Different strokes

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Yesterday we explained how a single ended power amplifier worked and what the differences are between our traditional power amplifier output stage and this unique topology. Today I want to wrap up the background series on traditional amp topologies so we can move on to learn about Class D - a topology that is still analog - but way different than anything we've discussed so far. Several readers have asked me to explain the term "push, pull" and since we covered the opposite of push pull yesterday, I'll explain what that means and then tie them all together with the different classes of amps. If you've been following along in the series you'll remember I have referred to the output stage of both Class A, B and A/B as having two transistors called a complimentary pair. One transistor handles the upper part of the output and the other other the lower. This arrangement is also known as push pull and because I was trying to keep things simple and easy to understand, I didn't go into details. It's probably somewhat misleading suggesting that each of the two transistors in our complimentary output handles half the waveform because, what's really happening, is one is pushing current while the other is pulling current. Imagine for a moment you're operating a hand pump for water. The handle on the pump moves up and down and when you push the lever up, it pulls water out of the well. When you apply force in the downward direction, you force the water you sucked up out of the well into a pipe for delivery to a storage tank. This is a good metaphor of push pull: applying equal energy to pull something up and push some out. Now imagine that same well, you pull the lever up and load the water, but then you simply release the handle and let gravity slowly push it down to get the water out. This is what a single ended amplifier does and, as you can see, it isasymmetrical: meaning it pulls up but it doesn't push down. Here's a picture I grabbed of a push pull output stage: In this drawing the circle on the far left labeled VS3 is the incoming audio signal, the triangle labeled X0P1 is the voltage amplifier and the two transistors, Q21 and Q22 are the push pull output stage. Q21 pushes up and Q22 pulls down. If this were used in a class B mode, both transistors would be off when there's no signal. In class AB mode, both would be always on, even when there's no signal, perhaps 10% of their total capability. In class A mode both would be on 100% of the time, either creating heat or sending power to the loudspeaker. Here is a picture of a single ended output stage: Sorry it isn't more complete, I had to grab and go this morning. Just mentally add the signal source and voltage amplifier of the first picture to the left of this drawing to complete the picture. Notice there is but one device and, where it says Vout is where you would connect your speaker. Notice it is the same as Q21 in our first picture? It's missing Q22. What that means is it can push but it cannot pull - so any music falling below the center line is asymmetrically feeding the lower half of the signal to the speaker. To compensate for this designers run a lot of current through this device constantly and, like Nelson Pass's Aleph amplifier of yesterday, that can mean either the ability to fry an egg on the amp or very few watts available to send to the speaker to keep the heat in check. One last thing: many of you have asked what a SET amplifier is. SET stands for Single Ended Triode and it is exactly what we've been talking about. Replace the single transistor with a type of vacuum tube called a triode and you have it.
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Paul McGowan

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