High current mirror ?

forr · 17th June 2009, 10:09 AM

Hi,
Can current mirrors using 2, 3 or 4 transistors be made to reliably hold currents up to 100 mA ?

jcx · 17th June 2009, 02:15 PM

degeneration provides feedback that increases ratio control in the face of device variations - the higher the Vdrop across the emitter Rs the more feedback correction you get

you can also inrease device gain with Sziklai/CFP - the cost is the voltage burden of ~1V

Elvee · 17th June 2009, 02:21 PM

The two-transistor version risks some problems of thermal tracking, but the 3 and 4 transistors should be OK, with devices of suitable size.

forr · 17th June 2009, 08:01 PM

Hi,
Thanks for your replies.
A Sziklai pair has some appeal.
Do you think of of the input of the CM with a single transistor connected as a diode, as usual, mirrored by the controling transistor of the Sziklai pair, which then can provide current gain as you said ?
I feel silly to have never thought of such a scheme but I am not aware of anybody having ever used it. I do not see any reason why it should not work (I can't imagine how a Sziklai can be used as the input of a CM).
My initial question comes from the fact I have not seen CM's used with more than 10 mA per branch.

Wavebourn · 17th June 2009, 08:04 PM

Quote:

Originally posted by forr
Hi,
Can current mirrors using 2, 3 or 4 transistors be made to reliably hold currents up to 100 mA ?

Yes. I use 11 of Toshiba2SC1815 transistors between 2 copper plates for such a purpose.

syn08 · 17th June 2009, 08:16 PM

Quote:

Originally posted by Wavebourn

Yes. I use 11 of Toshiba2SC1815 transistors between 2 copper plates for such a purpose.

YMMV, but the output impedance will be anywhere but good. Ro=VA/Ic where VA is the Early voltage. Ro is the same, disregarding the number of 2SC1815 you are paralleling.

Wavebourn · 17th June 2009, 08:19 PM

Quote:

Originally posted by syn08

YMMV, but the output impedance will be anywhere but good. Ro=VA/Ic where VA is the Early voltage. Ro is the same, disregarding the number of 2SC1815 you are paralleling.

I'm concerned about accuracy, especially with thermal effects in mind. I can't reveal all details here, JC is reading.

Eva · 17th June 2009, 08:40 PM

How about a servo loop with two sense resistors, an op-amp and a buffer transistor? (probably MOSFET to avoid base current).

Times have changed...

forr · 17th June 2009, 08:47 PM

Hi Eva,

Quote:

Originally posted by Eva
How about a servo loop with two sense resistors, an op-amp and a buffer transistor? (probably MOSFET to avoid base current).

Times have changed...

I have thought of that before posting the question, buf it probably won't be as fast as a simple scheme and may introduce stability problems for what I have in mind.

syn08 · 17th June 2009, 08:49 PM

Quote:

Originally posted by Wavebourn
I can't reveal all details here, JC is reading.

Do you really think JC and perhaps other guys in the industry are ripping off design ideas from this forum? I don't think so, but if God forbid this proves to be right, and if it would be worth of ( unlikely

) I may decide to legally pull some eyes. Been there, done that. BTW, under the US law patenting is not really required to protect an idea. You can make a case in court if you keep track of your stuff.

17th June 2009, 10:09 AM	#1
forr diyAudio Member Join Date: Dec 2004 Location: Next door	High current mirror ? Hi, Can current mirrors using 2, 3 or 4 transistors be made to reliably hold currents up to 100 mA ?

17th June 2009, 08:40 PM	#8
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	How about a servo loop with two sense resistors, an op-amp and a buffer transistor? (probably MOSFET to avoid base current). Times have changed... __________________ I use to feel like the small child in The Emperor's New Clothes tale

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17th June 2009, 02:15 PM	#2
jcx diyAudio Member Join Date: Feb 2003 Location: ..	degeneration provides feedback that increases ratio control in the face of device variations - the higher the Vdrop across the emitter Rs the more feedback correction you get you can also inrease device gain with Sziklai/CFP - the cost is the voltage burden of ~1V

17th June 2009, 02:21 PM	#3
Elvee diyAudio Member Join Date: Sep 2006	The two-transistor version risks some problems of thermal tracking, but the 3 and 4 transistors should be OK, with devices of suitable size.

17th June 2009, 08:01 PM	#4
forr diyAudio Member Join Date: Dec 2004 Location: Next door	Hi, Thanks for your replies. A Sziklai pair has some appeal. Do you think of of the input of the CM with a single transistor connected as a diode, as usual, mirrored by the controling transistor of the Sziklai pair, which then can provide current gain as you said ? I feel silly to have never thought of such a scheme but I am not aware of anybody having ever used it. I do not see any reason why it should not work (I can't imagine how a Sziklai can be used as the input of a CM). My initial question comes from the fact I have not seen CM's used with more than 10 mA per branch.

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