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Class D amplifiers, first proposed in , have become increasingly popular in recent years. What are Class D amplifiers? How do they compare with other kinds of amplifiers? Why is Class D of interest for audio? Find the answers to all these questions in the following pages.
The goal of audio amplifiers is to reproduce input audio signals at sound-producing output elements, with desired volume and power levels—faithfully, efficiently, and at low distortion. Audio frequencies range from about 20 Hz to 20 kHz, so the amplifier must have good frequency response over this range less when driving a band-limited speaker, such as a woofer or a tweeter. A straightforward analog implementation of an audio amplifier uses transistors in linear mode to create an output voltage that is a scaled copy of the input voltage.
The forward voltage gain is usually high at least 40 dB. If the forward gain is part of a feedback loop, the overall loop gain will also be high. Feedback is often used because high loop gain improves performance—suppressing distortion caused by nonlinearities in the forward path and reducing power supply noise by increasing the power-supply rejection PSR.
In a conventional transistor amplifier, the output stage contains transistors that supply the instantaneous continuous output current. Compared with Class D designs, the output-stage power dissipation is large in even the most efficient linear output stages. This difference gives Class D significant advantages in many applications because the lower power dissipation produces less heat, saves circuit board space and cost, and extends battery life in portable systems.
Linear-amplifier output stages are directly connected to the speaker in some cases via capacitors. If bipolar junction transistors BJTs are used in the output stage, they generally operate in the linear mode, with large collector-emitter voltages.
The output stage could also be implemented with MOS transistors, as shown in Figure 1. The amount of power dissipation strongly depends on the method used to bias the output transistors. The Class A topology uses one of the transistors as a dc current source, capable of supplying the maximum audio current required by the speaker. Good sound quality is possible with the Class A output stage, but power dissipation is excessive because a large dc bias current usually flows in the output-stage transistors where we do not want it , without being delivered to the speaker where we do want it.
The Class B topology eliminates the dc bias current and dissipates significantly less power. Its output transistors are individually controlled in a push-pull manner, allowing the MH device to supply positive currents to the speaker, and ML to sink negative currents. This reduces output stage power dissipation, with only signal current conducted through the transistors.
The Class B circuit has inferior sound quality, however, due to nonlinear behavior crossover distortion when the output current passes through zero and the transistors are changing between the on and off conditions. The small dc bias current is sufficient to prevent crossover distortion, enabling good sound quality. Some control, similar to that of the Class B circuit, is needed to allow the Class AB circuit to supply or sink large output currents.
Unfortunately, even a well-designed class AB amplifier has significant power dissipation, because its midrange output voltages are generally far from either the positive or negative supply rails. Thanks to a different topology Figure 2 , the Class D amplifier dissipates much less power than any of the above. Its output stage switches between the positive and negative power supplies so as to produce a train of voltage pulses.
Since most audio signals are not pulse trains, a modulator must be included to convert the audio input into pulses. The frequency content of the pulses includes both the desired audio signal and significant high-frequency energy related to the modulation process. A low-pass filter is often inserted between the output stage and the speaker to minimize electromagnetic interference EMI and avoid driving the speaker with too much high frequency energy.
The filter Figure 3 needs to be lossless or nearly so in order to retain the power-dissipation advantage of the switching output stage. The filter normally uses capacitors and inductors, with the only intentionally dissipative element being the speaker.
Significant differences in power dissipation are visible for a wide range of loads, especially at high and moderate values. At the onset of clipping, dissipation in the Class D output stage is about 2.
Note that more power is consumed in the Class A output stage than is delivered to the speaker—a consequence of using the large dc bias current. These best-case values for Class A and Class B are the ones often cited in textbooks. The differences in power dissipation and efficiency widen at moderate power levels. This is important for audio, because long-term average levels for loud music are much lower by factors of five to 20, depending on the type of music than the instantaneous peak levels, which approach P LOAD max.
Under this condition, mW is dissipated inside the Class D output stage, vs. These differences have important consequences for system design. For power levels above 1 W, the excessive dissipation of linear output stages requires significant cooling measures to avoid unacceptable heating—typically by using large slabs of metal as heat sinks, or fans to blow air over the amplifier.
If the amplifier is implemented as an integrated circuit, a bulky and expensive thermally enhanced package may be needed to facilitate heat transfer. These considerations are onerous in consumer products such as flat-screen TVs, where space is at a premium—or automotive audio, where the trend is toward cramming higher channel counts into a fixed space. For power levels below 1 W, wasted power can be more of a difficulty than heat generation. If powered from a battery, a linear output stage would drain battery charge faster than a Class D design.
In the above example, the Class D output stage consumes 2. For simplicity, the analysis thus far has focused exclusively on the amplifier output stages. However, when all sources of power dissipation in the amplifier system are considered, linear amplifiers can compare more favorably to Class D amplifiers at low output-power levels.
The reason is that the power needed to generate and modulate the switching waveform can be significant at low levels. Thus, the system-wide quiescent dissipation of well-designed low-to-moderate-power Class AB amplifiers can make them competitive with Class D amplifiers.
Class D power dissipation is unquestionably superior for the higher output power ranges, though. Figure 3 shows a differential implementation of the output transistors and LC filter in a Class D amplifier. This H-bridge has two half-bridge switching circuits that supply pulses of opposite polarity to the filter, which comprises two inductors, two capacitors, and the speaker.
Each half-bridge contains two output transistors—a high-side transistor MH connected to the positive power supply, and a low-side transistor ML connected to the negative supply. The diagrams here show high-side p MOS transistors. High-side n MOS transistors are often used to reduce size and capacitance, but special gate-drive techniques are required to control them Further Reading 1. For a given V DD and V SS , the differential nature of the bridge means that it can deliver twice the output signal and four times the output power of single-ended implementations.
Full-bridge circuits do not suffer from bus pumping, because inductor current flowing into one of the half-bridges flows out of the other one, creating a local current loop that minimally disturbs the power supplies. The lower power dissipation provides a strong motivation to use Class D for audio applications, but there are important challenges for the designer. These include:.
The output transistor size is chosen to optimize power dissipation over a wide range of signal conditions. But this requires large transistors with significant gate capacitance C G. The gate-drive circuitry that switches the capacitance consumes power— CV 2 f , where C is the capacitance, V is the voltage change during charging, and f is the switching frequency.
Conductive losses will dominate power dissipation and efficiency at high output power levels, while dissipation is dominated by switching losses at low output levels.
To protect against dangerous overheating, temperature-monitoring control circuitry is needed. In simple protection schemes, the output stage is shut off when its temperature, as measured by an on-chip sensor, exceeds a thermal-shutdown safety threshold, and is kept off until it cools down. The sensor can provide additional temperature information, aside from the simple binary indication about whether temperature has exceeded the shutdown threshold.
By measuring temperature, the control circuitry can gradually reduce the volume level, reducing power dissipation and keeping temperature well within limits—instead of forcing perceptible periods of silence during thermal-shutdown events.
Excessive current flow in the output transistors : The low on resistance of the output transistors is not a problem if the output stage and speaker terminals are properly connected, but enormous currents can result if these nodes are inadvertently short-circuited to one another, or to the positive or negative power supplies.
If unchecked, such currents can damage the transistors or surrounding circuitry. Consequently, current-sensing output-transistor protection circuitry is needed.
In simple protection schemes, the output stage is shut off if the output currents exceed a safety threshold. In more sophisticated schemes, the current-sensor output is fed back into the amplifier—seeking to limit the output current to a maximum safe level, while allowing the amplifier to run continuously without shutting down.
In these schemes, shutdown can be forced as a last resort if the attempted limiting proves ineffective. Effective current limiters can also keep the amplifier running safely in the presence of momentarily large transient currents due to speaker resonances.
Undervoltage : Most switching output stage circuits work well only if the positive power supply voltages are high enough. Problems result if there is an undervoltage condition, where the supplies are too low.
This issue is commonly handled by an undervoltage lockout circuit, which permits the output stages to operate only if the power supply voltages are above an undervoltage-lockout threshold. It is therefore important to avoid situations in which both MH and ML are on simultaneously, as this would create a low-resistance path from V DD to V SS through the transistors and a large shoot-through current. At best, the transistors will heat up and waste power; at worst, the transistors may be damaged.
Break-before-make control of the transistors prevents the shoot-through condition by forcing both transistors off before turning one on. The time intervals in which both transistors are off are called nonoverlap time or dead time.
Clicks and pops , which occur when the amplifier is turning on or off can be very annoying. Unfortunately, however, they are easy to introduce into a Class D amplifier unless careful attention is paid to modulator state, output-stage timing, and LC filter state when the amplifier is muted or unmuted.
Signal-to-noise ratio SNR : To avoid audible hiss from the amplifier noise floor, SNR should typically exceed 90 dB in low-power amplifiers for portable applications, dB for medium-power designs, and dB for high-power designs. This is achievable for a wide variety of amplifier implementations, but individual noise sources must be tracked during amplifier design to ensure a satisfactory overall SNR.
Distortion mechanisms: These include nonlinearities in the modulation technique or modulator implementation—and the dead time used in the output stage to solve the shoot-through current problem.
Information about the audio signal level is generally encoded in the widths of the Class D modulator output pulses. Adding dead time to prevent output stage shoot-through currents introduces a nonlinear timing error, which creates distortion at the speaker in proportion to the timing error in relation to the ideal pulse width.
The shortest dead time that avoids shoot-through is often best for minimizing distortion; see Further Reading 2 for a detailed design method to optimize distortion performance of switching output stages. Other sources of distortion include: mismatch of rise and fall times in the output pulses, mismatch in the timing characteristics for the output transistor gate-drive circuits, and nonlinearities in the components of the LC low-pass filter.
Power-supply rejection PSR : In the circuit of Figure 2, power-supply noise couples almost directly to the speaker with very little rejection.
This occurs because the output-stage transistors connect the power supplies to the low-pass filter through a very low resistance. The filter rejects high-frequency noise, but is designed to pass all audio frequencies, including noise.
See Further Reading 3 for a good description of the effect of power-supply noise in single-ended and differential switching output-stage circuits.
If neither distortion nor power-supply issues are addressed, it is difficult to achieve PSR better than 10 dB, or total harmonic distortion THD better than 0. Even worse, the THD tends to be the bad-sounding high-order kind.
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Understanding Audio Signals. It is generally used in the context of analog signals or alternating current AC signals. The RMS value is the amount of current or voltage which is equal to its equivalent direct current DC.
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