"As an RF Engineer, work involves power amplifier more or less. Power amplifier can be said to be a barrier that many RF engineers can't get around. Function, classification, performance index, circuit composition, efficiency improvement technology, development trend... Do you know anything about RF power amplifier? Come and make up lessons!
Two key indicators of RF PA: power and linearity
In RF power amplifier, power efficiency (PAE) is defined as the ratio of the difference between output signal power and input signal power to DC power consumption, that is, PAE = (prfout - prfin) / PDC = (prfout - prfin) / (VDC * IDC)
Function of RF power amplifier RF PA
RF power amplifier RF PA is the main part of the transmitting system, and its importance is self-evident. In the transmitter's forward circuit, the RF signal generated by the modulation oscillating circuit is very small. It needs a series of amplification buffer, intermediate amplification stage and final stage power amplifier stage to obtain enough RF power before it can be fed to the antenna to radiate. In order to obtain enough RF output power, RF power amplifier must be used. Power amplifier is often the most expensive, power consuming and inefficient device of fixed equipment or terminal.
After the modulator generates the RF signal, the RF modulated signal is amplified to sufficient power by the RFPA, and then transmitted by the antenna through the matching network.
Fig. 1 block diagram of transmitting system
The function of the amplifier is to amplify and output the input content. The contents of input and output, which we call "signal", are often expressed as voltage or power. For a "system" such as an amplifier, its "contribution" is to raise what it "absorbs" to a certain level and "output" to the outside world. This "contribution to promotion" is the "significance" of the existence of the amplifier. If the amplifier can have good performance, it can contribute more, which reflects its own "value". If there are some problems in the initial "mechanism design" of the amplifier, after starting to work or working for a period of time, not only can it no longer provide any "contribution", but there may be some unexpected "shocks". This "shock" is disastrous for the outside world or the amplifier itself.
Classification of RF PA for RF power amplifier
According to different working conditions, power amplifiers are classified as follows:
Fig. 2 classification of power amplifiers
The working frequency of RF power amplifier is very high, but the relative frequency band is narrow. RF power amplifier generally adopts frequency selection network as load loop. RF power amplifiers can be divided into three working states: a (a), B (b) and C (c) according to different current conduction angles. The conduction angle of class a amplifier current is 360 °, which is suitable for small signal and low power amplification. The conduction angle of class B amplifier current is equal to 180 °, and the conduction angle of class C amplifier current is less than 180 °. Both class B and class C are suitable for high-power working state. The output power and efficiency of class C working state are the highest among the three working states. Most RF power amplifiers work in class C, but the current waveform distortion of class C amplifier is too large, so it can only be used for load resonant power amplification with tuning loop. Because the tuning loop has filtering ability, the loop current and voltage are still close to sinusoidal waveform, and the distortion is very small.
In addition to the above working states classified according to the current conduction angle, there are class D amplifier and class E amplifier that make the electronic devices work in the switching state. The efficiency of class D amplifier is higher than that of class C amplifier.
Performance index of RF PA
The main technical index of RF power amplifier RF PA is output power and efficiency. How to improve output power and efficiency is the core of the design goal of RF power amplifier. Usually in RF power amplifier, the fundamental frequency or a certain harmonic can be selected by LC resonant circuit to realize undistorted amplification. Generally speaking, there are probably the following indicators for the evaluation of amplifiers:
Gain. This is the ratio between input and output, representing the contribution of the amplifier. Good amplifiers contribute "output" as much as possible within their "own ability". Operating frequency. This represents the carrying capacity of the amplifier for signals of different frequencies. Working bandwidth. This determines the extent to which the amplifier can make a "contribution". For a narrowband amplifier, its own design, even if there is no problem, its contribution may be limited. Stability. Each transistor has a potential "unstable region". The "design" of the amplifier needs to eliminate these potential instabilities. There are two kinds of amplifier stability, potential instability and absolute stability. The former may be unstable under specific conditions and environment, while the latter can ensure stability under any circumstances. The stability problem is important because instability means "oscillation". At this time, the amplifier will not only affect itself, but also output unstable factors. Maximum output power. This index determines the "capacity" of the amplifier. For "large systems", it is hoped that they can output more power at the expense of a certain gain. Efficiency. Amplifiers must consume a certain "energy" and achieve a certain "contribution". The ratio of contribution to consumption is the efficiency of the amplifier. It is a good amplifier that can contribute more and consume less. Linearity. Linearity characterizes the correct response of the amplifier to a large number of inputs. The deterioration of linearity means that the amplifier will "distort" or "distort" the input in the state of excessive input. A good amplifier should not exhibit this "abnormal" nature.
The following contents: circuit composition, stability and efficiency improvement of RF power amplifier
Circuit composition of RF power amplifier RF PA
There are different types of amplifiers. To simplify, the circuit of the amplifier can be composed of the following parts: transistor, bias and stability circuit, input and output matching circuit.
1. Transistors
There are many kinds of transistors, including the invention of transistors with various structures. In essence, the transistor works as a controlled current source or voltage source, and its working mechanism is to convert the DC energy without content into a "useful" output. DC energy is obtained from the outside, consumed by transistors and converted into useful components. A transistor can be regarded as "one unit". Different transistors have different "capabilities", such as their ability to withstand power, which is also due to their ability to obtain DC energy; For example, its reaction speed is different, which determines how wide and high the frequency band it can work in; For example, its impedance facing the input and output terminals is different, and its external response ability is different, which determines the difficulty of matching it.
2. Bias and stabilization circuit
Bias and stabilization circuits are two different circuits, but they are often difficult to distinguish, and the design objectives converge, so they can be discussed together.
The transistor needs to work under certain bias conditions, which we call the static operating point. This is the foundation of the transistor and its own "positioning". Each transistor has a certain positioning for itself. Its different positioning will determine its own working mode, and there are different performance in different positioning. There is little fluctuation on the write positioning point, which is suitable for small signal operation; Some positioning points fluctuate greatly, which is suitable for high-power output; Some positioning points require less and release pure, which is suitable for low-noise work; At some fixed points, the transistor always hovers between saturation and cut-off and is in the on-off state. An appropriate offset point is the basis for normal operation.
The stabilizing circuit must be before the matching circuit, because the transistor needs to exist the stabilizing circuit as a part of itself and then contact with the outside world. In the view of the outside world, the transistor with a stable circuit is a "new" transistor. It makes certain "sacrifices" and obtains stability. The mechanism of stabilizing the circuit can ensure the smooth and stable operation of the transistor.
3. Input output matching circuit
The purpose of matching circuit is to select an acceptable way. For transistors that want to provide greater gain, the way is overall acceptance and output. This means that through the interface of matching circuit, the communication between different transistors is more smooth. For different amplifier types, matching circuit is not only a design method of "overall acceptance". Some small tubes with small DC and shallow foundation are more willing to block when accepting to obtain better noise performance. However, they cannot block too much, otherwise their contribution will be affected. For some giant power tubes, they need to be cautious in output, because they are more unstable. At the same time, a certain reservation will help them play more "undistorted" energy.
Realization of RF PA stability of RF power amplifier
Each transistor is potentially unstable. Good stability circuits can be integrated with transistors to form a "sustainable working" mode. The realization of stable circuit can be divided into two kinds: narrowband and wideband.
Narrow band stabilization circuit is to carry out certain gain consumption. This stabilization circuit is realized by adding a certain consumption circuit and selective circuit. This circuit makes the transistor contribute only in a small frequency range. Another kind of broadband stability is the introduction of negative feedback. This circuit can work in a wide range.
The root of instability is positive feedback. The idea of narrowband stability is to curb some positive feedback. Of course, this also inhibits the contribution. And if negative feedback is done well, there are many additional gratifying advantages. For example, negative feedback may prevent transistors from matching, and they can communicate well with the outside world without matching. In addition, the introduction of negative feedback will improve the linear performance of the transistor.
Efficiency improvement technology of RF PA
There is a theoretical limit to the efficiency of transistors. This limit varies with the selection of the offset point (static operating point). In addition, the poor design of peripheral circuits will greatly reduce its efficiency. At present, engineers have few ways to improve efficiency. There are only two kinds: envelope tracking technology and Doherty technology.
The essence of envelope tracking technology is to separate the input into two types: phase and envelope, and then amplify them by different amplification circuits. In this way, the two amplifiers can focus on their respective parts, and their cooperation can achieve the goal of higher efficiency utilization.
The essence of Doherty technology is that it uses two similar transistors, only one works at a small input, and works in a high efficiency state. If the input increases, both transistors operate at the same time. This method is based on the tacit understanding between the two transistors. The working state of one transistor will directly determine the working efficiency of the other.
Testing challenges for RF PA
Power amplifiers are very important components in wireless communication system, but they are nonlinear, which will lead to spectrum proliferation and interfere with adjacent channels, and may violate the out of band radiation standard enforced by laws and regulations. This characteristic will even cause in band distortion, increase the bit error rate (BER) and reduce the data transmission rate of the communication system.
Under the peak to average power ratio (PAPR), the new OFDM transmission format will have more occasional peak power, which makes the PA difficult to be segmented. This will reduce the spectrum shielding compliance, expand the EVM of the whole waveform and increase BER. In order to solve this problem, design engineers usually deliberately reduce the operating power of PA. Unfortunately, this is a very inefficient method, because if the PA reduces the operating power by 10%, 90% of the DC power will be lost.
Nowadays, most RF PAS support multiple modes, frequency range and modulation modes, which makes more test items. Thousands of test projects are not uncommon. The application of new technologies such as crest factor reduction (CFR), digital predistortion (DPD) and envelope tracking (ET) helps to optimize PA efficiency and power efficiency, but these technologies will only make the test more complex and greatly prolong the design and test time. Increasing the bandwidth of RF PA will increase the bandwidth required for DPD measurement by 5 times (possibly more than 1 GHz), resulting in further increase of test complexity.
According to the trend, in order to increase efficiency, RF PA components and front-end modules (FEM) will be more closely integrated, while a single FEM will support a wider range of frequency bands and modulation modes. Integrating envelope tracking power supply or modulator into FEM can effectively reduce the overall space demand inside mobile devices. In order to support a larger operating frequency range, a large number of filter / duplexer slots will increase the complexity of mobile devices and the number of test items.
Market situation of mobile phone RF module power amplifier (PA)
The field of mobile phone power amplifier is an element that cannot be integrated in mobile phones at present. The performance, occupied area, call quality, strength and battery life of mobile phones are determined by the power amplifier.
How to integrate these different frequency bands and standard power amplifiers is an important topic in the industry. At present, there are two schemes: one is the fusion architecture, which integrates RF power amplifiers PA of different frequencies; Another architecture is the integration along the signal link, that is, the integration of PA and duplexer. The two schemes have their own advantages and disadvantages and are suitable for different mobile phones. With the fusion architecture, PA has a high degree of integration. It has obvious size advantages for more than 3 frequency bands and obvious cost advantages for 5-7 frequency bands. The disadvantage is that although PA is integrated, the duplexer is still quite complex, and there is switching loss during PA integration, which will affect the performance. For the latter architecture, the performance is better. The integration of power amplifier and dual power amplifier can improve the current characteristics, save tens of milliamps of current, which is equivalent to prolonging the talk time by 15%. Therefore, the suggestion of insiders is that when the frequency band is greater than 6 (excluding 2G, it refers to 3G and 4G), the fusion architecture is adopted, while when the frequency band is less than 4, the scheme pad of PA and duplexer integration is adopted.
Original link: https://www.eeboard.com/news/pa/
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