When we walked along the Hi-Fi and audio system from the listener’s ears to the music source, we had reached the amplifier. In our previous article, we first introduced distortion and how certain amplifier characteristics affect it. Here we will continue to study the amplifier itself along this path. What types of audio amplifier circuits will you encounter, and what are their relative advantages and disadvantages?
Some amplifier basics
If you know something about transistors, then it may be a three-terminal device whose output pins form part of a voltage divider, and its state depends on what is presented to its input pins. Electronic art Think of it as a cartoon character. He is standing inside a bipolar transistor, watching the ammeter on the base while adjusting the variable resistance between the collector and the emitter.
Properly biased within its conduction range, the transistor can behave as a linear device, where the voltage divider moves in a linear relationship in response to the input, so the output voltage is an amplified version of the output voltage. This is the simplest transistor amplifier because of the different types The amplifier is represented by a letter category, so it is called a Class A amplifier.
The linearity of a Class A amplifier makes it a good choice for audio designers looking for low distortion, but it also has a drawback. The role of the voltage divider means that no matter what state it is in, it always passes current, so the transistor must always be able to dissipate this power in the form of heat. This makes Class A amplifiers significantly less efficient, so an amplifier that is sufficient to drive the speakers must also emit the same power as it provides to the speakers, but as heat. Therefore, Class A power amplifiers require additional cooling to avoid overheating and consuming unnecessary power. There are Class A audio power amplifiers on the market, but they are still uncommon.
Have your cake and eat it: Class AB
The inefficiency of Class A comes from the continuous conduction of its transistors, which is likely to reduce the bias to the point where the transistors are in the off position, but only conduct when a signal appears and pushes them to conduct. This type of amplifier is called a Class B amplifier, and it usually only amplifies a part of the input waveform. It solves the inefficiency problem of Class A, but it can cause serious distortion of its output, which is why you will not use it as an audio amplifier in a simple form, except in some very early tube radios.
Given that Class B circuits can amplify half of the waveform period without distortion, an obvious improvement is to combine the two; one amplifies the upper half of the period and the other amplifies the lower half. The idea is that the complete amplified waveform can be reconstructed from two amplifiers and produce low distortion and efficient results.
This is effective to a certain extent, but such a circuit still retains some distortion, because it is almost impossible to achieve the point where two waveforms meet without some kind of interruption. This small glitch is called Cross distortionThe solution to this problem comes from clever biasing, which operates in almost all ranges of class B transistors, but gives them enough bias to operate at the midpoint of class A operation, at which the two amplifiers Half transferred from one to the other. This arrangement is called a Class AB amplifier, which provides very low distortion and very important power efficiency, and constitutes the vast majority of analog Hi-Fi amplifiers.
You might want us to now turn to the next letter of the alphabet and describe a class C amplifier here, but the final audio configuration is a class D amplifier. (Class C amplifiers are switching square wave amplifiers with high efficiency but large distortion. They are not good for audio, but can be used in RF power amplifiers, where the LC circuit filters out the harmonics generated.)
All-digital audio amplifier: Class D
Most Hackaday readers are familiar with the concept of pulse width modulation, that is, the behavior of changing the energy delivered to the load by sending pulses with a variable on-time to off-time ratio to the load. This is how many microcontrollers produce pseudo-analog outputs for linear control of LED brightness or motor speed, to name just two examples. Given a sufficiently high switching frequency, PWM can be used to encode rapidly changing analog signals, such as audio, so the PWM stream can be fed to a high-power buffer to produce audio output.
A practical Class D amplifier uses PWM in this way, and its output transistor only works as a high-speed switch. Similar to Class C amplifiers, but the difference is that Class D amplifiers switch at many times the signal frequency, while Class C amplifiers operate at their signal frequency. Class D amplifiers generate high-power PWM pulse trains, which are converted into high-power audio-driven speakers through a low-pass filter network. The advantage of a Class D amplifier is that it can provide extremely high efficiency, which means that it can be built smaller and lighter than linear circuits, and requires less thermal management.
There are also several types of amplifiers worth mentioning. Class E and Class F are more RF amplifiers that rely on pulsed resonant LC networks with short pulses to obtain output. Class G and Class H are variants of Class AB amplifiers. Change the PSU to minimize the continuous current. You may come across ICs that provide Class G and Class H, but from an audio point of view, they can be seen as a more efficient version of Class AB amplifiers.
We have learned about your Hi-Fi system and the circuit topology behind the amplifier in every audio production device you own. So the question is: which is better, type A, type AB, or type D? The answer is not static, because the performance of an amplifier does not necessarily depend on its principle, but on how its designer implements it. It is possible to make a really bad or fine-sounding audio amplifier in all of the above topologies, so it’s best not to care too much about it. Class A amplifiers give you bragging rights and keep your house warm, Class AB amplifiers are what you can find in most Hi-Fi splitters, and Class D amplifiers will be lighter and cooler. Amplifier: There, we fixed it!