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Old 7th May 2009, 12:39 AM   #21
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Thank you Peter;
though they appear to be large, I don't think they will be happy on 1A current. I have in mind a MOSFET current mirror with relatively high ratio (cascoded, actually): I am going to finish my Festival powered speakers I postponed to design as desired the last year (hybrid, class A+C+C).

I will rather use some Toshiba devices bolted together...
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Old 7th May 2009, 01:47 PM   #22
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Does it need to be a "Mirror", where one current is slave to
the other? Or just two current sources that exactly match?

If only matching is the concern: Alephs share some of the
same benefits as cascodes. Collectors move in response
to error correction as needed. But the control transistors
never directly see what swings at the drains. Early effect
is present, but has no influence over regulated currents.
Collectors are not in the path of the regulated current.

Never any need for matched bipolars to pull a full Ampre
in the circuit described above. Though MOSFETs involved
may easily exceed that figure.

On the other hand: If you need a mirror to copy or multiply
an arbitrary variable current to the other channel, I havn't
figured how one could do that with Alephs yet.
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Old 7th May 2009, 02:14 PM   #23
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by kenpeter
Does it need to be a "Mirror", where one current is slave to
the other? Or just two current sources that exactly match?
That was the gist of my question a few posts back. We are talking about tubes. I assumed that the goal was to balance DC in the output transformer. The original post suggested use of a single thermaltrak device, and resistors. If that works well, this device is robust enough and the application appears uncomplicated. Then discussion started on various ways to improve it, until we are talking about more complex systems that don't involve the original idea.

So what is the goal here? What problem are we trying to solve. I feel like I'm missing an important point somewhere (wouldn't be the first time).

Sheldon

edit: How closely matched to they have to be? I can't imagine it's that difficult to get well within a percent or so.
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Old 7th May 2009, 04:31 PM   #24
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Quote:
Originally posted by kenpeter
Does it need to be a "Mirror", where one current is slave to
the other? Or just two current sources that exactly match?
It needs to be a mirror with current ratio about 10 and high output dynamic resistance.
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Old 7th May 2009, 04:50 PM   #25
Jeb-D. is offline Jeb-D.  United States
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Quote:
So what is the goal here? What problem are we trying to solve. I feel like I'm missing an important point somewhere
You were right. DC current matching.

Quote:
How closely matched to they have to be? I can't imagine it's that difficult to get well within a percent or so.
I don't think it has to be extremely precise. Figure that the OPT is not perfectly symmetric and the current mirror is only handling DC. This is why I prefer to keep it simple.
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Old 7th May 2009, 06:37 PM   #26
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Jeb, don't forget that the diode and the transistor of the Sanken thermal tracks only track if the diode current is 2.5mA and the transistor current is 40mA....

Jan Diddeen
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Old 7th May 2009, 07:00 PM   #27
Jeb-D. is offline Jeb-D.  United States
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Thanks Jan.
The part I'm trying is ON-Semi NJL3281D. The data sheet didn't really have any info regarding the diode section. I'll just breadboard it up when the samples arrive. If it doesn't work out, I'll just use the parts for a different project.
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Old 8th May 2009, 02:35 PM   #28
JoshK is offline JoshK  Canada
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I really like this idea. Please let us know how it turns out.
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Old 8th May 2009, 04:42 PM   #29
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Quote:
Originally posted by Jeb-D.
Thanks Jan.
The part I'm trying is ON-Semi NJL3281D. The data sheet didn't really have any info regarding the diode section. I'll just breadboard it up when the samples arrive. If it doesn't work out, I'll just use the parts for a different project.
OK. The On-semi's don't have a specific spec of at what current they track IIRC so you need to find that out yourself. I'd be interested in your results. Keep us posted!

Jan Didden
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Old 8th May 2009, 04:57 PM   #30
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Speaking of current sources in cathodes, do they really need to track each other?
What if to make them as current mirrors that reflect the current set by a trimpot from a voltage source?

By the way, does anybody know of transistor pairs thermally tracking each other, in which one of transistors can work on 1A before the beta droop region?

I need much more precision current mirroring that diode+transistor pairs allow.
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