A brief analysis of the power amplifier in RF chips

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A brief analysis of the power amplifier in RF chips [Copy link]

1. RF chip market

According to Yole Development, the value of RF front-end chips in 2G smartphones is $0.9, which has risen sharply to $3.4 in 3G smartphones. The value of RF front-end chips in smartphones supporting regional 4G standards has reached $6.15, and high-end LTE smartphones have reached $12-15, which is 17 times that of RF front-end chips in 2G smartphones. It is estimated that by 2023, mobile phone radio frequency (RF) front-end modules and components will reach $35 billion, with a compound annual growth rate of 14% from 2017 to 2023.

The growth rates of various mobile phone RF front-end components vary, such as antenna tuners.

The compound annual growth rate of RF power amplifiers and low-noise amplifiers (PAs & LNAs) is only 1%.

The number of PA chips required for 4G multi-mode and multi-frequency mobile phones has increased to 5-7. StrategyAnalytics predicts that there may be as many as 16 PAs in mobile phones in the 5G era. In terms of process materials, gallium arsenide PA is currently the mainstream, and CMOS PA is only used in the low-end market due to the influence of parameter performance. 4G, especially LTE cat16, 4x20MHZ carrier aggregation technology such as Qualcomm, requires high Q value for PA linearity, and will further rely on gallium arsenide PA. At the same time, according to Qorvo's forecast, with the popularization of 5G, gallium arsenide PA below 8GHz will still be the mainstream, but gallium nitride above 8GHz is expected to become the main force in the mobile phone market. With the complexity of wireless communication protocols and the continuous evolution of RF front-end chip design, PA design manufacturers often integrate functions such as switches or duplexers with power amplifier circuits in one chip package to form a combination of multiple functions. According to actual conditions, there are multiple PA chip types with complex functions such as TxM (PA+Switch), PAD (PA+Duplexer), MMPA (multi-mode multi-frequency PA), etc.

2. What is an RF Power Amplifier?

A power amplifier is an amplifier that amplifies the input signal and provides sufficient power to the load. 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 RF signal power generated by the modulated oscillation circuit is very small, and it needs to go through a series of amplifications (buffer stage, intermediate amplifier stage, final power amplifier stage) to obtain sufficient RF power before it can be fed to the antenna for radiation. In order to obtain a sufficiently large RF output power, an RF power amplifier must be used. After the modulator generates the RF signal, the RF modulated signal is amplified by the RF PA to sufficient power, and then transmitted by the antenna through the matching network.

The function of the amplifier is to amplify the input content and output it. The input and output content are called "signals", which are often expressed as voltage or power. The main technical indicators of RF power amplifiers are output power and efficiency. How to improve output power and efficiency is the core of RF power amplifier design goals. Usually in RF power amplifiers, LC resonant circuits can be used to select the fundamental frequency or a certain harmonic to achieve distortion-free amplification. In addition, the harmonic components in the output should be as small as possible to avoid interference with other channels.

According to different working states, power amplifiers can be divided into: linear power amplifiers and switching power amplifiers.

The operating frequency of linear power amplifiers is very high, but the relative frequency band is relatively narrow. RF power amplifiers generally use frequency selection networks as load circuits. Linear RF power amplifiers can be divided into three working states according to the different current conduction angles: A, B, and C. The conduction angle of the current of the Class A amplifier is 360°, which is suitable for small signal low-power amplification. The conduction angle of the current of the Class B amplifier is equal to 180°, and the conduction angle of the current of the Class C amplifier is less than 180°. Both Class B and Class C are suitable for high-power working states. The output power and efficiency of the Class C working state are the highest among the three working states. Most 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 for resonant power amplification using a tuned circuit as a load. Because the tuned circuit has filtering capabilities, the loop current and voltage are still close to the sine waveform, and the distortion is very small.

Switching Mode PA (SMPA) makes electronic devices work in a switching state. Common types include Class D amplifiers and Class E amplifiers. Class D amplifiers are more efficient than Class C amplifiers. SMPA drives active transistors into a switching mode. The working state of the transistor is either on or off. There is no overlap in the time domain waveforms of its voltage and current, so the DC power consumption is zero, and the ideal efficiency can reach 100%.

In general, 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.

3. Power Amplifier Process

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

Although the CMOS process is relatively mature, Si CMOS power amplifiers are not widely used. In terms of cost, although the silicon wafers of the CMOS process are relatively cheap, the layout area of the CMOS power amplifier is relatively large. In addition, the R&D cost of the complex design of the CMOS PA is relatively high, making the overall cost advantage of the CMOS power amplifier not so obvious. In terms of performance, the performance of CMOS power amplifiers in terms of linearity, output power, efficiency, etc. is poor, and the inherent disadvantages of the CMOS process are: high knee voltage, low breakdown voltage, and low resistivity of the CMOS process substrate.

4. Power Amplifier Development Trend

British research company Technavio said that the global power amplifier market has three major development trends: wafer size increases; start-ups adopt CMOS technology; the demand for high-speed amplifiers in the defense field is gradually increasing; and the use of InGaP technology to achieve low power consumption and high efficiency of power amplifiers.

Wafer size is getting bigger. The semiconductor industry has witnessed changes in wafer size over the past 40 years. Gallium arsenide (GaAs) wafer size has increased from 50mm to 150mm, reducing manufacturing costs by 20% to 25%. Currently, the industry usually uses 150mm wafers to manufacture power amplifiers. It is predicted that 150mm wafers will continue to be used because manufacturers such as Taiwan's Win Semiconductors are still investing heavily in upgrading and building new 150mm factories. The industry is developing 200mm wafer technology and is expected to be able to trial production by the end of 2018. Researchers at Stanford University are studying how to reduce the price of 200mm GaAs wafers so that they can compete with silicon wafers in the market at a lower price. At the same time, this also puts forward the demand for mask inspection equipment and wafer manufacturing equipment.

Startups adopt CMOS technology. Some startups, such as Acco Semiconductor, are increasingly adopting CMOS technology. Acco Semiconductor has invested $35 billion to expand its CMOS-based RF power amplifier business, taking advantage of the huge demand for RF power amplifiers in mobile phones and IoT products. Currently, most power amplifiers use silicon germanium (SiGe) or GaAs technology, not CMOS. But according to the report, CMOS-based processes can help achieve low-cost, high-performance power amplifiers.

Defense needs high-speed amplifiers. The military needs to use spectrum more efficiently and use more mobile devices to communicate. Therefore, Technavio said that the military requires high-speed power amplifiers. The Defense Advanced Research Projects Agency (DARPA) of the United States has made progress in the terahertz electronics project, that is, Northrop Grumman has developed a solid-state power amplifier and a traveling wave tube amplifier, which are the only two terahertz frequency products. Power amplifiers in the terahertz band can be used in many fields, including high-resolution security imaging, high-data rate communications, anti-collision radar, long-range hazardous chemicals and explosives detection systems, etc. The high-speed operation of these devices requires the use of high-speed amplifiers.

Low power consumption and high efficiency of power amplifiers are achieved using InGaP technology. InGaP is particularly suitable for high-frequency applications that require relatively high power output. Improvements in the InGaP process have led to higher yields and a higher degree of integration, allowing more functions to be integrated into the chip. This simplifies system design, reduces raw material costs, and saves board space. Some InGaP PAs also use multi-chip packages that include CMOS control circuits. Today, front-end WLAN modules that integrate PAs and low-noise amplifiers (LNAs) on the receiving end and combine RF switches can be packaged in a compact package. For example, the InGaP-Plus process proposed by ANADIGICS can integrate bipolar transistors and field-effect transistors on the same InGaP chip. This technology is being used in new CDMA and WCDMA power amplifiers with improved size and PAE (power added efficiency).

5. Main indicators of power amplifier

Operating frequency range. Generally speaking, it refers to the linear operating frequency range of the amplifier. If the frequency starts at DC, the amplifier is considered a DC amplifier.

Gain. Working gain is the main indicator to measure the amplification ability of an amplifier. Gain is defined as the ratio of the power transmitted from the output port of the amplifier to the load to the power actually transmitted from the signal source to the input port of the amplifier. Gain flatness refers to the range of change of the amplifier gain within the entire operating frequency band at a certain temperature, and is also a main indicator of the amplifier.

Output power and 1dB compression point (P1dB). When the input power exceeds a certain value, the transistor's gain begins to decrease, and the final result is that the output power reaches saturation. When the amplifier's gain deviates from a constant or is 1dB lower than other small signal gains, this point is the famous 1dB compression point (P1dB).

Efficiency. Since the power amplifier is a power component, it needs to consume supply current. Therefore, the efficiency of the power amplifier is extremely important for the efficiency of the entire system. Power efficiency is the ratio of the RF output power of the power amplifier to the DC power supplied to the transistor.

Intermodulation distortion. Intermodulation distortion refers to the mixed components produced by two or more input signals with different frequencies passing through the power amplifier. This is caused by the nonlinear characteristics of the power amplifier.

Third-order intermodulation cutoff point (IP3). IP3 is also an important indicator of amplifier nonlinearity. When the output power is constant, the greater the output power at the third-order intermodulation cutoff point, the better the linearity of the amplifier.

Dynamic range. The dynamic range of an amplifier generally refers to the difference between the minimum detectable signal and the maximum input power in the linear operating area. Naturally, the larger this value is, the better.

Harmonic distortion. When the input signal increases to a certain level, the power amplifier will generate a series of harmonics due to working in the nonlinear region. For high-power amplifier systems, filters are generally required to reduce harmonics to below 60dBc.

Input/output standing wave ratio. Indicates the matching degree between the power amplifier and the whole system. The deterioration of input/output ratio will lead to the fluctuation of system gain and the deterioration of group delay. However, it is more difficult to design a power amplifier with a high standing wave ratio. In general systems, the input standing wave ratio of the power amplifier is required to be lower than 2:1.

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