Linear amplifiers are amplifiers with a linear output stage, in which there exists a voltage drop across the output transistors to generate the correct output voltage. Even though most of these amplifiers use some sort of switching, they are not to be confused with switching amplifiers. Switching amplifiers are amplifiers with a switching output stage. This means that the transistors in the output stage have a switch function. Any simultaneous occurrence of voltage across and current through these transistors is undesirable.
The class D principle
A typical class D amplifier consists of a modulator that converts an analogue or digital audio signal into a high frequency Pulse Width Modulated (PWM) or Pulse Density Modulated (PDM) signal followed by the output stage, often a half bridge power switch (Picture 1). The output of the switches is either high or low, and changes at a frequency that is much higher than the highest audio frequency. Typical values are between 200 kHz and 500 kHz. The frequency spectrum of the PWM signal in the audio band is the same as the frequency spectrum of the audio signal. An LC filter filters out the high frequency switching components, so that the audio signal is available at the output of the filter. Ideally, the switches do not dissipate and neither does the filter, so the efficiency can be very high.
Picture 1: Principle of PWM amplifier
For a 10 kHz sinewave, a switching frequency of 350 kHz, and a filter with a 30 kHz Butterworth characteristic, the signals look like Picture 2. In this case, the audio frequency is close to the corner frequency of the filter, so some phase shift can be observed between the PWM signal and the audio signal.
Picture 2: Class D output signal (before and after the filter)
Output stage
Picture 3 shows a typical class D output stage. It is a class AD stage, which is used for most class D amplifiers. It is a simple inverter. When the input signal is positive, M2 conducts. When it is negative, M1 conducts.
Picture 3: A typical class D output stage
The diodes D1 and D2 are needed because the transistors are unidirectional switches. Suppose the output signal is positive, and the output current Io is also positive. When M1 is switched on, this is OK, but when M2 is switched on, the coil in the output filter still tries to keep the current Io, forcing the output voltage below -VS, causing D2 to conduct. With DMOS transistors as switches, the intrinsic diodes can be used. However, the intrinsic diode of a DMOS transistor can have a long recovery time (several hundred ns) or cause latch-up. In that case external (shottky) diodes are a solution, although not a desirable one. It is also possible to build DMOS transistors with a fast-recovery intrinsic diode.
Switching speed
High switching speeds are necessary to keep switching losses small. Typical values of today’s integrated designs are tens of nanoseconds. Because of the large gate-source capacitances of M1 and M2, this leads to large peak currents. Also, the high speed switching in combination with wires and (gate) capacitances can cause ringing, overshoot, and delays. For a low distortion it is important that the switching times of M1 and M2 are equal. Tuneable coils between M1 and M2 can provide a solution. However, both the fact that these coils can not be integrated and that they need to be tuned make this an unattractive solution. With high speed switching, the risk of common conduction of M1 and M2 increases. The introduction of a "dead zone" in which both transistors are turned off is a common solution, although this introduces extra distortion in the audio signal. Another option is a handshake procedure to check if the other transistor is turned off.
Power supply
In pure feed-forward systems a stable power supply is extremely important, because any deviation from the nominal value shows up in the output signal. For an output signal of 16 bit accuracy, the power supply should have a 16 bit stability. Common solutions are feedback from the pulsed output or feed-forward correction by referring the triangle waveform to the supply voltage. Another supply issue arises from the use of NMOS devices that are preferable thanks to the lower R on per area. The gate of M1 needs a voltage that is higher than VS. A bootstrap capacitor or a charge-pump can provide such a voltage.
Cross-over distortion
M1 and M2 have a certain Ron resistance. D1 and D2 have a certain voltage drop when conducting. Suppose the output current is positive. During conduction of M1, the voltage will be a little lower than VDD because of Ron1. During conduction of D2, the voltage will be a little lower than -VS due to the voltage drop. So all the time the voltage is lower than it should be. When the output current is negative, the same reasoning shows that the output voltage is too high. This results in crossover distortion. It can be solved by connecting the transistors to a tap of the output inductor or a separate supply voltage.
Class BD output stage
An alternative to the class AD stage is the class BD stage. In class BD there are three possible output voltages: positive, negative, and zero. There are several ways in which this can be implemented, but the simplest one is shown in Picture 4.
Picture 4: Class BD modulator with output filter
In quiescent, the signal at A and B is the same PWM signal with 50 % duty cycle. The signal A-B across the filter is therefore zero. For a positive output voltage, the duty cycle of A is increased and that of B decreased. The difference signal A-B is now a voltage that varies between 0 and VS. Similarly, for negative output voltages, A-B varies between 0 and -VS. The pulse frequency of A-B is doubled compared to A and B, which is favourable for speed requirements. Balanced current design has the same qualities and the topologies are very similar. The difference between a bridge class BD stage and a bridge class AD stage is subtle. The topologies are exactly the same. In the class AD case, however, A is always the inverse of B, so that A-B alternates between -VS and VS.
Resonant output stage
A way to generate the high frequency pulses for a PDM modulator is to use a quasi-resonant converter. This converter gives 1 bit each time it is switched on. The bit is not a squarewave, but the positive half of a sinewave. This is irrelevant, as long as the area under the signal is the same each time. For the topology in Picture 5 (Lf and Cf are the output filter) this is true, virtually independent of output current and voltage. Switching occurs when the current is zero, giving better efficiency and lower switching noise. The large number of filter components make this topology not very attractive for use with integrated circuits.
Picture 5: Quasi-resonating converter
Summary and Conclusions
The design of a class D output stage is not a trivial matter. In general, an output stage will not be able to preserve the exact frequency content of its input signal. To summarise the limitations that were encountered, it is easy to start with an important audio amplifier specification: low distortion. With feedback directly from the switched output, very good high power PWM signals can be generated. The output filter, however, introduces additional distortion and deviations of the specified frequency transfer when non-resistive loads are connected. Feedback after the filter is difficult, and high feedback factors can not be realised. Even when these problems are overcome, the filter prevents further integration because for sufficient suppression of the carrier frequency, typically a fourth order filter is necessary. It is not possible to eliminate the external two coils and two capacitors without introducing a much larger switching residue.
Linear amplifiers have a low complexity, can have a low distortion, but show limited possibilities for reduction of the dissipation. To reduce the dissipation to very low values, complex switching schemes are necessary, and a large number of external elcos makes such a solution little attractive. Switching amplifiers can have a very low dissipation, but suffer from switching noise at the output, an external filter, a load dependent frequency transfer and difficulties in achieving a low distortion. The idea that a mix of these two systems may be beneficial is not new, and there are a lot of possibilities for such combinations.
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