Integrated Technical Education Cluster Banna - At AlAmeeria ‎ © Ahmad El J-601-1448 Electronic Principals Lecture #8 Power Amplifiers Instructor: Dr. Ahmad El-Banna December 2014 Banna Agenda - Introduction © Ahmad El Series-Fed Class A Amplifier 2014 Dec Transformer-Coupled Class A Amplifier , Lec#8 Class B Amplifier Operation & Circuits 1448 , Amplifier Distortion - 601 - Power Transistor Heat Sinking J 2 Class C & Class D Amplifiers INTRODUCTION 3 J-601-1448 , Lec#8 , Dec 2014 © Ahmad El-Banna Banna Amplifier Classes - • In small-signal amplifiers, the main factors are usually amplification linearity and magnitude of gain. • Large-signal or power amplifiers, on the other hand, primarily provide sufficient power © Ahmad El to an output load to drive a speaker or other power device, typically a few watts to tens of watts. • The main features of a large-signal amplifier are the circuit’s power efficiency, the 2014 maximum amount of power that the circuit is capable of handling, and the impedance Dec matching to the output device. , • Amplifier classes represent the amount the output signal varies over one cycle of operation for a full cycle of input signal. Lec#8 Power Amplifier Classes: 1. Class A: The output signal varies 1448 , for a full 360° of the input signal. - • Bias at the half of the supply 601 - J 2. Class B: provides an output signal varying over one-half the input 4 signal cycle, or for 180° of signal. • Bias at the zero level Banna Amplifier Efficiency - Power Amplifier Classes … 3. Class AB: An amplifier may be biased at a dc level above the zero-base-current level of class B and above one-half the supply voltage level of class A. © Ahmad El 4. Class C: The output of a class C amplifier is biased for operation at less than 180° of the cycle and will operate only with a tuned (resonant) circuit, which provides a full 2014 cycle of operation for the tuned or resonant frequency. Dec , 5. Class D: This operating class is a form of amplifier operation using pulse (digital) signals, which are on for a short interval and off for a longer interval. Lec#8 • The power efficiency of an amplifier, defined as the ratio of power output 1448 , to power input, improves (gets higher) going from class A to class D. - 601 - J 5 SERIES - FED FED CLASS A AMPLIFIER A CLASS 6 J-601-1448 , Lec#8 , Dec 2014 © Ahmad El-Banna Banna SERIES-FED CLASS A AMPLIFIER - • DC Bias Operation © Ahmad El 2014 Dec , Lec#8 • AC Operation 1448 , - 601 - J 7 Banna Power Considerations - • The power drawn from the supply is © Ahmad El • Output Power • 2014 Efficiency Dec , Lec#8 • Maximum Efficiency 1448 , - 601 - J 8 N.B.: Example 9 J-601-1448 , Lec#8 , Dec 2014 © Ahmad El-Banna AMPLIFIER TRANSFORMER - COUPLED COUPLED CLASS A A CLASS 10 J-601-1448 , Lec#8 , Dec 2014 © Ahmad El-Banna Banna Transformer Action - • A transformer can increase or decrease voltage or current levels according to its © Ahmad El turns ratio a=N1:N2 • The impedance connected to one side of 2014 a transformer can be made to appear Dec either larger or smaller (step up or step , down) at the other side of the transformer. Lec#8 • Impedance Transformation 1448 , • - Voltage Transformation 601 - J • Current Transformation 11 Banna Operation of Amplifier Stage - • Signal Swing and Output AC Power © Ahmad El 2014 Dec , Lec#8 • Efficiency 1448 , - 601 • - power loss J 12 • Maximum Theoretical Efficiency • Check EXAMPLE 12.4 ! CLASS CLASS B AMPLIFIER AMPLIFIER OPERATION 13 J-601-1448 , Lec#8 , Dec 2014 © Ahmad El-Banna Banna Push–Pull Amplifier - • Class B operation is provided when the dc bias leaves the transistor biased just off, the transistor turning on when the ac signal is © Ahmad El applied. • This is essentially no bias, and the transistor conducts current for only one-half of the signal 2014 cycle. Dec , • Connection of push–pull amplifier to load Lec#8 • The current drawn from a single 1448 , power supply has the form of a full- - wave rectified signal • whereas that drawn from two power 601 - supplies has the form of a half-wave J rectified signal from each supply. 14 Banna Efficiency & Power Consideration - • Maximum Power Considerations © Ahmad El 2014 • Dec Efficiency , Lec#8 1448 , - 601 - J • Power Dissipated by Output Transistors 15 CLASS CLASS B AMPLIFIER CIRCUITS AMPLIFIER B 16 J-601-1448 , Lec#8 , Dec 2014 © Ahmad El-Banna Phase - Splitter Circuits Splitter 17 J-601-1448 , Lec#8 , Dec 2014 © Ahmad El-Banna Banna Class B Amplifier Circuits - • Transformer-Coupled Push–Pull Circuits © Ahmad El 2014 Dec , Lec#8 1448 , - 601 - J 18 Transformers are bulky ! Banna Class B Amplifier Circuits.. - • Complementary-Symmetry Circuits • Complementary-symmetry push–pull circuit using Darlington transistors. © Ahmad El Needs two separate voltage supplies! 2014 Dec , Lec#8 Biasing the transistors in class AB improves this operation 1448 , - 601 - J 19 o higher output current o lower output resistance. Banna Class B Amplifier Circuits… - • Quasi-Complementary Push–Pull Amplifier © Ahmad El o In practical power amplifier circuits, it 2014 is preferable to use npn transistors Dec for both high-current-output devices. , o The push–pull operation is achieved by using complementary transistors Lec#8 (Q 1 and Q2) before the matched npn output transistors ( Q and Q ). 3 4 1448 , - o R can be adjusted to minimize 2 601 - crossover distortion. J o It is the most popular form of power 20 amplifier • Quasi-complementary push–pull transformerless power amplifier. Example 21 J-601-1448 , Lec#8 , Dec 2014 © Ahmad El-Banna AMPLIFIER AMPLIFIER DISTORTION 22 J-601-1448 , Lec#8 , Dec 2014 © Ahmad El-Banna Banna Amplifier Distortion - • A pure sinusoidal signal has a single frequency at which the voltage varies positive and negative by equal amounts. Any signal varying over less than the full 360° cycle is considered to have distortion. © Ahmad El • Distortion can occur because the device characteristic is not linear, in which case non- linear or amplitude distortion occurs. • Distortion can also occur because the circuit elements and devices respond to the input 2014 signal differently at various frequencies, this being frequency distortion. • Dec One technique for describing distorted but period waveforms uses Fourier analysis , • Harmonic Distortion Lec#8 A signal is considered to have harmonic • Power of a Signal Having Distortion distortion when there are harmonic frequency components 1448 , - 601 - J A : amplitude of the fundamental frequency 1 23 An : amplitude of the nth frequency component • Total Harmonic Distortion POWER POWER TRANSISTOR HEAT HEAT TRANSISTOR SINKING 24 J-601-1448 , Lec#8 , Dec 2014 © Ahmad El-Banna Banna Power Transistor Heat Sinking - • The maximum power handled by a particular device and the © Ahmad El temperature of the transistor junctions are related since the power dissipated by the device 2014 causes an increase in temperature at the junction of the device. Dec , Lec#8 1448 , - 601 - J 25 CLASS C & CLASS CLASS & C D AMPLIFIERS 26 J-601-1448 , Lec#8 , Dec 2014 © Ahmad El-Banna Banna Class C Amplifier - • Although class A, class AB, and class B amplifiers are most used as power amplifiers, © Ahmad El class D amplifiers are popular because of their very high efficiency. • Class C amplifiers, although not used as audio 2014 amplifiers, do find use in tuned circuits as in Dec communications. , Lec#8 • The tuned circuit in the output, however, will 1448 , provide a full cycle of output signal for the - fundamental or resonant frequency of the tuned 601 - circuit ( L and C tank circuit) of the output. J • This type of operation is therefore limited to use at one fixed frequency, as occurs in a 27 communications circuit, for example. Banna Class D Amplifier - © Ahmad El 2014 Dec , Lec#8 • Class D amplifier is designed to operate with digital or pulse-type signals. 1448 , • An efficiency of over 90% is achieved, making it - desirable in power amplifiers. 601 - • It is necessary to convert any input signal into a J pulse-type waveform before using it to drive a large power load and to convert the signal back 28 into a sinusoidal-type signal to recover the original signal. Banna - • For more details, refer to: • Chapter 12, Electronic Devices and Circuits, Boylestad. © Ahmad El • The lecture is available online at: 2014 • https://speakerdeck.com/ahmad_elbanna Dec • For inquires, send to: , • [email protected] Lec#8 1448 , - 601 - J 29 .