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Old 17th June 2009, 11:09 AM   #1
forr is offline forr  France
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Default High current mirror ?

Hi,
Can current mirrors using 2, 3 or 4 transistors be made to reliably hold currents up to 100 mA ?
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Old 17th June 2009, 03:15 PM   #2
jcx is offline jcx  United States
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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
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Old 17th June 2009, 03:21 PM   #3
Elvee is offline Elvee  Belgium
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The two-transistor version risks some problems of thermal tracking, but the 3 and 4 transistors should be OK, with devices of suitable size.
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Old 17th June 2009, 09:01 PM   #4
forr is offline forr  France
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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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Old 17th June 2009, 09:04 PM   #5
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Default Re: High current mirror ?

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.
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Old 17th June 2009, 09:16 PM   #6
syn08 is offline syn08  Canada
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Default Re: Re: High current mirror ?

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.
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Old 17th June 2009, 09:19 PM   #7
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Default Re: Re: Re: High current mirror ?

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.

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Old 17th June 2009, 09:40 PM   #8
Eva is offline Eva  Spain
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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...
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Old 17th June 2009, 09:47 PM   #9
forr is offline forr  France
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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.
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Old 17th June 2009, 09:49 PM   #10
syn08 is offline syn08  Canada
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Default Re: Re: Re: Re: High current mirror ?

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.
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