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BC847BS (dual NPN) as current mirror ?

According to data sheets, these things are not matched pairs. Its a shame because they are cheap and multi sourced.

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http://61.222.192.61/mccsemi/up_pdf/BC847BS(SOT-363).PDF

I'm wondering what risk I'm taking using them as current mirrors in production. I measured Vbe offset of a few BC847BS's on hand (manufactured by NXP)

(mV)

0.58 0.74 0.36 0.30 0.40 0.61 0.32

Test circuit: Ic_q1 = Ic_q2 = 1.00mA

+10.6V *------------. | | 10.018k 10.035k | | *--. *--. | | | | c | c | b-' (Q1) b-' (Q2) e e | | *------------' | GND

Seems quite reasonable considering the data sheet specifies a maximum of

120mV Vbe offset voltage between the transistors. This sample had a Ic_Q1 / Ic_Q2 match better than 3%.
Reply to
Adam S
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Why dont you just use the MBT3904DW1T1G cheaper

1000 - 2499 $0.029 500 - 999 $0.033

From newark

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Reply to
Hammy

120mV ??? Surely you jest! 120mV implies 100:1 current range for the same Vbe!

5mV is 21% mismatch.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

This would be the perfect case for contacting the app engineers there. Maybe they can furnish some data but you'd have to get it in writing. I had a similar case with diode matching and that mfg sent me stacks of final testing data. This allowed me to calculated our theoretical yield which calculated to better than 99.9% but ended up being 100%.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
Reply to
Joerg

The NXP datasheet says: "Closely matched current gain." To me, that implies "made from chips closely located on the same wafer."

None of the other datasheets make that claim.

That situation *is* guaranteed here though (DMMT3904W):

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Reasonably priced at DigiKey, too--15 cents @ qty 3,000.

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?name=DMMT3904W-FDITR-ND

Cheers, James Arthur

Reply to
James Arthur

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Hello Adam,

Is it possible to include a test during production? If yes, you certainly might consider BC847BS If not and mismatch may lead to an unreliable circuit, you may consider BCM847 (NXP and Infineon). They are more expensive, but matched (delta Vbe < 2mV, guaranteed).

I had the same discussion with a client. Testing the current source in combination with BC847BS or only ICT with BCM847? For them using BCM847 was commercially the best option.

Note that some difference in dissipation may lead to more then 2mV additional Vbe.

Best regards,

Wim PA3DJS

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please remove abc from the address when replying directly.

Reply to
Wimpie

f

TRANSISTORS ON THE SAME CHIP WILL BE MATCHED 99 % OF THE TIME I MEASURED MANY AND THIS IS TRUE WILL

Reply to
will

(DMMT3904W):

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3,000.http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=DMMT...

Right, that's why I mentioned it.

Cheers, James Arthur

Reply to
James Arthur

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