PA knowledge that RF engineers must master: all you need to know is here!

As an RF engineer, your job involves power amplifiers to some extent. Power amplifiers can be said to be a hurdle that many RF engineers cannot bypass. Function, 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 the class!

As an RF engineer, your job involves power amplifiers to some extent. Power amplifiers can be said to be a hurdle that many RF engineers cannot bypass. Function, 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 the class!

Two Key Metrics for RF PAs: Power and Linearity

PA knowledge that RF engineers must master: all you need to know is here!

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)

Function of RF Power Amplifier RF PA

The RF power amplifier RF PA is the main part of the transmission system, and its importance is self-evident. In the front-end circuit of the transmitter, the power of the RF signal generated by the modulation 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 enough RF power before feeding. radiate to the antenna. In order to obtain a sufficiently large RF output power, a RF power amplifier must be used. Power amplifiers tend to be the most expensive, power-hungry, and least-efficient devices in fixed equipment or terminals.

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

PA knowledge that RF engineers must master: all you need to know is here!

Figure 1 Block diagram of the transmitting system

The function of the amplifier is to amplify the input content and output it. What is input and output, what we call “signals”, is often expressed as voltage or power. For a “system” like an amplifier, its “contribution” is to raise what it “absorbs” to a certain level and “output” it to the outside world. This “improved contribution” is the “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 can it not provide any “contribution”, but there may be some unexpected “oscillation”. Oscillation”, both for the outside world and the amplifier itself, is catastrophic.

Classification of RF Power Amplifier RF PA

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

PA knowledge that RF engineers must master: all you need to know is here!

Figure 2 Classification of power amplifiers

The working frequency of the radio frequency power amplifier is very high, but the relative frequency band is relatively narrow, and the radio frequency power amplifier generally adopts the frequency selection network as the load loop. The RF power amplifier can be divided into three 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 state, and the output power and efficiency of Class C working state are the highest among the three working states. Most of the RF power amplifiers work in class C, but the current waveform of the class C amplifier is too distorted, so it can only be used to amplify the resonant power using the tuned loop as the load. Due to the filtering capability of the tuned loop, the loop current and voltage are still close to sinusoidal 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 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 the design goal of RF power amplifier. Usually in the RF power amplifier, the fundamental frequency or a certain harmonic can be selected by the LC resonant circuit to achieve distortion-free amplification. Generally speaking, the evaluation of amplifiers probably has the following indicators:

gain. This is the ratio between input and output and represents the contribution of the amplifier. A good amplifier contributes as much “output” as possible within its “range of its own capabilities”. working frequency. This represents the ability of the amplifier to carry signals of different frequencies. working bandwidth. This determines how far the amplifier can “contribute”. For a narrowband amplifier, the contribution of its own design may be limited, if not problematic. stability. Every transistor has a potential “region of instability”. The “design” of the amplifier needs to eliminate these potential instabilities. There are two types of amplifier stability, potential instability and absolute stability. The former may be unstable under certain conditions and circumstances, while the latter is guaranteed to remain stable under any circumstances. The issue of stability is important because instability means “oscillation”, at which time the amplifier not only affects itself, but also outputs unstable factors. maximum output power. This metric determines the “capacity” of the amplifier. For “big systems”, hopefully they can put out more power at the expense of a certain gain. efficient. Amplifiers must consume a certain amount of “energy” and also achieve a certain “contribution”. The ratio of its contribution to consumption is the efficiency of the amplifier. Able to contribute more and consume less is a good amplifier. linear. Linearity characterizes the correct response of the amplifier to a large number of inputs. Deterioration in linearity means that the amplifier will “distort” or “twist” the input in a state of excess input. A good amplifier should not exhibit this “malformed” quality.

The following content: circuit composition, stability and efficiency improvement method of RF power amplifier

The 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: transistors, bias and stabilization circuits, and input and output matching circuits.

1. Transistor

There are many kinds of transistors, including transistors of various structures that have been invented. Essentially, the transistor’s job is to act as a controlled current or voltage source, which works by converting the energy of DC without content into a “useful” output. DC energy is obtained from the outside world, consumed by transistors, and converted into useful components. A transistor, we can think of it as “a unit”. 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 reaction speed is different, which determines how wide and high it can work. On the frequency band; for example, its impedance facing the input and output ends is different, and its external response ability is different, which determines the difficulty of matching it.

2. Bias and stabilization circuit

Biasing and stabilization circuits are two different circuits, but can be discussed together because they are often indistinguishable and have similar design goals.

The operation of the transistor needs to be under a certain bias condition, which we call the static operating point. This is the foundation of the transistor’s foothold, its own “positioning”. Each transistor has a certain positioning for itself, and its 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 and are suitable for high-power output; some positioning points have less demand and pure release, and are suitable for low-noise work; Transistors are always on and off between saturation and cutoff. A proper bias point is the basis for normal operation.

The stabilization circuit must be before the matching circuit, because the transistor needs to have the stabilization circuit as a part of itself, and then come into contact with the outside world. In the eyes of the outside world, with the addition of the transistor for the stabilization circuit, it is a “brand new” transistor. It made certain “sacrifices” and gained stability. The mechanism of stabilizing the circuit can ensure the smooth and stable operation of the transistor.

3. Input and output matching circuit

The purpose of the matching circuit is to select an acceptable way. For transistors that want to provide greater gain, the path 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 only a “full acceptance” design method. Some small tubes with small DC and shallow foundation are more willing to do a certain amount of blocking when they are accepted to obtain better noise performance, but they cannot block too much, otherwise their contribution will be affected. For some giant power tubes, you need to be cautious when outputting, because they are more unstable, and at the same time, a certain reservation helps them to exert more “undistorted” energy.

Implementation of RF PA Stability for RF Power Amplifiers

Every transistor is potentially unstable. Good stable circuits can be fused with transistors to form a “sustainable working” mode. The implementation of stabilization circuits can be divided into two types: narrowband and wideband.

The narrow-band stabilization circuit is to carry out a certain gain consumption. This stable circuit is achieved by adding a certain consumption circuit and a selective circuit. This circuit allows the transistor to contribute only within a small frequency range. Another broadband stabilization is the introduction of negative feedback. This circuit can work in a wide range.

The source of instability is positive feedback, and the idea of ​​narrow-band stabilization is to contain part of the positive feedback. Of course, this also inhibits the contribution. And negative feedback, done well, has many additional delightful advantages. Negative feedback, for example, may keep transistors free from matching, allowing them to interface well with the outside world without matching. In addition, the introduction of negative feedback improves the linearity performance of the transistor.

Efficiency Improvement Technology of RF Power Amplifier RF PA

There is a theoretical limit to the efficiency of a transistor. This limit varies with the choice of bias point (static operating point). In addition, poorly designed peripheral circuits will greatly reduce their efficiency. At present, engineers have few ways to improve efficiency. Here are only two: envelope tracking technology and Doherty technology.

The essence of envelope tracking technology is to separate the input into two types: phase and envelope, which are then amplified by different amplifier circuits. In this way, the two amplifiers can focus on their respective parts, and the cooperation of the two 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 it works in a high-efficiency state. If the input increases, both transistors work simultaneously. The basis of this method is that the two transistors must cooperate tacitly. The working state of one transistor directly determines the working efficiency of another.

Test Challenges for RF PAs

Power amplifiers are very important components in wireless communication systems, but they are inherently nonlinear, causing spectral proliferation that interferes with adjacent channels, and may violate out-of-band emission standards mandated by law. This characteristic can even cause in-band distortion, which increases the bit error rate (BER) of the communication system and reduces the data transmission rate.

Under peak-to-average power ratio (PAPR), the new OFDM transmission format will have more occasional peak power, making the PA less easily segmented. This will reduce spectral mask compliance and expand the EVM and BER of the entire waveform. To solve this problem, design engineers often deliberately reduce the operating power of the PA. Unfortunately, this is a very inefficient method, as the PA reduces the operating power by 10% and loses 90% of the DC power.

Most of today’s RF PAs support multiple modes, frequency ranges and modulation modes, resulting in more test items. 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 techniques only complicate testing and significantly extend Design and test time. Increasing the bandwidth of the RF PA will result in a 5x increase in the bandwidth required for DPD measurements (possibly in excess of 1 GHz), further increasing the complexity of the test.

According to the trend, in order to increase efficiency, RF PA components and front-end modules (FEMs) will be more tightly integrated, and a single FEM will support a wider range of frequency bands and modulation modes. Integrating an envelope tracking power supply or modulator into a FEM can effectively reduce the overall space requirement inside a mobile device. The massive increase in filter/duplexer slots to support a wider operating frequency range increases 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 a component that cannot be integrated in mobile phones at present. 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 formats is an important topic that the industry has been studying. At present, there are two schemes: one is the fusion architecture, which integrates the RF power amplifier PA of different frequencies; the other architecture is the integration along the signal chain, that is, the PA and the duplexer are integrated. Both schemes have their own advantages and disadvantages and are suitable for different mobile phones. Fusion architecture, high integration of PA, has obvious size advantage for more than 3 frequency bands, and also has obvious cost advantage when there are 5-7 frequency bands. The disadvantage is that although the PA is integrated, the duplexer is still quite complex, 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 about tens of milliamps of current, which is equivalent to extending the talk time by 15%. Therefore, the advice of industry insiders is that when there are more than 6 frequency bands (not counting 2G, which refers to 3G and 4G), the fusion architecture is used, and when the frequency is less than four frequency bands, the PA and duplexer integration scheme PAD is used.

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