Amplifier Classes Explained
Not all amplifiers are the same. Generally, amplifiers are classified according to their circuit configuration and method of operation, and as such Amplifier Classes are used to differentiate between them. Amplifier classes range from entirely linear operation (for use in high-fidelity signal amplification) with low efficiency, to entirely non-linear (where faithful reproduction is not so important) operation with high efficiency, while others are a compromise between the two.
Amplifier Classes are mainly lumped into two basic groups. The classically controlled conduction angle amplifier forming amplifier classes A, B, AB and C, which are defined by the length of their conduction state over some portion of the output waveform, such that the output stage transistor operation lies somewhere between being â€œfully-ONâ€ and â€œfully-OFFâ€, and the so-called â€œswitchingâ€ amplifier classes of D, E, F, G, S, T etc, that are constantly being switched between â€œfully-ONâ€ and â€œfully-OFFâ€.
The most commonly available amplifier classes are those that are used as audio amplifiers , mainly A, B, AB and C and to keep it simple, it is these amplifier classes we will look at here in this amplifier classes tutorial. Class A Amplifier
Class A Amplifiers are the simplest in design, and probably the best sounding of all the amplifier classes due to their low signal distortion. The class A amplifier has the highest linearity over the other amplifier classes and as such operates in the linear portion of the characteristics curve. This means that the output stage whether using a bipolar, mosfets or IGBT device, is never driven fully into its cut-off or saturation regions. Class A Amplifier
To achieve high linearity and gain, the output stage is biased â€œONâ€ (conducting) all the time and operates at a constant current equal to or greater then the current which the load (usually a loudspeaker) requires to produce the largest output signal. The output device conducts through 360 degrees of the output waveform. Then the class A amplifier is equivalent to a current source.
Since a class A amplifier operates in the linear region, the transistors base (or gate) DC biasing voltage should by chosen properly to ensure correct operation and low distortion. However, as the output device is â€œONâ€ at all times, it is constantly carrying current, which represents a continuous loss of power in the amplifier.
Due to this continuous loss of power class A amplifiers create tremendous amounts of heat adding to their very low efficiency at around 30%, making them impractical for high-power amplifications. Therefore, due to the low efficiency and over heating problems of Class A amplifiers, more efficient amplifier classes have been developed. Class B Amplifier
Class B amplifiers were invented as a solution to the efficiency and heating problems associated with the class A amplifiers. The basic class B amplifier uses two complimentary transistor devices (one NPN and one PNP transistor connected in common collector mode) in its output stage configured in a â€œpush-pullâ€ arrangement, with each device amplifying only half of the output waveform.
In the class B amplifier, there is no standing bias current as its quiescent current is zero, therefore its efficiency is much higher than that of the class A amplifier. When the input signal goes positive, the positive biased device conducts while the negative device is switched off. Likewise, when the input signal goes negative, the positive device switches off while the negative biased device turns on and conducts the negative portion of the signal. Class B Amplifier
Therefore, each transistor device of the class B amplifier only conducts through 180 degrees of the output waveform in strict time alternation, but as the output stage has devices for both halves of the signal waveform the two halves are combined together to produce the full linear output waveform.
This push-pull design of amplifier is obviously more efficient than Class A, at about 50%, but the problem with the class B amplifier design is that it can create distortion at the zero-crossing point of the waveform due to the transistors dead band of input base voltages from -0.7V to +0.7V, making it unsuitable for precision amplifier applications. Class AB AmplifierAs its name suggests, the Class AB Amplifier is a combination of the two class A and class B type amplifiers above, and is currently one of the most common types of power amplifier design. The class AB amplifier is a variation of a class B amplifier as described above, except that both devices are allowed to conduct at the same time around the crossover point eliminating the crossover distortion problems of the pure class B amplifier.
The two transistors have a very small bias voltage, typically at 5 to 10% of the quiescent current to bias the transistors just above cut-off. In this case, the transistor will be â€œONâ€ for more than half a cycle, but less than a full cycle of the input signal. Then in a class AB amplifier design each of the push-pull transistors 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 AB Amplifier
The advantage of this small bias voltage is that the crossover distortion created by the class B amplifier characteristics is overcome, without the inefficiencies of a the class A amplifier design. So the class AB amplifier is a compromise between class A and class B in terms of efficiency and linearity, with efficiencies reaching about 50% to 60%. Class C AmplifierThe Class C Amplifier design has the greatest efficiency but the poorest linearity of the classes of amplifiers. 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.
However, the class C amplifier is heavily biased so that the output current is zero for more than one half of an input sinusoidal signal cycle. In other words, the conduction angle for the transistor is significantly less than 180 degrees, at around 90 to 120 degrees.
This form of biasing gives a much improved efficiency of around 80% to the amplifier, but very heavy distortion of the output signal. Therefore, class C amplifiers are not suitable for use as audio amplifiers. Class C Amplifier
Class C amplifiers 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.
Then we have seen that the quiescent DC operating point (Q-point) of an amplifier determines the amplifier classification. By setting the position of the Q-point at half way on the load line of the amplifiers characteristics curve, the amplifier will operate as a class A amplifier. By moving the Q-point lower down the load line changes the amplifier into a class AB, B or C amplifier.
Then the class of operation of the amplifier with regards to its DC operating point can be given as: Amplifier Classes and Efficiency
As well as audio amplifiers there are a number of high efficiency Amplifier Classes relating to switching amplifier designs that use different switching techniques to reduce power loss and increase efficiency. Some amplifier class designs listed below use RLC resonators or multiple power-supply voltages to reduce power loss.