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How a class ab amplifier works

14/03/2022

Class AB is the type of amplifier that, until recently, was used in Hi-Fi equipment many times more often than any other. Now the menacing shadow of class D amplifiers is already hanging over it, occupying an increasing share of the Hi-Fi market, but so far class AB models are still in the majority and they are not going to give up so easily. In class AB, both tube and transistor circuits can work, but if we talk about the vast majority, class AB is more associated with the era of transistor Hi-Fi.

Principle of operation of a class ab amplifier

From the very designation of class AB, it is easy to conclude that this mode is a hybrid of class A and class B. We have already figured out how class A amplifiers work, but we did not have time to get acquainted with class B, so let's start with it. And to begin with, let's recall the logic that guided the creator of the class A amplifier. In order to be able to reproduce both positive and negative half-waves with the help of one active element, he applied a midpoint shift (quiescent current) to the middle of the lamp's working area.

The creators of class B amplifiers reasoned differently: “If one tube or one zero-bias transistor can reproduce only one half-wave of the signal, why not add another active element to the circuit, placing it in a mirror image to reproduce the other half-wave?

This is quite logical, because in this scenario, both transistors operate with zero bias. While there is a positive half-wave at the input of the amplifier, one transistor works, and when it comes time to reproduce the negative half-wave, the first transistor closes completely and the second one turns on instead. 
class ab amplifier

If we compare class B with class A, the most obvious advantage is that in class B each wave covers the full operating range of the transistor (or lamp), while in class A both half-waves are reproduced by one active element. This means that a class B amplifier will be twice as powerful as a class A amplifier assembled on the same transistors.

The second, slightly less obvious, but very important plus of class B is zero bias currents. When the input signal is zero, the current flowing through the transistors is also zero, which means that there is no waste of energy, and the energy efficiency of the circuit is many times higher than in class A.

However, the main disadvantage of a class B amplifier also follows from this fact. The moment the transistor is turned on after a completely closed state is accompanied by a slight delay, therefore, when the sound signal passes the zero point, when one transistor has already closed, the second transistor does not have time to instantly pick up the baton, and in at this very transition point, there are small time delays.

In practice, this is expressed in the special dislike of the amplifier for quiet music, as well as in the poor transmission of microdynamics. And although history knows successful implementations of class B, for example, the legendary Quad 405, the problems of this mode of operation have not gone away. The same 405th not only pleased with its energetic and muscular sound, but also had a clear tendency to paint the sound picture with large strokes, on a large scale, without exchanging for trifles.

In order to keep all the advantages of class B and solve the problem of transients, the engineers went to the trick. They turned on both transistors with a bias, as is done in class A, but the bias was chosen to be significantly smaller: so as to cover only those moments when the transistor is close to closing, thereby removing transients from the working zone.

This allowed the class AB amplifier to silently cross the zero point, and also gave another extremely useful effect. With a small signal amplitude that falls within the quiescent current offset, such an amplifier operates in class A and, only when the amplitude goes beyond the offset value selected by the manufacturer, does it switch to AB mode.

 

With more power and better energy efficiency, class AB amplifiers are much less capricious when choosing acoustics. They do not need high sensitivity and are easier to get along with the complex crossovers used in multiband speakers. It would be fair to say that the vast majority of passive speaker systems on the market today are designed to work with an average class AB transistor amplifier.

Minuses

The objective disadvantages of class AB can only be seen against the background of even more technically advanced classes G, H or D, which we will talk about a little later. The list of complaints can only include subjective reviews from connoisseurs of class A, which, in general, come down to the fact that class AB does not sound so clean, detailed and elegant. To assess the validity of these claims, consider the circuitry of class AB amplifiers in more detail, in terms of sound quality.

Peculiarities

One of the practical problems of class B and AB amplifiers is the selection of pairs of transistors operating in the same amplification channel. Being mirrored in the circuit, two transistors must be completely identical to each other. Otherwise, the positive and negative half-wave signals will not be reproduced symmetrically, and this will significantly increase the overall level of distortion.

In real life, absolute identity is an abstract concept; rather, it makes sense to talk about the degree of similarity or, in technical language, about the limits of permissible deviations of transistors from given characteristics. The more similar two transistors are to each other, the lower the level of distortion, and the more their joint work approaches what we have in class A, when both half-waves are reproduced by one transistor.

Realizing that even with the strictest selection in terms of parameters, there will still be differences between two transistors in a pair (albeit in extremely small values), we are forced to admit that, all other things being equal, one of the same transistors operating in class A will sound slightly cleaner and slightly better than a pair in class AB.

A completely different situation emerges when it comes to working at a large signal amplitude and at a load requiring high power. Having a high efficiency class AB needs a less powerful and cumbersome power supply than a class A amplifier, and here fans of single-cycles are forced to recognize the absolute and unconditional superiority of class AB.

What's more, designers have much more freedom to experiment with power supplies, controlling the character and dynamics of the sound by adjusting the performance of the transformer and capacitors. For example, you can install a transformer with multiple power reserves so that it does not go out of optimal operation at signal peaks, or use improved capacitors that can instantly deliver high current.

Another subtlety: when working in class A, transistors emit a large amount of heat, which can adversely affect the quality of their work, especially when the load increases. In class AB, transistors heat up to a lesser extent, as a result of which they quickly come into operation and are less at risk of overheating, which reduces the sound quality when the amplifier is operated at high volume.

Practice

Defending the honor of class AB amplifiers in the comparative listening was the powerful Atoll Signature series two-box amplifier, consisting of an AM200 power amplifier and a PR300 preamplifier. The power amplifier of interest to us is built in full accordance with the above theoretical calculations.

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