basic concept
The radio frequency power amplifier ( RF PA) is the main part in the transmission system, and its importance is self-evident. In the pre-stage circuit of the transmitter, the radio frequency signal power generated by the modulation oscillator circuit is very small, and it needs to go through a series of amplification (buffer stage, intermediate amplification stage, final power amplification stage) to obtain sufficient radio frequency 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. After the modulator generates a radio frequency signal, the radio frequency modulated signal is amplified to sufficient power by the RF PA, 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 "signals," are often expressed as voltages or power. For a "system" such as an amplifier, its "contribution" is to raise a certain level of what it "absorbs" and "output" to the outside world. If the amplifier can have good performance, then it can contribute more, which reflects its own "value". If there are certain problems with 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 there may be some unexpected "shocks", which are still amplifiers to the outside world. In itself, it is catastrophic.
The main technical indicators of RF power amplifiers are output power and efficiency. How to improve output power and efficiency is the core of the design goal of RF power amplifiers. 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. In addition, the harmonic content in the output should be as small as possible to avoid interference with other channels.
Classification
According to different working conditions, power amplifiers are classified as follows:
The operating frequency of traditional linear 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: A (A), 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 distortion of Class C amplifiers is too large, so they can only be used for power amplification 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.
Switching Mode PA (Switching Mode PA, SMPA) enables electronic devices to work in a switching state. The common ones are Class D (D) amplifiers and Class E (E) amplifiers. The efficiency of Class D amplifiers is higher than that of Class C amplifiers. SMPA drives the active transistor into switch mode, the working state of the transistor is either on or off, and there is no overlapping phenomenon in the time domain waveform of its voltage and current, so the DC power consumption is zero, and the ideal efficiency can reach 100 %.
Traditional linear power amplifiers have high gain and linearity but low efficiency, while switching power amplifiers have high efficiency and high output power but poor linearity. See the table below for details:
circuit composition
There are different types of amplifiers. Simplified, the circuit of the amplifier can be composed of the following parts: transistors, bias and stabilization circuits, and input and output matching circuits.
1-1. 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" output. DC energy is obtained from the outside world, and the transistor consumes it and converts it into useful components. Different transistors have different "capabilities", such as their ability to withstand power, which is also due to their different ability to obtain DC energy; for example, their response speed is different, which determines how wide and high it can work. In the frequency band; for example, the impedances facing the input and output ports are different, and the external response capabilities are different, which determines the difficulty of matching it.
1-2. Bias circuit 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 and are suitable for small signal work; some positioning points have large fluctuations and are suitable for high-power output; some positioning points have less demand and pure release, which is suitable for low-noise work; some positioning points, transistors Always hovering between saturation and cutoff, it is on and off. An appropriate bias point is the basis for normal operation. When designing a broadband power amplifier, or when the operating frequency is high, the bias circuit has a great influence on the circuit performance. At this time, the bias circuit should be considered as a part of the matching circuit.
There are two types of bias networks, passive and active. Passive networks, or self-biasing networks, typically consist of resistive networks that provide the transistors with the proper operating voltage and current. Its main drawback is that it is very sensitive to the parameter changes of the transistor and has poor temperature stability. Active bias networks can improve the stability of the quiescent operating point and also provide good temperature stability, but they also have some problems, such as increased circuit size, increased difficulty in circuit layout, and increased power consumption.
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, with the transistor of the stabilization circuit, it is a "brand new" transistor. It makes certain "sacrifices" to gain stability. Mechanisms that stabilize the circuit keep the transistors running smoothly and steadily.
1-3. 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. However, 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.
Typical impedance matching networks include L matching, π-shaped matching and T-shaped matching. Among them, L matching is characterized by simple structure and only two degrees of freedom L and C. Once the impedance transformation ratio and resonant frequency are determined, the Q value (bandwidth) of the network is also determined. An advantage of the π-shaped matching network is that no matter what kind of parasitic capacitance is connected to it, it can be sucked into the network, which also leads to the widespread application of the π-shaped matching network, because in many practical situations, the dominant The status parasitic element is the capacitance. T-shaped matching, when the parasitic parameters of the power supply end and the load end are mainly inductive, T-shaped matching can be used to absorb these parasitic parameters into the network.
How to ensure the stability of 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. Of course, this also suppresses the contribution. Negative feedback, done well, has many additional gratifying advantages. For example, negative feedback may prevent transistors from being matched, neither needing to be matched to interface well with the outside world. In addition, the introduction of negative feedback will improve the linear performance of the transistor.
Efficiency Improvement Technology of RF PA
Transistor efficiency has a theoretical limit. This limit varies with the selection of the bias point (static operating point). In addition, if the peripheral circuit is not well designed, its efficiency will be greatly reduced. At present, 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. In this way, 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, frequency ranges, 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.
Changes in semiconductor materials:
Ge (germanium), Si (silicon) → → → GaAs (gallium arsenide), InP (indium phosphide) → → → SiC (silicon carbide), GaN (gallium nitride ), SiGe (silicon germanium), SOI ( Silicon on insulating layer) → → → carbon nanotube (CNT) → → → graphene (Graphene).
At present, the mainstream process of power amplifier is still GaAs process. In addition, GaAs HBT, gallium arsenide heterojunction bipolar transistor. Among them, HBT ( heterojunction bipolar transistor) is a bipolar transistor composed of a gallium arsenide (GaAs) layer and an aluminum gallium arsenide (AlGaAs) layer .
Although the CMOS process is relatively mature, the application of Si CMOS power amplifiers is not widespread. In terms of cost, although the silicon wafer of the CMOS process is relatively cheap, the layout area of the CMOS power amplifier is relatively large. In addition, the complex design of the CMOS PA requires a high R&D cost, so the overall cost advantage of the CMOS power amplifier is not so obvious. In terms of performance, CMOS power amplifiers have poor performance in terms of linearity, output power, efficiency, etc., coupled with the inherent shortcomings of CMOS technology: higher knee point voltage, lower breakdown voltage, and resistance of CMOS technology substrates. The rate is lower.
Carbon nanotubes (CNTs) are considered ideal materials for nanoelectronic devices due to their small physical size, high electron mobility, high current density, and low intrinsic capacitance.
Graphene, a zero-bandgap semiconductor material, will become a popular material for the next generation of radio frequency chips because of its high electron mobility, nanometer-scale physical size, excellent electrical and mechanical properties.
Linearization Technology of RF PA
The nonlinear distortion of the RF power amplifier will cause it to generate new frequency components, such as the second harmonic and double-tone beat frequency for the second-order distortion, and the third harmonic and multi-tone beat frequency for the third-order distortion. If these new frequency components fall within the passband, they will cause direct interference to the transmitted signal, and if they fall outside the passband, they will interfere with signals of other channels. For this reason, it is necessary to linearize the radio frequency power amplifier, which can better solve the problem of signal spectrum regeneration.
The principle and method of the basic linearization technology of RF power amplifier is nothing more than taking the amplitude and phase of the input RF signal envelope as a reference, comparing it with the output signal, and then generating appropriate corrections. The power amplifier linearization techniques that have been proposed and widely used include power back-off, negative feedback, feed-forward, pre-distortion, envelope elimination and restoration (EER), and linear amplification with nonlinear components (LINC). More complex linearization techniques, such as feedforward, predistortion, envelope cancellation and restoration, and linear amplification using nonlinear components, have a better effect on improving the linearity of the amplifier. However, linearization techniques that are relatively easy to implement, such as power back-off and negative feedback, have limited improvements in linearity.
2-1. Power back-off
This is the most commonly used method, that is, to use a higher-power tube as a low-power tube, in fact, to improve the linearity of the power amplifier at the expense of DC power consumption.
The power fallback method is to change the input power of the power amplifier from the 1dB compression point (the amplifier has a linear dynamic range, within this range, the output power of the amplifier increases linearly with the input power. As the input power continues to increase, the amplifier gradually enters In the saturation zone, the power gain begins to decline, and the output power value when the gain drops to 1dB lower than the linear gain is usually defined as the 1dB compression point of the output power, expressed by P1dB.) Backwards 6-10 decibels, working at a distance At a level less than the 1dB compression point, the power amplifier is kept away from the saturation region and enters the linear working region, thereby improving the third-order intermodulation coefficient of the power amplifier. In general, when the fundamental wave power is reduced by 1dB, the third-order intermodulation distortion is improved by 2dB.
The power back-off method is simple and easy to implement without adding any additional equipment. It is an effective method to improve the linearity of the amplifier. The disadvantage is that the efficiency is greatly reduced. In addition, when the power falls back to a certain level, when the third-order intermodulation reaches below -50dBc, continuing to fall back will no longer improve the linearity of the amplifier. Therefore, it is not enough to completely rely on power back-off when the linearity requirement is very high.
2-2. Predistortion
Predistortion is to add a nonlinear circuit in front of the power amplifier to compensate the nonlinear distortion of the power amplifier.
The advantage of predistortion linearization technology is that there is no stability problem, it has a wider signal frequency band, and it can handle signals containing multiple carriers. The cost of pre-distortion technology is low. Several carefully selected components are packaged into a single module and connected between the signal source and the power amplifier to form a pre-distortion linear power amplifier. The power amplifier in the handheld mobile station has adopted pre-distortion technology, which reduces the intermodulation product by only a few dBs with a small number of components, but it is a critical few dBs.
Predistortion technology is divided into two basic types: RF predistortion and digital baseband predistortion. RF predistortion is generally implemented by analog circuits, which have the advantages of simple circuit structure, low cost, and easy high-frequency and broadband applications. The disadvantages are that the spectral regeneration components are less improved and the high-order spectral components are difficult to cancel.
Due to the low operating frequency, digital baseband predistortion can be implemented with digital circuits , and has strong adaptability. It can also offset high-order intermodulation distortion by increasing the sampling frequency and increasing the number of quantization orders. It is a promising method. . This predistorter consists of a vector gain adjuster that controls the magnitude and phase of the input signal based on the contents of a look-up table (LUT). The magnitude of the predistortion is controlled by the input of the look-up table. A vector gain regulator, once optimized, will provide a non-linear characteristic opposite to that of a power amplifier. Ideally, the output intermodulation product at this time should be equal to the output amplitude of the two-tone signal through the power amplifier but opposite in phase, that is, the adaptive adjustment module is to adjust the input of the look-up table, so that the difference between the input signal and the output signal of the power amplifier is minimized . Note that the envelope of the input signal is also an input of the look-up table, the feedback path is to sample the distorted output of the power amplifier, and then send it to the adaptive adjustment DSP through A/D conversion , and then update the look-up table.
2-3. Feedforward
Feedforward technology originated from "feedback". It should be said that it is not a new technology. It was proposed by Bell Laboratories in the United States as early as the 1920s and 1930s. Conceptually it's all about "feedback" except that the calibration (feedback) is applied to the output.
The feed-forward linear amplifier forms two loops through couplers , attenuators, synthesizers, delay lines, and power dividers. After the RF signal is input, it is divided into two paths by a power divider. All the way into the main power amplifier, due to its nonlinear distortion, in addition to the main frequency signal that needs to be amplified, there is third-order intermodulation interference at the output. A part of the signal is coupled from the output of the main power amplifier, and the main carrier frequency signal of the amplifier is canceled through the loop 1, so that only the anti-phase third-order intermodulation component remains. After the third-order intermodulation component is amplified by the auxiliary amplifier, the intermodulation component generated by the non-linearity of the main amplifier is canceled through the loop 2, thereby improving the linearity of the power amplifier.
The feed-forward technique offers the advantages of high calibration accuracy without the disadvantages of instability and bandwidth limitation. Of course, these advantages are exchanged for high cost. Since the output calibration has a large power level, the calibration signal needs to be amplified to a higher power level, which requires an additional auxiliary amplifier, and requires the auxiliary amplifier itself. The distortion characteristics should be above the index of the feedforward system.
Feed-forward power amplifiers have very high requirements for offsetting, and the matching of amplitude, phase, and time delay must be obtained. If there are power changes, temperature changes, and device aging, etc., the offset will disappear. For this reason, adaptive offset technology is considered in the system, so that the offset can keep up with the changes of internal and external environments.