BC847BS (dual NPN) as current mirror ?

[Adam S]

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

http://www.fairchildsemi.com/ds/BC/BC847BS.pdf
http://www.nxp.com/acrobat_download/datasheets/BC847BS_2.pdf
http://www.diodes.com/datasheets/ds30222.pdf
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]

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

http://www.fairchildsemi.com/ds/BC/BC847BS.pdf
http://www.nxp.com/acrobat_download/datasheets/BC847BS_2.pdf
http://www.diodes.com/datasheets/ds30222.pdf
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%.

Why dont you just use the MBT3904DW1T1G cheaper

1000 - 2499 $0.029
500 - 999 $0.033

From newark

http://canada.newark.com/88H4687/se...-SEMICONDUCTOR-MBT3904DW1T1G&_requestid=73013

 

[Joerg]

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

http://www.fairchildsemi.com/ds/BC/BC847BS.pdf
http://www.nxp.com/acrobat_download/datasheets/BC847BS_2.pdf
http://www.diodes.com/datasheets/ds30222.pdf
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%.

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]

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

http://www.fairchildsemi.com/ds/BC/BC847BS.pdf
http://www.nxp.com/acrobat_download/datasheets/BC847BS_2.pdf
http://www.diodes.com/datasheets/ds30222.pdf
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%.

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):
http://www.diodes.com/datasheets/ds30311.pdf

Reasonably priced at DigiKey, too--15 cents @ qty 3,000.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=DMMT3904W-FDITR-ND

Cheers,
James Arthur

 

[Eeyore]

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

http://www.fairchildsemi.com/ds/BC/BC847BS.pdf
http://www.nxp.com/acrobat_download/datasheets/BC847BS_2.pdf
http://www.diodes.com/datasheets/ds30222.pdf
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)

Putting a couple in parallel can help average out the mismatches. But only
statistically of course.

Can you afford a supermatch pair ?

Graham

 

[Wimpie]

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

http://www.fairchildsemi.com/ds/BC/...22.192.61/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%.

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
www.tetech.nl
please remove abc from the address when replying directly.

 

[will]

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):http://www.diodes.com/datasheets/ds30311.pdf

Reasonably priced at DigiKey, too--15 cents @ qty 3,000.http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=DMMT...

Cheers,
James Arthur

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

 

[James Arthur]

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

Right, that's why I mentioned it.

Cheers,
James Arthur