What is a Class A Amplifier?

A Class A amplifier is a high gain amplifier with high linearity. It has a conduction angle of 360°, meaning that the amplifier remains active for the entire duration of the cycle and uses the complete input signal. If you see the figure below, the amplifier remains active for the whole Sinewave cycle. This is the most common type of amplifier topology as it uses just one switching transistor (Bipolar, FET, IGBT, etc.) within the amplifier design. The single output transistor is biased around the Q-point (operating point) within the middle of the load line, due to which it is never driven into the cut-off or saturation regions, thereby allowing it to conduct current over the full 360° of the input cycle.  

Class A amplifiers have low efficiency (< 40%) but good signal reproduction and linearity. They have high fidelity (the output signal is similar to the input signal except in amplitude) and are totally immune to crossover distortion.  Even though the class A power amplifier has a handful of good features, they’re not the prime choice because the active elements (transistors) are in forward bias all the time, so some current flows through them even though no input signal is present. 

Working Principle of a Class A Amplifier

The most important component in this amplifier is the NPN transistor, which aims to amplify the AC input that is fed to it. In circuit (I), the output will always be Vcc, so a resistor (RL) is added between them. The voltage drop across the resistor is directly proportional to the current flowing through it, and the current through the resistor is equivalent to the collector's current. The collector current depends on the base current, which depends on the input signal's voltage. Therefore, any change in the input voltage will change the output voltage. 

When the input voltage is high, the current is high; hence, the voltage drop is high, and the output is low. Similarly, when the input turns low, the output goes high. The output has a phase shift of 180° , but it looks distorted as the gain of the transistor is very high and it's getting saturated. In order to limit the gain, a feedback resistor (RE) is needed between the emitter and the ground.  The voltage drop across the resistor is directly proportional to the current flowing through it. Thus, when the base current increases, the collector current also increases, which increases the voltage drop across the resistors. As the supply voltage is fixed, the voltage at the other end rises and increases the output voltage which provides negative feedback for the output, thus limiting the gain of the amplifier. 

The gain is more dependent on these two resistors than the transistor's gain, which helps control the amplifier's total gain, which is given by Rc to Re.  A capacitor is added to remove the DC bias and get the amplified input signal. Also, a transistor requires 0.7 volts or greater to turn the base on. To do that, a voltage divider is added to provide the required bias to the voltage of the signal. Now, a complete waveform of the signal can be obtained at the output, but the transistor is always on, and the DC bias voltage can interfere with the source of the input signal. Therefore, a capacitor (C1) is added here to block DC from any direction. 

Applications of a Class A RF Amplifier

In the RF domain, they’re used for RF signal amplification where high linearity is required. They are also used in the front-end stages of communication receivers to amplify weak incoming signals, as they are able to maintain signal integrity and provide low signal distortion which is required in wireless receivers.

These amplifiers are also used in AM and FM broadcasting of high-quality signals and are most most often used in premium audio systems and professional audio equipment where sound quality is an important factor. They produce minimal distortion with sound clarity and warmth. 

Advantages of Class A Amplifiers

Class A amplifiers provide the best linearity and the lowest signal distortion. They provide very good signal reproduction as the Q-point (or the operating point) is halfway between cut-off and saturation. They offer a broad and flat frequency response, making them suitable for various applications. Also, they’re simpler circuits as compared to other classes of amplifier 

Disadvantages of Class A Amplifiers

The Class A amplifiers are less efficient as compared to Class B, Class AB, and Class D amplifiers as they produce more heat and consume more power. Due to more heat it requires larger heatsinks and high-quality components, making them bulkier and more expensive.