Whack A Mole

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Whack A Mole

Engineering is like the game where you try and beat down critters as they pop up. Change one thing and something else goes differently.

Feedback is a great example: increase or decrease its amount in an amplifier and a whole list of changes happen, some expected, others not so much. For example, in a power amplifier, we can increase negative feedback to lower output impedance. The two go nicely hand-in-hand. Once the output impedance of an amplifier is lowered, its damping factor goes up in direct proportion thus speakers are more easily controlled.

Yet, those same speakers don’t always sound as good as designs with less feedback and higher impedance.

As the engineer whacks one mole down—lowers output impedance—another mole pops up in unexpected places. The impacts of negative feedback are too many and too technical to list here, but they include lowered gain, increased bandwidth, potential for instability, masking of destructive non-linear distortions in devices, increase in higher order harmonics while reducing lower orders, changing the nature of distortion (both IM and harmonic) from simple to complex, and so on.

The bottom line is engineers should practice common sense approaches to problems by exercising a few conservative design practices.

Whacking too many of the same moles might cause the unexpected mole-hordes to overwhelm designers.

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Paul McGowan

Founder & CEO

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