Class-D Amplification Explained

If you don't know anything about electronics, don't worry. Well, don't worry much - We're not going any deeper than an existing understanding of how a battery and bulb work (of course, a little knowledge of audio signals won't go amiss).

In the beginning was the single-ended Class-A amplifier, as shown in Figure 1. We've simplified the schematic to show only the output device, which is where the differences between the classes are defined. In a simple amplifier like this, the audio input signal - a small alternating current (AC) - flowing into the base of the transistor ('b') controls a larger direct current (DC) flowing from the output of the amplifier's power supply through the collector ('c') and emitter ('e') to earth. The parts of the circuit we have left out, 'bias' the transistor so that when there is no input signal, the output voltage (ie. the voltage at the collector) is half the total supply voltage. This allows the output voltage to vary both up and down to an equal extent, to recreate the AC waveform of the input. If the voltage with no input-signal present was to be anything other than halfway between zero and the full power-supply voltage, then inevitably one half of the waveform would run out of volts before the other, limiting the amount of amplification available before the waveform would be clipped.

You Push, I'll Pull

Figure 2 shows an alternative strategy, in the form of a push-pull amplifier output stage. One transistor 'pulls' the voltage up on the positive half-cycle of the waveform. The other transistor 'pushes' the voltage down on the negative half-cycle. Well, that's the kindergarten explanation. Let's look in a little more detail...

In this version, we have shown both a positive supply rail and a negative supply rail, as well as an earth exactly in between in voltage; zero volts in fact. A single-ended (positive- or negative-only) power supply can be used, but a dual-rail supply is better, as no DC-blocking output capacitor is necessary. This is because, when there's no signal, both terminals of the loudspeaker are at zero volts, so no current flows and there is no DC to be blocked. You will notice that the transistors are slightly different to each other. The upper transistor (Q1) is what we call 'npn', meaning that it will conduct between collector and emitter for a positive voltage at the base. The lower transistor (Q2) is 'pnp', meaning that it will conduct between collector and emitter for a negative voltage at the base. If you're into electronics already, you will have noticed that there is another difference between this and Figure 1. In Figure 2, the loudspeaker is connected to the emitters of the transistor, rather than the collector of the transistor in Figure 1. This means that all the voltage amplification has to precede this stage. This part of the circuit is responsible for delivering a high current to the loudspeaker. But don't worry too much about that; it doesn't affect my explanation of the amplifier classes.

In this configuration, a high input voltage will cause Q1 to conduct, bringing the output voltage close to the positive supply-rail voltage. A zero input-voltage will cause neither transistor to conduct. The output is at zero volts. With neither transistor in conduction, clearly no current is available for the loudspeaker. But since there is no voltage across the terminals of the loudspeaker, it doesn't need any (a handy coincidence). When the input voltage is low, Q2 conducts, allowing the voltage at the output to descend almost to that of the negative supply-rail. From this, you can see that Q1 handles the positive half-cycles of the waveform and Q2 handles the negative half-cycles. This is Class B.

The beauty of this arrangement is that it is much more efficient. When the input signal is zero, there is no current flowing either through the loudspeaker or through the transistors. The maximum theoretical efficiency for a sine-wave input is 78.5 percent - a vast improvement over Class A.

But there is a fly in the ointment. A transistor will conduct hardly at all if the voltage on the base is less than 0.6 volts (minus 0.6 volts for a pnp transistor). So input voltages between -0.6 and +0.6 volts will not stir either transistor into conduction. Figure 3 shows the consequence. That flat spot in the middle of the waveform is called 'crossover distortion' and is an intrinsic feature of Class B. Fortunately, there is an answer, and that is to 'bias' the input to the two transistors, as in Figure 4. Those two new components between the bases of the transistors are diodes. Their effect is to separate the standing voltages on the bases by 1.2 volts, thus overcoming the intrinsic 'inertia' of the transistors. The input signal now only has to twitch and the transistors will respond. This is a simplification of a real-world circuit, but only slightly so. In a real circuit, the voltages on the bases of the transistors would have to be slightly further apart, and adjustable to set the 'quiescent current' (the constant current when no input signal is present).

The benefit here is that crossover distortion is almost eliminated, at the expense of a slight standing current when the signal is at zero level. It is interesting to note that the biasing could be arranged so that the transistors carried a very high current for a zero input signal. When the input moved, this current would be diverted through the load. Guess what? This is Class A again. It's a push-pull Class-A output stage, no more efficient than a single-ended Class A, but more practical to implement, and entirely lacking in crossover distortion, which is why it is admired by high-end hi-fi enthusiasts. The compromise situation of biasing the output transistors so that they are just in conduction is called Class AB. Class AB is far and away the most common type of amplifier. It's reasonably efficient, and its sound quality is excellent, surpassed only by Class-A amplifiers that run as warm as an Aga and cost the earth - both to buy and to run.

To round off this section, Figure 5 shows a simple Class-C amplifier. It drives a resonant load and is thus very efficient at the resonant frequency of the load - over 90 percent. However, since it only works over a narrow range of frequencies, it is entirely unsuitable for audio. Class-C amplification is actually used in radio transmission.

Now that we know how classes A, B, AB and C work, we can look at Class D. Clearly, classes A to C are all in the same family, but Class D is completely different. In Classes A, B and AB, the problem is lack of efficiency. Some power is wasted, and we would prefer that it could be sensibly employed in driving the loudspeakers to ever-higher sound pressure levels - or, at least, not converted to heat. Where power is wasted is where a transistor is in partial conduction. When a transistor is fully conducting, it's like a piece of wire, and a piece of wire loses hardly any power. When a transistor is fully off, it doesn't conduct at all, and if it doesn't conduct at all, there's no power to waste. It's the in-between stages that cause the problem, where the transistor wastes power and gets hot. So what if we could find a way for transistors to be used only in their fully-on or fully-off states. If that were possible, no power would be lost. But is it possible...?

It is, and the solution is what we call Class D. Figure 6 shows a simplified Class-D amplifier. First, let's look at the similarities between this and what we've already discussed. You can see two transistors, in push-pull configuration, as before. The transistors look slightly different because they are MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) rather than 'ordinary' transistors. The output transistors have to be fast, so that they can switch very quickly between fully on and fully off. It also helps for the on and off states to be 'really' on and off. The closer the transistors can get to full conduction or full non-conduction, the greater the efficiency of the amplifier will be. Clearly, though, as well as the similarities there are some differences.

Going further, if the output is filtered to remove the high frequencies and sharp corners of the pulse waveform, the original input signal will be reconstructed, exactly the same shape as it was, but bigger. The net result is an amplified signal that you couldn't distinguish from that produced by a conventional Class-AB power amplifier.

But how is the pulse waveform produced? OK, it isn't simple, but it isn't rocket science either. First we need a circuit building-block known as a comparator. A comparator has two inputs: let's call them Input A and Input B. When Input A is higher in voltage than Input B, the output of the comparator will go to its maximum positive voltage. When Input A is lower in voltage than Input B, the output of the comparator will go to its maximum negative voltage. Figure 7 shows how the comparator operates in a Class-D amplifier. One input (input A in our example) is supplied with the signal to be amplified. The other input (input B) is supplied with a precisely generated triangle wave. When the signal is instantaneously higher in level than the triangle wave, the output goes positive. When the signal is instantaneously lower in level than the triangle wave, the output goes negative. The result is a chain of pulses where the pulse width is proportional to the instantaneous signal level. Magically simple! We call it 'pulse width modulation', or PWM. And that's all there is to it. You now understand how a Class-D amplifier works, and if anyone tries to pull the wool over your eyes and convince you that the 'D' stands for 'digital', you can tell them how wrong they are, with confidence. Class D is not digital.