How to choose an alternative for a specific BJT transistor?

Question

I'm trying to reproduce some circuits with electronic pieces that I've at home. Unfortunately I miss some recommended transistors so I'm trying to understand how to replace them with an alternative transistor and overcome this problem.

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How can I choose the right transistor alternative for my circuits. I know that there will be a lot of parameters depending from my circuit but which are the most important parameters that are mandatory? and are there criteria that could be helpful for choosing the right alternative?

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My specific case

In my case I don't have a 2N2222 transistor. In alltransistors.com I've found that I can use BFX95, BSW63, ECG123A, MM531 that are different alternatives that I don't find in other site where it seems that 2N3904 is the right one. I don't know which are the parameters they look at. For example I'm try to amplify a signal and send it to a loud speaker (with a 100 Ohm resistor) in a circuit with a load lower than 6 V, can I use a C945 that as the same Ueb but uses an hfe of 130 instead of 100 of 2N2222? What will happen?

Answer 1

Look at the obvious things first: how much voltage and current must the transistor handle in the circuit? How much power must it dissipate? Any transistor that meets these specs will probably work to some degree.

Then look at lower-level details such as current transfer ratio (does the circuit only work with a certain minimum gain?) and capacitance values (especially in high-frequency circuits).

Does the circuit exploit some particular feature of the specified transistor? If so, you'll need to verify that your replacement transistor behaves similarly.

Answer 2

If that is a 100 ohm resistor in series with the speaker (not 100% clear in the diagram) and the supply voltage is nominally 6V then the maximum current through the transistor is going to be about 60mA - this could rise to 100mA on a larger supply voltage so be aware of that.

In the circuit you've drawn Hfe variations between one transistor and another won't account for much of a change in performance. I'd expect most NPN transistors to perform equally as good for this design.