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Old 5th June 2013, 11:39 AM   #3361
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IIRC there is not much benefit from more than 5% Ic matching of the LTP, after that the 3rd harmonics dominate and don't cancel. In most of the designs I see here the LTP is very lightly loaded to minimize distortion from the VAS+OPS, so I wonder if its own contribution to THD is significant.
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Old 5th June 2013, 12:21 PM   #3362
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Originally Posted by Bob Cordell View Post
Trim... the current mirror resistors is indeed a much better approach
Why?

Best wishes
David
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Old 5th June 2013, 02:27 PM   #3363
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there's the classic http://users.ece.gatech.edu/~lanterm...ors/AN-222.pdf

somewhere I think I've seen a suggestion for audio to trim the diff pair current balance/degen R? for minimum distortion - then inject external offset adj if needed
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Old 5th June 2013, 05:49 PM   #3364
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Originally Posted by Rafael L View Post
I used your tip, output transistors and drives the same heatsink, Predrives are in board with its own VBE multiplier.
Without compensation VBE in Predrives error is 20% for 10C, with compensation error is of only 5% for 10C which is reasonable for a simple circuit VBE multiplier.

Circuit VBE multiplier and simulation are attached, Thanks for the help.



In his book has several circuits VBE multiplier, but from what I've seen, none of them you write for use with 2 VBE sensor.
How did you get the 2N5551 thermally contacted to the pre-driver? I would use TO126 Vbe multiplier and TO126 predriver mounted together.

I'll have to sdd the 2 VBE sensor variation to the book.

Cheers,
Bob
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Old 5th June 2013, 10:23 PM   #3365
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Originally Posted by Dave Zan View Post
Why?

Best wishes
David
Hi David,

If the trimmer is done in an optimal way, where only the smallest current necessary is passed through the pot wiper to give the trimmer adequate authority, then I guess the only advantage might be that the pot used on the current mirror emitter resistor circuit has very little signal on its pins - the whole pot is largely at an AC ground. When used in the tail of an LTP, the entire pot has the common mode signal on it, which is essentially the input signal in a conventional amplifier design.

I would avoid having to use such a trimmer in either location.

Cheers,
Bob
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Old 5th June 2013, 10:26 PM   #3366
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A good amplifier should have no trimmers in it except for the output stage bias trimmer.
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Old 5th June 2013, 10:28 PM   #3367
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Originally Posted by Bob Cordell View Post
How did you get the 2N5551 thermally contacted to the pre-driver? I would use TO126 Vbe multiplier and TO126 predriver mounted together.
I use TO-92 in Predriver (thermal coupling TO-92/TO-92), also do this in the current mirror.

The only ones TO-126 that I would use would 2SA1381/KSA1381, but they are not available to me here. My second option would be to use MJE243/253, but the datasheet reports a difference in gain +- 120% NPN for PNP, in my case, I found best TO-92.

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There is no real problem with mounting the predrivers, drivers and output transistors all on the main heat sink, but it may get a bit crowded. I think that a key objective is that the predrivers and drivers be isothermal with the portion of the bias spreader that is for them.
Predrivers and drives or only drives even heatsink in the output transistors, if you use ThermalTrak, diode sensor compensate at the same temperature as the internal transistor, Predrivers/drives or drives, give error in compensation due the different temperature mounted even heatsink output TRs.
Now understand why chose to ride on separate heatsink for Predrivers and drives.
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Old 6th June 2013, 12:18 AM   #3368
wahab is offline wahab  Algeria
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Originally Posted by michaelkiwanuka View Post
In the two-transistor current source that you call a "negative feedback current source", the series derived feedback signal is amplified by the sensing transistor before being shunt applied to the current source transistor's base.

This amplification of the derived feedback signal greatly increases the output impedance of the current source, so much so that there is no need to use a cascode current source in its place.
Yes but current will be highly temp dependent compared
to the zener referenced current source.
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Old 6th June 2013, 01:04 AM   #3369
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Default MOSFET models

Bob, in section 11.9 of your book, you recommend the EKV models when simulating THD in MOSFET power amps.

Are your EKV models publicly available?

I note the models in Cordell-models.txt for

* 2SK1530 2SJ201
* IRFP240 IRFP9240
* 2SK134 2SJ49
* 2SK1056 2SJ162

are marked VDMOS
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Old 6th June 2013, 01:47 AM   #3370
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Originally Posted by Rafael L View Post
I use TO-92 in Predriver (thermal coupling TO-92/TO-92), also do this in the current mirror.

The only ones TO-126 that I would use would 2SA1381/KSA1381, but they are not available to me here. My second option would be to use MJE243/253, but the datasheet reports a difference in gain +- 120% NPN for PNP, in my case, I found best TO-92.



Predrivers and drives or only drives even heatsink in the output transistors, if you use ThermalTrak, diode sensor compensate at the same temperature as the internal transistor, Predrivers/drives or drives, give error in compensation due the different temperature mounted even heatsink output TRs.
Now understand why chose to ride on separate heatsink for Predrivers and drives.
Hi Rafael,

TO-92 devices are OK for VAS and predriver in smaller amplifiers, but in general it is better to use good TO-126 devices for these locations. The 1381/3503 are great devices, I agree.

Cheers,
Bob
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