An Amplifier is an electronic device that increases the amplitude of an electrical signal. This effect is called amplifier gain.
Amplifiers are grouped into classes and types, and have historically changed from vacuum tube (valve) technology to semiconductor transistor based devices. Valves operated under the principle of thermionic emission from a hot filament in a vacuum controlled by a control grid. They are still used today in high power applications like radar. Semiconductor Amplifiers operate by the diffusion of charge carriers across a semiconductor junction controlled by an electric field and are the most common amplifiers readily available today.
Amplifiers are categorized in respect to their ability to effectively amplify either a voltage or current input signal. A voltage amplifier amplifies the input voltage signals and produces a larger output voltage. Current amplifiers, also known as transimpedance amplifiers, sense the currents at the inputs, and output a voltage to the load. Transconductance and Transresistance Amplifiers swap this effect – an input voltage or current produce a larger output current or voltage respectively.
One key performance parameter of an amplifier is its linearity, or its ability to accurately represent the input with the same shape signal in the time domain at the output. Other key performance parameters include the slew rate, frequency response, noise, and gain bandwidth just to name a few.
The class of Amplifier represents the amount of variation an output signal has over one cycle if the input is a sinusoidal waveform. This provides tradeoffs between linearity and efficiency. The more linear an amplifier is, the lower the distortion product. Efficiency of an amplifier is the measure of power supplied to the amplifier compared to the resulting amplified output. Amplifiers that are designed for higher efficiency increase the amount of time the output stage is fully on or fully off, which may increase distortion to the signal.
Classifications of amplifiers are A, AB, B, C, D and E. Class A is the most linear and class C is the least linear. Class C is not suitable for audio applications and typically is used in products like RF transmitters. Classes D and E are switching amplifiers that can utilize output circuit stages containing harmonic resonators, multiple supply rail switching, complimentary output techniques, and PWM and delta-sigma modulation to increase efficiency and reconstruct the input waveform at the output.