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I'm working on a design which will use a BJT to switch 2 PMOS gates low to power 2 separate loads. I am going to drive the transistor from a NOR gate.

From my transistors datasheet, I thought I could extrapolate the current gain graphs with the assumption that they are worst case. In my situation:

Ib ~= (5 - 0.7)/1000 = 4.3 mA (5 V from NOR gate, 0.7 Vbe drop, 1k resistor)

Ic ~= 12 / 10000 = 1.2 mA

DC Gain from Datasheet

However after probing around a couple copies of my PCB, I'm seeing very different results on each board... Things like the voltage at the base ranging from 0.6 - 4 V, etc. The voltage at the collector also sometimes work like I expect (~0 V) and sometimes is the full 12 V as if the BJT is completely off.

Edit (2016-06-28) here are the different Vbe and Vce values for the BJT:

Table of transistor values

So it definitely seems like there is either a manufacturing issue or blown transistor. I'll keep investigating.

End edit

Can we not extrapolate the graphs? Any ideas would be appreciated. Alternatively, I wonder if it was a PCB manufacturing / assembly issue.

Circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

NOR Gate: http://www.ti.com/lit/ds/symlink/sn74lvc1g27.pdf

BJT: http://www.mccsemi.com/up_pdf/MMBT2222A(SOT-23).pdf

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    \$\begingroup\$ If Vbe=4V there's either a broken connection (soldering error?) or a blown transistor. Either way, 12V at the collector is likely. \$\endgroup\$
    – user16324
    Jun 28, 2016 at 16:33
  • \$\begingroup\$ Thanks Brian, will do some more continuity testing and diode testing on the BJT to see. \$\endgroup\$
    – Jim
    Jun 28, 2016 at 16:40

1 Answer 1

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Transistor parameters vary widely with temperature. That can cause a wide variation in the dc current gain between Ic and Ib.

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    \$\begingroup\$ I don't think this helps much with answering the question. Can you provide examples of nominal variation to improve your answer? \$\endgroup\$ Jun 28, 2016 at 20:19
  • \$\begingroup\$ That dc current gain or beta varies widely not only with temperature and also with every device during manufacture. Check the 2N3904 datasheet. There is such a curve there. Beta can increase by 50% from 25 deg C to 125 deg C. Just a suggestion to check the temperature though. \$\endgroup\$ Jun 29, 2016 at 9:49

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