A2 Amplifiers with BJT

Topic

This same bipolar transistor—already described in the context of advanced semiconductor device models—amplifies current: a small current injected into its base terminal controls a much larger current flowing between its other two terminals, governed by an amplification ratio specific to each transistor. To correctly amplify a signal without distortion, the transistor must first be biased: this involves establishing a stable operating point—using resistors and fixed voltage sources—within the region where the transistor behaves most linearly, neither fully cut off nor fully saturated.

The common-emitter configuration, the most common setup for voltage amplification, inverts the input signal in addition to amplifying it. Beyond this basic configuration, there are various amplification "classes" that define the fraction of the signal cycle during which the transistor conducts; these represent a fundamental trade-off between linearity and energy efficiency. Class A conducts throughout the entire cycle and offers the best linearity but wastes most of the energy as heat—the same Joule heating effect described by the resistor model and Ohm's Law. Class B conducts for only half the cycle, sacrificing some linearity for significantly higher efficiency. Class AB strikes a balance between the two and is the most widely used in high-quality audio amplifiers. Finally, Class D—which operates via rapid switching rather than linear amplification—achieves the highest efficiency of all and currently dominates the fields of high-power audio amplifiers and motor controllers.

Thus, this module—which began by illustrating how a small current controls a larger one—concludes by showing that amplification never comes for free: each of these four classes strikes a different balance between faithful signal reproduction and minimizing energy waste as heat, and none can maximize both simultaneously.

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