International Journal of Applied Engineering Research ISSN 0973-4562 Volume 9, Number 2 (2014) pp. 257-276 © Research India Publications http://www.ripublication.com
An Overview of RF Power Amplifier Techniques and Effect of Transistor Scaling on its Design Parameters VeeraiyahThangasamy *1, Noor Ain Kamsani2, MohdNizar Hamidon3, Muhammad Faiz Bukhori4 1, 2, 3
Department of Electrical and Electronics Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Malaysia.*
[email protected] 4 Department of Electrical, Electronics & Systems Engineering, UniversitiKebangsaan Malaysia, 43600 Bangi, Malaysia.
Abstract A strong market growth for wireless systems has taken place over the last two decades, driven by persistent demands of ever smaller form-factor, increased power-efficiency and reliability. Radio frequency (RF) power amplifier (PA) is a critical block in a wireless transceiver, consuming most power and affecting battery life. Battery life is directly affected by the linearity and efficiency characteristics of the integrated PA. In this paper, we critically review key theories and techniques that are used in RF power amplifier designs, with emphasis on efficiency and linearity improvement. The device’s physical size, material and processes greatly affect the power characteristics of PAs. Hence, we also critically review how scaling, semiconductor materials and technologies relate to RF power amplifier designs, with emphasis on power and frequency. Keywords – RF power amplifier, linearity, efficiency, peak-to-average power ratio (PAPR), Scaling, current gain cut-off frequency (fT).
I. INTRODUCTION Modern communication systems such as mobile phones, satellite links, and radio broadcasts apply a wide range of frequencies from the HF to millimeter wavelength. The transmitter output power of these systems range from a few milliwatts to megawatts. Various types of modulation schemes such as QPSK, GMSK and OFDM are in use although multilevel QAM is increasingly popular for emerging applications. The choice of a particular PA technique is often a challenging consideration between efficiency, linearity and bandwidth requirements; because no single PA technique
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suits all the applications. RF power amplifier (PA) converts the dc-power of the supply into RF output power. A transmitter consists of one or more stages of PAs and additional circuits including signal generators, modulators, frequency converters, processors and power supplies. The power amplifier may be a single stage or multistage depending on the output power level required. In this paper, basic PA topologies of class-A, -B, -C, -D, -E, and -F are reviewed and their performance discussed in section II. Different architectures are employed to improve the efficiency and linearity of PAs. These architectures include envelope elimination and restoration (EER), envelope tracking (ET), Doherty, and outphasing; all of which are discussed and theirs output power against efficiency is compared in section III.In section IV, the effects of transistor scaling on PA’s power and frequency characteristics are discussed. How different semiconductor materials affect the PA’s power and frequency are reviewed in section V. The various conventional transistor technologies such as CMOS, HBT, and HEMT are discussed in section VI, and finally a conclusion is drawn in section VII.
II. RF POWER AMPLIFIER TOPOLOGIES A transistor is a key device in any power amplifier circuit. The biasing circuit and the drive signal at the input of PA will determine the different transistor topologies of operation. Fig.2 shows the ideal output and transfer characteristics of a NMOS transistor of a single-ended PA circuit of Fig.1 with the location of quiescent point for different topologies of operation. VDD
Idrain Drive and Bias
Q1
+ Vds -
Output Filter Io + Vo -
---------> Id
Vgs4
RFC
Vgs3 Vgs2 Vgs1
RL
------->Vds
---------> V d s
Vgs5
Idc
Class C o Vth
o
Class A
Class AB Class B Voc ------->Vgs
Fig.1. Single-ended PA using NMOS Fig.2. Ideal output and transfer transistor characteristics of NMOS transistor. A.LINEAR MODE PAs Class-A: In this topology, the quiescent point is set in the middle of the transfer characteristics as shown in Fig.2, and the transistor remains in active region at all time, that is, Vgs is maintained between Vth and Voc(open channel voltage). As a result, the drain current and the drain voltage waveform are sinusoidal as shown in Fig.3a. Both cycles of the input signal appear at the output and hence the amplifier is highly linear and the gain is high. However the efficiency is poor because of the saturation voltage of the transistor. The theoretical efficiency is 50 %. To obtain the
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optimum efficiency close to the theoretical value, the design for optimum load resistance (Ropt) is essential [1].The class-A topology is perfectly linear and there is no harmonics developed by the transistor during the amplification process, and hence class-A PAs can be operated until the maximum frequency capability (fmax) of the transistor.The class-A operation is well suited for varying envelope modulated input signals (such as QAM) because of its high linearity. Class-B: The transistor is biased such that the quiescent point lies at the threshold point (Vth) of the characteristics as shown in Fig.2, so that the transistor is ON only for half the cycle of the input signal, and the drain current is half sinusoidal as shown in Fig.3b. Full sinusoidal drain current can be obtained by connecting two transistors in push-pull manner in which each transistor conducts for half the cycle of the input.The efficiency of ideal class-B PA is 78.5% which is higher than in class-A operation. The pair of transistors in push-pull connection should be perfectly matched to obtain an output current of smooth sinusoidal. However, cross-over distortion occurs in class-B when the input signal crosses zero where both the transistors are in their cut-off state. This cross-over distortion degrades the linearity. Class-AB: The poor linearity in class-B is improved by compromising the efficiency. In this mode, the transistor is biased between class-A and –B, and the efficiency lies between 50-78.5%. The cross-over distortion associated with class-B is reduced in class-AB and hence the linearity is improved. Class C: For PAs intended for narrow band application, it is possible to reduce the bias voltage below the cut-off voltage of the transistor and the transistor is said to be in class-C operation. In this class of operation, the transistor is active less than half the cycle of the input signal and thus the linearity is lost, but the efficiency theoretically increases to 78.5-100%. Class-C operation produces higher order harmonics because of nonlinearity of operation. Third harmonic can be controlled by bias control and the second harmonic is limited by employing the differential structure [2]. Class A input
Class B input
Class A output
Class B output 2
2
Vin
V in
Vq
V
1 Vth
ds
Vq,Vth
V ds
1
Id Id 0
90 180 270 360 450 540
0
0
90
180 270 360 450 540 = 360
90
180 270 360 450 540
(a)
0
90
180 270 360 450 540 =180
(b)
Class AB input
Class AB output
Class C input
Class C output 2
2
Vin Vq
Vds
1
Vth
Vin
Vth
Vds 1
Vq
Id
Id 0
90
180 270 360 450 540
0
90 180 270 360 450 540 =180-360
0
90
180 270 360 450 540
0
90
180 270 360 450 540
(c) (d) Fig.3. Waveforms ofideal class-A, -B, -AB, -C power amplifiers.
