOSFET output... Could be be good also...
Good idea MiiB.
I will try this when I get time.

This might be the best approach. We all know what Laterals can do.

I have the SPICE models of 2SK1058 and 2S162
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Old 14th November 2012, 02:11 PM   #129
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Originally Posted by lineup View Post
I will try this when I get time.
This might be the best approach. We all know what Laterals can do.
I have the SPICE models of 2SK1058 and 2S162
Cool, we might put some circuits from the discrete opamp open design thread to drive the buffer. I myself have some opamp projects waiting for this kind of job.
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Old 14th November 2012, 04:01 PM   #130
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Quote:
Originally Posted by lineup View Post
Try to setup a diamond normal power buffer,
and you see what distortion figures you get.
That is what I call disappointing


I agree with homemodder regarding a Blameless with negative feedback and TMC.
But then you have global negative feedback over your ears.

This buffer approach try to get away from GNFB.
And so, like Struth understands we get 2 distinct parts with only local feedback.

Whats wrong with GNFB ??

How can you define as being local feedback. Here you have two GNFB loops, Im not sure which is worse.
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