While this amplifier configuration provides very linear performance and the best signal fidelity from input to output, it is also the least efficient of the amplifier classes, with typically less than 50% drain efficiency for the amplifier’s transistors. Its transistors are powered or biased in the “on” state, drawing current and using power 100% of the time.
#Power amplifier circuit full#
Class-A AmplifiersĬlass-A amplifiers are designed to operate with active devices in full 360° conduction. For a pulsed signal with a short duty cycle, an amplifier with the full device conduction state may not be needed, and an amplifier with high efficiency can help save power applied to the amplifier and improve efficiency.įigure 1: Different amplifier classes use their active semiconductor devices with full or partial conducting modes to achieve different levels of efficiency as needed for different types of applications and waveforms. An amplifier with active devices in a full 360° conduction state, or “always on,” will use more power and have less efficiency than an amplifier with the same devices that draw current at only a 180° conduction angle. In general, the class of an amplifier refers to the portion of the waveform being amplified for which an amplifier’s transistors are “ON” or in their conduction state. and a hybrid configuration known as Class-AB.The different amplifier classes are denoted by capital letters, with different configurations from Class-A through Class-T, although the most common configurations are: Learn the Alphabet: Differences Between Amplifier Classes Real-world examples are examined to illustrate typical performance. This article explains the basic characteristics and differences between the most common RF amplifier classes (A, AB, B, and C). In other applications such as radar where pulsed signals are used, amplifier efficiency, gain and output power may be more important and high-linearity may not be needed.ĭifferent classes of amplifiers were developed to help users differentiate the various configurations and operating modes that result in different balances of linearity, efficiency, and other parameters. A PA with poor linearity will generate unacceptable levels of intermodulation distortion (IMD) when boosting the multitone signals commonly found in such systems. An amplifier with good linearity preserves the AM and PM relationships. For example, some input signals commonly found in communications systems with amplitude modulation (AM) and/or phase modulation (PM), require maintaining the amplitude and phase relationships at the input of an amplifier through to the higher-amplitude output signals. Priority depends on the nature of the application and the signals involved. At the same time, poor linearity means the amplifier will operate more in the non-linear region of its power curve, which can create distortion in the form of harmonics and intermodulation products. Lower efficiency results in power converted to heat in PA semiconductor junctions, which must be dissipated to avoid performance degradation and protect the amplifier and its active devices from overheating.
In reality, though, designers make tradeoffs between efficiency and linearity depending on the requirements of a given application. Ideally, a PA operates with high efficiency, so most of its applied power supply is used to boost the amplitude of an input signal, and high linearity, so output power is directly proportional to input power over most of the input power range. Different signal types have different amplification needs in terms of output power, gain, efficiency, linearity, and other performance parameters. Signals may be continuous wave (CW) or many forms of pulsed waveforms, with different pulse widths and duty cycles.
Power amplifiers (PAs) boost input signals using different amplification schemes depending upon application requirements and the nature of the signals to be boosted.