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Old 6th Dec 2017, 10:33 am   #2
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 3,365
Default Re: Matched Transistors

hFE varies wildly with temperature, collector current, planetary alignment, etc. Any circuit designer that relies on a particular value of hFE is committing commercial suicide - such a circuit wouldn't have made it past a design review.

Certainly, worrying about a 1% difference is pointless. I'd be happy with 50%, frankly. The key thing is making sure that neither transistor has a hFE that is lower than the minimum spec for that device, because that implies a transistor that is in some way faulty, albeit not quite dead yet.

The acid test is to see how the circuit performs. Check the output waveform with a 'scope - make sure it's a symmetrical sine wave with no premature clipping or crossover distortion. Job done
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