Tag: Class D Amplifier Audio Amplifier
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The purpose of the audio amplifier is to reproduce the input audio signal on the audible output component to provide the required volume and power levels - to ensure reproducible fidelity, high efficiency and low distortion. In the face of this task, Class D amplifiers show many advantages.
Audio refers to a frequency range of about 20 Hz to 20 kHz, so an audio amplifier must have excellent frequency response characteristics in this band (the frequency range with better frequency response characteristics can be narrower when driving a limited bass and tweeter) ). The demand for power capability varies greatly. The specific indicators depend on the application requirements, from the mW level of the headset to the number of W on the TV or PC, to the "mini" home stereo, car audio, and the highest It is a more powerful home and commercial audio system for theaters and auditoriums with powers of hundreds of W or more.
The most straightforward, analog implementation of an audio amplifier is to have the transistor operate in linear mode, allowing the output voltage to vary with the input signal voltage in a certain ratio. The forward voltage gain is often very high (at least 40 dB). If the forward gain is part of the feedback loop, the total loop gain will also be high. Feedback is often used in circuits because of the high loop gain that provides higher performance—suppressing the distortion caused by the nonlinearity of the forward path and reducing power supply noise by increasing the power supply rejection (PSR).
In conventional transistor amplifiers, the transistors on the output stage need to provide a continuous output current at all times. A variety of implementations that the sound system can take include Class A, Class AB, and Class B. Among these circuits, even the most efficient linear output stage, the power dissipation is large compared to the class D amplifier. This difference reflects the fact that Class D amplifiers offer significant advantages in many applications because less power dissipation means lower heat generation, board space and cost savings, and extended battery operating time in portable systems.
Power dissipation occurs in all linear output stages because the generation of Vout inevitably results in non-zero IDS and VDS on at least one of the output transistors. The amount of power dissipation is largely determined by the biasing method of the output transistor.
The Class A architecture uses a transistor in the architecture as a DC current source that provides the maximum audio current required by the speaker. Class A output stage can provide good sound quality, but because the output stage transistor tends to flow a large DC bias current (which we don't want), this current cannot be supplied to the speaker (this is instead We hope that) will result in excessive power dissipation.
Class B loudspeakers cancel the DC bias current and the power dissipated is greatly reduced. The output transistors are separately controlled in a push-pull manner such that one of the devices supplies a forward current to the speaker and the other absorbs the negative current. This reduces the power dissipation of the output stage, with only the signal current flowing through the transistor. However, the sound quality of the Class B circuit is poor because it operates in a non-linear state (crossover distortion) when the output current crosses zero and the transistor switches between on and off states.
Class AB output circuits are a compromise between Class A and Class B circuits that have a certain DC bias current, but this current is much less than the current used in a pure Class A design. The small DC bias current is sufficient to prevent crossover distortion, thus ensuring good sound quality. Power dissipation, although between Class A and Class B, is generally closer to Class B. Class AB circuits must take some control mechanism similar to Class B circuits in order to be able to provide or absorb large output currents. A different topology - the emergence of Class D amplifiers, is fortunate, it consumes The power is much lower than any other circuit. Its output stage switches back and forth between positive and negative supplies to create a voltage pulse train. This waveform is advantageous for reducing power dissipation because the output transistor has zero current when no switching action occurs, and the voltage across it is low when the current is turned on, so the IDS x VDS value is smaller.
Because most audio signals are not pulse trains, the audio input must be converted to pulses by a modulator. The frequency components of these pulses include both the desired audio signal and the significant high frequency energy introduced by the modulation process.
A low-pass filter is often placed between the output stage and the speaker to minimize electromagnetic interference (EMI) and to avoid excessively high frequency energy in driving the speaker. The filter should be lossless (or nearly lossless) in order to guarantee the low power dissipation of the switching output stage. The filter is usually composed of a capacitor and an inductor, and the only power dissipating component that is intentionally introduced is the speaker itself.
Clicks and pops
To ensure the overall excellent sound quality of the Class D amplifier, several problems must be solved.
Turning the amplifier on and off can be accompanied by annoying clicks and popping sounds. Unfortunately, these noises are easily introduced in Class D amplifiers unless the modulator's state, output stage timing, and LC filter state are carefully regulated during the amplifier's mute and non-silent state switching.
In order to avoid amplifier background noise that can be heard by the human ear, the signal-to-noise ratio (SNR) of low-power amplifiers in portable applications tends to be greater than 90 dB, while amplifiers for medium- and high-power designs are designed. The SNR should be 100dB and 110dB or more, respectively. This can be done with many types of amplifier implementations, but individual noise sources should be tracked in the amplifier design to ensure a satisfactory overall SNR.
The mechanisms that generate distortion include modulation techniques or non-linearities in modulator implementations, as well as "dead times" introduced at the output stage to prevent shoot-through current problems.
Information about the strength of the audio signal is typically encoded by the width of the output pulse of the class D modulator. In order to prevent the shoot-through current of the output stage from introducing a dead zone, a non-linear timing error is introduced, which in turn produces a distortion amount on the speaker that is proportional to the timing error relative to the ideal pulse width. In order to minimize distortion, the dead time introduced to avoid straight-through should be as short as possible.
Other sources of distortion include mismatch in rise and fall times of the output pulse, mismatch in timing characteristics of the output transistor gate drive circuit, and nonlinearity of the components of the LC low pass filter.
In terms of power supply fluctuation rejection, power supply noise can be directly coupled to the speaker with little suppression. This is so because the output stage transistors connect the power supply directly to the low-pass filter through a small resistor. The filter rejects high frequency noise but passes all audio components, including noise.
If distortion and power problems are not solved, it is difficult to achieve a PSR better than 10dB or a total harmonic distortion better than 0.1%. To make matters worse, THD belongs to the high-order component that makes an unpleasant sound.
Fortunately, there are two solutions to these problems. If the audio designer uses feedback with a very high loop gain (as with many linear amplifier designs), the circuit performance will be greatly improved. Feedback from the LC filter input will greatly improve the PSR and attenuate all non-LC filter distortion. The nonlinearity of the LC filter can be attenuated by incorporating the speaker into the feedback loop. In a well-designed closed-loop Class D amplifier, designers can achieve high fidelity audio quality: PSR > 60 dB, THD < 0.01%.
System cost
What are the important factors that affect the total cost of a sound system using a Class D amplifier? How can designers minimize costs?
The active components of a Class D amplifier are the switching output stage and the modulator. The cost of this circuit can be roughly equivalent to the cost of an analog linear amplifier, and the other components of the system are where the real cost is.
The lower power dissipation of a Class D amplifier can save the cost (and space) of the cooling device (heat sink or fan). Class D integrated circuit amplifiers can use a smaller, less expensive package than linear amplifiers. When playing with a digital audio source, the analog linear amplifier needs to convert the audio signal into an analog form through a digital-to-analog converter. Analog-to-digital Class D amplifiers also require digital-to-analog converters, but digital input circuits can effectively integrate a D/A converter function.
On the other hand, the main disadvantage of the class D amplifier in terms of cost is that it uses an LC filter. This component - especially the inductor - will take up board space and increase cost. In high power amplifiers, the total system cost is still competitive, as the enormous cost savings achieved in terms of cooling devices can offset the increase in the cost of the LC filter. However, in cost-sensitive, low-power applications, the cost of the inductor is enormous. In extreme cases, such as low-cost amplifiers for mobile phones, the cost of an amplifier IC may be lower than the cost of a total LC filter. In addition, the board space occupied by the LC filter is a problem for small form factor applications, even without considering cost.
In order to solve these problems, sometimes people simply cancel the LC filter, that is, use a filterless amplifier. This saves cost and space, although the benefits of low-pass filtering are not available. Without a filter, EMI and high-frequency power dissipation can increase to an unacceptable level unless the speaker is inductive, placed in close proximity to the amplifier, and the circuit ring area is small and the power level is low. Although this can often be done in portable applications such as cell phones, this technology is not suitable for more powerful systems, such as home stereos.
Another approach is to reduce the LC filter components of each channel as much as possible. This can be achieved by using a single-ended, half-bridge output stage circuit that requires half the number of inductors and capacitors for a differential, full-bridge circuit. However, if a half-bridge requires a bipolar power supply, the cost of generating a negative power supply is unacceptably high, unless some negative power supply is used for additional considerations—or the amplifier has enough Audio channels to share the cost increase associated with a negative power supply. On the other hand, a half-bridge power supply can also be powered by a single power supply, but the output power must be reduced, and it often requires a large DC blocking capacitor.
The output stage of a Class D amplifier switches between positive and negative supplies, producing a series of voltage pulses instead of producing a linear output like A, B, and AB amplifiers in traditional audio applications. Its output drives a speaker through a passive LC low-pass filter. The output transistor has zero current when turned off, and the voltage drop during turn-on is small, which makes the power dissipation of the class D amplifier much lower than other solutions. As a result, Class D amplifiers are less power consuming, take up less board space, and extend the battery life of portable systems, making them ideal for audio applications.
Sound designers need to carefully consider a variety of details when developing high-performance Class-D audio amplifiers, including choice of output transistor size, output stage protection, modulation techniques, and filter topology. In addition to the expected sound quality, EMI reduction and system cost are also factors to consider.
Fortunately, commercially available integrated circuit products can be used to implement the entire Class D amplifier, including gain-programmable amplifiers, modulators, and output stages to reduce the workload of the audio designer and reduce time-to-market . Evaluation boards, PC board layout and a reasonable bill of materials allow people to quickly design cost-effective sound systems without having to re-develop the main part of the Class D amplifier.
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