Output Stages and Power Amplifiers
14 Output Stages and Power Amplifier Low output resistance To deal with relatively large signals: small-signal approximation and models Linearity: total harmonic distortion (THD) – rms value of the harmonic components of the output signal To deliver the required amount of power to the load in an efficient manner, which implies low power dissipation.
14 Output Stages and Power Amplifier 14.1 Classification of Output Stages Type A Conduction Angle: 360 Type B Conduction Angle: 180
14 Output Stages and Power Amplifier 14.1 Classification of Output Stages (cont.) Type AB Conduction Angle: (180, 360) Type C Conduction Angle: <180 Type A, B, AB: op amps and audio power amplifier Type C: RF power amplification (mobile phone, radio, and TV)
14.2 Class A Output Stage 14.2.1 Transfer Characteristics
14.2 Class A Output Stage 14.2.2 Signal Waveforms vCE1=VCC - vo Neglecting VCEsat Neglecting VCEsat How about an open/short output?
14.2 Class A Output Stage 14.2.2 Signal Waveforms (cont.) vCE1=VCC - vo When RL = , iC1 = I, vCE1_max = 2VCC, PD1=2VccI When RL = 0, iC1 , PD1 , Q1 may burn up. Short Circuit Protection
14.2 Class A Output Stage 14.2.4 Power-Conversion Efficiency Typical: 10%~20%
14.3 Class B Output Stage 14.3.1 Circuit Operation PUSH and PULL
14.3 Class B Output Stage 14.3.2 Transfer Characteristics
14.3 Class B Output Stage 14.3.3 Power-Conversion Efficiency
14.3 Class B Output Stage 14.3.4 Power Dissipation TRADEOFF?
14.3 Class B Output Stage 14.3.5 Reducing Crossover Distortion 0.7 V dead band is reduced to: 0.7/A0 Another option: Class AB stage
14.3 Class B Output Stage 14.3.6 Single-Supply Operation
14.4 Class AB Output Stage 14.4.1 Circuit Operation For vI = 0, vo = 0 When vI goes positive
14.4 Class AB Output Stage 14.4.1 Circuit Operation (cont.) For small vI: both transistors conduct As vI increases, or decrease, one of the two transistors takes over the operation Crossover distortion will be eliminated
Homework: Exercise: 14.1, 14.2, 14.3, 14.4, 14.5, 14.7, 14.8, 14.9