POWER AMPLIFIERS Suresh P. Nair [AIE, ME, (PhD)] MIEEE Professor & Head Department of Electronics and
Engineering Royal College of Engineering and Technology Chiramanangad PO, Akkikkavu, Thrissur, Kerala,
India Topics: Topics Power Amplifiers: Class A,B,AB,C,D and S Power Amp - Harmonic Distortion & Efficiency Wide Band Amplifiers Broadbanding Techniques Low & High Frequency Compensation Cascode Amplifier Broadbanding using inductive Load PUBLIC ADDRESS SYSTEM (PAS): PUBLIC ADDRESS SYSTEM (PAS) Basic public address system block diagram.: Basic public address system block diagram. The complete public address/paging system before packaging.: The complete public address/paging system before packaging. Power Amplifier Board: Power Amplifier Board Test bench setup for the power amplifier board.: Test bench setup for the power amplifier board. Power Amplifiers Basics & Classifications: Power Amplifiers Basics & Classifications Class A Class B Class AB Class C Class D Class S PA Basics: PA Basics The term amplifier is very generic. In general, the purpose of an amplifier is to take an input signal and make it stronger (or in more technically correct terms, increase its amplitude ) There are many different types of amplifiers, each with a specific purpose in mind. PA Basics ….: PA Basics …. Some other Amplifiers you may run across: op amp, signal amp, RF (radio frequency amp), instrumentation amp. This lecture will focus on audio power amplifiers . Audio power amplifiers are those amplifiers which are designed to drive loudspeakers . PA Basics ….: PA Basics …. The purpose of a power amplifier, in very simple terms, is to take a signal from a source device and make it suitable for driving a loudspeaker . Ideally, the ONLY thing different between the input signal and the output signal is the strength of the signal. PA Basics ….: PA Basics …. In mathematical terms, if the input signal is denoted as S , the output of a perfect amplifier is X*S , where X is a constant (a fixed number). The "*" symbol means ”multiplied by". No amplifier does exactly the ideal . PA Basics ….: PA Basics …. But many do a very good job if they are operated within their advertised power ratings . Output signal of all amplifiers contain additional (unwanted) components that are not present in the input signal ; these additional characteristics may be lumped together and are generally known as distortion . Distortion Types: Distortion Types There are many types of distortion; however the two most common types are known as: 1. Harmonic distortion and 2.Intermodulation distortion PA Basics ….: PA Basics …. All power amplifiers have a power rating , the units of power are called watts . The power rating of an amplifier may be stated for various load impedances ; the units for load impedance are ohms . The most common load impedances are 8 ohms, 4 ohms, and 2 ohms PA Basics ….: PA Basics …. The power output of a modern amplifier is usually higher when lower impedance loads (speakers) are used. If an amplifier is rated at 100 watts, then the output can be anything between zero and this maximum rated value. PA Basics ….: PA Basics …. Power amplifiers get the necessary energy for amplification of input signals from the AC wall outlet to which they are plugged into. If you had a perfect amplifier , all of the energy the amplifier took from the AC outlet would be converted to useful output (to the speakers) PA Basics ….: PA Basics …. In the real world no amplifier is 100% efficient , so some of the energy from the wall outlet is wasted. The vast majority of energy wasted by an amplifier shows up in the form of heat . Heat is one of the biggest enemies to electronic equipment, so it is important to ensure adequate air flow around equipment. PA Basics ….: PA Basics …. Power is not really something that can be “amplified”. Voltage and current can be amplified. The term “power amplifier” although technically incorrect has become understood to mean an amplifier that is intended to drive a load (such as a speaker, a motor, etc). Slide 20: Power = current times voltage (for resistive loads), this is where the term “power” amplification comes from. Functional blocks of an amplifier : Functional blocks of an amplifier All power amplifiers have: 1.A Power supply 2.A n input stage 3.A n output stage 1.Power Supply: 1.Power Supply The primary purpose of a power supply in a power amplifier is to take the 120 V AC power from the outlet and convert it to a DC voltage. The very best of amplifiers have two totally independent power supplies , one for each channel (they do share a common AC power cord though). 2. Input Stage: 2. Input Stage The general purpose of the input stage of a power amplifier (sometimes called the "front end") is to receive and prepare the input signals for "amplification" by the output stage. Two types: 1.Balanced Input 2.Single Ended Input 2. Input Stage:
2. Input Stage Balanced inputs are much preferred over single ended inputs when interconnection cables are long and/or
to noisy electrical
because they provide very good noise rejection . The input stage also contains things like input level controls.
3.Output Stage: 3.Output Stage The portion which actually converts the weak input signal into a much more powerful "replica" which is capable of driving high power to a speaker. This portion of the amplifier typically uses a number of "power transistors" (or MOSFETs) and is also responsible for generating the most heat in the unit. The output stage of an amplifier interfaces to the speakers. Amplifier Classes : Amplifier Classes Introduction: Introduction The Class of an amplifier refers to the design of the circuitry within the amp . For audio amplifiers, the Class of amp refers to the output stage of the amp. Classes: Classes Collector current waveforms for transistors operating in (a) class A, (b) class B, (c) class AB, and (d) class C amplifier stages. Types of Classes: Types of Classes CLASS A CLASS B CLASS AB CLASS C CLASS D CLASS S Slide 31: Class-A: Output device(s) conduct through 360 degrees of input cycle (never switch off) - A single output device is possible. The device conducts for the entire waveform in Figure 1 Class-B: Output devices conduct for 180 degrees (1/2 of input cycle) - for audio, two output devices in "push-pull" must be used (see Class-AB) Class-AB: Halfway (or partway) between the above two examples (181 to 200 degrees typical) - also requires push-pull operation for audio. The conduction for each output device is shown in Figure 1. Figure 1 - The Sinewave Cycle Slide 32: Class-C: Output device(s) conduct for less than 180 degrees (100 to 150 degrees typical) - Radio Frequencies only - cannot be used for audio! This is the sound heard when one of the output devices goes open circuit in an audio amp! See Figure 1, showing the time the output device conducts Class-D: Quasi-digital amplification . Uses pulse-width-modulation of a high frequency (square wave) carrier to reproduce the audio signal - although my original comments were valid when this was written, there have been some very significant advances since then. There are some very good sounding Class-D amplifiers being made now, and they are worthy of an article of their own. Figure 1 - The Sinewave Cycle CLASS “A”: CLASS “ A ” TOPICS Introduction Transfer Characteristics Signal Waveforms Power Dissipation Power – Conversion Efficiency A Class-A amp maintains the same current through the transistors, therefore ensuring that they remain in their most linear region at all times Class A Output Stage - Recap: Class A Output Stage - Recap Class A output stage is a simple linear current amplifier. It is also very inefficient, typical maximum efficiency between 10 and 20 %. Only suitable for low power applications. High power requires much better efficiency. Transfer Characteristics: Transfer Characteristics Transfer Characteristics: Transfer Characteristics Basic class A amplifier operation. Output is shown 180∞ out of phase with the input (inverted).: Basic class A amplifier operation. Output is shown 180∞ out of phase with the input (inverted). Maximum class A output occurs when the Q-point is centered on the ac load line.: Maximum class A output occurs when the Q-point is centered on the ac load line. Q-point closer to cutoff.: Q-point closer to cutoff. Q-point closer to saturation.: Q-point closer to saturation. FIGURE: FIGURE FIGURE 9-30 Class A power amplifier with correct output voltage swing.: FIGURE 9-30 Class A power amplifier with correct output voltage swing. FIGURE 9-31 Oscilloscope displays showing output voltage for the amplifier in Figure 9-30, for several types of failures.: FIGURE 9-31 Oscilloscope displays showing output voltage for the amplifier in Figure 9-30, for several types of failures. Why is class A so inefficient ?: Why is class A so inefficient ? Single transistor can only conduct in one direction. D.C. bias current is needed to cope with negative going signals. 75 % (or more) of the supplied power is dissipated by d.c. Solution : eliminate the bias current. Class A: Class A Class A amplifiers have very low distortion (lowest distortion occurs when the volume is low) They are very inefficient and are rarely used for high power designs. The distortion is low because the transistors in the amp are biased such that they are half "on" when the amp is idling Class A: Class A As a result of being half on at idle, a lot of power is dissipated in the devices even when the amp has no music playing! Class A amps are often used for "signal" level circuits (where power requirements are small) because they maintain low distortion. Class-A Benefits: Class-A Benefits The first is circuit simplicity . The signal is subjected to comparatively little amplification , resulting in an open loop gain which is generally fairly low. This means that very little overall feedback is used, so stability and phase should be excellent over the audio frequencies. Do not require any frequency compensation . Class-A Benefits: Class-A Benefits No cross over distortion No switching distortion Lower harmonic distortion in the voltage amplifier Lower harmonic distortion in the current amplifier No signal dependent distortion from the power supply Constant and low output impedance Simpler design CLASS “B”: CLASS “ B ” TOPICS Introduction Circuit Operation Transfer Characteristics Power – Conversion Efficiency Power Dissipation Reducing Crossover Distortion Single – Supply Operation Circuit Operation: Circuit Operation Basic class B amplifier operation (noninverting).: Basic class B amplifier operation (noninverting). Common-collector class B amplifier.: Common-collector class B amplifier. Class B push-pull ac operation.: Class B push-pull ac operation. Transfer Characteristics: Transfer Characteristics Crossover Distortion: Crossover Distortion Illustration of crossover distortion in a class B push-pull amplifier. The transistors conduct only during the portions of the input indicated by the shaded areas.: Illustration of crossover distortion in a class B push-pull amplifier. The transistors conduct only during the portions of the input indicated by the shaded areas. Transformer coupled push-pull amplifiers. Q1 conducts during the positive half-cycle; Q2 conducts during the negative half-cycle. The two halves are combined by the output transformer.: Transformer coupled push-pull amplifiers. Q 1 conducts during the positive half-cycle; Q 2 conducts during the negative half-cycle. The two halves are combined by the output transformer. Biasing the push-pull amplifier to eliminate crossover distortion.: Biasing the push-pull amplifier to eliminate crossover distortion. Class B Output Stage: Class B Output Stage Q 1 and Q 2 form two unbiased emitter followers Q 1 only conducts when the input is positive Q 2 only conducts when the input is negative Conduction angle is, therefore, 180° When the input is zero, neither conducts i.e. the quiescent power dissipation is zero Class B Current Waveforms: Class B Current Waveforms I out I C1 I C2 time time time Class B Efficiency: Class B Efficiency Average power drawn from the positive supply: I C1 Phase, q A/R L 0 p 2p A sin( q ) Slide 68: By symmetry, power drawn from +ve and –ve supplies will be the same. Total power, therefore: Load power: Efficiency: Power Dissipation: Power Dissipation To select appropriate output transistors, the maximum power dissipation must be calculated. Just need to find the maximum value of P D to select transistors/heatsinks Slide 70: 0 5 10 15 0 0.5 1 1.5 Peak Output Amplitude, A [V] Power [W] P L P S P D E.g. V S = 15 V, R L = 100 W 15 0 1.5 Maximum Power Dissipation: P D is a quadratic function of A, Maximum Power Dissipation maximum when: Efficiency / Power Dissipation: Efficiency / Power Dissipation Peak efficiency of the class B output stage is 78.5 %, much higher than class A. Unlike class A, power dissipation varies with output amplitude. Remember, there are two output devices so the power dissipation is shared between them. Design Example: Design Example Design a class B amplifier which will deliver up to 25 W into a 4 W load. Supply voltages must be larger than A max so choose V s = 15V. Slide 74: Each of the two output transistors must be able to safely dissipate up to 5.7 Watts. Using a TIP120 & TIP 125: But, with q JC = 1.92 °C/W i.e. Either two heatsinks rated at less than 20°C/W are required or a single heatsink rated at less than 10°C/W. Slide 75: Suggested heatsink Dimensions, 50mm x 50mm x 9.5mm Accommodates two devices Rating 6.5°C/W Cost 60p inc VAT Cross-Over Distortion: Cross-Over Distortion A small base-emitter voltage is needed to turn on a transistor Q 1 actually only conducts when v in > 0.7 V Q 2 actually only conducts when v in < -0.7 V When 0.7 > v in > -0.7, nothing conducts and the output is zero. i.e. the input-output relationship is not at all linear. Actual Input-Output Curve: Actual Input-Output Curve v in v out +V BE -V BE Crossover Distortion Effect of Cross-Over Distortion: Effect of Cross-Over Distortion Class B Summary: Class B Summary A class B output stage can be far more efficient than a class A stage (78.5 % maximum efficiency compared with 25 %). It also requires twice as many output transistors… …and it isn’t very linear; cross-over distortion can be significant. Class B:
Class B Class B amplifiers are used in low cost designs or designs where sound
is not that important. Class B amplifiers are significantly more efficient than class A amps. They suffer from bad distortion when the signal level is low (the distortion in this region of operation is called "crossover distortion").
Class B: Class B Class B is used most often where economy of design is needed. Before the advent of IC amplifiers, class B amplifiers were common in clock radio circuits, pocket transistor radios , or other applications where quality of sound is not that critical. Slide 82: CLASS AB Class AB: Class AB Class AB is probably the most common amplifier class currently used in home stereo and similar amplifiers. Class AB amps combine the good points of class A and B amps. They have the improved efficiency of class B amps and distortion performance that is a lot closer to that of a class A amp. Eliminating crossover distortion in a transformer-coupled push-pull amplifier. The diode compensates for the base-emitter drop of the transistors and produces class AB operation.: Eliminating crossover distortion in a transformer-coupled push-pull amplifier. The diode compensates for the base-emitter drop of the transistors and produces class AB operation. Load lines for a complementary symmetry push-pull amplifier. Only the load lines for the npn transistor are shown.: Load lines for a complementary symmetry push-pull amplifier. Only the load lines for the npn transistor are shown. Single-ended push-pull amplifier.: Single-ended push-pull amplifier. A Darlington class AB push-pull amplifier.: A Darlington class AB push-pull amplifier. FIGURE 9-32 A Class AB push-pull amplifier with correct output voltage.: FIGURE 9-32 A Class AB push-pull amplifier with correct output voltage. FIGURE 9-33 Incorrect output waveforms for the amplifier in Figure 9-32.: FIGURE 9-33 Incorrect output waveforms for the amplifier in Figure 9-32. Class AB: Class AB With such amplifiers, distortion is worst when the signal is low , and generally lowest when the signal is just reaching the point of clipping. Class AB amps use pairs of transistors , both of them being biased slightly ON so that the crossover distortion (associated with Class B amps) is largely eliminated. Slide 97: CLASS C Class C: Class C Class C amps are never used for audio circuits . They are commonly used in RF circuits . Class C amplifiers operate the output transistor in a state that results in tremendous distortion (it would be totally unsuitable for audio reproduction). FIGURE 9-22 Basic class C amplifier operation (non inverting).: FIGURE 9-22 Basic class C amplifier operation (non inverting). FIGURE 9-23 Basic class C operation.: FIGURE 9-23 Basic class C operation. FIGURE 9-24 Class C waveforms.: FIGURE 9-24 Class C waveforms. FIGURE 9-25 Tuned class C amplifier.: FIGURE 9-25 Tuned class C amplifier. FIGURE 9-26 Resonant circuit action.: FIGURE 9-26 Resonant circuit action. FIGURE 9-27 Tank circuit oscillations. Vr is the voltage across the tank circuit.: FIGURE 9-27 Tank circuit oscillations. V r is the voltage across the tank circuit. FIGURE 9-28 Tuned class C amplifier with clamper bias.: FIGURE 9-28 Tuned class C amplifier with clamper bias. FIGURE 9-29 Clamper bias action.: FIGURE 9-29 Clamper bias action. Class C: Class C However, the RF circuits where Class C amps are used, employ filtering so that the final signal is completely acceptable. Class C amps are quite efficient . Slide 109: CLASS D Class D: Class D Class D amplifiers use a completely different method of amplification as compared to Class A, B and AB. Due to improvements in the speed, power capacity and efficiency of modern semiconductor devices, applications using Class D amps have become affordable for the common person. Class D: Class D Class A,B and AB operate the semiconductor devices in the linear mode, Class D amplifiers operate the output semiconductor devices as switches (ON or OFF). In a Class D amplifier, the input signal is compared with a high frequency triangle wave , resulting in the generation of a Pulse Width Modulation (PWM) type signal. Class D: Class D This signal is then applied to a special filter that removes all the unwanted high frequency by-products of the PWM stage. The output of the filter drives the speaker. Class D amps are (today) most often found in car audio subwoofer amplifiers. Class D: Class D Very good efficiency due to the fact that the semiconductor devices are ON or OFF in the power stage, resulting in low power dissipation in the device as compared to linear amplifier classes (i.e. A,B and AB) One notable disadvantage of Class D amplifiers is that they are fairly complicated and special care is required in their design Class D: Class D Due to the high frequencies that are present in the audio signal (as a result of the PWM stage), Class D amps used for car stereo applications are often limited to subwoofer frequencies , however designs are improving all the time. They will also be small and lightweight compared to the class AB Other Classes: Other Classes Other Classes: Other Classes There are a number of other classes of amplifiers, such as: CLASS G CLASS H CLASS S etc…. Other Classes: Other Classes Most of these classes are actually clever variations of the class AB design They result in higher efficiency. Slide 121: CLASS S Class S: Class S Slide 124: Collector current waveforms for transistors operating in (a) class A, (b) class B, (c) class AB, and (d) class C amplifier stages.