I am trying to replace several transistors on my PCBs due to chip shortages. One such unit is a particular NPN transistor datasheet I cannot get my hands on these little guys anymore. But I figured this shouldn't be too hard to replace, but first I need to understand a few things about transistors and sizing the resistors around them before I select a candidate. This is long overdue for me because I have been using them with little understanding.
I am not using the unit for an amplifier but just as a switch. Above is an example circuit. Let's say there is 30mA going to the collector and the GPIO is at 3.3V. I look at the datasheet for this particular transistor and see that based on the collector emitter saturation voltage Ic/Ib = 10 which is beta. If my collector current will be 30mA then I need 3mA on the base? This means if I have a 3.3V pin on the base then I need a 3.3V/0.003A = 1.1K resistor but it would probably be a good idea to lower resistance slightly to ensure proper current. I am not sure if this is the correct procedure.
Furthermore how do I know if a 3.3V GPIO can be on the base? I imagine there are some transistors that require a greater voltage on the base but how do I know? Is there a max and min voltage with respect to the collector? This NPN transistor is on the ground side of the circuit but I imagine I couldn't connect it to the ground side of a 1000V circuit, but I have no real justification for thinking this.
So this brings me to the root of the issue which is I am looking at this potential replacement transistor datasheet2 It appears to have a similar beta, but its reference saturation conditions are much higher although it is still Ic/Ib = 10. And I don't understand how to fully evaluate whether or not this is a replacement candidate for the example circuit.