Ch13_15 - Chapter 13 Output Stages and Power Amplifiers...

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Unformatted text preview: Chapter 13 Output Stages and Power Amplifiers 13.1 General Considerations 13.2 Emitter Follower as Power Amplifier 13.3 Push-Pull Stage 13.4 Improved Push-Pull Stage 13.5 Large-Signal Considerations 13.6 Short Circuit Protection 13.7 Heat Dissipation 13.8 Efficiency 13.9 Power Amplifier Classes 1 Why Power Amplifiers? Drive a load with high power. Cell phone needs 1W of power at the antenna. Audio system needs tens to hundreds Watts of power. Ordinary Voltage/Current amplifiers are not equipped for such applications 2 CH 13 Output Stages and Power Amplifiers Chapter Outline 3 CH 13 Output Stages and Power Amplifiers Power Amplifier Characteristics Experiences small load resistance. Delivers large current levels. Requires large voltage swings. Draws a large amount of power from supply. Dissipates a large amount of power, therefore gets hot. 4 CH 13 Output Stages and Power Amplifiers Power Amplifier Performance Metrics Linearity Power Efficiency Voltage Rating 5 CH 13 Output Stages and Power Amplifiers Emitter Follower Large-Signal Behavior I As V in increases V out also follows and Q 1 provides more current. 6 CH 13 Output Stages and Power Amplifiers Emitter Follower Large-Signal Behavior II However as V in decreases, V out also decreases, shutting off Q 1 and resulting in a constant V out . 7 CH 13 Output Stages and Power Amplifiers Example: Emitter Follower 1 1 ln 0.5 211 out in T out L S in out V V V I V R I V V V mV - + = = - ( 29 1 1 1 1 1 ln 0.01 390 C in T C L S C in I V V I I R I I I V mV = +- 8 CH 13 Output Stages and Power Amplifiers Linearity of an Emitter Follower As Vin decreases the output waveform will be clipped, introducing nonlinearity in I/O characteristics. 9 CH 13 Output Stages and Power Amplifiers Push-Pull Stage As V in increases, Q 1 is on and pushes a current into R L . As V in decreases, Q 2 is on and pulls a current out of R L . 10 CH 13 Output Stages and Power Amplifiers I/O Characteristics for Large V in For positive V in , Q 1 shifts the output down and for negative V in , Q 2 shifts the output up. V out =V in-V BE1 for large +V in V out =V in +|V BE2 | for large -V in 11 CH 13 Output Stages and Power Amplifiers Overall I/O Characteristics of Push-Pull Stage However, for small V in , there is a dead zone (both Q 1 and Q 2 are off) in the I/O characteristic, resulting in gross nonlinearity. 12 CH 13 Output Stages and Power Amplifiers Small-Signal Gain of Push-Pull Stage The push-pull stage exhibits a gain that tends to unity when either Q1 or Q2 is on....
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This note was uploaded on 06/18/2010 for the course CE 01 taught by Professor Bazgei during the Spring '09 term at University of California, Santa Cruz.

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Ch13_15 - Chapter 13 Output Stages and Power Amplifiers...

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