Generally, an amplifier is any device that will convert one signal (often with a very small amount of energy, a few milliwatt) into a another signal (often with a larger amount of energy e.g. several hundred watts).
In popular use, the term today usually refers to an electronic amplifier, often as in audio applications. The relationship of the input to the output of an amplifier — usually expressed as a function of the input frequency — is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain. A related device that emphasizes conversion of signals of one type to another (for example, a light signal in photons to a DC signal in amperes) is a transducer, or a sensor. However, a transducer does not amplify power.
Amplifier Classes:
Most mobile amplifiers use complementary transistor pairs to drive the speakers. In this configuration there is a transistor (or group of transistors) which conducts current from the positive power supply voltage for the positive half of the audio waveform and a different transistor (or group of transistors) which conducts current from the negative power supply voltage for the negative half of the waveform. There are some amplifiers which use the same transistor(s) to drive both the positive and the negative halves of the waveform.
NOTE:Amplifiers in classes A, B, and AB operate their output transistors in a 'linear' mode. Class 'D' amplifiers operate their outputs in 'switch' mode.
Class A
100% of the input signal is used (conduction angle Θ = 360° or 2π). Where efficiency is not a consideration, most small signal linear amplifiers are designed as Class A, which means that the output devices are always in the conduction region. Class A amplifiers are typically more linear and less complex than other types, but are very inefficient. This type of amplifier is most commonly used in small-signal stages or for low-power applications (such as driving headphones).
Class B
50% of the input signal is used (Θ = 180° or π). In Class B, there are two output devices (or sets of output devices), each of which conducts alternately for exactly 180 deg (or half cycle) of the input signal.
Class AB
More than 50% but less than 100% is used. (181° to 359°, π < Θ < 2π). Class AB amplifiers are a compromise between Class A and B, which improves small signal output linearity; conduction angles vary from 180 degrees upwards, selected by the amplifier designer. Usually found in low frequency amplifiers (such as audio and hi-fi) owing to their relatively high efficiency, or other designs where both linearity and efficiency are important (cell phones, cell towers, TV transmitters).
Class AB1 applies to tube or transistor amplifiers in class AB where the grid or base is more negatively biased than it is in class A.
Class AB2 applies to tube or transistor amplifiers in class AB where the grid or base is often more negatively biased than in AB1, and the input signal is often larger. When the drive is high enough to make the grid or the base more positive, the grid or base current will increase. It is possible depending on the level of the signal input for the amplifier to move from class AB1 to AB2.
Class C
Less than 50% is used (0° to 179°, Θ < π). Popular for high power RF amplifiers, Class C is defined by conduction for less than 180° of the input signal. Linearity is not good, but this is of no significance for single frequency power amplifiers. The signal is restored to near sinusoidal shape by a tuned circuit, and efficiency is much higher than A, AB, or B classes of amplification.
Class D
Main article: Switching amplifier
These use switching to achieve a very high power efficiency (more than 90% in modern designs). By allowing each output device to be either fully on or off, losses are minimized. A simple approach such as pulse-width modulation is sometimes still used; however, high-performance switching amplifiers use digital techniques, such as sigma-delta modulation, to achieve superior performance. Formerly used only for subwoofers due to their limited bandwidth and relatively high distortion, the evolution of semiconductor devices has made possible the development of high fidelity, full audio range Class D amplifiers, with S/N and distortion levels similar to their linear counterparts.
Other classes
There are several other amplifier classes, although they are mainly variations of the previous classes. For example, Class H and Class G amplifiers are marked by variation of the supply rails (in discrete steps or in a continuous fashion, respectively) following the input signal. Wasted heat on the output devices can be reduced as excess voltage is kept to a minimum. The amplifier that is fed with these rails itself can be of any class. These kinds of amplifiers are more complex, and are mainly used for specialized applications, such as very high-power units. Also, Class E and Class F amplifiers are commonly described in literature for radio frequencies applications where efficiency of the traditional classes deviate substantially from their ideal values. These classes use harmonic tuning of their output networks to achieve higher efficiency and can be considered a subset of Class C due to their conduction angle characteristics.
In popular use, the term today usually refers to an electronic amplifier, often as in audio applications. The relationship of the input to the output of an amplifier — usually expressed as a function of the input frequency — is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain. A related device that emphasizes conversion of signals of one type to another (for example, a light signal in photons to a DC signal in amperes) is a transducer, or a sensor. However, a transducer does not amplify power.
Amplifier Classes:
Most mobile amplifiers use complementary transistor pairs to drive the speakers. In this configuration there is a transistor (or group of transistors) which conducts current from the positive power supply voltage for the positive half of the audio waveform and a different transistor (or group of transistors) which conducts current from the negative power supply voltage for the negative half of the waveform. There are some amplifiers which use the same transistor(s) to drive both the positive and the negative halves of the waveform.
NOTE:Amplifiers in classes A, B, and AB operate their output transistors in a 'linear' mode. Class 'D' amplifiers operate their outputs in 'switch' mode.
Class A
100% of the input signal is used (conduction angle Θ = 360° or 2π). Where efficiency is not a consideration, most small signal linear amplifiers are designed as Class A, which means that the output devices are always in the conduction region. Class A amplifiers are typically more linear and less complex than other types, but are very inefficient. This type of amplifier is most commonly used in small-signal stages or for low-power applications (such as driving headphones).
Class B
50% of the input signal is used (Θ = 180° or π). In Class B, there are two output devices (or sets of output devices), each of which conducts alternately for exactly 180 deg (or half cycle) of the input signal.
Class AB
More than 50% but less than 100% is used. (181° to 359°, π < Θ < 2π). Class AB amplifiers are a compromise between Class A and B, which improves small signal output linearity; conduction angles vary from 180 degrees upwards, selected by the amplifier designer. Usually found in low frequency amplifiers (such as audio and hi-fi) owing to their relatively high efficiency, or other designs where both linearity and efficiency are important (cell phones, cell towers, TV transmitters).
Class AB1 applies to tube or transistor amplifiers in class AB where the grid or base is more negatively biased than it is in class A.
Class AB2 applies to tube or transistor amplifiers in class AB where the grid or base is often more negatively biased than in AB1, and the input signal is often larger. When the drive is high enough to make the grid or the base more positive, the grid or base current will increase. It is possible depending on the level of the signal input for the amplifier to move from class AB1 to AB2.
Class C
Less than 50% is used (0° to 179°, Θ < π). Popular for high power RF amplifiers, Class C is defined by conduction for less than 180° of the input signal. Linearity is not good, but this is of no significance for single frequency power amplifiers. The signal is restored to near sinusoidal shape by a tuned circuit, and efficiency is much higher than A, AB, or B classes of amplification.
Class D
Main article: Switching amplifier
These use switching to achieve a very high power efficiency (more than 90% in modern designs). By allowing each output device to be either fully on or off, losses are minimized. A simple approach such as pulse-width modulation is sometimes still used; however, high-performance switching amplifiers use digital techniques, such as sigma-delta modulation, to achieve superior performance. Formerly used only for subwoofers due to their limited bandwidth and relatively high distortion, the evolution of semiconductor devices has made possible the development of high fidelity, full audio range Class D amplifiers, with S/N and distortion levels similar to their linear counterparts.
Other classes
There are several other amplifier classes, although they are mainly variations of the previous classes. For example, Class H and Class G amplifiers are marked by variation of the supply rails (in discrete steps or in a continuous fashion, respectively) following the input signal. Wasted heat on the output devices can be reduced as excess voltage is kept to a minimum. The amplifier that is fed with these rails itself can be of any class. These kinds of amplifiers are more complex, and are mainly used for specialized applications, such as very high-power units. Also, Class E and Class F amplifiers are commonly described in literature for radio frequencies applications where efficiency of the traditional classes deviate substantially from their ideal values. These classes use harmonic tuning of their output networks to achieve higher efficiency and can be considered a subset of Class C due to their conduction angle characteristics.
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