NPN transistor replacement verification

Question

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.

Answer 1

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.

The base-emitter junction will drop about 0.9 volts when driving a current of around 3 mA into the base as per this: -

Hence, to calculate the base series resistor, you need to subtract 0.9 volts from the GPIO drive voltage of 3.3 volts then divide by 3 mA. So, I calculate that the resistor value is (3.3 - 0.9)/0.003 = 800 Ω.

Furthermore how do I know if a 3.3V GPIO can be on the base?

That is true, you cannot assume that a GPIO can deliver 3 mA into a load without that GPIO voltage dropping slightly so, you'd need to look in the data sheet for the GPIO driver or, just assume that it might drop to 3 volts when delivering 3 mA. Now, the series base resistor value becomes (3.0 - 0.9)/0.003 = 700 Ω. But, I'd probably go for 680 Ω.

I imagine there are some transistors that require a greater voltage on the base but how do I know?

The data sheet will provide that information.

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.

As an example, the MMBT3904 data sheet tells you that the maximum collector-base voltage is 60 volts. It's a fundamental maximum limit specified in all BJT and MOSFET data sheets.

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.

You need 30 mA collector current so start there and look at DC current gain in figure 2 for 30 mA - it's still pretty high at around 250 with a half decent level of collector-emitter saturation voltage so, I'd make a reasonable jump and assume that it might be ten times lower i.e. to draw 30 mA into the collector (with the BJT acting nearly as a switch) would need a base current of about 1.2 mA.

Can you take it from here?