BC847BS (dual NPN) as current mirror ?

Question

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%.

Hammy · Accepted Answer

Why dont you just use the  MBT3904DW1T1G cheaper1000 - 2499 	$0.029500 - 999 	$0.033From newark

Jim Thompson · Answer

120mV ???  Surely you jest!  120mV implies 100:1 current range for thesame Vbe!5mV is 21% mismatch.                                        ...Jim Thompson-- | James E.Thompson, P.E.                           |    mens     || Analog Innovations, Inc.                         |     et      || Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    || Phoenix, Arizona  85048    Skype: Contacts Only  |             || Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  || E-mail Icon at  |    1962     |                      America: Land of the Free, Because of the Brave

Joerg · Answer

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%.

James Arthur · Answer

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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Cheers, James Arthur

Wimpie · Answer

Hello Adam,Is it possible to include a test during production?If yes, you certainly might consider BC847BSIf not and mismatch may lead to an unreliable circuit, you mayconsider BCM847 (NXP and Infineon). They are more expensive, butmatched (delta Vbe < 2mV, guaranteed).I had the same discussion with a client. Testing the current source incombination with BC847BS or only ICT with BCM847? For them usingBCM847 was commercially the best option.Note that some difference in dissipation may lead to more then 2mVadditional Vbe.Best regards,WimPA3DJSplease remove abc from the address when replying directly.

will · Answer

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

James Arthur · Answer

(DMMT3904W):?name=DMMT...Right, that's why I mentioned it.Cheers,James Arthur

BC847BS (dual NPN) as current mirror ?

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