Many amplifiers exist that don’t fall into any of the standard amplifier categories.  These could be combination ICs, such as devices integrating general purpose op-amps with comparators, or other specialized amplifiers.

 

Amplifiers are grouped into classes.  These classes represent the amount of variation an output signal has over one cycle if the input is a sinusoidal waveform. The reason for these variations is to provide amplification at different tradeoffs between linearity and efficiency. Linearity is a measure of an amplifiers output in proportion to its inputs.  Efficiency of an amplifier is the measure of how much power needs to be supplied to the amplifier from its power supply for 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. This introduces distortion characteristics to the signal. In many cases, these distortion characteristics can be filtered out as they tend to be higher frequency harmonic components. Class A devices can be designed to have good linearity.  However, various degrees of linearity exist for the different classes of amplifiers: AB, B, and C (class C amplifiers are not linear in any topology). Class C are not suitable for audio applications and are typically used in products such as RF transmitters, where the pulse style output can be reconstructed to a continuous waveform by the output circuit. Class D, E and above are a special set of classes called switching amplifiers where techniques for efficient output are used like PWM. Switching amplifiers can make use of special output circuits containing harmonic resonators, multiple supply rail switching, complimentary output techniques and delta-sigma modulation to increase efficiency and reconstruct the waveform.

 

Amplifiers can require heat sinks and may also require filtering to remove undesired noise. They are typically specified by signal gain, output power, and their stability driving various load impedances. Stability of an amplifier is very important and is merited by its phase margin. The phase margin marks the difference from 180 degrees of the phase of the open-loop transfer function when the gain is at unity.