amplifier classes


(n.) A system of classification based on the relationship between the output voltage and the input voltage of audio power amplifiers. Each class is defined by the design of the output stage. The classification system is based on the amount of time the output devices operate during one complete cycle of signal swing. This is also defined in terms of output bias current, which is the amount of current flowing in the output devices with no applied signal. With the exception of class A, it is assumed that a simple output stage consists of two complementary devices, one with positive polarity and the other with negative polarity, using tubes or any type of transistor.

  • class A In this design, both output devices conduct continuously for the entire cycle of signal swing, or the bias current flows in the output devices at all times. The amplifier classes 20 identifying factor in class A operation is that both devices are always on. There is no condition where one or the other is turned off. For this reason class A amplifiers are not complementary designs. They are single-ended designs with a single type of polarity output. Class A is the most inefficient of all power amplifier designs, averaging about 20% efficiency, which means that it draws five times as much power from the source as it delivers to the load Class A amplifiers are heavy and run hot because they are constantly operating at full power. The benefit is class A designs are the most linear, with the least amount of distortion.
  • class B This design is the opposite of class A. The two output devices are never allowed to be on at the same time, or the bias is set so that current flow in an output device is zero when not stimulated with an input signal. Each output device operates for exactly one half of a complete sinusoidal signal cycle. Class B designs have high efficiency but poor linearity. The extra time it takes to turn the output devices on and off results in high crossover distortion. These designs are restricted to power-consumption critical applications such as battery-operated equipment and communications audio.
  • class AB This design is a blending of class A and class B. Both output devices are allowed to be on at the same time, as in class A, but only slightly overlap. Only a small amount of current is allowed to flow through both devices simultaneously, unlike the full load current class A designs. However, enough current is allowed to keep each device operating so they respond instantly to the input demand. This eliminates the non-linearity of class B design, without the inefficiency of class A design. With 50% efficiency and excellent linearity, class AB is the most popular audio amplifier design.
  • class AB plus B In this design, two pairs of output devices are used. One pair operates class AB, while the other pair operates class B.
  • class C This design is used exclusively in the broadcast industry for radio frequency (RF) transmission. The output devices take turns operating. Each is pulsed on for a percentage of the half cycle, instead of operating continuously for the entire half cycle. It is a very efficient design capable of massive output power. Radio frequency –tuned circuits overcome the distortion created by the pulsed operation of Class C designs.
  • class D This design is referred to as a switching power amplifier. The output devices are rapidly switched on and off several times for each cycle. Theoretically, since the output devices are either on or off, they do not dissipate any power. Class D operation approaches 90% efficiency. This design is similar to the original “class S” designs.
  • class E This class is designed for rectangular input pulses, not sinusoidal audio waveforms. The output load is a tuned circuit, with the output voltage resembling a damped single pulse. Normally, class E employs a single transistor driven to act as a switch.

The following amplifier classifications are generally agreed upon, but they do not have any amplifier classes _ 21 official status:

  • class F This is a class of tuned power amplifiers, and the load is a tuned resonant circuit. The circuit may be tuned for one or more harmonic frequencies, as well as the carrier frequency. Designs in this group are also known as “biharmonic,” “polyharmonic,” “Class DC,” “singleended Class D,” “High-efficiency Class C,” and “multiresonator.”
  • class G In this design, the power supply voltage is changed from a lower level to a higher level when larger output swings are required. Typically, a switch connects a single class AB output stage to two power supply rails. The output stage is generally connected to the lower supply voltage, but automatically switches to the higher rails for large signal peaks (rail-switching). Using two power supplies improves efficiency enough to allow significantly more power at a given size and weight. Class G is common in pro audio applications.
  • class H This design improves on class G by modulating the higher power supply voltage by the input signal. This allows the power supply to track the audio input and provide just enough voltage for optimum operation of the output devices. It is sometimes called a tracking power amplifier. The efficiency is comparable to class G designs.
  • class S First invented in 1932, this design is used for both amplification and amplitude modulation. It is similar to class D; however, the pulse width modulation (PWM) voltage waveform is applied to a low-pass filter that allows only the slowly varying average voltage component to appear across the load. Class D amplifiers also operate in this fashion.
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