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Old 6th Dec 2017, 2:21 pm   #7
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 4,624
Default Re: Matched Transistors

The input impedance of the stage, whether emitter follower or common-emitter, is proportional to hfe. So, if the hfe's of the two transistors in a push-pull amplifier are widely different, there will be different loads on the driver stage on the two half-cycles. This could give rise to asymmetric distortion.

So, while agreeing with mhenessey that a circuit which relies on a particular value of hfe is a bad design, it is still good practice to match them if you want best performance.

Being mounted on the same board or chassis, they'll be at the same temperature, see the same planetary alignment, have the same collector currents on symmetric waveforms, everything will track!
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