On Or Off?

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In yesterday’s post, I suggested our discussion of the various degrees of amplifier bias—classes A, AB, and B—apply mostly to solid state amplifiers. Several of my learned readers pointed out that’s not technically correct, and indeed it is not. The mode of operation for any amplification device is not dependant on the type, but the implementation. Those facts not withstanding, I still stand by my statement for purposes of this discussion, and I would ask my more technically minded readers to look the other way and give us some poetic license in the interests of clarity.

I am doing my best to explain a complex subject and I fear too many side roads get us lost.

It’s news to many readers that modern day solid state amplifiers divide the incoming signal in two: a positive half and a negative half. (An exception to this would include SET amps with only one output device). Here is the same circuit diagram I posted yesterday.

When the positive half-cycle is conducting (on) the negative transistor (TR2) is off, and then vice versa. This is a classic class B amplifier stage where one-half of the amp’s output is on while the other is off. Here’s a better picture describing the output when this happens.

Note the green half of the upper and lower sine wave and their straight line when off. It kind of looks like a rounded measuring spoon.

But here is where this discussion starts to come together. This next picture shows what happens when we put the upper and lower halves of the input signal together—the amplifier’s output.

Doesn’t look like the input signal, right? This is called crossover distortion (remembering that distortion means deviation from the original).

How would this sound? Harsh. Ugly. Which is why no modern power amplifier runs class B. In order to fix this, we need to add something.

Tomorrow, we spice this up with a little A.

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Paul McGowan

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