P O L I T E C N I C O D I T O R I N O CORSO DI LAUREA MAGISTRALE IN INGEGNERIA ELETTRONICA TELECOMMUNICATION ELECTRONICS AUDIO AMPLIFIERS

Transcription

1 P O L I T E C N I C O D I T O R I N O CORSO DI LAUREA MAGISTRALE IN INGEGNERIA ELETTRONICA ANALOG AND TELECOMMUNICATION ELECTRONICS AUDIO AMPLIFIERS An overview of the most commonly used class of amplifiers in audio applications Professor: DANTE DEL CORSO Student: JACOPO ABRAMO,

2 INTRODUCTION... 3 CLASS-A... 4 CLASS-B... 4 CLASS-AB... 5 CLASS-A EXAMPLE: Death-of-Zen... 6 CLASS-D: Digital Amplifier... 7 How class-d works... 7 Circuit Topology... 8 Efficiency... 9 CLASS-XD: Crossover Distortion... 10

3 AUDIO AMPLIFIERS An overview of the most commonly used class of amplifiers in audio applications INTRODUCTION To project a power amplifier requires, beside a good knowledge about electronics, a bit of creativity and imagination. It could almost be considered an art. This document illustrates some of the most known amplifier classes in the world of electronics applied for audio applications. For each class of amplifiers described below we could talk for more than just an hour, spanning on so many layers and topics that probably we could loose ourselves even in philosophical problems such as Is high efficiency preferable to very low signal distortion? and so on. We will try to discuss as much as possible for each of them, trying to respect time limits. It must be kept in mind that a good audio amplifier must fulfill many specification that do not always refer to the quality of the output sound. An audio designer has to take into account that many factors concur to generate distortion and/or contribute to loose efficiency, for example a power supply affected by high noise, the introduction of protection circuit against short circuit currents, a correct heat dissipation and so on. In Figure 1: general block diagram of a power amplifier

4 CLASS-A The class-a amplifier can be described with the following describing circuit. Figure 2: basic example of a class-a amplifier The output signal is amplified of a certain factor, depending on the design specification, for the whole period of the signal itself. The drawback of this circuit is the low efficiency, due to the fact that even without any applied input, the amplifier is still dissipating lots of unusable power. The class-a efficiency is around 25%. This is a reasonably acceptable approximation when speaking of input sinusoidal. When we speak of audio signals, which do not have an expected behavior in time, efficiency can even drop down to 1%. This could bring us to say why we should ever use this class of amplifiers in audio application. The main reason audio designer choice this class is because of distortion: since the signal is amplified for the whole period, the output has a very low THD, which is the main goal when reproducing, for example, a music signal. CLASS-B The class-b amplifier can be described with the following describing circuit. Figure 3: basic example of a class-b amplifier

5 An improvement of the class-a, this class of amplifiers removes the drawback of the low efficiency due to the fact that now there are two transistors, T1 and T2: each of them conducts only when the input signal is higher than V be (for T1) and lower than V be (for T2). The operational amplifier connected to the input of the circuit is needed to remove the crossover distortion caused by the fact that until the is greater than V be or lower than V be, both T1 and T2 are turned off. Beside this, the efficiency of this circuit is much higher (c.a. 78%) compared to the class-a. The higher efficiency comes from the fact that when no input signal is applied, both transistors are inhibited. It is important to notice that even if the insertion of the operational amplifier removes apparently removes the cross-over distortion between V be and V be, there is still a little portion of distortion not detectable when considering a sinusoidal wave as input, but has an higher impact when speaking of audio signals. CLASS-AB The class-ab amplifier can be described with the following circuit. Figure 4: basic example of a class-ab amplifier This type of amplifier differs from the class-b previously described because in this case to remove the crossover distortion, a constant voltage is applied to the B-E junction for the transistor T1, as well as a constant voltage to the E- B junction for the transistor T2. There are many ways to obtain this, i.e. using diodes or a V be multiplier (this is a more accurate way to have the correct V be and V eb voltage drops required to have T1 and T2 in conduction). The efficiency lowers a bit because of static current that flows through the transistor even when they do not drive any signal, but it still comparable to the efficiency of a class-b.

6 CLASS-A EXAMPLE: Death-of-Zen The Death-of-Zen Class-A amp designed by Nelson Pass seems to have become popular between audio amateurs. It satisfies the requirement of many amplifier builders: it is simple, stable, and very tolerant of layout and component variations. The sonic characteristics will also appeal to many, due to the valve-like sound (or tube-like, if you prefer). Hundreds of DoZ amplifiers have been made, with comparatively few reported problems. Figure 5: Death-of-Zen amplifier schematic V R1 is used to set the DC voltage at the +V e of C 3 to 1 the supply voltage (20 V for a 40 V supply), by setting the voltage 2 at the base of Q1. The 100 μf capacitor ensures that no supply ripple gets into the input. Q1 is the main amplifying device, and also sets the gain by the ratio of R 9 and R 4. Gain 22 db, providing an input sensitivity of about 1V for full output. Q4 is the buffer for the output transistor Q5, and modulates the current in Q2 and Q3. V R2 is used to set quiescent current, which I found needs to be about 1.7 A for best overall performance. C 4 and R 6 are part of a bootstrap circuit, which ensures that the voltage across R6 remains constant. If the voltage is constant, then so is the current, and this part of the circuit ensures linearity as the output approaches the +V e supply. Be very careful - if you accidentally set V R2 to minimum resistance the amplifier will probably self-destruct - more or less immediately. As the supply voltage changes with normal variations in AC mains voltage, the quiescent current also shifts. This is not desirable, and is solved with the addition of a resistor and a zener diode. If you are using a regulated supply, this mod is not needed. A quick calculation example for the zener rating and resistance are in order, so it is properly understood. The maximum zener current for a given voltage is calculated as I z = P z V z, where I z is the zener current, P z is the power rating and V z is the voltage drop.

7 CLASS-D: Digital Amplifier Class-D amplifiers have become one of the most widely used class of commercial audio amplifier. At first, class-d amplifiers used BJT technology since the MOSFET technology was still under development. The fields of application for Class-D amplifiers can be divided into two areas: low and high power outputs. The low power field reaches from few milliwatts to around 5 W (for example small headphones), while the high power applications go from around 80 W to 1400 W (home theather systems, sub-woofers, etc.). How class-d works Class-D performs in a different way than the ones we just described. It bases its operation in switching output devices between 2 states, namely "ON" and "OFF". Before discussing the topology specific details, we can say that in the ON state a given amount of current flows through the device while theoretically no voltage is present from drain to source (almost every Class-D will use MOSFETs), hence power dissipation is theoretically zero. In the OFF state, voltage will be the total supply rails as it behaves like an open-circuit, and no current will flow (it will not be actually 0 A flowing through the MOSFETs, but it is indeed a very small current). To represent a signal wave by using an ON-OFF MOSFETs condition, a circuit that generates a modulated pulse is needed. This technique is known as PWM: Pulse Width Modulation. A certain circuit has in input the input audio signal and has in output a modulated square-wave with fixed frequency but changing duty cycle. Figure 6: circuit generating a square wave with changing duty cycle As you can see in figure 4, the implementation of PWM is done by using a block called Precision Triangle Wave Generator, whose output is compared to the input signal. When the input signal has a voltage higher than the triangle wave, the output of the comparator goes to V cc ; viceversa, when the triangle wave is higher than the input signal, the comparator goes to GND. Triangle wave generator has to be very precise and linear (meaning the slope of triangle wave has to be constant) to prevent distortion being introduced at this stage of the amplifier. Assuming this is the case, the generated PWM may be theoretically free from distortion, but this assumes zero switching times and no glitching of any kind in the output devices. These assumptions are very far from the truth in the real world, and so Class-D output stages in general produce an averaged output with considerable distortion.

8 The PWM generated is used to drive a series of MOSFET, which form the driving stage of the amplifier, in cascade configuration: the output of the driving stage is switched between V al and V al when the comparator output is low or high respectively. The signal then passes through an LC filter, which reconstructs the audio signal and then goes to the speaker. There are other ways to obtain a PWM in input to the driving stage: a digital input coming from a DSP/computer; a Sigma-Delta modulator (even of higher order). Circuit Topology There are two ways to design the driving stage of the amplifier: half-bridge and full bridge. Figure 7: Class-D with half-bridge topology The half-bridge topology shown in figure 7 is the simplest configuration for a Class-D amplifier: it requires only two MOSFET devices connected to the comparator, and the output is then filtered. With this topology the output has a voltage swing between V al and V al, the last one being the reference potential itself GND. It has a simple circuit, with only two MOSFETs; they are usually used to design high power amplifier since they have an higher efficiency then the full-bridge in terms of effective used power.

9 Figure 8: Class-D with half-bridge topology In figure 8 we have instead the full-bridge topology, which compared to the half-bridge topology is different in: the number of transistors (4 MOSFETS compared to the 2 of the half-bridge); single rail power supply. It allows twice the voltage-swing across the load, and therefore theoretically four times the output power, and also permits the amplifier to run from one supply rail without the need for bulky output capacitors of doubtful linearity. Efficiency Ideally, Class-D amplifiers should have 100% efficiency since it uses a switch-technology (V = 0 if ON, I = 0 if OFF). Instead, there is a power dissipation due to the fact that during the switching from ON to OFF state (and from OFF to ON state) are slightly conducting for a short time. There are also some problems that need to be taken in account that reduce the efficiency of the power amplifier: the output transistors have non-zero resistance, even when they are turned hard on. The order is of hundreds of mω, and can double as the device temperature rises (this causes R 2 I losses, with I the current flowing in the transistors); the output devices have non-zero times for switching ON and OFF. When the FET is turning ON or OFF, it has an intermediate value of resistance which again causes R 2 I losses. It is essential to minimize the stray inductance in drain and source circuits, since it does not only extends the switching time but also causes voltage transients at turn-off that can overstress the FETs. the flyback pulses generated by an inductive load can cause conduction of the parasitic diodes that are part of the FET construction. These diodes have long reverse recovery times and more current will flow than its necessary. To prevent this, apply Schottky clamp diodes between the output and the supply rail. These turn on at a lower voltage than the parasitic FET and deal with flyback pulses. the shoot-through : it happens that both FETs are in conduction at the same time; this implies a low impedance path between the supply rails, which is very brief, but large amount of currents can bring to lots of unwanted heating. To avoid this, introduce a dead-time circuit that avoids this situation. The purpose of the output filter is to prevent radiation of switching frequencies for amplifiers that have external speaker cables, and also to improve efficiency. The inductance of a loudspeaker coil alone will in general be low enough to allow some of the switching frequency energy to pass through it to ground, causing significant losses. While some low-power integrated applications have no output filter at all, most Class-D amplifiers have a second-order LC filter between the amplifier output and the loudspeaker. In some cases a fourthorder filter is used. The Butterworth alignment is usually chosen to give maximal flatness of frequency response.

10 CLASS-XD: Crossover Distortion When we consider Class-B, it is clear that it would be better if the crossover region were anywhere else rather than where it is. If we can displace the crossover point away from its zero-crossing position, then the amplifier output will not traverse it until the output reaches a certain voltage level. Below this level the performance is pure Class-A; above it the performance is optimal Class-B, the only difference being that crossover discontinuities on the THD residual are no longer evenly spaced. The harmonic structure of the crossover distortion produced is not significantly changed, as explained in more detail below. The central idea of the Crossover Displacement principle is the injection of an extra current, either fixed or varying with the signal, into the output point of a conventional Class-B amplifier. A box called the Displacer draws a controlled current from the output node and sinks it into the negative rail; sourcing current from the positive rail and injecting it into the output would be equally valid. The displacer current may be constant, in which case the displacer is simply a constant-current source, or it may vary with the signal. The latter improves both efficiency and linearity. In either case the Displacer is working in Class-A and never turns off. The displacement current does not directly alter the output-voltage because the output stage has an inherently low output impedance, which is further reduced by the global negative feedback. What it does do is alter the pattern of current flowing in the output devices. Figure 9: amplifier with Displacer block. The block can be also connecte between V+ and the output