- Different amplifier designs exist. Between amplifier classes, there is a distinct difference in how their output stages are set up and work. Linearity, signal gain, efficiency, and power output are the primary operational qualities of an ideal amplifier, however, in practical amplifiers, there is always a trade-off between these various features.
- To drive a loudspeaker load, massive signal or power amplifiers are typically used in the output stages of audio amplifier systems. The impedance of a typical loudspeaker ranges from 4 to 8, hence a power amplifier needs to be able to deliver the high peak currents needed to drive the speaker’s low impedance.
Classification of Amplifier
- There are significant variations among different types of amplifiers, particularly in Power Amplifiers. Therefore, we shall examine many classes of amplifiers below.
- The performance and attributes of an amplifier are identified by its class. When current is passed through different kinds of power amplifiers, they respond in various ways. Amplifiers are given several letters or alphabets that stand in for their classes by their criteria. Amplifiers can be divided into various classes, such as A, B, C, AB, D, E, F, and T, among others. The classes of audio amplifiers that are most frequently used are A, B, AB, and C.
- Other Classes are contemporary amplifiers that drive the output load using switching topologies and PWM (Pulse Width Modulation) technology. To distinguish them as a new class of amplifiers, upgraded versions of conventional classes are occasionally given a letter designation. For example, a class G amplifier is a modified Class B or Class AB amplifier.
- When current is routed through the amplifier, the classes of the amplifier represent the input cycle proportion. The sinusoidal wave conduction in the amplifier input gives rise to the input cycle, which is the conduction angle. The Amplifiers’ time during a whole cycle is directly proportional to this conducting angle. During a cycle, the conduction angle will be 360 degrees if the amplifier is always ON. Therefore, if an amplifier offers a 360-degree conduction angle, it used the entire input signal and the active element conducted for the full 100% of a sinusoidal cycle.
CLASS A AMPLIFIER
- A class A amplifier is characterized by a 360° conduction angle. Operation of Class A tiny signals is linear. Halfway between saturated current and pinch-off is the bias point. Think Class A when you hear the terms “gain block” or “linear amplifier.” 100% of the waveform of the input signal is used in the output signal.
- The quiescent point of a Class A amplifier is chosen to be in the middle of the load line to accomplish this property, as shown in the waveform below.
- As previously mentioned, a 360-degree conduction angle indicates that the amplifier device is always active and using the entire input signal An perfect class A amplifier is displayed in the figure below.
- There is only one active component, a transistor, as can be seen in the figure. The bias of the transistor is constantly ON. Class A amplifiers offer improved high frequency and feedback loop stability as a result of its never switch off characteristic. Aside from these benefits, building a Class A amplifier is simple and requires just one device component and a small number of parts.
- It has various limits despite its benefits and great linearity. The constant conducting feature of the class A amplifier results in a significant amount of power loss. Additionally, Class A amplifiers produce noise and distortion because of their great linearity. To prevent unwanted noise and reduce distortion, the power supply and bias architecture require careful component selection.
CLASS B AMPLIFIER
- In class B, the transistor only operates for one-half of the signal cycle. Therefore, the DC power consumed in Class B with no stimulation by an input signal is optimum for a FET-based amplifier and very minimal for a bipolar amplifier. The maximum efficiency for Class B according to theory is 78.5%.
- Push-pull amplifiers, also known as “complementary amplifiers,” work using transistors with opposite polarity (PNP and NPN) yet nonetheless deliver the complete waveform. The distortion caused by the threshold voltages that the transistors must all cross can be lessened by a bias circuit on the input.
- An ideal Class B amplifier setup is represented in the image above. It consists of two active devices that are biased sequentially throughout the positive and negative halves of a sinusoidal wave, pushing or pulling the signal to an enhanced level from both the positive and the negative side. When we combine the results, we get a full cycle across the output.
- When compared to the 25–30% efficiency of a Class A amplifier, it theoretically offers more than 60% efficiency because each device was turned on or became active for half of the cycle. The graph of input and output signals for each device is shown in the image below. The efficiency of a Class B amplifier is no higher than 78%. This class has a low heat sink space, which reduces heat dissipation.
- But this class has some restrictions as well. The crossover distortion is a very significant restriction in this class. There is a mismatch (cross over) at the region where two halves are merged as two devices provide each half of the sinusoidal waves that are combined and linked across the output. This is because when one device completes a half-cycle, the other one must nearly immediately supply the same amount of power. Since the other device is fully dormant while the active device is running, fixing this problem in a class A amplifier is challenging. The output signal is distorted as a result of the fault. This restriction makes it a serious failure for applications using precision audio amplifiers.
Class AB Amplifier
- The Class AB Amplifier, as its name suggests, combines the “Class A” and “Class B” types of amplifiers that we have just examined. One of the most widely utilized varieties of audio power amplifier design at the moment is the AB classification of the amplifier.
- The class AB amplifier is an adaptation of the class B amplifier as previously described, with the exception that both devices are permitted to conduct simultaneously around the waveforms crossover point, hence removing the crossover distortion issues of the prior class B amplifier.
- Each device conducts a modest quantity of input on a subsequent sinusoidal half-cycle rather than ceasing conduction immediately after finishing the first half of the waveform. The crossover mismatch during the dead zone is significantly decreased by using this biasing technique.
- However, because the devices’ linearity is weakened in this arrangement, efficiency is decreased. While still higher than that of a standard Class A amplifier, the efficiency is lower than that of a Class B amplifier system. Additionally, the diodes must be properly selected, have the same rating, and be positioned as near as feasible to the output device. To prevent distortion across the output, designers of particular circuits frequently include tiny value resistors to provide consistent quiescent current across the device.
Class C Amplifier
- There is a Class C amplifier in addition to the Class A, B, and AB amplifiers. It’s a conventional amplifier that operates differently from the other classes of amplifiers. Class C amplifiers are tuned amplifiers that can operate in either tuned or untuned modes.
- A class-C amplifier uses less than half of the input signal (conduction angle: 180°). A tuned circuit must be used as a load since distortion is significant. Radio-frequency applications have an efficiency of 80%.
- Class C amplifiers are frequently used in high-frequency sine wave oscillators and some types of radio frequency amplifiers despite their significant audio distortion because the pulses of current they produce at their output can be transformed into complete sine waves of a specific frequency by the use of LC resonant circuits in their collector circuit.