Overview

An amplifier is an active device that has the ability to amplify voltage, current, or both, at zero frequency (a DC amplifier), low frequencies (an audio amplifier), or high frequencies (an RF amplifier). Since power is P = VI, then power amplification is only a normal outcome of this capability, since raising the current and/or the voltage will create power amplification.

AC amplifiers, whether for high or low frequencies, operate by allowing a small fluctuating external input signal to control a much greater DC output bias current. This small input signal changes the amplitude of the larger bias current, with the varying bias current then sent through a high-value output impedance or resistance component, which creates an AC output voltage due to V = IR. Depending on the amplifier s designated purpose, these output components may be composed of either a resistor, inductor, or tuned circuit.

There are assorted circuit configurations to allow an amplifier to achieve different frequency responses, input and output impedances, gains, and phase shifts. Various bias circuits can be adopted to produce amplification at different efficiency and thermal stability levels, while special coupling methods can be applied to match impedances and filter out undesired frequencies with other stages or loads.

Amplifier circuit configurations. Amplifiers come in three different basic flavors, each with its own distinct application and capability. They are referred to as common-base, common-collector, and common-emitter amplifiers, depending on whether the base, collector, or emitter is common...