### Author Topic: Current mirror fail?  (Read 256 times)

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#### Lucky-Luka

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##### Current mirror fail?
« on: September 20, 2019, 06:35:07 pm »
Hi all
I just wanted to realize a current mirror circuit.
Something went wrong: the load current keep rising...
It looks like there is something that I haven't understood of the theory behind the circuit.
Can anyone help me?
https://youtu.be/umwotfRn-ow

Cheers
Memento audere semper.

#### Zero999

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##### Re: Current mirror fail?
« Reply #1 on: September 20, 2019, 07:25:16 pm »
It would help if you posted a schematic, rather than a video.

Were the transistors matched?

They're obviously not in thermal contact with one another.

In short, this is exactly what you should expect to happen. The transistors need have a similar hFE and VBE at a given current and be at the same temperature, in order for a current mirror to work properly. Adding emitter resistors may help and glue the two transistors together. The VBE can be matched using the diode test function on a multemeter. Matchign the hFE is more tricky, but not as critical as the VBE.

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#### Lucky-Luka

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##### Re: Current mirror fail?
« Reply #2 on: September 20, 2019, 08:11:23 pm »
Hi
Q1 Vbe is 0.68V and Q2 Vbe is 0.75V.
You said "The VBE can be matched using the diode test function on a multemeter" but I haven't understood how I can actually do the matching in order to have a better current mirror circuit.
Cheers
Memento audere semper.

#### Zero999

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##### Re: Current mirror fail?
« Reply #3 on: September 20, 2019, 08:52:34 pm »
The VBE must be matched without the transistors in the circuit.

Set the multimeter to diode test. Connect the base and collector to positive lead and the emitter to the negative lead. Repeat the process with several different transistors until a matched pair is found.

The transistors will need to be at the same temperature to work, so glue them together. 15mA might be too higher current, because it will cause more heating in the transistor driving the LED, than the one with the resistor, as it will have a higher voltage across it.

Emitter degradation resistors can also help. Try putting a 470R resistor between each of the transistors' emitters and 0V.

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#### Lucky-Luka

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##### Re: Current mirror fail?
« Reply #4 on: September 22, 2019, 07:56:13 pm »
Thanks for the tips.
Glue them together? Isn't glue insulating?
Cheers
Memento audere semper.

#### Zero999

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##### Re: Current mirror fail?
« Reply #5 on: September 22, 2019, 08:16:21 pm »
Thanks for the tips.
Glue them together? Isn't glue insulating?
Cheers
Yes, compared to metal, most adhesives are thermal insulators, but they're much better thermal conductors, than air. I recommend cyanoacrylate, commonly sold as superglue, but be careful, it bonds human skin very well.

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