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opamp biasing class A

Kevin Weddle

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Hi Kevin,
Don't you think those extremists are ridiculous?
They take an opamp with a frequency response from DC to green light and that has an extremely low distortion, and add class-A loading to its output to reduce its distortion! Just for an ordinary headphones amp, and this opamp already has its output buffered from the headphones load.
Ridiculous! How much distortion can you hear? 0.1%?

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  • 2 weeks later...

Hey, where's my pic? It must have gotten stuck on the old server. Here it is again. Look at the extremely low distortion of the opamp, parts per million! Those extremists are adding output bias current to it to reduce that distortion.

Anyway, opamps have such a high gain that most of it is used as negative feedback which reduces their distortion tremendously. The OPA3132 opamp has a gain of 130dB, which is 3.16 million! All opamps, except for the old low-power LM324 and LM358, operate their output transistors in class-AB, for low crossover distortion.

Negative feedback can't reduce crossover distortion because the output voltage has a discontinuity, a step in it. The LM324 and LM358 operate their output transistors in class-B, for low power, and therefore have lots of crossover distortion. Their datasheet recommends adding an output bias current resistor, just like in Kevin's article, to eliminate their crossover distortion when they are used for audio.


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