"In the traditional high fidelity system, the technical specifications of audio amplifier always emphasize the sound quality, but rarely consider the degree of power loss. However, with the growth of portable high fidelity in the audio industry, the shortcomings of traditional amplifier devices, especially its low efficiency, have become an urgent problem to be solved.
Traditionally, audio playback devices use so-called class AB amplifiers, which have low distortion and produce high sound quality. However, the operation mode of class AB amplifier explains the reason for its low efficiency: the internal voltage of the amplifier will decrease as the output voltage decreases. The transistor of the amplifier will consume too much power, so the efficiency of the system will decrease as the output speaker power decreases.
For high fidelity equipment powered by power supply, this is not a big problem; However, for battery powered audio devices, such as mobile phones and MP3 players, this is a considerable problem, because the power consumption of audio amplifiers accounts for a large proportion in the whole system. Taking MP3 player as an example, the power consumption of audio amplifier accounts for 80% of the overall power consumption.
Therefore, audio equipment designers have been looking for ways to strengthen class AB topology. The question to be discussed in this paper is whether the power saving effect achieved by using new technologies such as g or h is worth it? If the system designer uses class G or class H amplifiers, is the difference in power consumption caused by using class G or class H amplifiers large enough to affect the overall power budget?
System requirements for portable audio equipment
The audio amplifier used by handheld devices usually drives a 16? Or 32? These two impedances often consume most of the power budget of the equipment. This means that any improvement in the efficiency of the power amplifier can significantly improve the efficiency of the whole equipment and battery life. For more information, please log in to the electronic enthusiast website（ http://www.elecfans.com )
As we can see, the most important parameter affecting the efficiency of traditional audio amplifiers is the peak output power. This is mainly determined by the type of headset used by the equipment: compared with headphones, earplug type has lower peak power requirements, but the typical output power range of the two channels is 4MW each, and the total power can be as high as 2 × 30mW。
Right 32? For impedance headphone speakers, if the output power is 30MW, an amplifier output swing of ± 1.38vpk is required. The amplifier stage for this application will require an additional voltage space of 100-200mv. Therefore, the power supply voltage of the headphone amplifier will be 2 × 1.5V = 3.0V
In order to avoid using an output DC decoupling capacitor that is too large for the application, a charge pump is generally used to generate the negative power rail required by the headphone amplifier so that the audio output operates near the battery ground point. This configuration is the "true ground" headphone amplifier. It uses 1.5V positive power supply- The 1.5V power rail comes from the charge pump.
The most commonly used battery type is lithium-ion battery, which generally produces 3.6V output. The efficient DC / DC step-down converter can convert the battery output to positive 1.5V power supply without significant loss. This is a common configuration of class AB amplifier. See Figure 1 for typical system block diagram.
Figure 1: true ground headphone amplifier.
(DC / DC step-down converter, charge pump for negative power supply, class AB amplifier control unit, earphone amplifier)
The high-quality DC-DC converter can convert the lithium-ion battery voltage from 3.7V to a fixed 1.5V output voltage with an efficiency of up to 93%. This method is certainly more efficient than when the amplifier transistor consumes 2.2V (3.7V battery voltage - 1.5V working voltage).
However, this does not mask the fact that transistors still consume a lot of power except at high output voltage levels. To solve this problem, we need to change the power configuration of the amplifier itself, which is the reason for developing class G and class H amplifiers.
Matching input voltage and output voltage
The power value of the audio amplifier is the peak power value. In fact, the time required for this maximum supply voltage is very short; The audio signal has a wide dynamic range. Most of the time, the output voltage is lower than 0.5V, while the power supply voltage of the amplifier can reach 1.5V. The difference between the output voltage and the supply voltage comes from the part consumed by the internal amplifier transistor, which is the main cause of power loss.
In order to solve this problem, the power supply voltage of class G and class H amplifiers should meet the required output power to a certain extent. Class G amplifiers generally have two supply voltage levels. The higher supply voltage level is determined by the maximum output power required. The lower supply voltage level is determined by the minimum supply voltage at which the amplifier can operate and which is higher than the threshold of total harmonic distortion (THD).
Compared with Class G amplifier, class H amplifier can smoothly adjust the output voltage according to the requirements of the output signal. Therefore, unlike class G amplifiers, class H amplifiers are not limited to two or three fixed output voltages. It can smoothly convert the minimum allowable supply voltage from the step-down converter to any discrete output voltage.
Therefore, the class H amplifier can operate at a power supply voltage that closely matches the actual output voltage; Reduce the difference between power supply voltage and output voltage and reduce the required power consumption（ (see Figure 2)
Figure 2: power consumption for different amplifier configurations
In fact, since audio equipment operates at the minimum allowable power supply voltage most of the time, the power consumption of class G and class H amplifiers and the proportion of headphones in the total power consumption of the system are more or less determined by the power supply at the lower level of the total harmonic distortion threshold. We call this important parameter, that is, the minimum supply voltage, vsupmin.
Figure 2 shows the differences in power consumption between different types of amplifiers. The positive and negative voltages of each amplifier are indicated by black dotted lines. Class G amplifiers can support two different output voltages: 1.5V and 1.2V (vsupmin). On the other hand, class H amplifiers support additional voltage levels between 1.5V and 1.2V. Class G and class H amplifiers can significantly reduce power consumption.
In addition, compared with class AB amplifiers, the implementation using class G or class H topology is not very complex, and the cost does not increase much (Fig. 3 is a simplified block diagram of class G / class H amplifiers). The main difference from class AB amplifiers is that the DC / DC converter no longer has a fixed output voltage. The variable voltage output needs to add a feedback signal between the amplifier output stage and the DC / DC converter to adjust the output voltage according to the audio signal.
Figure 3: block diagram of class G / h amplifier.
(DC / DC step-down converter, charge pump for negative power supply, class AB amplifier control unit, headphone amplifier, class G / h sensor)
The degree of difference between the best and worst class H or G amplifiers
Considering cost and availability, headphone amplifiers are generally manufactured by CMOS technology. Although the ideal value of vsupmin depends on the circuit design of the amplifier, in practice, it often depends on the minimum threshold of CMOS technology used to manufacture the amplifier.
In today's amplifier design, vsupmin is defined as 2 × VTH+Vdsat。 If the CMOS process for manufacturing the device can support a lower vsupmin value by realizing a lower transistor threshold, a better result will be obtained. For example, if the amplifier stage operates at a minimum supply voltage of ± 1.2V, the power consumption is increased by 30% compared with an amplifier operating at ± 0.9V playing the same audio signal.
This is the commitment of Austrian microelectronics special lowvt CMOS process. These processes have been used to produce new as3561 devices. The as3561 is a class H amplifier that provides a supply voltage as low as ± 0.9 V to the headphone amplifier. With high-efficiency DC-DC converter and adaptive charge pump, it can minimize power consumption: 2 x 0.1MW, the battery voltage is 3.6V, and the typical current consumption is only 1.7ma. The power consumption difference between this high-efficiency architecture and class G and class AB architectures is shown in Figure 4.
Figure 4: power consumption comparison of class h, class AB and class G
conclusion
Compared with the widely used class AB amplifiers, class G and class H audio amplifiers can dynamically adjust the power supply voltage and greatly improve the power efficiency. However, the analysis of the devices that work at the total harmonic distortion threshold of the amplifier most of the time shows that the class H amplifier manufactured by low voltage threshold technology can save an additional 30% of power., Technology Zone
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