What is a Class AB Amplifier?

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Class AB Amplifiers combine the best features of A and B Classes.

Class AB amplifiers are hybrids, taking the best from both parent categories.

It helps to know what A and B classes are to appreciate why AB is important for car audio.

We covered Class D amplifiers last time. What’s with these other letters?

You might think there is a grading system, judging by the appearance of the categories themselves. But there’s not.

No one amp class is flat-out better than another. And not all amplifiers are the same. There are many unique and varying applications and reasons to choose one amp over another.

The distinguishing factors of an amplifier category include linearity, efficiency, and power output.

The amplifier classes describe the amplification topology.

That is, how the amps function and provide amplitude to the source signal.

This article will focus on the power supplies, efficiency, and how the categories differ among A, B, and AB.

As far as power goes, amplifiers put out less than what is put into them. They’re fed a low voltage input signal from a source like your phone or radio.

They increase the amplitude of this input signal while attempting to produce a faithful signal in the output stage. Their efficiency is a ratio of its output divided by its power draw.

What power isn’t used is converted into heat. There is a balancing act between fidelity and efficiency, which results in a slew of amp classes.

Classes A, B, and AB are analog amplifiers. Analog amps have a level of warmth, or coloration that some audiophiles find preferable to the output of digital amplifiers. Digital amps are less prone to signal coloration, which is why they’re perfect for the substage.

Analog amplifiers are particularly useful for Mids and Highs due to this coloration.

What is the difference between Class A and B Amplifiers?

Class A amplifiers have a clean output with very low distortion.

Their transistor or FET has a constant bias. This means that the FET is active for the entire input cycle. The power supply is constant with no duty cycle.

It provides ideal linearity or a high fidelity replica of the input signal. This produces a linear waveform or a high quality signal with little to no distortion.

The signal quality is also the greatest weakness of A Class. Since its power supply is on at all times, a great amount of energy is wasted in operation.

That is power which turns to heat.

A Class amp efficiency levels can be as low as 25%.

Even when A Class amplifiers are idling and producing no power, their power draw remains steady. That creates a lot of warmth, with up to 75% of the power drawn by the amp turning to heat.

Can you imagine 75% of the power in your car stereo system turning to heat? That would not make for happy amplifiers- Or passengers.

To solve this, more efficient amps have been designed.

Class B amplifiers are much more efficient but do not have the same sound quality.

They improve on the design of Class A by replacing the single FET with two active FETs that work on a push-pull cycle.

This push-pull splits the cycle between the power supplies. One power supply handles the negative voltage and the other transistor handles the positive voltage.

Because of this dual power supply, Class B Amplifiers are capable of using 60-70% of input power with efficiency. That efficiency comes at the cost of sound quality in Class B amplification.

B Class amps produce crossover distortion.

This is because of a dead zone in the crossover of duty cycles. Crossover distortion isn’t something to filter out with high and low pass filters but is an issue with the design itself.

Crossover distortion is caused by a dropoff in the signal handling in the dead zone between the two power supplies. The slight delay between cycles creates a nonlinear waveform.

The minor and brief pause in the signal chain translates into unpleasant distortion. While their efficiency greatly improves on Class A power handling, Class B amplifiers cannot produce a faithful audio signal due to their nonlinear signal handling.

Efficiency doesn’t count for much if the signal isn’t a faithful replica of the input.

AB Class Amplifiers

Fidelity without compromise.

Class AB is a union of the two designs for the best of both worlds. It uses some of the strengths of both Class A and B Amplifiers while also improving on the disadvantages of both categories.

Class AB use two FETs or transistors, like in B Class.

The two FETs are made to work like both parent transistors. They still run on duty cycles like that of Class B but work more like the A-Class FET.

Class AB amplifiers bias the duty cycles, causing the FETs cycles to overlap at the crossover point. This overlap provides better control over the cycle’s crossover.

Where the cycles would be crossing over with dead zones for efficiency in B Class, AB FETs are forward biased, with the cycles slightly overlapping.

This overlap allows for more consistent, near linear operation. It makes up for the sound quality deficit between the parent classes.

AB FETs have no dead zone like Class B amplifiers.

Because of this, crossover distortion is reduced to lower levels than B Class amps are capable of. This creates a smoother, more faithful tone than B Class is capable of.

By using two transistors and biasing both to operate at the same time, the crossover phase of AB class is reduced and distortion is minimized as low as possible.

While neither parent grouping is ideal for car audio, Class AB amplifiers provide the best of both worlds while reducing the issues both A and B Class Amps create. They retain the configuration of B Class efficiency and mimic the A Class linearity of the output signal by biasing the transistors above the threshold.

AB Class amplifiers are very useful in high powered amplification.

Class AB is ideal for car audio, as they can handle high frequencies without distorting.

Ready for your own Class AB Amplifier?

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Need help installing your amplifier? Check out our installation guide here.