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-   -   what happens if transistors are not matched? especially output stage.. (http://www.diyaudio.com/forums/solid-state/223630-what-happens-if-transistors-not-matched-especially-output-stage.html)

rhythmsandy 15th November 2012 12:42 PM

what happens if transistors are not matched? especially output stage..
 
if there are 1 pair, 3 pair, 5 pair and 14 pair....
what are the disadvantages? what happens if they are not matched.

the reason why this question popped out is I see alot of amplifiers based on LME49810 ebay which is sold for about less than 100 USD i really dont think they are matched transistors..

now if they are not matched how does it reflects the performance and stability?...

Boscoe 15th November 2012 12:54 PM

They will not share current well even if they have emitter resistors. One pair it doesn't really matter.

I doubt the transistors on those ebay amps are even real.

Jay 15th November 2012 12:56 PM

Source resistors will help share the current. In most amplifiers, unmatched transistors is usually not an issue. I have never matched my transistors.

The disadvantage of unmatched paralleled transistors is in the "coherency" of the sound, where single pair is better in this area because of no matching issue. But paralleled transistors will give lower output impedance or more ability to drive speaker or better bass.

So, parallel is most often better than a single pair. But matched paralleled is even better, but requires too many transistors to test. I only have done and experienced this with mosfet output.

Darlington transistor will have much greater issue when paralleled because it consists of 2 transistors in one package. But running high current should reduce the issue imo.

dry joint 15th November 2012 01:15 PM

What happens is normality, its how almost all amps with paralleled outputs have been built. Buying hundreds or thousands of transistors to get a reasonably matched 4, 8, 16 etc would be astonomical.

rhythmsandy 15th November 2012 02:01 PM

so generally what is the ratio if I buy 200 transistors from digikey or so how many transistors will match in VDS and hfe both...

bobodioulasso 15th November 2012 02:25 PM

Matching cannot be avoided for mosfets.
For BJTs, coming from same batch may suffice and is generally OK.

rhythmsandy 15th November 2012 02:33 PM

its for BJTs not mosfets...

bobodioulasso 15th November 2012 02:44 PM

I think all these sellers use same batch, not proceed to matching.
I t can become a problem if you push the amplifier to it's limits because some transistors will draw more current than others and fail first.
For home use it's not generally a problem as we do not go to the limits.

rhythmsandy 15th November 2012 02:47 PM

but generally what percent of the max limit we can go before it fails..

bobodioulasso 15th November 2012 02:56 PM

As it's random, i would give a random answer of eighty per cent.


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