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The Real Facts for Class A vs. AB Amplifiers for EMC Applications
This application note from Instruments for Industry (IFI) compares Class A and Class AB amplifiers specifically within the context of Electromagnetic Compatibility (EMC) testing. The paper details how Class A amplifiers prioritize linearity at the expense of efficiency, leading to high heat dissipation and reduced reliability – particularly problematic with high VSWR common in EMC setups. Conversely, Class AB amplifiers offer a compromise between linearity and efficiency and incorporate features like “Bias-Plus Class AB” biasing (Class A operation up to -3dB below P1dB, then transitioning to Class AB) and 50% transistor derating (“3dB-over-silicon”) for improved reliability. IFI’s analysis shows that harmonic distortion is virtually equivalent between the two classes within EMC testing parameters (-25 to -35 dBc for 2nd harmonics, -22 to -30dBc for 3rd harmonics prior to P1dB).