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Talking About COFDM PA Power Amplifier

COFDM PA Power Amplifier

As a professional supplier of wireless video and data transceivers, many customers will ask about power amplifiers to increase the coverage of wireless transmitters and enhance wireless signal strength. The power amplifier can be said to be a hurdle that many RF engineers cannot avoid. Fungsi, classification, performance index, circuit composition, efficiency improvement technology, development trend... Do you know everything you need to know about RF power amplifiers? Come make up lessons!

Two key specifications for RF PAs: power and linearity

In RF power amplifiers, power efficiency (PAE) is defined as the ratio of the difference between the output signal power and the input signal power to the power consumption of the DC power supply, namely:
PAE = (PRFOUT - PRFIN)/PDC = (PRFOUT - PRFIN)/(VDC*IDC)

Functions of RF Power Amplifier RF PA

Radio Frequency Power Amplifier RF PA is the main part of the transmission system, and its importance is self-evident. In the pre-stage circuit of the transmitter, the RF signal power generated by the modulating oscillator circuit is very small, and it needs to go through a series of amplification-buffer stage, intermediate amplification stage, and final power amplification stage to obtain sufficient RF power before feeding radiate to the antenna. In order to obtain a sufficiently large radio frequency output power, a radio frequency power amplifier must be used. Power amplifiers are often the most expensive, most power-hungry, and least efficient components of a stationary installation or terminal.
After the modulator generates the radio frequency signal, the radio frequency modulated signal is amplified to sufficient power by the RFPA, passed through the matching network, and then emitted by the antenna.
The function of the amplifier is to amplify the input content and output it. The input and output, which we call "sinyal," are often expressed as voltages or power. For a "sistem" such as an amplifier, its "contribution" is to raise a certain level of what it "absorbs" dan "keluaran" to the outside world. Ini "improvement contribution" adalah "meaning" of the existence of the amplifier. If the amplifier can have good performance, then it can contribute more, which reflects its own "value". If there are certain problems in the initial "mechanism design" of the amplifier, then after starting to work or working for a period of time, not only will it not be able to provide any "contribution", but some unexpected "shocks" may occur. "Terkejut" is disastrous to the outside world or to the amplifier itself.

Classification of RF Power Amplifier RF PA

According to different working conditions, 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 frequency selection networks as load circuits. RF power amplifiers can be divided into three types of working states: SEBUAH (SEBUAH), B (B), and C (C) according to the current conduction angle. The conduction angle of the Class A amplifier current is 360°, which is suitable for small signal low power amplification. The conduction angle of the Class B amplifier current is equal to 180°, and the conduction angle of the Class C amplifier current is less than 180°. Both Class B and Class C are suitable for high-power working conditions, and the output power and efficiency of Class C working conditions are the highest among the three working conditions. Most RF power amplifiers work in Class C, but the current waveform of Class C amplifiers is too distorted, so they can only be used to amplify power by using a tuned circuit as a load resonance. Due to the filtering ability of the tuning loop, the loop current and voltage are still close to sinusoidal waveforms with little distortion.
In addition to the above working states classified according to the current conduction angle, there are also Class D (D) amplifiers and Class E (E) amplifiers that make electronic devices work in the switching state. The efficiency of Class D amplifiers is higher than that of Class C amplifiers.

Performance index of radio frequency power amplifier RF PA

The main technical indicators of radio frequency power amplifier RF PA are output power and efficiency. How to improve output power and efficiency is the core of the design goal of radio frequency power amplifier. Usually in the RF power amplifier, the fundamental frequency or a certain harmonic can be selected by the LC resonant circuit to realize undistorted amplification. Secara umum, there are probably the following indicators in the evaluation of amplifiers:
- memperoleh. This is the ratio between input and output and represents the contribution of the amplifier. A good amplifier is to contribute as much "keluaran" as possible within its "range of its own capabilities".
-frekuensi kerja. This represents the carrying capacity of the amplifier for different frequency signals.
- bandwidth yang bekerja. This determines how much range the amplifier can "contribute". For a narrow-band amplifier, even if its own design is no problem, its contribution may be limited.
-stability. Every transistor has potential "regions of instability." Itu "Desain" of the amplifier needs to eliminate these potential instabilities. There are two types of amplifier stability, potentially unstable and absolutely stable. The former may appear unstable under certain conditions and environments, while the latter can guarantee stability under any circumstances. The question of stability is important because instability means "oscillation", when the amplifier not only affects itself, but also outputs unstable factors.
- Daya keluaran maksimum. This indicator determines the "capacity" of the amplifier. Untuk "big systems", it is hoped that they can output more power at the expense of certain gain.
-efisiensi. Amplifiers must consume a certain amount of "energy" and also achieve a certain amount of "contribution". The ratio of its contribution to consumption is the efficiency of the amplifier. A good amplifier is one that contributes more and consumes less.
- linier. Linearity characterizes the correct response of the amplifier to a large number of inputs. A deterioration in linearity means that the amplifier "distorts" atau "distorts" the input in the presence of excess input. A good amplifier should not exhibit this "freaky" nature.

Circuit Composition of RF Power Amplifier RF PA

There are different types of amplifiers. Sederhana, the circuit of the amplifier can be composed of the following parts: transistors, bias and stabilization circuits, and input and output matching circuits.

1. Transistor

There are many kinds of transistors, including transistors with various structures that have been invented. Essentially, a transistor works as a controlled current or voltage source by converting the energy of an empty direct current into a "useful" keluaran. DC energy is obtained from the outside world, and the transistor consumes it and converts it into useful components. A transistor, we can regard it as "a unit". Different "capabilities" of different transistors, such as their ability to withstand power are different, which is also due to their ability to obtain DC energy; sebagai contoh, their response speed is different, which determines how wide and high it can work In the frequency band; sebagai contoh, the impedances facing the input and output ports are different, and the external response capabilities are different, which determines the difficulty of matching it.

2. Bias and stabilization circuit

Biasing and stabilization circuits are two different circuits, but because they are often difficult to distinguish and the design goals converge, they can be discussed together.
The operation of the transistor needs to be 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, and different positioning will determine its own working mode, and there are also different performances in different positioning. Some positioning points have small fluctuations, which are suitable for small signal work; some positioning points have large fluctuations, which are suitable for high-power output; some positioning points have less demand, pure release, and are suitable for low-noise work; some positioning points, Transistors are always hovering between saturation and cutoff, in a switching state. An appropriate bias point is the basis for normal operation.
The stabilization circuit must be before the matching circuit, because the transistor needs the stabilization circuit as part of itself, and then contacts the outside world. In the eyes of the outside world, the transistor with the stabilization circuit is a "Barang baru" transistor. It makes certain "sacrifices" to gain stability. Mechanisms that stabilize the circuit keep the transistors running smoothly and steadily.

3. Input and output matching circuit

The purpose of the matching circuit is to select an accepted mode. For those transistors that want to provide more gain, the approach is to accept and output across the board. This means that through the interface of the matching circuit, the communication between different transistors is smoother. For different types of amplifiers, the matching circuit is not the only design method that is "accepted in its entirety". Some small tubes with small DC and shallow foundation are more willing to do a certain amount of blocking when receiving to obtain better noise performance. Namun, the blocking cannot be overdone, otherwise it will affect its contribution. For some giant power tubes, you need to be cautious when outputting, because they are more unstable, and at the same time, a certain amount of reservation helps them to exert more "undistorted" energy.

Realization of Stability of RF Power Amplifier RF PA

Every transistor is potentially unstable. Good stabilizing circuits can be fused with transistors to form a "continuous work" mode. The implementation of stabilization circuits can be divided into two types: narrow-band and wide-band.
The narrowband stabilization circuit consumes a certain amount of gain. This stable circuit is realized by adding certain consumption circuits and selective circuits. This circuit allows the transistor to contribute only a small frequency range. Another broadband stabilization is the introduction of negative feedback. This circuit can work over a wide range.
The source of instability is positive feedback, and the idea of narrow-band stability is to curb some of the positive feedback. Tentu saja, this also suppresses the contribution. Negative feedback, done well, has many additional gratifying advantages. Sebagai contoh, negative feedback may prevent transistors from being matched, neither needing to be matched to interface well with the outside world. Sebagai tambahan, the introduction of negative feedback will improve the linear performance of the transistor.

Efficiency Improvement Technology of RF Power Amplifier RF PA

Transistor efficiency has a theoretical limit. This limit varies with the selection of the bias point (static operating point). Sebagai tambahan, if the peripheral circuit is not well designed, its efficiency will be greatly reduced. Saat sekarang, there are not many ways for engineers to improve efficiency. There are only two kinds here: 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 separately by different amplifier circuits. Lewat sini, the two amplifiers can focus on their respective parts, and the cooperation of the two amplifiers can achieve the goal of higher efficiency utilization.
The essence of Doherty technology is: using two transistors of the same type, only one works when the input is small, and works in a high-efficiency state. If the input increases, both transistors work simultaneously. The basis for the realization of this method is that the two transistors should cooperate with each other tacitly. The working state of one transistor will directly determine the working efficiency of the other.

Testing Challenges for RF PAs

Power amplifiers are very important components in wireless communication systems, but they are inherently non-linear, causing spectral growth phenomena that interfere with adjacent channels, and may violate statutory-mandated out-of-band emission standards. This characteristic can even cause in-band distortion, which increases the bit error rate (BER) and reduces the data transmission rate of the communication system.
Under the peak-to-average power ratio (PAPR), the new OFDM transmission format will have more sporadic peak power, making the PA difficult to be segmented. This degrades spectral mask compliance and increases EVM and BER across the waveform. To solve this problem, design engineers usually deliberately reduce the operating power of the PA. Unfortunately, this is a very inefficient approach, since the PA reduces 10% of its operating power and loses 90% of its DC power.
Most of today's RF PAs support multiple modes, rentang frekuensi, and modulation modes, making more test items available. Thousands of test items are not uncommon. The use of new technologies such as crest factor reduction (CFR), digital predistortion (DPD) and envelope tracking (ET) can help optimize PA performance and power efficiency, but these technologies will only make the test more complicated and greatly prolong the test time. Design and test time. Increasing the bandwidth of the RF PA will result in a five-fold increase in the bandwidth required for DPD measurements (possibly exceeding 1 GHz), further increasing test complexity.
According to the trend, in order to increase efficiency, RF PA components and front-end modules (FEM) will be more closely integrated, and a single FEM will support a wider range of frequency bands and modulation modes. Integrating an ET power supply or modulator into the FEM can effectively reduce the overall space requirements inside the mobile device. Increasing the number of filter/duplexer slots to support a larger operating frequency range will increase the complexity of mobile devices and the number of test items.

Mobile Phone RF Module Power Amplifier (PA) Market Situation

The field of mobile phone power amplifiers is currently a component that cannot be integrated in mobile phones. Mobile phone performance, footprint, call quality, mobile phone strength, and battery life are all determined by the power amplifier.
How to integrate these power amplifiers of different frequency bands and standards is an important subject that the industry has been studying. sekarang, there are two solutions: one is the fusion architecture, which integrates RF power amplifiers PA of different frequencies; the other architecture is the integration along the signal chain, itu adalah, the PA and the duplexer are integrated. Both schemes have advantages and disadvantages, and are suitable for different mobile phones. Converged architecture, high integration of PA, has obvious size advantage for more than 3 frequency bands, and obvious cost advantage for 5-7 frequency bands. The disadvantage is that although the PA is integrated, the duplexer is still quite complicated, and there is switching loss when the PA is integrated, and the performance will be affected. For the latter architecture, the performance is better. The integration of the power amplifier and the duplexer can improve the current characteristics, which can save tens of milliamperes of current, which is equivalent to extending the talk time by 15%. Karena itu, industry insiders suggest that when there are more than 6 frequency bands (excluding 2G, referring to 3G and 4G), a converged architecture is adopted, and when less than 4 frequency bands are used, PAD, a solution integrating PA and duplexer, is used.
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