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Old 3rd November 2006, 11:29 AM   #1
TGRANT is offline TGRANT  United States
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Default Novice Question on Transistor Replacements

Novice Question on Transistor Replacements

Iíve seen a number of amp designs that use small signal transistors like BC559, BC560, BC 546 and 2n5551 for the input stages. Iíve 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?

Iíve read pros and conís, and though Iíd hate to build something that doesnít work or for which I would have to have tweak the bias and degeneration resistors to get it to work (Iím not that knowledgeable yet). On the other hand, Iíd rather not try to stock a bunch of different small signal transistors if I donít have to. Iím 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 wouldnít make different transistors if they didnít need them, and every circuit has unique facets to itís design, but I have been lead to believe that the minor variations between say a BD547 and a 2n3904 arenít that important in simple circuits like a LTP input stage or a current mirror. (Or have I missed something big time?)

Any thoughts?
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Old 3rd November 2006, 02:46 PM   #2
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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
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Old 3rd November 2006, 06:33 PM   #3
AndrewT is offline AndrewT  Scotland
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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.
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Old 3rd November 2006, 07:28 PM   #4
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If parameters match, go for it and make a test circuit amp.
It will not blow up unless the Vce are out of range....
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Old 3rd November 2006, 07:46 PM   #5
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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.
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Old 4th November 2006, 10:42 AM   #6
TGRANT is offline TGRANT  United States
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Thanks to all who replyed. Helpful info.
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Old 4th November 2006, 10:51 AM   #7
lineup is offline lineup  Sweden
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Quote:
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
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