# How to choose an alternative for a specific BJT transistor?

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.

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?

# 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?

• 2N2222 has a continuous collector current rating of 1A while 2N3904 has a rating of 200mA. It can only be used as an alternative in cases where the collector current is below that, and in some cases it may not be possible. – alexan_e Mar 6 '14 at 21:34
• Post the circuit if you want a definitive answer dude - the words that describe it (in your question) are not enough for anyone to be sure of how to choose a different device or, even if the circuit suits the existing device. – Andy aka Mar 6 '14 at 23:49
• @Andyaka I've tried to drew it with the editor but is to big, tomorrow I will scan it... – G M Mar 6 '14 at 23:51
• @Andyaka added! – G M Mar 7 '14 at 11:10

• Yes, along with $V_{CEO}, V_{BEO}$. – Dave Tweed Mar 7 '14 at 0:13