Can anyone post some great links to learn about transistors?

[tele_guitarist]

Specifically, I want to know if any npn can be substitued for another, and the same for the pnp. Also, what is the difference between the two?

Thanks!

[Sic]

here is a pretty decent link Electronic Tutorial Has some info about transistors down the page a bit...

doesn't have all the info you are looking for, but its a starting point

[zener]

http://www.101science.com/transistor.htm
http://www.electronics-tutorials.com/basics/transistors.htm

I don't know if this would help
http://nte01.nteinc.com/nte/NTExRefSemiProd.nsf/$$Search?OpenForm

The NPN I know are MPSA18, BC549, BC109, BC108. 2N5088, 2N5089, 2N3904,  2N2222 and 2N4401. The latter three are of less gain, AFAIK.

[niftydog]

http://www.allaboutcircuits.com/

http://www.tpub.com/electronics.htm

http://www.oersted.dtu.dk/personal/ldn/javalab/Circuit15.html

Specifically, I want to know if any npn can be substitued for another, and the same for the pnp. Also, what is the difference between the two?

You need to match up as close as possible the different characteristics of a transistor in order to get good results.

the difference between and npn and a pnp is in their basic construction and each type has advantages and disadvantages.  Also, an npn requires a positive bias voltage, whereas a pnp requires a negative bias voltage...

without going right into it...  loads of stuff out there, google is your friend, my friend!