Proceedings of Asia-Pacific Microwave Conference 2007
A High-Efficiency SiC MESFET Power Amplifier Based on Class-F Configuration 1
Yong-Sub Lee1, Mun-Woo Lee1, Sung-Woo Jung2, and Yoon-Ha Jeong1,2 Department of Electronic and Electrical Engineering, Pohang University of Science and Technology 2 National Center for Nanomaterials Technology (NCNT) San 31, Hyoja-Dong, Nam-Gu, Pohang, Gyungbuk 790-784, Republic of Korea E-mail:
[email protected]
Abstract-This paper reports a high efficiency power amplifier (PA), which is based on the class-F configuration using the composite right/left-handed transmission lines as the harmonic trap. Also, the compensation elements with the series capacitor and shunt inductor are used to compensate for the internal parasitic components of the packaged active device in the class-F PA design. To verify this method, a class-F PA is designed using a 10-W PEP SiC MESFET and tested for a continuous wave at WCDMA band of 2.14 GHz. From the measured results, the power-added efficiency (PAE) of 70.1% with a gain of 10.4 dB is achieved at an output power of 40.4 dBm.
id
DCB vd
Fund Even Odd
Ropt Short Open Current
Harmonic Control Network
Voltage
Ropt T/2
T
t
Figure 1. Ideal circuit and drain current and voltage waveforms of the class-F PA.
INTRODUCTION
The switching-mode power amplifiers (PAs) have recently received attention to improve efficiency because the high efficiency PAs decrease power consumption and heat sink requirement in wireless transmitters. Among various switching-mode PAs, the class-F PA has been widely investigated and developed. For class-F operation, the PA adopts the harmonic control method at output matching network in the saturation condition. The efficiency increases as the number of controlled harmonics increases. In practice, only second and third harmonics are controlled due to design complexity and circuit size [1]-[5]. For the active device in the class-F PA, commercial packaged transistors show the internal parasitic capacitors, inductors, and resistors caused by the package, bond wires, and interconnection. Because parasitic components prevent the class-F PA from accurately controlling the harmonic impedances, their effects should be eliminated or compensated in the PA design procedure [4]. In this paper, we report a high efficiency RF PA, which is based on the class-F configuration. The composite right/lefthanded transmission lines (CRLH-TLs) are used as a harmonic trap because they can provide the suitable harmonic impedances for the class-F operation by inserting the additional λ/4 microstrip line before the CRLH-TLs. Also, the compensation elements with the series capacitor and shunt inductor are used to compensate for the internal parasitic components of the packaged active device. To verify these methods, the class-F PA is designed with a SiC MESFET device and tested for a 2.14-GHz single tone. The measured results show the superior performance.
1-4244-0749-4/07/$20.00 @2007 IEEE.
RFC
I.
id, vd
147
II. CLASS-F PA DESIGN USING CRLH-TL A. Ideal Class-F PA Fig. 1 shows the output equivalent circuit of the ideal classF PA. The basic operation principle of the class-F PA has been well described in previous studies [1]-[3]. Ideal class-F PA is expected to appear a half-sine wave and a rectangular wave for drain current and voltage, respectively, in the time domain plots. To achieve ideal class-F drain current and voltage waveforms, it is necessary for the harmonic control network to provide short and open circuit impedances for even and odd harmonics, respectively. Theoretically, the drain efficiency of 100% can be achieved. In practice, although only second and third harmonics are controlled due to circuit size and complexity of the harmonic control network, it is possible to achieve efficiency of over 80% [1],[2]. B. CRLH-TL In [6], the CRLH-TL is the combination of an LH-TL and a RH-TL. When the CRLH-TL is modeled as a periodic structure with infinitesimally small-dimension unit cells, each unit cell consists of a right-handed series inductance (LR) and shunt capacitor (CR) and a left-handed series capacitor (CL) and shunt inductor (LL). For the balanced case (LRCL=LLCR), the phase response of the CRLH-TL can be expressed as
φCRLH = φ RH + φLH
(1)
φRH ≈ − Nω LR C R
(2)
φLH ≈
N ω LL C L
(3)
Zout1
Zout2
Cpgd
λ /4 Lg
50Ω
l1
LH-TL RH-TL
2CL
l2
Cgs
2CL LL
Cgd Rgd
Rg
intrinsic
Gate
RH-TL
Cpg
Vgs Rgs
gmV gs
rds
Rd
Ld
Drain
Cds Cpd
Ls
CRLH-TL
Rs
Figure 2. Schematic of output harmonic control circuit using the CRLH-TL.
Source
Table I
extrinsic
Figure 4. Practical equivalent circuit model of the packaged MESFET device.
DESIGN PARAMETER OF THE CRLH-TL l1(o) l2 (o) CL (pF) LL (nH) 80.3 84.7 1 2.4
S(2,1)
3fc 2fc
fc
Zout2
Zout1
50*S(1,2)
2fc
fc
-8
3fc
-6
-4
-2
0
2
4
6
8
S(2,2)
S(1,1)
freq (100.0MHz to 5.000GHz) freq (2.140GHz to 8.560GHz)
freq (2.140GHz to 8.560GHz)
Figure 5. Simulated and measured S-parameters at VDD=48V and I ds=250mA from 100 MHz to 5 GHz.
(a) (b) Figure 3. Simulated results of harmonic impedances for the CRLH-TL (a) without and (b) with the λ/4 TL.
where N is the number of unit cells. The equivalent lumped element models of the LH-TL and RH-TL exhibit positive and negative phase responses, respectively, as shown in (2) and (3). It is worth noting that the phase response of the CRLH-TL can be manipulated to yield electrical length of ±90˚ at two arbitrary frequencies. Therefore, these properties make the CRLH-TL use as a harmonic tuner for the class-F PA [5]. The characteristic impedance of the CRLH-TL can be expressed as follows.
Z oRH =
LR LL = Z oLH = = Z oCRLH CR CL
freq (100.0MHz to 5.000GHz)
(4)
Fig. 2 shows the schematic of the CRLH-TL using lumped element model. The LH-TL section consists of a T-type unit cells with series capacitors (CL) and shunt inductor (LL). The values of CL and LL can be determined by (4) with the characteristic impedance of 50Ω. The RH-TL sections are simply two transmission lines on each side of the LH-TL section, which can be realized by using the microstrip line length corresponding to the RH-TL phase in (2). The harmonic impedances of the CRLH-TL were examined using Agilent ADS simulator. Table I shows the parameter values of the CRLH-TL. From the simulated results, the CRLH-TL produces a short for all harmonics, as shown in Fig. 3(a). When the additional λ/4 microstrip line is inserted in front of the harmonic control network, the short impedance should be transformed to a short and an open for even and odd
harmonics, respectively, as shown in Fig. 3(b). These attributes are very suitable to the harmonic impedance conditions for the class-F operation. C. Internal Parasitic Components of the packaged active device Fig. 4 shows the practical equivalent circuit model of the packaged MESFET, which has not only intrinsic but also extrinsic parasitic components by the package, bond wires, and interconnection. For the ideal class-F operation, the harmonic control should be performed after the current source in order to reduce the power consumption in the transistor and achieve high efficiency by minimizing the overlap between the current and voltage waveforms in the class-F PA design. So, the internal parasitic components should be compensated or contained in the class-F PA design procedure. Fig. 5 shows the good agreement between the simulated and measured S-parameters. From the results, the shunt capacitance (Cd), resistance (Rd), and inductance (Ld) seen at the drain are about 1.92pF, 3.4Ω, and 0.46nH, respectively. Here, the capacitance (Cd) is the sum of all capacitances seen at the drain and can be expressed as follows.
Cd ≅C ds + C pd + C gd + C pgd
(5)
III. IMPLEMENTATION AND EXPERIMENTAL RESULTS A main PA was designed and implemented with Cree CRF24010 SiC MESFET having 10 W PEP at 2.14 GHz. The RF35 (εr=3.5, H=0.5 mm) circuit board has been used as the substrate and provides a width of 1.1 mm for all TLs with 50Ω characteristic impedance.
148
VGS
λ /4
VDD
λ/4 Ccom
IMN
λ /4
OMN
Lcom
l1
Compensation elements
2CL 2CL
Figure 8. Photograph of the implemented class-F PA using the CRLH-TL with compensation elements.
LL
l2
Figure 6. Full schematic of the class-F PA using the CRLH-TL with compensation elements. 100
Without Compensation With Compensation
40
80
35 30
Pout
60
25
Drain Efficiency
20
40
15 10
Drain Efficiency [%]
Output Power, Pout [dBm] / Gain [dB]
45
20
5
(a) output power
Gain
0
0
10
15
20
25
30
35
Input Power, Pin [dBm]
Figure 7. Simulated output power, gain, and drain efficiency of the class-F PA using the CRLH-TL without or with compensation elements at class-C bias scheme as a function of input power.
Fig. 6 shows the full schematic of the class-F PA using the CRLH-TL with compensation elements. The series capacitor (Ccom) is used to compensate for the package-induced resistance (Rd) and inductance (Ld). Here, the shunt inductor (Lcom) is used to compensate for the parasitic capacitance (Cd). At first, we simulated the class-F PA without or with compensation elements. For each case, all control parameters are optimized separately. The results are shown in Fig. 7, which are the output power, gain, and drain efficiency according to input power. By compensating for the internal parasitic components of the packaged active device, the efficiency improvement over about 20% is achieved in the saturation region. The photograph of the implemented class-F PA is shown in Fig. 8. In the CRLH-TL, two series capacitors (CL) and a shunt inductor (LL) are a capacitance of 1pF and an inductance of 2.7nH, respectively, which are implemented by SMT chip components. The lengths of upper (l1) and lower (l2) microstrip lines in the RH-TL are λ/4.85 (17.5mm) and λ/4.53 (18.7mm), respectively. The series capacitor (Ccom) and shunt inductor (Lcom) in the compensation circuit are a capacitance of 1.8pF and an inductance of 1.8nH, respectively. The series TLs and shunt capacitors are used in the input and output matching networks (IMN and OMN).
(b) drain efficiency Figure 9. Measured output power and drain efficiency characteristics of the class-F PA using the CRLH-TL with compensation elements according to gate and drain bias voltages.
The measured output power and drain efficiency characteristics according to gate and drain bias voltages using a 2.14-GHz single tone are shown in Fig. 9. The class-F PA using the CRLH-TL shows the flat response of output power and drain efficiency according to drain bias voltage. It is especially critical that high efficiency characteristics are maintained for a wide range of drain bias, because a flat response of the drain efficiency according to drain bias is required in the case of the drain bias modulation applications, such as envelope elimination and restoration (EER). At the VGS (class-C bias voltage) lower than a pinch-off voltage of 12.6V, high output power and drain efficiency are achieved.
149
consist of the series capacitor and shunt inductor, are inserted in front of the CRLH-TLs in order to compensate for the internal parasitic components of the packaged active device. For experimental validations, the class-F PA is implemented with a SiC MESFET device and tested using a continuous wave at WCDMA band of 2.14 GHz. The measured results show that the drain efficiency of 70.1% with a gain of 10.4 dB is achieved at an output power of 40.4 dBm. From the measured results, this work confirms that an efficiency enhancement of the RF PA with the class-F configuration can be obtained by compensating for internal parasitic components in the packaged active device.
100
Pout
40
90
Drain Efficiency
35
80 70
30
PAE
25
60
VDD=35V VGS=-14V Vpincn-off=-12.6V
20 15
50 40 30
10
Effieicny [%]
Output Power, Pout [dBm] / Gain [dBm]
45
20
Gain 5
10 0
0 20
22
24
26
28
30
32
34
Input Power, Pin [dBm]
ACKNOWLEDGMENT
Figure 10. Measured output power, gain, and drain efficiency as a function of input power.
This work was supported by the BK21 program of Korea.
Fig. 10 shows measured output power, gain, and drain efficiency characteristics according to input power at VDD=35V and VGS=-14V (class-C bias point). The maximum PAE of 70.1% is obtained at a Pout of 40.4 dBm with a gain of 10.4 dB, which is the drain efficiency of 77.1%. The even harmonic levels are suppressed below -40 dBc, and especially the significant suppression of below -64 dBc is obtained for the odd harmonics. From the excellent performance in the wide saturation region (input power range of 26 dBm to 34 dBm), it is worth noting that the high output power and efficiency of the PA with the class-F harmonic tuner and class-C bias scheme can be suitable for linear amplification using nonlinear components (LINC).
REFERENCES [1] [2] [3] [4]
[5]
[6]
IV. CONCLUSIONS In this paper, we have reported a high efficiency RF PA, which adopts the class-F configuration using composite right/left-handed transmission lines (CRLH-TLs) as the harmonic trap. Also, the compensation elements, which
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F. H. Raab, “Class-F Amplifiers with Maximally Flat Waveforms,” IEEE Trans. Microwave Theory & Tech., vol. 45, no. 11, pp. 2007-2012, November 1997. F. H. Raab, “Maximum Efficiency and Output of Class-F Power Amplifier,” IEEE Trans. Microwave Theory & Tech., vol. 49, no. 6, pp. 1162-1166, June 2001. M. Albulet, RF Power Amplifiers, Atlanta, GA: Noble Publishing Co, 2001. H. C. Park, G. Ahn, S. C. Jung, C. S. Park, W. S. Nah, B. Kim, and Y. Yang, “High-Efficiency Class-F Amplifier Design In the Presence of Internal Parasitic Components of Transistors,” Proc. of 36th European Microwave Conf., pp. 184-187, September 2006. A. Dupuy, K. M. K. H. Leong, and T. Itoh, “Class-F power amplifier using a multi-frequency composite right/left-handed transmission line harmonic tuner,” IEEE MTT-S Int Microwave Symp Dig, pp. 2023-2026, June 2005. Lin, M. Devincentis, C. Caloz, and T. Itoh, “Arbitray dual-band components using composite right/left-handed transmission lines,” IEEE Trans. Microwave Theory & Tech., vol. 52, no. 4, pp. 1142-1149, April 2004.
