A neat trick
In yesterday's post on amplifier biasing, we saw the reason why class B output stages are not used in the modern power amplifier. Distortion, and not the sonically acceptable kind. Here's what the output signal looked like: Not pretty, but there's an easy fix. Just add a couple of 5-cent diodes (among other ways) to make sure the two output transistors never turn off. The results are free of crossover distortion. Keeping the two output transistors always on has a name: class AB—the most common output biasing scheme in modern audio amplifiers. In fact, I'll bet the power amplifier you have at home uses class AB biasing. (The odds are in my favor). We began this mini series of posts asking the question, "How much is enough?" In the case of class AB, that's up to the designer. Choices vary from just enough to lower distortion or more to increase the low-level signal area that runs in class A (always on regardless of signal level). Tomorrow we look at these choices to see what differences they make and what trade offs happen.
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