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-   -   Current Mirror Discussion (http://www.diyaudio.com/forums/solid-state/133018-current-mirror-discussion.html)

AKSA 10th November 2008 05:45 AM

Now the thread issue is resolved, perhaps offering up an idea would be in order.

Current Mirrors. They rely on two transistors with identical Vbes and emitter degeneration, delivering an output current on one side driven from a sensing current on the other side.

However, there's a problem. The sensing side configures the transistor as a diode, but the output side configures its device as a transistor.

These two devices therefore have different transfer functions. That is, the absence of a depletion layer on the diode connected side means that the output current will not precisely track because the Vbe's differ with current.

How to fix this?

Instead of running a simple link from collector to conjoined bases, connect the collector on the sensing side with a forward biased diode to the two bases. Bases remain joined, as before.

This way both transistors are operating with depletion layers, viz, as transistors, and the transfer functions are much closer to being identical. Thus, linearity is improved. The Widlar current mirror achieves this, but by adding another transistor. This is simpler.

Hugh

gaetan8888 10th November 2008 06:03 AM

2 Attachment(s)
Quote:

Originally posted by AKSA
Now the thread issue is resolved, perhaps offering up an idea would be in order.

Current Mirrors. They rely on two transistors with identical Vbes and emitter degeneration, delivering an output current on one side driven from a sensing current on the other side.

However, there's a problem. The sensing side configures the transistor as a diode, but the output side configures its device as a transistor.

These two devices therefore have different transfer functions. That is, the absence of a depletion layer on the diode connected side means that the output current will not precisely track because the Vbe's differ with current.

How to fix this?

Instead of running a simple link from collector to conjoined bases, connect the collector on the sensing side with a forward biased diode to the two bases. Bases remain joined, as before.

This way both transistors are operating with depletion layers, viz, as transistors, and the transfer functions are much closer to being identical. Thus, linearity is improved. The Widlar current mirror achieves this, but by adding another transistor. This is simpler.

Hugh


Hello Hugh

A 1N4148 would do or need a faster diodes ?

Thank

Bye

Gaetan

It should look like that;

AKSA 10th November 2008 06:10 AM

Yup, Gaetan,

You got it!!

4148 is fine, plenty fast enough.....

Hugh

ostripper 10th November 2008 06:13 AM

Ahh, a subtle truth is gained.:)

?? A question , what diode , general purpose(1n4001??)
and, would one HEAR a difference.

I guess I'll be "floatin" my current mirror bases to add a diode
in a few days. Thanks hugh.

AKSA 10th November 2008 06:14 AM

Yes Ostripper,

There is a difference sonically. Far superior decay, which I take to mean more resolution. Translates to slightly improved imaging, too.

Hugh

PMA 10th November 2008 06:16 AM

1N4001 is not a candidate (slow, different V-A). 1N4148 is a "general purpose" as well.

lumanauw 10th November 2008 01:29 PM

Hi, AKSA,

I once tought about this about current mirror. The sensing side has almost constant Vce=0V6. But the mirror side has bigger Vce. They both will have different Early effect voltage. How far this will affect the perfectness of a current mirror?

GK 10th November 2008 01:37 PM

2 Attachment(s)
Quote:

Originally posted by ostripper
Ahh, a subtle truth is gained.:)

Hardly.

That diode only has potential to make the current mirror linearity worse – in fact I just proved my hunch in LTspice – at 20kHz the LTP non-linearity of a “Blameless” increased significantly (~50%).

It is sub optimal to add a diode in the signal path of a linear circuit like this such that it is forward biased with a current as low as the base currents of the current-mirror transistors.

If you want to better the current mirror by any significant amount then your options are the 4 transistor Widlar or the 3-transistor version attached below. Note the 470-ohm resistor? This ensures high frequency linearity by developing an adequate quiescent emitter current for Q16.

And no, this 470-ohm resistor is not a potential fix for the diode fudge.

Cheers,
Glen

ostripper 10th November 2008 07:15 PM

Quote:

By G. kleinschmidt : That diode only has potential to make the current mirror linearity worse – in fact I just proved my hunch in LTspice – at 20kHz the LTP non-linearity of a “Blameless” increased significantly (~50%).
Confirmed... But the "truth" I was mainly interested in was
the psychoacoustical one. (how will it sound??)

I guess the only way to see would be to actually add
the diode (or the widlar circuit) to the real thing, crank it
up , and let the ears have at it.
So, it comes down to linearity vs. acoustics.


Would it not be nice to be able to listen to the output
of LT spice :cool: Lt spice w/ headphone jack....

Quote:

By balaboo : ego massaging
Give me a break.. a compliment will get you much more than an
attack. Like maybe a friend.

OS

AKSA 10th November 2008 09:02 PM

Kleinschmidt,

I expected your usual whining comment. You have PSpiced it, not built it, never heard it..... get on with it man, do the work, and learn to trust your ears.

This gem is found in a 1999 Halcro US patent, so I would suggest you refer your august arguments with Bruce Candy. After all, his amps are reputedly amongst the best in the world, and they boast extremely low distortion, which would be mandatory for you. Good enough for him and the world, it seems. Where do you stand? Where is your commercial amplifier? How many patents in this area do you hold?

One can only speculate on the state of mind of someone who appears to know it all, and is so certain of everything he does and says. Upbringing, genetics, who knows. The contempt you radiate is self-damaging, and a complete waste of energy, particularly for one who in his own mind at least, is very clever. Time you behaved with a dignity befitting your intellect......


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