Amplifier / Transmitter Classes

Ever wonder what it means when someone mentions how their amplifier is classed? See below to get an idea and hopefully clear up some confusion on what the different classes mean. This is a list of the most popular classes and how they work however, there are several others too!

Class A Transmitters

A Class A transmitter circuit can be constructed using a single transistor or a pair of transistors.

In a single transistor Class A transmitter, the transistor is biased so that it is always conducting a steady amount of current, even when there is no signal being transmitted. This means that the amplifier is always "on" and using power, which can result in lower efficiency. However, this design also results in a very pure and accurate signal output, with minimal distortion.

In a transistor pair (Push-Pull) Class A transmitter, two transistors are used to amplify the signal. One transistor is biased to conduct current during the positive half of the input signal, while the other is biased to conduct current during the negative half of the input signal. This configuration allows for a higher output power, while still maintaining the high signal quality and low distortion characteristics of a single transistor Class A transmitter.

To visualize this, think of each transistor in the pair as a light switch, with one switch controlling the power during the positive half of the input signal and the other switch controlling the power during the negative half of the input signal. When the input signal is positive, the first transistor turns on and conducts current, while the second transistor remains off. When the input signal is negative, the second transistor turns on and conducts current, while the first transistor remains off. This allows for a continuous output signal that is amplified by both transistors.

Overall, Class A transmit circuits are simple and effective, and can be constructed using either a single transistor or a transistor pair configuration. They are often used in low-power applications where high signal quality is important.

Class B Transmitters

In a Class B transmit circuit, the amplifier is designed to only conduct current during half of the input signal cycle. Specifically, the amplifier conducts current during the positive half of the input signal and turns off during the negative half of the input signal. This design results in a more efficient use of power, as the amplifier is not conducting current when there is no signal to amplify.

To achieve this, a Class B transmit circuit typically uses a pair of complementary transistors, one PNP and one NPN, each biased to conduct during opposite halves of the input signal. This configuration is known as a push-pull amplifier. The input signal is split into two halves, and each half is amplified by one of the transistors in the push-pull configuration. The output signals are then combined to produce the amplified output signal.

In this Class, think of each transistor in the pair as a light switch, with one switch controlling the power during the positive half of the input signal and the other switch controlling the power during the negative half of the input signal. When the input signal is positive, the PNP transistor turns off and the NPN transistor turns on, conducting current. When the input signal is negative, the NPN transistor turns off and the PNP transistor turns on, conducting current. This allows for a continuous output signal that is amplified by both transistors.

Overall, Class B transmit circuits are more efficient than Class A circuits, as they only use power when there is a signal to amplify. However, they can result in more distortion in the output signal due to the switching behavior of the transistors. Therefore, Class B transmit circuits are often used in applications where efficiency is important, such as in power amplifiers for audio systems, but not where signal quality is paramount.

Class AB Transmitters

Class AB transmit circuits are a hybrid between Class A and Class B circuits. They are designed to reduce the power consumption of Class A circuits while also reducing the distortion of Class B circuits.

In a Class AB transmit circuit, the amplifier conducts current during a portion of the input signal cycle that extends beyond the positive and negative half-cycles. Specifically, the amplifier conducts a small amount of current when there is no input signal and then conducts more current when there is a signal to amplify. This design allows for some power savings compared to Class A circuits, as the amplifier is not always conducting current. However, it also reduces distortion compared to Class B circuits, as the amplifier is still conducting current during part of the cycle when the input signal is close to zero.

To achieve this, a Class AB transmit circuit typically uses a push-pull amplifier with biasing that allows the transistors to operate in Class A mode for a portion of the cycle and in Class B mode for the rest of the cycle. This configuration is designed to minimize distortion while still achieving good efficiency.

Class AB transmit circuits can also use a single transistor, but the configuration is a bit different compared to Class A and Class C circuits. In a Class AB circuit with a single transistor, the transistor is biased to conduct a small amount of current when there is no input signal and then conduct more current when there is a signal to amplify, just like in a push-pull Class AB circuit. However, the biasing is designed to keep the transistor operating in the linear region for a portion of the cycle, rather than switching off completely like in a Class C circuit. This linear region of operation helps to reduce distortion in the output signal.

The biasing of the single transistor in a Class AB circuit can be accomplished using a variety of techniques, such as a bias resistor network or a bias voltage applied to the transistor's base. The key is to ensure that the transistor stays in the linear region for a portion of the cycle, even as the input signal varies.

Overall, Class AB circuits with a single transistor are less common than push-pull Class AB circuits, but they can still provide a good compromise between efficiency and distortion reduction. They are often used in low-power applications, where the use of a single transistor can simplify the circuit design and reduce cost.

In this Class, think of each transistor in the pair of a push-pull circuit as a light switch, with one switch controlling the power during the positive half of the input signal and the other switch controlling the power during the negative half of the input signal. However, instead of switching off completely during the negative half of the input signal, the transistor stays on at a lower level of current to reduce distortion in the output signal.

Overall, Class AB transmit circuits are a good compromise between the high signal quality of Class A circuits and the efficiency of Class B circuits. They are often used in audio amplifiers, where distortion reduction is important, but power consumption is also a concern.

Class C Transmitters

Class C transmit circuits are another type of amplifier circuit commonly used in electronics. In a Class C circuit, the amplifier conducts current for less than half of the input signal cycle. Specifically, the amplifier conducts current for only a small portion of the cycle when the input signal is at its peak. This design results in the highest efficiency of all the amplifier classes but can also be prone to having the most distortion. Luckily however, the availability of much higher quality transistors, components and better, more carefully thought-out electronics practices are reducing these distortions considerably.

To achieve this, a Class C transmit circuit typically uses a single transistor that is biased to conduct current only when the input signal is at its peak. The transistor is designed to act as a switch that only conducts current during the positive or negative peak of the input signal. This results in a pulsed output signal with high amplification during the peak but minimal amplification during the rest of the cycle.

Finally, in this Class, think of the transistor as a light switch that only turns on when the input signal is at its peak. The transistor then turns off during the rest of the cycle. This design allows for high efficiency, as the amplifier is only conducting current when the input signal is at its peak. However, it can also result in high distortion, as the output signal is a pulsed waveform that is not a faithful reproduction of the input signal.

Overall, Class C transmit circuits are used in applications where high efficiency is paramount, such as in radio frequency (RF) amplifiers. However, they may not be suitable for applications where high signal quality is required, due to the possibility of a high amount of distortion they can introduce.