Class B

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Yesterday we covered Class A and what it means when it comes to a power amplifier. It's been pointed out in our comments section that Class A biasing doesn't have to apply only to a power amp output stage and, in fact, it is used extensively in the input circuits of most amps, preamps and DACS. Indeed, any circuit can qualify as Class A as long as the current to that stage never varies under any signal condition. As an example, all PS Audio amps and preamps are all class A on their inputs and our DACS are all Class A on their outputs. The important take away, however, is what we learned about power amplifier output stages: class A draws a constant amount of power, twice what is ever delivered to the speaker, and that power is used to either heat up the amplifier or drive the speaker itself. The amount of power it consumes from the wall is constant, regardless of how loud it plays. Class B, on the other hand, is exactly the opposite. A Class B output stage draws only what is being delivered to the loudspeaker and never any more. This type of stage is very efficient, unlike Class A, but has very high distortion and is almost never used in an audio amplifier. It's important to understand the difference between a Class A and a Class B configuration because tomorrow we'll learn what the combination of Class A and B looks like - and this arrangement is used in the vast majority of all audio power amps. If we remember back to what an amplifier does in past posts, we learned that an amp should duplicate its input signal with a larger version at its output. Using a sine wave as an example, there is a top half and a bottom half to the sine wave. In most amplifier circuits one type of transistor is used to amplify the top half and another type the bottom. Here's a picture of one device amplifying only the one half: This shows a nice sine wave (on the left) coming into a transistor amplifier. Coming out of this amplifier is only 1/2 of the sine wave. The next picture shows two transistors, as is typical in an amplifier, one handling the top and the other the bottom of the sine wave. Don't worry that it looks a little technical, it doesn't matter. What's important to understand is that in this typical output stage (class A or B are the same) is that one device handles the top and the other device handles the bottom. This arrangement is called a complimentary pair. In a class A amplifier, neither of the two output devices ever turns off - the opposite is true in a Class B. Once the Class B output stage is finished with making its half of the sine wave, it shuts off and waits for the next time it's needed. The advantage to this type of amplifier is efficiency. You can imagine that it is very efficient because it is only on when it is delivering power to the speaker. The problem with Class B is distortion and that happens at the very point where one transistor turns off and the other turns on. The area where this happens is called the crossover point and the type of distortion that is created is called Crossover Notch Distortion. Here's a picture of what that looks like. See the bottom sine wave in this picture? There you see where the two transistors meet and are off. This is bad distortion because any music down in that area is going to be lost because the transistors aren't on. What music is found down in that area? All the low level detail in the music - the quiet parts - the parts of the music that are critical to sounding live - all the inner details, the background noises, the subtle nuances - all gone or distorted. So tomorrow I'll explain Class A/B which solves this problem. Before we end today, I'll give a you a brief glimpse into what we'll cover tomorrow. Just imagine this Class B amplifier and then put a little bit of Class A "always on" into the mix. When you do this, the point where the two transistors meet is now always on - and voila! The distortion is gone and the inner detail returns. Details tomorrow.
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Paul McGowan

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