Basic concepts

Radio frequency power amplifier (RF PA) is the main part of transmission 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. After a series of amplification (buffer stage, intermediate amplification stage, and last stage power amplification stage), enough RF power can be obtained before it can be fed to the antenna to radiate. In order to obtain enough RF output power, RF power amplifier must be used. After the modulator generates the RF signal, the RF modulated signal is amplified to sufficient power by the RF PA, and then transmitted by the antenna through the matching network.

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. If the amplifier can have good performance, it can contribute more, which reflects its own “value”. If there are some problems with the amplifier, not only can it no longer provide any “contribution” after starting to work or working for a period of time, but there may be some unexpected “shock”. This “shock” is disastrous for the outside world or the amplifier itself.

The main technical index of RF power amplifier is output power and efficiency. How to improve output power and efficiency is the core of RF power amplifier design goal. Usually in RF power amplifier, the fundamental frequency or a certain harmonic can be selected by LC resonant circuit to realize undistorted amplification. In addition, the harmonic component 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 working frequency of traditional linear power amplifiers is very high, but the relative frequency band is narrow. RF power amplifiers generally use 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.

Switching mode PA (SMPA) enables electronic devices to work in the switching state. The common ones are class D (d) amplifier and class E (E) The efficiency of class D amplifier is higher than that of class C amplifier. SMPA drives the active transistor to the switching mode. The working state of the transistor is either on or off. There is no overlap between the time-domain waveforms of voltage and current, so the DC power consumption is zero and the expected efficiency can reach 100%.

Traditional linear power amplifier has high gain and linearity, but low efficiency, while switching power amplifier has high efficiency and high output power, but poor linearity. See the following table for details:

Circuit composition

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. 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 circuit 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. Some positioning points have small fluctuation and are 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. When designing broadband power amplifier, or when the working frequency is high, the bias circuit has a great impact 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 offset networks, passive networks and active networks. Passive networks (i.e. self biased networks) are usually composed of resistance networks to provide appropriate operating voltage and current for transistors. Its main defect is that it is very sensitive to the changes of transistor parameters and has poor temperature stability. Active bias network can improve the stability of static operating point and good temperature stability, but it also has some problems, such as increasing the circuit size, increasing the difficulty of circuit layout and increasing the power consumption.

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.

Typical impedance matching networks include l-matching, π – matching and T-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. One advantage of π – shaped matching network is that no matter what kind of parasitic capacitance is connected to it, it can be absorbed into the network, which also leads to the universal application of π – shaped matching network, because in many practical situations, the dominant parasitic element is capacitance. T-shape matching, when the parasitic parameters at the power supply end and load end are mainly inductive, T-shape matching can be used to absorb these parasitic parameters into the network.

How to ensure the stability of RF PA

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.

Test challenges faced by 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.

Changes in semiconductor materials:

Ge, Si → GaAs, InP → SiC, Gan, SiGe, SOI → CNT → graphene.

At present, the mainstream process of power amplifier is still GaAs process. In addition, GaAs HBT, GaAs Heterojunction bipolar transistor. HBT (heterojunction bipolar transistor) is a bipolar transistor composed of gallium arsenide (GaAs) layer and aluminum gallium arsenic (AlGaAs) layer.

Although CMOS technology has been relatively mature, Si CMOS power amplifier is not widely used. In terms of cost, although the silicon wafer of CMOS process is relatively cheap, the layout area of CMOS power amplifier is relatively large, coupled with the high R & D cost invested in the complex design of CMOS PA, the overall cost advantage of CMOS power amplifier is not so obvious. In terms of performance, CMOS power amplifier has poor performance in linearity, output power and efficiency, coupled with the inherent disadvantages of CMOS process: high knee point voltage, low breakdown voltage and low resistivity of CMOS process substrate.

Carbon nanotubes (CNTs) are considered to be ideal materials for nano electronic devices because of their small physical size, high electron mobility, high current density and low intrinsic capacitance.

Graphene, a zero band gap semiconductor material, will become a hot material for the next generation of RF chips because of its high electron migration rate, nano physical size, excellent electrical and mechanical properties.

Linearization technology of RF PA

The nonlinear distortion of RF power amplifier will produce new frequency components, such as second harmonic and two tone beat for second-order distortion, third harmonic and multi tone beat for third-order distortion. If these new frequency components fall in the passband, they will directly interfere with the transmitted signal. If they fall outside the passband, they will interfere with the signals of other channels. Therefore, the of RF power amplifier should be linearized, which can better solve the problem of signal spectrum regeneration.

The principle and method of the basic linearization technology of RF power amplifier is to take the amplitude and phase of the input RF signal envelope as a reference, compare it with the output signal, and then produce appropriate correction. At present, the proposed and widely used power amplifier linearization technology includes power fallback, negative feedback, feedforward, predistortion, envelope elimination and recovery (EER), and linear amplification (Linc) using nonlinear elements. More complex linearization techniques, such as feedforward, predistortion, envelope elimination and recovery, and linear amplification using nonlinear elements, have a better effect on the linearity of the amplifier. The linearization technologies that are easy to implement, such as power fallback and negative feedback, have limited improvement on linearity.

1. Power fallback

This is the most commonly used method, that is, selecting a high-power tube as a low-power tube is actually to improve the linearity of the power amplifier at the expense of DC power consumption.

The power fallback method is to compress the input power of the power amplifier from 1dB to the point (the amplifier has a linear dynamic range in which the output power of the amplifier increases linearly with the input power. As the input power continues to increase, the amplifier gradually enters the saturation region and the power gain begins to decrease. Generally, the output power value when the gain decreases to 1dB lower than the linear gain is defined as the 1dB compression point of the output power, expressed in p1db.) Back off 6-10 decibels and work at a level far less than 1dB compression point to make the power amplifier far away from the saturation region and enter the linear working region, so as to improve the third-order intermodulation coefficient of the power amplifier. Generally, when the fundamental wave power is reduced by 1dB, the third-order intermodulation distortion is improved by 2dB.

The power fallback 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 fallback reaches a certain degree and the third-order cross modulation reaches below – 50dbc, the linearity of the amplifier will not be improved by continuing the fallback. Therefore, it is not enough to rely solely on power fallback in cases where high linearity is required.

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 it has no stability problem, has a wider signal frequency band and can process signals with multiple carriers. Predistortion technology is the first mock exam. It is a low-cost preamplifier technology. It is packaged by a few carefully selected components into a single module. Predistortion technology has been adopted in the power amplifier of handheld mobile station. It reduces the intermodulation product by several dB with only a few components, but it is a key few dB.

Predistortion technology is divided into two basic types: RF predistortion and digital baseband predistortion. RF predistortion is generally realized by analog circuit, which has the advantages of simple circuit structure, low cost, easy high-frequency and broadband applications. The disadvantages are less improvement of spectrum regeneration components and difficult cancellation of high-order spectrum components.

Digital baseband predistortion can be realized by digital circuit because of its low working frequency. It has strong adaptability, and can offset high-order intermodulation distortion by increasing sampling frequency and quantization order. It is a promising method. The predistorter consists of a vector gain regulator, which controls the amplitude and phase of the input signal according to the contents of the look-up table (LUT), and the size of predistortion is controlled by the input of the look-up table. Once the vector gain regulator is optimized, it will provide a nonlinear characteristic opposite to the power amplifier. Ideally, the output intermodulation product should be equal to the output amplitude of the two tone signal through the power amplifier, but the phase is opposite, that is, the adaptive adjustment module is to adjust the input of the look-up table, so as to minimize the difference between the input signal and the output signal of the power amplifier. Note that the envelope of the input signal is also an input of the look-up table. The feedback path samples the distorted output of the power amplifier, and then sends it to the adaptive adjustment DSP through a / D transformation to update the look-up table.

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. Except that calibration (feedback) is added to the output, it is completely “feedback” in concept.

The feedforward linear amplifier consists of two loops through coupler, attenuator, synthesizer, delay line and power divider. After the RF signal is input, it is divided into two channels through the power divider. All the way into the main power amplifier, due to its nonlinear distortion, the output end not only has the main frequency signal to be amplified, but also has third-order intermodulation interference. 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 cancelled through loop 1, leaving only the inverted third-order intermodulation component. After the third-order intermodulation component is amplified by the auxiliary amplifier, the intermodulation component generated by the nonlinearity of the main amplifier is cancelled through loop 2, so as to improve the linearity of the power amplifier.

Feedforward technology not only provides the advantages of higher quasi precision, but also has no disadvantages of instability and bandwidth limitation. Of course, these advantages are obtained at a high cost. Due to the large power level in the output calibration, the calibration signal needs to be amplified to a higher power level, which requires an additional auxiliary amplifier, and the distortion characteristic of the auxiliary amplifier itself should be above the index of the feedforward system.

The offset requirements of feedforward power amplifier are very high, and the matching of amplitude, phase and time delay is required. The offset failure will be caused by power change, temperature change and device aging. Therefore, the adaptive cancellation technology is considered in the system, so that the cancellation can keep up with the changes of internal and external environment.

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