Amplifier An electrical circuit that produces an output that is a replica of the input. The output may be scaled or have increased drive, or it may provide isolation (so changes in output conditions do not affect the input or other outputs. It may perform other transformations (e.g. filtering or logarithmic drive).
Amplifier Class Amplifier circuit types are divided into “classes” which describe whether the amplifier operates in a linear or switching mode, and any techniques used to restore linearity of output.
Class A The simplest type of amplifier, class A amplifiers are those in which the output transistors conduct (i.e. do not fully turn off) irrespective of the output signal waveform. This type of amplifier is typically associated with high linearity but low efficiency.
Class AB Class AB amplifiers combine Class A and Class B to achieve an amplifier with more efficiency than Class A but with lower distortion than class B.
This is achieved by biasing both transistors so they conduct when the signal is close to zero (the point where class B amplifiers introduce non-linearities). The transistors transition to class B operation for large excursions.
So, for small signals both transistors are active, acting like a class A amplifier. For large signal excursions, only one transistor is active for each half of the waveform, acting like a class B amplifier.
Class B Class B amplifiers are those in which the output transistors only conduct during half (180 degrees) of the signal waveform. To amplify the entire signal two transistors are used, one conducting for positive output signals and the other conducting for negative outputs.
Class B amplifiers are much more efficient than class A amplifiers, but have high distortion due to the crossover point when the two transistors transition from on to off.
Class C A class C amplifier is a form of switching amplifier in which the transistors are on for less than a half cycle (less than 180 degrees) — often, much less. For instance, the transistor may be on only during the top 10% of the signal excursion, delivering just a pulse.
Class C amps are very efficient because the transistors are off most of the time and when they are on, they are in full conduction. They deliver high distortion and are often used in RF circuits, where tuning circuits restore some of the original signal and reduce distortion. They are also used in low-fidelity applications where the distortion is not important, such as a siren speaker driver.
Class D Class D amplifiers are those that output a switching waveform, at a frequency far higher than the highest audio signal that needs to be reproduced. The low-pass filtered, average value of this waveform corresponds to the actual required audio waveform.
Class D amplifiers are highly efficient (often up to 90% or higher) since the output transistors are either fully turned on or off during operation. This completely eliminates the use of the linear region of the transistor which is responsible for the inefficiency of other amplifier types. Modern Class D amplifiers achieve fidelity comparable to class AB.
Class G Class G amplifiers are similar to class AB amplifiers except they use two or more supply voltages. When operating at low signal levels, the amplifier uses a lower supply voltage. As the signal level increases, the amplifier automatically picks the appropriate supply voltage.
Class G amplifiers are more efficient than class AB amplifiers since they use the maximum supply voltage only when required, while a class AB amplifier always uses the maximum supply voltage.
Class H Class H amplifiers modulate the supply voltage to the amplifier output devices so that it is never higher than necessary to support the signal swing. This reduces dissipation across the output devices connected to that supply and allows the amplifier to operate with an optimized class AB efficiency regardless of output power level.
Class H amplifiers are generally more complex than other designs, with extra control circuitry required to predict and control the supply voltage.