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/sci/ - Science & Math

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15035041 No.15035041 [Reply] [Original]

Can somebody explain to electric brainlet why a current mirror even works?

Say those are three discrete transistors of the same type. Now let the BE voltage of Q1 be exactly 0.7V while the BE of Q2 be 0..698V due to manufactural differences. Shouldn't the the current in the base node split like 1:4 in favour of the transistor with the slightly lower BE voltage? And assuming both transistors having the same gain the collector current have the same ratio?

>> No.15035062

>>15035041
>Now let the BE voltage of Q1 be exactly 0.7V while the BE of Q2 be 0..698V due to manufactural differences. Shouldn't the the current in the base node split like 1:4 in favour of the transistor with the slightly lower BE voltage?
yes, that's why you match them. and it's not quite as bad as 1:4 at 2mV difference, korg and moog specifications say 1mV is good enough

>> No.15035106
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15035106

but even if it's not 1:4, let it be 1:2 or 1:1.5. and still with a higher current the BE diode in the transistor with the lower voltage will heat up more and faster and thus the resistance will decrease more which will increase the current event more etc.

and with discrete elements it's basically impossible to balance their temperature on the same value with only passive measurements.

And all this applies even more the higher the current mirrored is.

>> No.15035173

>>15035106
that's why you thermally couple them
>with discrete elements it's basically impossible
dumbass

>> No.15035232

okay, at least I now know for sure that you have no idea what you're talking about

>> No.15036644

Aren't current mirror transistors usually placed into one IC?