A Brief Discussion on the Types and Structures of Audio Amplifiers

The difference between "KALAOK amplifier" and general amplifier is that "KALAOK amplifier" has reverberator from BBD analog reverberation to DIGETAL (digital reverberation), pitch shifter, microphone amplifier. In order to meet the market demand, some manufacturers combine various functions including AV amplifier and KALAOK amplifier into one, which is called "comprehensive amplifier". This is a hodgepodge amplifier, which has everything but can't do anything well. It is a low-end amplifier that does not pursue sound quality but only pursues rich functions.

As the name suggests, "special power amplifiers" are amplifiers used in special occasions, such as alarms, car low-voltage power amplifiers, etc., which will not be introduced here.

Audio power amplifier, as the name implies, is an amplifier that amplifies the power of audio signals. From the early simple Class A and Class B, it has developed to the current Class G and even Class W. The input and output of audio has also evolved from the early pure analog signals to the current digital/analog coexistence. The efficiency is getting higher and higher, the harmonic distortion is getting smaller and smaller, and the fidelity is getting higher and higher. This article analyzes the development of power amplifiers from the perspective of structure and basic characteristics (part of the content in the article comes from the Internet).

Definition of power amplifier

The power amplifier, also known as the PA, is commonly known as the "amplifier". It is the most basic device in the audio system. Its task is to amplify the weak electrical signal from the signal source to drive the speaker to make sound.

Its main function is to amplify the weak signal input from the sound source equipment and generate a large enough current to drive the speaker to reproduce the sound. Due to considerations of power, impedance, distortion, dynamics, and different usage ranges and control adjustment functions, different power amplifiers have different internal signal processing, circuit design and production processes.

Classification of power amplifiers

There are many ways to classify power amplifiers, which are generally divided according to the different conduction methods of the power amplifier tubes. Usually they are divided into Class A (Class A), Class B (Class B), Class AB (Class A and B), Class D (Class D), and later developed Class G, Class H and other types.

Class A Amplifier

The characteristic of a Class A amplifier is that current always flows in its output circuit regardless of whether there is an input signal, and this type of amplifier usually operates within the linear range of the characteristic curve to ensure that the amplified signal is not distorted.

So its advantages are: low distortion, the smaller the signal, the higher the fidelity. The biggest disadvantage is low efficiency, which is only 25% at most. When there is no input signal, the power consumption is not reduced at all, which is extremely unsuitable for power amplification. However, due to its high fidelity, some high-end audio equipment still uses Class A amplifiers.

Since the current loss of Class A amplifier is always large regardless of whether there is a signal input or not, it will generate a lot of heat. Therefore, when using Class A amplifier, a good heat dissipation environment is required. The figure below is a schematic waveform of the working range of Class A amplifier, as well as the general implementation methods of Class A amplifier, which are "common collector" and "common emitter".

Class A amplifier working range

The output amplitude of the Class A power amplifier is Vp, the output load average power is PL, the power input power is Ps, and the working efficiency is η, then the following expression can be obtained:

PL=Vp*Vp/(2*Rl); Ps=2*Vcc*Iq; η=Pl/Ps, so it can be inferred that when Vp=VCC and Vp=IQ*RL, the Class A amplifier has the maximum working efficiency, which is 25%.

Class B Amplifier

Class B power amplifier is an amplifier whose operating point is at the extreme of the characteristic line. When there is no signal input, the output end consumes almost no power. According to the definition, the static operating point is 0, and the signal is connected to the original emitter follower through a PNP type BJT, forming the so-called "complementary emitter follower" also known as "Class B push-pull amplifier".

Its operating principle is that during the positive half cycle of Vi, Q1 is turned on and Q2 is turned off, so a positive half cycle sine wave is formed at the output end of Figure 4; similarly, when Vi is the negative half cycle, Q1 is turned off and Q2 is turned on, resulting in a negative half cycle sine wave at the output end, as shown in the dotted part of Figure 4.

Since the Class B push-pull amplifier does not consume power when there is no input signal, it has a higher maximum efficiency of up to 78% than the Class A amplifier. However, since a portion of the signal amplitude range of the push-pull amplifier is in the nonlinear region of the characteristic line, it causes severe distortion, as shown below. We call this distortion "Cross-Over Distortion".

Class B amplifier implementation

Class B amplifier working range

Assuming the output signal is Vp*sinωt, the output load average power PL, the power input power is Ps, and the working efficiency is η, we can get:

PL=Vp*Vp/(2*Rl); Ps=2*Vcc*Vp/(π*Rl); η=Pl/Ps. When Vp=VCC, the Class B amplifier has the highest working efficiency, 78.5%.

Class AB amplifier

The crossover distortion of the Class B push-pull amplifier mentioned above is caused by the fact that when the signal size is between -0.6V < Vi < 0.6V, both Q1 and Q2 cannot be turned on. Therefore, if we add two 0.6V voltages between the Vbe of Q1 and Q2, so that when the input signal size is between ±0.6V, Q1 and Q2 can also be turned on to reduce distortion, this situation is a Class AB amplifier, as shown in the figure above.

Although the distortion produced by the Class AB amplifier is smaller than that of the Class B amplifier, the price paid for this improvement is the waste of static power consumption and the loss of efficiency. Therefore, the efficiency of the Class AB amplifier will be between Class A and Class B.

Main points of distinction Class A amplifier Class B amplifier Class AB amplifier

Working point position Load line midpoint Load line cutoff point Between load line midpoint and cutoff point

Distortion: The minimum distortion is slightly higher than Class AB. There is crossover distortion, which can be eliminated.

Power transfer efficiency is the lowest, below 50% efficiency is about 50% to 78.5% efficiency is slightly lower than Class B

Main Applications Low-distortion low-power amplifier High-power amplifier General audio amplifier

Class D Amplifier

The Class A, Class B, and Class AB amplifiers mentioned above can all be considered analog amplifiers. This is because their input and output are both analog sound electrical signals, which are amplified by analog amplifiers and do not involve modulation, filtering, encoding, or decoding. Class D amplifiers can be called the simplest digital amplifiers (some people also call them PWM amplifiers, which are not strictly digital amplifiers).

The Class D amplifier receives the analog audio signal and compares it with the triangle wave generated by the internal triangle wave generator. The result is a pulse width modulated signal (PWM), which is then amplified and restored to an analog audio signal. Therefore, the Class D amplifier simulates the analog audio amplitude with the pulse width, and its information transmission process is analog, non-quantized, and non-coded. And due to the current limitations of device performance, PWM amplifiers cannot use too high a sampling frequency, and the performance indicators cannot reach the Hi-Fi level. The efficiency of Class D amplifiers can generally reach more than 80%~90%. Due to its high efficiency, the requirements for environmental heat dissipation performance are greatly reduced, so Class D amplifiers have become the mainstream in current portable products.

Class D amplifier implementation and PWM waveform

For Class D amplifiers, the comparator and the triangular wave signal form a fixed-frequency PWM circuit, which modulates the audio input signal with a triangular wave signal (the frequency of the triangular wave is much higher than the audio input signal, and the frequency of the triangular wave is generally between 25KHz and 1.5MHz).

The larger the input signal amplitude, the wider the generated PWM wave pulse width.

When the Class D amplifier is working, the output P-type and N-type power switch tubes are in the switching state. Ideally, the on-resistance of the power switch tube is 0Ω, and there is no voltage loss. When turned off, the switch tube resistance is infinite and no current flows. Therefore, the efficiency of the Class D amplifier can reach 100% in theory. However, in practical applications, due to the limitations of device characteristics (such as switching speed, leakage current, on-resistance is not zero, etc.), the actual working efficiency can reach more than 90%. The general design architecture of the Class D amplifier is shown in the figure below. In the actual design, protection circuits such as over-temperature protection and over-current protection are also added.

Class G Amplifier

In order to improve the efficiency of the power amplifier, the G-class power amplifier was developed. The G-class power amplifier was proposed by Hitachi in 1976. Its main principle is to provide multiple power supply voltages for the power amplifier and select the required power supply voltage according to the size of the input audio signal. When the input signal is low, a small power supply voltage is provided to the circuit, and vice versa, a high power supply voltage is provided. Since the audio signal has a very high peak rate (Peak-to-Mean Ratio), the flexible selection of the power supply voltage of the G-class power amplifier can effectively reduce power consumption and improve efficiency. Therefore, the G-class power amplifier has been increasingly widely used in high-power audio power amplifier systems in recent years.