There are 4 types of amplifier the class A, class B, class AB and class D.
Class A is a type of amplifier where the output is as much represented as to the input. A type of amplifier which is a hard working amplifier it is because it is working all the time. The transistor on these amplifier is always ON and there is no condition that it is turned off. That is why these type of amplifier is inefficient due to waste a lot of power in the circuit. These use the same single transistor (Bipolar, FET, IGBT, etc) connected in common emitter configuration for both halves of the waveform with the transistor always having current flowing through it, even if it has no base signal. This means that the output stage whether using a Bipolar, MOSFET or IGBT device, it is never driven fully into its cut-off or saturation regions but instead has a base biasing Q-point in the middle of its load line.
These are most common type of amplifier class due to mainly their simple design. Class A literally means the best class of amplifier due mainly to their low signal distortion levels and are probably the best sounding of all the amplifier classes mentioned. The class A amplifier has the highest linearity over the other amplifier classes and as such operate in the linear portion of the characteristic curve. With regards to its output it has achieve a high linearity and gain. Since it operate in the linear region, the transistor base DC biasing voltage should by chosen properly to ensure correct operation and low distortion. The output device is ON at all times, it is constantly carrying current, which represents a continuous loss of power in the amplifier. Due to power loss these make it impractical for high power amplification.
Class B is a type of amplifier where the positive and negative halves wave of the signal are working on different of way on the circuit. The output devices continually or constantly turn on and off. Its input signal must be larger in order to drive the transistor appropriately. The output stage employs two output devices and each side amplifies each half of the wave. It uses two transistors either bipolar of FET for each half of the waveform with its output stage configured in a push and pull type arrangement. So that each transistor device amplifies only half of the output waveform. There is no DC base bias current as its quiescent current is zero, so that the dc power is small and therefore its efficiency is much higher than that of the class A amplifier. When the input signal goes positive, The positive biased of the transistor conducts while the negative of transistor is switched OFF. When the input signal goes negative, the positive transistor switches ‘OFF’ while the negative biased transistor turns ON and conducts the negative portion of the signal. Thus the transistor conducts only half of the time, either on positive or negative half cycle of the input signal.
Class AB is a type of amplifier that operates has some of the best advantages of both class A and class B build in its benefits is similar comparable to class A and the efficiency similar to that of class B. The AB classification of amplifier is currently one of the most common used types of audio power amplifier design. The two transistors have a very small bias voltage, typically at 5 to 10% of the quiescent current to bias the transistors just above its cut-off point. Then the conducting device either bipolar of FET and will be ON for more than one half cycle, but much less than one full cycle of the input signal. In a class AB amplifier design each of the push and pull transistor is conducting for slightly more than the half cycle of conduction in class B, but much less than the full cycle of conduction of class A.
Class C is a type of amplifier that has the greatest efficiency but the poorest linearity of the classes of amplifiers compare to other class. The previous classes, A, B and AB are considered linear amplifiers, as the output signals amplitude and phase are linearly related to the input signals amplitude and phase.
Class C amplifier is heavily biased so that the output current is zero for more than one half of an input sinusoidal signal cycle with the transistor idling at its cut-off point. In other words, the conduction angle for the transistor is significantly less than 180 degrees, and is generally around the 90 degrees area. It is introduces a very heavy distortion of the output signal. Class C amplifiers are not suitable for use as audio amplifier. Due to heavy audio distortion, class C amplifier are commonly used in high frequency sine wave oscillators and certain types of radio frequency amplifiers, where the pulses of current produced at the amplifiers output can be converted to complete sine waves of a particular frequency by the use of LC resonant circuits in its collector circuit.
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