Function/classification/performance indicators of RF power amplifier RF PA

Power amplifiers can be said to be a hurdle that many RF engineers cannot avoid. Function, classification, performance indicators, circuit composition, efficiency improvement technology, development trends... The two key indicators of RF power amplifier RF PA are power and linearity.

In RF power amplifiers, power efficiency (PAE) is defined as the ratio of the difference between output signal power and input signal power to the power consumption of the DC power supply, i.e.:

PAE = (PRFOUT – PRFIN)/PDC = (PRFOUT – PRFIN)/(VDC*IDC)

Functions of RF power amplifier RF PA

The RF power amplifier (RF PA) is the main component of the transmission system, and its importance is self-evident. In the front stage circuit of the transmitter, the RF signal power generated by the modulation oscillation circuit is very small. It needs to go through a series of amplification buffer stages, intermediate amplification stages, and final power amplification stages to obtain sufficient RF power before it can be fed to the antenna for radiation. In order to obtain sufficient RF output power, an RF power amplifier must be used. Power amplifiers are often the most expensive, power consuming, and inefficient components of fixed equipment or terminals.

After the modulator generates the RF signal, the RF modulated signal is amplified to sufficient power by the RFPA, matched by the network, and then transmitted out by the antenna.

The function of an amplifier is to amplify the input content and output it. The content of input and output, which we call "signals", is often represented as voltage or power. For a "system" like an amplifier, its "contribution" is to elevate what it "absorbs" to a certain level and "output" it to the outside world. This' contribution to improvement 'is the' meaning 'of the existence of amplifiers. If an amplifier can have good performance, then it can contribute more, which reflects its own "value".

If there are certain problems with the initial "mechanism design" of the amplifier, then after starting to work or working for a period of time, not only can it no longer provide any "contribution", but it may also experience some unexpected "oscillations", which are catastrophic for both the outside world and the amplifier itself.

Classification of RF Power Amplifier RF PA

According to different working states, power amplifiers are classified as follows:

The operating frequency of RF power amplifiers is very high, but the frequency band is relatively narrow. RF power amplifiers generally use a frequency selective network as the load loop. RF power amplifiers can be classified into three working states: A (A), B (B), and C (C) according to the different conduction angles of the current. The conduction angle of Class A amplifier current is 360 °, suitable for small signal low-power amplification. The conduction angle of Class B amplifier current is equal to 180 °, while the conduction angle of Class C amplifier current is less than 180 °. Both Class B and Class C are suitable for high-power working conditions, with Class C having the highest output power and efficiency among the three working conditions. Most RF power amplifiers work in Class C, but the current waveform distortion of Class C amplifiers is too large and can only be used for load resonance power amplification using tuned circuits. Due to the filtering capability of the tuning circuit, the current and voltage of the circuit still approach a sinusoidal waveform with minimal distortion.

In addition to the working states classified by current conduction angle mentioned above, there are also Class D amplifiers and Class E amplifiers that operate electronic devices in a switching state. Class D amplifiers have higher efficiency than Class C amplifiers.

Performance indicators of RF power amplifier RF PA

The main technical indicators of RF power amplifier (RF PA) are output power and efficiency. How to improve output power and efficiency is the core of RF power amplifier design goals. Usually in RF power amplifiers, LC resonant circuits can be used to select the fundamental frequency or a certain harmonic, achieving distortion free amplification. Overall, there are roughly the following indicators for evaluating amplifiers:

Gain. This is the ratio between input and output, representing the contribution of the amplifier. A good amplifier should contribute as much output as possible within its own capabilities. Working frequency. This represents the amplifier's carrying capacity for signals of different frequencies. Working bandwidth. This determines the extent to which the amplifier can make a "contribution".

For a narrowband amplifier, even if its own design is not a problem, its contribution may be limited. Stability. Every transistor has a potential 'unstable region'. The design of amplifiers needs to eliminate these potential instabilities. The stability of an amplifier includes two types: potential instability and absolute stability. The former may experience instability under specific conditions and environments, while the latter can ensure stability in any situation. The reason why stability is important is that instability means "oscillation", in which case the amplifier not only affects itself, but also outputs unstable factors. Maximum output power. This indicator determines the 'capacity' of the amplifier.

For 'large systems', it is hoped that they can output greater power while sacrificing a certain amount of gain. Efficiency. Amplifiers consume a certain amount of energy and make a certain contribution. The ratio of its contribution to its consumption is the efficiency of the amplifier. A good amplifier is one that can contribute more and consume less. Linear. Linearity represents the correct response of an amplifier to a large number of inputs. Linear degradation indicates that the amplifier distorts or distorts the input in a state of excessive input. A good amplifier should not exhibit such "abnormal" properties.