26.1: Theory Overview

Last updated
Save as PDF

Page ID: 37327

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

The maximum output signal, or compliance, of a class B amplifier is determined by its AC load line. The peak to peak compliance is roughly equal to the total DC supply voltage(s). As two output devices are used, each conducting for half of the cycle, the quiescent current can remain low, unlike a class A amplifier. This results in vastly improved efficiency, theoretically up to 78.5%. The switchover from one transistor to the other is problematic and can result in crossover or notch distortion. To alleviate this, the transistors are given a small idle current so that each base-emitter junction is just about fully on. While resistors can be used to create this bias, trying to match the linear current-voltage characteristic of a resistor to the logarithmic characteristic of a PN junction is tricky. Consequently, another PN junction, namely a diode, is used instead. The diode will result in a more stable circuit which produces less notch distortion.