Novice Question on Transistor Replacements

[TGRANT]

Novice Question on Transistor Replacements

I’ve seen a number of amp designs that use small signal transistors like BC559, BC560, BC 546 and 2n5551 for the input stages. I’ve looked up the data sheets and they seem similar in many respects to small signal transistors like 2n2904, 2n4401. Are these relatively interchangable? I happen to have a few hundred of the 2n2904, 2n4401on hand (I have the PNP compliments as well). Is it reasonable to try substituting these or should I buy the others if the circuit calls for it?

I’ve read pros and con’s, and though I’d hate to build something that doesn’t work or for which I would have to have tweak the bias and degeneration resistors to get it to work (I’m not that knowledgeable yet). On the other hand, I’d rather not try to stock a bunch of different small signal transistors if I don’t have to. I’m guessing that any transistor with a B = 100 and Collector-Emitter Breakdown Voltage of about 40-60 should be usable for LTP input stage and the associated current mirrors and current sources for many designs floating around.

I know this question might not have an easy answer since they wouldn’t make different transistors if they didn’t need them, and every circuit has unique facets to it’s design, but I have been lead to believe that the minor variations between say a BD547 and a 2n3904 aren’t that important in simple circuits like a LTP input stage or a current mirror. (Or have I missed something big time?)

Any thoughts?

[Jan Dupont]

In most designs, you can use other transistors than stated (from input to output)....
However some designs are tweaked to a specific transistor pair, however you are still able to use other kinds
Just make sure that Ic, Hfe and Vce are similar or higher....

I have repaired many old amp, where the original transistors where obsolete. I just replaced (all) to newer types

[AndrewT]

Hi,
I would not go as far as saying they are interchangable, but you can certainly experiment with alternatives.

Some ancilliary components may have to be changed to compensate for different device parameters, but I suspect the majority of the circuit can remain unaltered and it will still work. Just not optimised.

Output offset voltage depends on input offset current and this is VERY dependant on device parameters. But easy to alter once measuring starts.
Just a thought, changing an LTP device for the SAME type will change the output offset, it's that critical.

[Jan Dupont]

If parameters match, go for it and make a test circuit amp.
It will not blow up unless the Vce are out of range....

[myhrrhleine]

I didn't look up the specs, so I'm answering without looking.

When comparing the transistors, be sure to consider ft, hfe, and noise.

[TGRANT]

Thanks to all who replyed. Helpful info.

[lineup]

Originally posted by TGRANT
Thanks to all who replyed. Helpful info.

Be sure to check
B C E
Pin Configuration

when you replace transistors with other types
.. but this is elementary, My Dear Watson