A power amplifier (also known as a power amplifier) is an amplifier that can generate maximum power output to drive a load (such as a speaker) under given distortion conditions. It plays a pivotal role in organizing and coordinating the entire audio system, and to some extent, determines whether the entire system can provide good sound quality.

 

Power Amplifiers Operating Principle

 

The power of a power supply is converted into a current that varies according to the input signal using the current control function of a transistor or the voltage control function of a field-effect transistor. Because sound is a wave of varying amplitude and frequency, that is, an AC signal current, the collector current of a transistor is always β times the base current in the amplification region. β is the transistor's current amplification factor. Applying this principle, if a small signal is injected into the base, the collector current will be equal to β times the base current. This signal is then isolated by a DC blocking capacitor, resulting in a larger current (or voltage) β times the original signal. This phenomenon is known as the transistor's amplification effect. This continuous current amplification completes power amplification.

 

Main Types of Amplifiers

 

Traditional digital voice playback systems involve two main processes:

1. Conversion of digital voice data into analog voice signals (achieved using a high-precision digital-to-analog converter (DAC));

2. Amplification of the analog signal using an analog power amplifier, such as Class A, Class B, and Class AB amplifiers. Since the early 1980s, numerous researchers have been developing various types of digital amplifiers that directly amplify digital voice data without analog conversion. These amplifiers are commonly referred to as digital power amplifiers or Class D amplifiers.

 

Class A Amplifier:

Operational State: The quiescent operating point (Q-point) is set exactly at the center of the load line. The transistor remains on throughout the entire input signal cycle (360°), with a conduction angle of 360°. The power supply continuously supplies a high, constant current, regardless of signal presence.

 

The main characteristics of a Class A amplifier are: the amplifier's Q-point is set near the midpoint of the load line, and the transistor remains on throughout the entire input signal cycle. The amplifier can operate in either single-transistor or push-pull configurations. Because the amplifier operates within the linear range of its characteristic curve, transient and alternating current distortion are minimal. The circuit is simple and easy to debug. However, efficiency is low, as transistors consume a lot of power, resulting in a theoretical maximum efficiency of only 25%, and significant nonlinear distortion. Therefore, efficiency is relatively low.

 

Class A Amplifier Advantages:

1. Optimal Linearity: Operating in the linear portion of the amplification region, distortion is theoretically minimized, making it the gold standard for high-fidelity (Hi-Fi) audio design.

2. Simple Design: The circuit structure is typically the simplest.

 

Class A Amplifier Disadvantages:

1. Extremely Low Efficiency: The theoretical maximum efficiency is only 25% (with resistive loads), and in practice, it is often even lower. This means that over 75% of the electrical energy is wasted as heat.

2. Severe Heat Generation: Requires a large and expensive cooling system.

3. High Power Consumption: High quiescent current makes it unsuitable for battery-powered devices.

 

Class B Amplifier:

Operating State: The Q point is set at the cutoff point, and the transistor conducts only during half a cycle (180°) of the input signal, with a conduction angle of 180°. A push-pull circuit consisting of two transistors is typically used to process the positive and negative half-cycle signals separately.

 

The main characteristic of a Class B amplifier is that its quiescent point is at (VCC, 0). When there is no input signal, the output consumes almost no power. During the positive half-cycle of V, Q1 turns on and Q2 turns off, resulting in a positive half-cycle sine wave at the output. Similarly, when V is a negative half-cycle, a sine wave is generated, requiring two transistors in push-pull operation. While this amplifier boasts high efficiency (78%), it also suffers from significant crossover distortion, as the amplifier operates in a nonlinear region. This distortion occurs when the signal is between -0.6V and 0.6V, preventing Q1 and Q2 from conducting. Consequently, this type of amplifier is gradually being abandoned by designers.

 

Advantages of Class B Amplifiers:

High efficiency: The theoretical maximum efficiency can reach 78.5%. When there is no signal, the quiescent current is almost zero, resulting in low power consumption. Disadvantages of Class B Amplifiers:

Crossover Distortion: When the signal crosses zero at the junction of the positive and negative half-cycles, the two transistors need to alternate between turning on and off. This process produces significant nonlinear distortion and poor sound quality.

 

Class AB Amplifier:

Operating Mode: A compromise between Class A and Class B. The Q-point is set slightly above the cutoff point, with each transistor on for slightly longer than half a cycle (between 180° and 360°). The conduction angle is typically greater than 180° but less than 360°.

 

The main characteristics of Class AB amplifiers are: the transistor on-time is slightly longer than half a cycle, requiring two transistors to operate in push-pull mode. This prevents crossover distortion. The high alternating distortion offsets even-order harmonic distortion. The amplifier also features high efficiency and low transistor power consumption.

 

Advantages of Class AB Amplifiers:

1. Moderate Efficiency: The efficiency is between Class A and Class B (typically 50%-70%), significantly higher than Class A. 2. Significantly Reduced Distortion: By providing a low bias current, the objectionable crossover distortion of Class B amplifiers is effectively eliminated, maintaining excellent linearity.

 

Disadvantages of Class AB Amplifiers:

1. Efficiency Still Lower Than Newer Architectures: Compared to switching amplifiers like Class D, efficiency still lags behind.

2. Still Requires a Heatsink: Although smaller than Class A, it still requires a heat sink design.

 

Class C Amplifier:

Operating State: The Q-point is set below the cutoff point, the transistor conduction time is less than half a signal cycle (<180°), and the conduction angle is less than 180°. The output is a severely distorted pulse train.

 

The main feature of a Class C amplifier is that the transistor operates only for a very short period of each input signal cycle. During circuit operation, a negative bias is usually applied to the amplifier transistor to prevent the transistor from operating in Class B mode. Its collector load is not a resistor but an LC parallel resonant circuit, hence the name "resonant amplifier circuit." Frequency selection is achieved by adjusting the capacitance of the capacitor or the inductor. Class C amplifiers have extremely high conversion efficiency, reaching 98%. However, because the load is a resonant circuit, the circuit often operates at high frequencies, resulting in significant distortion. Therefore, Class C amplifiers are not suitable for use as audio power amplifiers. Instead, they are widely used in the radio industry due to their selectivity, often as RF amplifiers, tuned amplifiers, and frequency multipliers.

 

Class C Amplifier Advantages:

Extremely high efficiency: Theoretical efficiency can reach over 85%.

 

Class C Amplifier Disadvantages:

Extreme distortion: The output waveform is completely different from the input waveform, making it unsuitable for amplifying signals that require high fidelity, such as audio.

 

Class D Amplifier:

Operational state: Fundamentally different. Transistors do not amplify linearly, but rather operate as switches, switching between fully on (low resistance) and fully off (high resistance) at high speed. The concept of conduction angle does not apply. Pulse-width modulation (PWM) technology is used, with the pulse duty cycle representing the amplitude of the analog signal.

 

A Class D (digital audio power) amplifier converts an input analog audio signal or PCM digital information into a PWM (pulse width modulation) or PDM (pulse density modulation) pulse signal. This PWM or PDM pulse signal is then used to control the on/off switching of a high-power switching device. This amplifier, also known as a switching amplifier, offers the distinct advantage of high efficiency. A digital audio power amplifier can also be considered a one-bit power digital-to-analog converter. The amplifier consists of four components: an input signal processing circuit, a switching signal formation circuit, a high-power switching circuit (half-bridge or full-bridge), and a low-pass filter (LC). Class D amplifiers, or digital amplifiers, utilize extremely high-frequency switching circuits to amplify audio signals.

 

Advantages of Class D amplifiers:

1. High efficiency, typically exceeding 85%.

2. Small size, significantly saving space compared to analog amplifiers.

3. No crackling noise during turn-on.

4. Low distortion and excellent frequency response. Fewer external components, facilitating design and debugging.

 

Disadvantages of Class D Amplifiers:

Electromagnetic Interference (EMI): High-speed switching generates high-frequency noise, requiring effective filtering and shielding.

Potential Distortion: Output stage dead time, switching nonlinearities, and filter phase distortion can affect sound quality. However, modern Class D chip technology is highly mature, and sound quality is comparable to Class AB.

 

Class A, B, and AB amplifiers are analog amplifiers, while Class D amplifiers are digital amplifiers. Class B and AB push-pull amplifiers offer higher efficiency and lower distortion than Class A amplifiers, as well as lower power consumption and improved heat dissipation in the power amplifier transistors. However, Class B amplifiers can produce alternating distortion during the transition between the on and off states of the transistors due to poor switching characteristics or improper circuit parameter selection. Class D amplifiers, on the other hand, offer high efficiency, low distortion, and a good frequency response curve. They also require fewer external components. Class AB and Class D amplifiers are the basic circuit forms of audio power amplifiers.

 

Class T Amplifier:

Operational: The core of a Class T amplifier lies in its modulation technology. Invented and marketed by Tripath, this technology combines adaptive delta-sigma modulation with PWM (pulse-width modulation) technology, though it also operates the power transistors at high frequencies. The Class T amplifier's power output circuit is identical to that of a PWM Class D amplifier, with the power transistors also operating in a switching state. Its efficiency is comparable to that of a Class D amplifier. However, it differs from ordinary Class D amplifiers in the following ways:

 

First, it does not use pulse width modulation. Instead, Tripath developed a digital power technology called Digital Power Processing (DPP), which is the core of the Class T amplifier. It applies adaptive and predictive algorithms used in communications technology to small signal processing. The input audio signal and the current entering the speaker are digitally processed by DPP and then used to control the on/off switching of the power transistors, resulting in high-fidelity linear amplification.

 

Second, the switching frequency of the power transistors is not fixed, so the power spectrum of the unwanted component is not concentrated in a narrow band on both sides of the carrier frequency, but is spread over a wide frequency band. This allows for audible sound details across the entire frequency band.

 

Furthermore, Class T power amplifiers offer a wider dynamic range and a flatter frequency response. The advent of DDP has taken power amplifiers to new heights in the digital age. In terms of high fidelity, their linearity surpasses, if not surpasses, that of traditional Class AB amplifiers.

 

Advantages of Class T Amplifiers:

1. "Digital efficiency, analog sound quality": This was the original marketing slogan for Class T amplifiers, and it accurately captures their greatest strengths. They successfully combine the high efficiency of Class D (typically reaching 80%-90%) with excellent sound quality approaching Class AB or even Class A.

2. Low distortion and high signal-to-noise ratio: Thanks to digital modulation technology, Class T amplifiers excel in key audio metrics such as total harmonic distortion (THD) and signal-to-noise ratio (SNR), far surpassing the less mature Class D amplifiers of earlier generations.

3. Low electromagnetic interference (EMI): Adaptive switching frequency (also known as "spread spectrum" technology) effectively reduces peak noise in the spectrum, simplifying EMC certification. 4. High Integration: Early Class T products (such as Tripath's TK2050 and TA2024) integrated complex control logic and power output into a single chip, greatly simplifying peripheral circuit design.

 

Disadvantages of Class T Amplifiers:

1. Patent Barriers and Single Supplier: Class T technology was initially exclusively controlled by Tripath. Following Tripath's bankruptcy in 2007, the technology's development stagnated. Although its patents have expired and other companies (such as Sanyo's "Digital Inverter Amp") have introduced similar technologies, their ecosystem and promotion efforts are far less robust than those of the mainstream Class D architecture.

2. Being Overtaken by Modern Class D Technology: The greatest challenge facing Class T technology comes from the rapid advancement of traditional Class D technology. Modern high-performance Class D amplifiers (such as TI's PurePath™ HD, ADI's Class D, and Infineon's MERUS™ series) utilize more advanced modulation schemes, faster switching speeds (GaN FETs), and sophisticated feedback techniques. Their sound quality now rivals or even surpasses that of Class T amplifiers of the time, while maintaining exceptionally high efficiency. 3. No longer a mainstream technology: Today, the term "Class T" is rarely mentioned in academia and industry. Its technical concepts and advantages have been absorbed and integrated into the broader category of "high-performance Class D" or "digital input amplifier."

 

Comparison of Different Amplifier Types

Type	Conduction Angle	Theoretical Maximum Efficiency	Advantages	Disadvantages	Main Applications
A类	360°	25%	Optimal linearity, extremely low distortion	Extremely low efficiency, high heat generation, and high cost	High-end hi-fi audio
B类	180°	78.5%	High efficiency	Severe crossover distortion	Rarely used alone
AB类	180°-360°	50-70%	Low distortion, moderate efficiency, and excellent value for money	Inefficient compared to Class D, requiring heat dissipation	Mainstream analog audio amplifiers
C类	<180°	>85%	Extremely high efficiency	Extreme distortion and nonlinearity	RF transmission, resonant circuits
D类	(Switch)	90-95%	Extremely high efficiency, low heat generation, and compact size	Potential for EMI generation and complex design	Consumer electronics, portable devices
T类	(Switch)	>80%	High efficiency + high sound quality, low EMI	Patent barriers and a weak technology ecosystem	High-end mini speakers, car amplifiers, etc. are now integrated into modern Class D amplifiers

 

 

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