Knowledge of power amplifiers in rf chips

I. rf chip market

According to Yole Development statistics, 2 g system, smart phone rf front-end chip in the value of $0.9, in the 3 g standard smartphone soared to $3.4, support regional 4 g system of smart phones in the value of rf front-end chip has reached $6.15, high-end LTE smartphone $12 to 15, is 2 g system rf front-end chip 17 times in the smart phone.It is expected that mobile RF front-end modules and components will reach $35 billion by 2023, with a compound annual growth rate of 14% in 2017-23.

Various Antenna tuners, for example, have varying degrees of growth

In the presence of these Switches, the growth rate of rf power amplifiers and low noise amplifiers (PAs & LNAs) is only 1%.

The number of PA chips needed for 4G multimode and multifrequency phones has risen to between five and seven, with StrategyAnalytics predicting as many as 16 PA chips in 5G era phones.As for process materials, gallium arsenide PA is the mainstream at present, and CMOS PA is only used in the low-end market due to the influence of parameter performance.4G, especially LTE cat16 and 4x20MHZ carrier polymerization technologies such as qualcomm, will further rely on gallium arsenide PA due to its high requirement for PA linearity and Q worthiness.Meanwhile, with the popularization of 5G, gallium arsenide PA below 8GHz is still the mainstream, but gallium nitride above 8GHz is expected to become the main force in the mobile phone market, according to Qorvo prediction.With the complexity of wireless communication protocols and the continuous evolution of rf front-end chip design, PA designers tend to integrate functions such as switches or duplexers and power amplifier circuits into a chip package, forming a variety of functional combinations.According to the actual situation, TxM (PA+Switch), PAD (PA+ Duplexer), MMPA (multimode multifrequency PA) and other types of PA chip with multiple composite functions.

What is a RF power amplifier

A power amplifier is one that amplifies the input signal and provides sufficient power to the load.RF power amplifier (RF PA) is an important part of transmission system.In the front-stage circuit of the transmitter, the power of the rf signal generated by the modulating oscillation 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 it can be fed to the antenna for radiation.In order to obtain sufficient rf output power, a rf power amplifier must be used.After the modulator generates the RF signal, the modulated RF signal is amplified to sufficient power by the RF PA, and then transmitted by the antenna through the matching network.

The function of an amplifier, that is, to amplify input and output.The contents of the input and output, which we call "signals", are often expressed as voltage or power.The main technical indexes of rf power amplifier 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, LC resonant circuit can be used to select the fundamental frequency or a harmonic, to achieve non-distortion amplification.In addition, the harmonic component of the output should be as small as possible to avoid interference with other channels.

According to the different working state, power amplifier can be divided into: linear power amplifier and switching power meter.

Linear power amplifier has high working frequency but narrow relative frequency band.Linear rf power amplifiers can be divided into A (A), B (B) and C (C) working states according to the current conduction Angle.Class a amplifier current conduction Angle is 360 °, is suitable for small signal low power amplifier, class b amplifier current conduction Angle is equal to 180 °, class c amplifier current conduction Angle 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 are the highest among the three working conditions.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 resonance power amplification with tuning loop.Because of the filter capability of the tuning circuit, the current and voltage of the circuit are still close to the sinusoidal waveform, and the distortion is very small.

Switching Mode PA (SMPA) is a type of power amplifier used to make electronic devices operate in Switching state. There are class D amplifiers and class E amplifiers, and the efficiency of class D amplifiers is higher than that of class c amplifiers.SMPA drives the active transistor into switching mode, and the working state of the transistor is either on or off. 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%.

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.

Process of power amplifier

At present, the mainstream process of power amplifier is still GaAs process.In addition, GaAs HBT, GaAs heterojunction bipolar transistor.HeterojuncTIon bipolartransistor (HBT) is a bipolar transistor composed of a GaAs layer and an AlGaAs layer.

Although CMOS technology is mature, Si CMOS power amplifier is not widely used.In terms of cost, although silicon wafers with CMOS technology are relatively cheap, the overall cost advantage of CMOS power amplifier is not so obvious due to the relatively large territory of CMOS power amplifier and the relatively high research and development cost invested in the complex design of CMOS PA.In terms of performance, CMOS power amplifier has poor performance in terms of linearity, output power, efficiency and so on. In addition, CMOS process has inherent disadvantages: higher knee point voltage, lower breakdown voltage, and lower resistivity of CMOS process substrate.

Development trend of power amplifier

Technavio, a British research firm, says there are three main trends in the global power amplifier market.Startups adopt CMOS technology;The need of high speed amplifier in the field of national defense is increasing gradually: the InGaP technology is used to realize the low power consumption and high efficiency of power amplifier.

Wafer size increases.The semiconductor industry has seen the size of wafers change over the past 40 years, with gallium arsenide (GaAs) wafers growing from 50mm to 150mm and manufacturing costs falling by 20-25%.Currently, power amplifiers manufactured in the industry usually use 150mm wafers.150mm wafers are expected to remain in use as manufacturers such as Taiwan's avem semiconductor continue to invest heavily in upgrading and building 150mm factories.The industry is developing 200mm wafer technology, which is expected to be ready for trial production by the end of 2018.Stanford university researchers are working to lower the price of 200mm GaAs wafers so they can compete with silicon wafers at lower prices.At the same time, this also puts forward the demand for mask inspection equipment and wafer manufacturing equipment.

Startups use CMOS technology.Some start-ups, such as Acco Semiconductor, are increasingly adopting CMOS technology.Seizing on the huge demand for radio frequency power amplifiers from mobile phones and the Internet of things, Acco Semiconductor has invested $35 billion to expand its cmos-based radio frequency power amplifier business.At present, most power amplifiers use silicon germanium (SiGe) or GaAs technology instead of CMOS.However, according to the report, cmos-based technology can help realize low-cost and high-performance power amplifiers.

The defense sector needs high-speed amplifiers.The military needs to use spectrum more efficiently and use mobile devices to communicate.As a result, Technavio says, the military sector requires high-speed power amplifiers.The defense advanced research projects agency (DARPA) has made progress in the terahertz electronics program, where northrop grumman developed solid-state power amplifiers and travelling-wave tube amplifiers, the only two terahertz frequency products.Terahertz power amplifiers can be used in many fields, including high-resolution safety imaging, high data rate communications, crash-avoidance radar, remote hazardous chemicals and explosive detection systems, etc. High-speed amplifiers are required for the high rate operation of these devices.

The InGaP process is used to realize the low power consumption and high efficiency of power amplifier.InGaP is particularly suited for high frequency applications that require fairly high power output.Improvements to the InGaP process have led to increased production and higher levels of integration, allowing chips to integrate more functions.This simplifies the system design, reduces the cost of raw materials, but also saves the board space.Some InGaP pas also use multichip packages that contain CMOS control circuits.Today, front-end WLAN modules with PA and low noise amplifier (LNA) integrated at the receiver and combined with RF switches are available in compact packages.For example, ANADIGICS 'ingap-plus process can integrate bipolar and field-effect transistors on the same InGaP chip.This technology is being used for new CDMA and WCDMA power amplifiers with improved size and PAE (power increase efficiency).

Main index of power amplifier

Working frequency range.In general, refers to the amplifier linear operating frequency range.If the frequency starts with DC, the amplifier is considered a DC amplifier.

Gain.Working gain is the main index to measure the amplification ability of amplifier.Gain is defined as the ratio of the power transmitted to the load by the amplifier output port to the power actually transmitted by the signal source to the amplifier input port.Gain flatness refers to the variation range of amplifier gain in the whole working frequency band under a certain temperature, and it is also a main index of the amplifier.

Output power and 1dB compression point (P1dB).When the input power exceeds a certain value, the gain of the transistor begins to decrease, and the final result is that the output power reaches saturation.This point is known as the 1dB compression point (P1dB) when the amplifier's gain is off constant or 1dB lower than the gain of other small signals.

Efficiency.Since the amplifier is a power element, it consumes the supply current.So the efficiency of the amplifier is very important for the efficiency of the whole system.Power efficiency is the ratio of the rf output power of the amplifier to the dc power supplied to the transistor.

Alternating modulation distortion.Ac distortion is the mixed component produced by two or more input signals of different frequencies passing through a power amplifier.This is due to the nonlinear nature of the amplifier.

Intermodulation cutoff point (IP3).IP3 is also an important indicator of power amplifier nonlinearity.When the output power is constant, the higher the output power of the third-order intermodulation cut-off point, the better the linearity of the amplifier will be.

Dynamic range.The dynamic range of a power amplifier is generally the difference between the minimum detectable signal and the maximum input power in a linear workspace.Of course, the bigger the better.

Harmonic distortion.When the input signal is increased to a certain extent, the amplifier will generate a series of harmonics due to its work in the nonlinear region.For high power amplifier systems, it is generally necessary to use a filter to reduce the harmonic below 60dBc.

Input/output VSWR.Indicates the matching degree of the power amplifier and the whole system.Worse input/output ratio will lead to worse gain fluctuation and group delay of the system.However, power amplifiers with high VSWR are difficult to design. In general systems, VSWR input of power amplifiers is required to be lower than 2:1.