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
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:
So it definitely seems like there is either a manufacturing issue or blown transistor. I'll keep investigating.
Can we not extrapolate the graphs? Any ideas would be appreciated. Alternatively, I wonder if it was a PCB manufacturing / assembly issue.