Amplifier circuits are classified into different classes, which include the following: classes A, B, AB and C for analog designs, and classes D and E/F for switching designs.
Class A - 100% of the input signal is used (conduction angle a = 360° or 2π):
Class A amplifiers amplify over the entire input cycle such that the output signal is an exact magnified copy of the input. They are not efficient (not more than 50% efficiency is attainable), since the amplifying device is always conducting whether an input signal is applied or not.
Class B - 50% of the input signal is used (a = 180° or π):
A Class B amplifier is one whose operating point is at an extreme end of its characteristic, so that either the quiescent current or the quiescent voltage is almost zero. If a sinusoidal input voltage is used, the amplification of a Class B amplifier takes place only for 50% of the cycle, e.g., the amplifying device is switched off half of the time. A Class B amplifier can attain an efficiency of up to 78.5%. However, a Class B amp exhibits a higher distortion than an equivalent Class A amp.
Class C - less than 50% is used (0° to 179°, a < π):
Class C amplifiers conduct less than 50% of the input signal, allowing it to reach 90% efficiency but resulting in high distortion at the output. Thus, in a Class C amp, the output current (or voltage) is zero for more than 50% of the input waveform cycle. Some applications, such as megaphones, can tolerate the high distortion of Class C amps. Class C amps can also be used in tuned RF applications, since the distortion can be significantly reduced by the tuned loads.
A class D amplifier is a power amplifier whose power devices are operated in on/off mode. The input signal is converted into a sequence of pulses whose average is directly proportional to the amplitude of the input signal. The frequency of the pulses is typically ten or more times the highest frequency of interest in the input signal. The output of the amplifier goes through a passive filter to remove unwanted spectral components, resulting in an amplified replica of the input. Power efficiency is the main advantage of a class D amplifier. Class D amplifiers were widely used to control motors, but they are also used as audio amplifiers.
Class E/F amplifiers are highly efficient switching power amplifiers used at radio frequencies. Class E/F amplifiers consists of two basic parts: 1) a 'perfect' switching device and 2) an impedance network consisting of resistive and reactive components. The switching device is 'on' during the zero-voltage crossing, and off during the zero-current crossing, such that it can not have both current flowing through it and a non-zero voltage across it at the same time, thereby minimizing its power dissipation. The impedance network, on the other hand, is set up such that the 'imaginary part' of the impedance is eliminated through proper matching of complex conjugates to attain resonance, leaving behind only its 'real part.' Thus, a Class E/F amp is very efficient because power loss only occurs in the real part (resistive component) of the impedance network. Classes E and F are distinguished from each other by their resonance topology.
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Amplifier circuits are classified into different classes, which include the following: classes A, B, AB and C for analog designs, and classes D and E/F for switching designs.
Class A - 100% of the input signal is used (conduction angle a = 360° or 2π):
Class A amplifiers amplify over the entire input cycle such that the output signal is an exact magnified copy of the input. They are not efficient (not more than 50% efficiency is attainable), since the amplifying device is always conducting whether an input signal is applied or not.
Class B - 50% of the input signal is used (a = 180° or π):
A Class B amplifier is one whose operating point is at an extreme end of its characteristic, so that either the quiescent current or the quiescent voltage is almost zero. If a sinusoidal input voltage is used, the amplification of a Class B amplifier takes place only for 50% of the cycle, e.g., the amplifying device is switched off half of the time. A Class B amplifier can attain an efficiency of up to 78.5%. However, a Class B amp exhibits a higher distortion than an equivalent Class A amp.
Class AB - more than 50% but less than 100% is used. (181° to 359°, π < a < 2π):
A Class AB amplifier is an amplifier that operates between the two extremes defined for Class A and B amplifiers.
Class C - less than 50% is used (0° to 179°, a < π):
Class C amplifiers conduct less than 50% of the input signal, allowing it to reach 90% efficiency but resulting in high distortion at the output. Thus, in a Class C amp, the output current (or voltage) is zero for more than 50% of the input waveform cycle. Some applications, such as megaphones, can tolerate the high distortion of Class C amps. Class C amps can also be used in tuned RF applications, since the distortion can be significantly reduced by the tuned loads.
Class D:
A class D amplifier is a power amplifier whose power devices are operated in on/off mode. The input signal is converted into a sequence of pulses whose average is directly proportional to the amplitude of the input signal. The frequency of the pulses is typically ten or more times the highest frequency of interest in the input signal. The output of the amplifier goes through a passive filter to remove unwanted spectral components, resulting in an amplified replica of the input. Power efficiency is the main advantage of a class D amplifier. Class D amplifiers were widely used to control motors, but they are also used as audio amplifiers.
Class E/F:
Class E/F amplifiers are highly efficient switching power amplifiers used at radio frequencies. Class E/F amplifiers consists of two basic parts:
1) a 'perfect' switching device and
2) an impedance network consisting of resistive and reactive components.
The switching device is 'on' during the zero-voltage crossing, and off during the zero-current crossing, such that it can not have both current flowing through it and a non-zero voltage across it at the same time, thereby minimizing its power dissipation. The impedance network, on the other hand, is set up such that the 'imaginary part' of the impedance is eliminated through proper matching of complex conjugates to attain resonance, leaving behind only its 'real part.' Thus, a Class E/F amp is very efficient because power loss only occurs in the real part (resistive component) of the impedance network. Classes E and F are distinguished from each other by their resonance topology.
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