Efficiently Amplified

Bumman, Kim,Junghwan, Moon,Ildu, Kim IEEE 2010 IEEE microwave magazine Vol.11 No.5

<P>Modern wireless communication systems have evolved to support increasing numbers of subscribers and provide higher data rate services within the limited frequency resources. As part of this evolution, many transmitted signals in the new standards, such as WCDMA, long term evolution (LTE), and worldwide interoperability for microwave access (WiMAX), now utilize a high peak-to-average power ratio (PAPR) caused by complex modulation schemes, generating the rapid change in the magnitude of signal as shown in Figure 1(a). In particular, as depicted in Figure 1(b), the PAPRs of the signals exceed 9 dB at 0.01% level of complementary cumulative distribution function. Use of high PAPR signals result in the power amplifier (PA) operating at a large enough back-off to satisfy the stringent linearity requirement. However, in this region, efficiency of the PA is very low. To bolster low efficiency in the back-off region, various efficiency boosting techniques have been considered over time. Recently, due to high-efficiency capabilities [1]–[13], supply voltage modulated PAs, such as envelope elimination and restoration (EER), hybrid-EER (H-EER), and envelope tracking (ET) technique, have received a lot of attention. Excellent experimental results have been reported using the various device technologies and modulated signals, as summarized in Table 1.</P>

Advanced Doherty Architecture

Kim, Bumman,Kim, Ildu,Moon, Junghwan IEEE 2010 IEEE microwave magazine Vol.11 No.5

<P>For modulated signals with a high peak-to-average power ratio (PAPR), the transmitter has to be operated with its average output power backed off for an acceptable linearity at the expense of low efficiency. To achieve high efficiency and high linearity at the same time, both an efficiency enhancement technique and a linearization technique should be utilized. A powerful and reliable linearization technique, digital predistortion (DPD), is currently the most favored method for the linearization of base-station amplifiers [1]–[3]. Possible efficiency enhancement techniques are the hybrid envelope elimination and restoration/envelope tracking technique (H-EER/ET) and the Do-herty technique.</P>

0.18 μm CMOS Power Amplifier with High Efficiency and Linearity

Jongchan Kang,Bumman Kim 대한전자공학회 2005 ITC-CSCC :International Technical Conference on Ci Vol.2005 No.4

Enhanced Hammerstein Behavioral Model for Broadband Wireless Transmitters

Junghwan Moon,Bumman Kim IEEE 2011 IEEE transactions on microwave theory and techniqu Vol.59 No.4

<P>A novel enhanced Hammerstein behavior model consisting of a weighted memoryless polynomial followed by a Volterra filter is proposed. The weighted polynomial is used for predicting the strong static nonlinear behaviors of the power amplifiers (PAs). Since the Volterra filter is employed only for the mild dynamic nonlinearities, the filter can be implemented with low nonlinear order. Thus, this proposed model is capable of predicting both the static and dynamic nonlinearities of RF PAs with the acceptable complexity. The modeling performance of the proposed model is assessed in terms of in-band and out-of-band errors, such as normalized mean square error and adjacent channel error power ratio, and it is compared with a conventional Hammerstein, an augmented Hammerstein, and a Volterra series with respect to computation complexities such as the number of floating point operations and coefficients. The excellent estimation capability of the enhanced Hammerstein model is validated by two kinds of PAs: Si lateral diffusion metal-oxide-semiconductor and GaN high electron-mobility transistor amplifiers. Furthermore, the proposed scheme is applied to the digital predistortion (DPD) to cancel the nonlinearities of the PAs. The modeling performances and DPD experimental results clearly demonstrate the superiority of the enhanced Hammerstein scheme: the computational complexity is comparable with the augmented Hammerstein behavioral model, but the modeling performance is similar to the Volterra filter, which is the most accurate model.</P>

A Parallel Power Amplifier With a Novel Mode Switching Control

Kichon Han,Bumman Kim IEEE 2008 IEEE microwave and wireless components letters Vol.18 No.3

<P>Two parallel operating power amplifiers (PAs) are controlled by a novel mode switch for high efficiencies at both the back-off power region and high power region. The mode switch is realized by the base-collector (BC) junction diode which reuses the dc current of the low power mode amplifier. A 836 MHz CDMA PA has been demonstrated using InGaP/GaAs heterojunction bipolar transistor with fully integrated matching component for small package and low cost. It shows a 13 mA idle current, 15.4% power added efficiency (PAE), ACPR1 at 16 dBm of the low power mode operation and a 40.5% PAE, ACPR1 at 28 dBm of the high power mode operation.</P>

Investigation of a Class-J Power Amplifier With a Nonlinear <tex> $C_{\rm out}$</tex> for Optimized Operation

Junghwan Moon,Jungjoon Kim,Bumman Kim IEEE 2010 IEEE transactions on microwave theory and techniqu Vol.58 No.11

<P>This paper presents the operation principle of Class-J power amplifiers (PAs) with linear and nonlinear output capacitors (C<SUB>out</SUB>s). The efficiency of a Class-J amplifier is enhanced by the nonlinear capacitance because of the harmonic generation from the nonlinear C<SUB>out</SUB>, especially the second-harmonic voltage component. This harmonic voltage allows the reduction of the phase difference between the fundamental voltage and current components from 45° to less than 45° while maintaining a half-sinusoidal shape. Therefore, a Class-J amplifier with the nonlinear C<SUB>out</SUB> can deliver larger output power and higher efficiency than with a linear C<SUB>out</SUB>. As a further optimized structure of the Class-J amplifier, the saturated PA, a recently-reported amplifier in our group, is presented. The phase difference of the proposed PA is zero. Like the Class-J amplifier, the PA uses a nonlinear C<SUB>out</SUB> to shape the voltage waveform with a purely resistive fundamental load impedance at the current source, which enhances the output power and efficiency. The PA is favorably compared to the Class-J amplifier in terms of the waveform, load impedance, output power, and efficiency. These operations are described using both the ideal and real models of the transistor in Agilent Advanced Design System. A highly efficient amplifier based on the saturated PA is designed by using a Cree GaN HEMT CGH40010 device at 2.14 GHz. It provides a power-added efficiency of 77.3% at a saturated power of 40.6 dBm (11.5 W).</P>

A Novel Digital Feedback Predistortion Technique with Memory Lookup Table

Junghwan Moon,Jangheon Kim,Bumman Kim 한국전자파학회JEES 2009 Journal of Electromagnetic Engineering and Science Vol.9 No.3

We have developed a novel digital feedback predistortion(DFBPD) linearization based on RF feedback PD for the wide bandwidth modulated signals. The wideband PD operation is carried out by combining the DFBPD and memory lookup table(LUT). To experimentally demonstrate the linearization performance of the proposed PD technique for wideband signal, a class-AB amplifier using an LDMOSFET MRF6S23140 with 140-W peak envelope power is employed at 2.345 ㎓. For a forward-link 2FA wideband code-division multiple-access signal with 10 ㎒ carrier spacing, the proposed DFBPD with memory LUT delivers the adjacent channel leakage ratio at an 10 ㎒ offset of ?56.8 ㏈c, while those of the amplifier with and without DFBPD are ?43.2 ㏈c and ?41.9 ㏈c, respectively, at an average output power of 40 ㏈m. The experimental result shows that the new DFBPD with memory LUT provides a good linearization performance for the signal with wide bandwidth.

Accurate dB-Linear Variable Gain Amplifier With Gain Error Compensation

Inyoung Choi,Heesong Seo,Bumman Kim IEEE 2013 IEEE journal of solid-state circuits Vol.48 No.2

<P>This paper describes use of a novel exponential approximation for designing dB-linear variable gain amplifiers (VGAs). The exponential function is accurately generated using a simple error-compensation technique. The dB-linear gain is controlled linearly by the gate voltage, resulting in a simple and robust VGA. The proposed dB-linear VGA fabricated in a 65-nm CMOS process achieves a total variable gain range of 76 dB and dB-linear range greater than 50 dB with ±0.5-dB gain error. Under a 1.2-V supply voltage, the current consumption of the VGA is 1.8 mA and that of the output buffer is 1.4 mA. The input-referred in-band noise density is 3.5 nV/√{Hz} and the in-band OIP3 is 11.5 dBm. Due to the very simple circuit topology, the total active area of the VGA and the output buffer is extremely small, 0.01 mm<SUP>2</SUP>.</P>

Impedance-Transforming Lumped-Baluns Consisting of Left-Handed Small Impedance Transformers

Hee-Ran Ahn,Bumman Kim 대한전자공학회 2007 ITC-CSCC :International Technical Conference on Ci Vol.2007 No.7

Based on the concept that a left-handed transmission line section has a negative electrical length, two Π -type lumped-element circuits are presented as equivalent circuits, and it is newly suggested that a lowpass filter type can also be its lumped-equivalent circuit under a certain condition. Applying the negative electrical length concept to Smith chart, LHCCTs (left-handed constant conductance impedance transformers) are derived and two are validated by measurements, adding 180o righthanded transmission lines to LHCCTs. The measured results show good agreement with predictions. As an application of the LHCCTs, impedance-transforming lumped baluns are suggested for arbitray phase differences between two outputs and simulated for a 40Ω-50Ω impedance transforming and 120˚ phase difference.

Equivalent Transmission-Line Sections for Very High Impedances and Their Application to Branch-Line Hybrids with Very Weak Coupling Power

Hee-Ran Ahn,Bumman Kim 한국전자파학회JEES 2009 Journal of Electromagnetic Engineering and Science Vol.9 No.2

As operating frequency is raised and as more integration with active and passive elements is required, it becomes difficult to fabricate more than 120 Ω characteristic impedance of a microstrip line. To solve this problem, an equivalent high impedance transmission-line section is suggested, which consists mainly of a pair of coupled-line sections with two shorts. However, it becomes a transmission-line section only when its electrical length is fixed and its coupling power is more than half. To have transmission-line characteristics(perfect matching), independently of coupling power and electrical length, two identical open stubs are added and conventional design equations of evenand odd-mode impedances are modified, based on the fact that the modified design equations have the linear combinations of conventional ones. The high impedance transmission-line section is a passive component and therefore should be perfectly matched, at least at a design center frequency. For this, two different solutions are derived for the added open stub and two types of high impedance transmission-line sections with 160 Ω characteristic impedance are simulated as the electrical lengths of the coupled-line sections are varied. The simulation results show that the determination of the available bandwidth location depends on which solution is chosen. As an application, branch-line hybrids with very weak coupling power are investigated, depending on where an isolated port is located, and two types of branch-line hybrids are derived for each case. To verify the derived branch-line hybrids, a microstrip branch-line hybrid with ?15 ㏈ coupling power, composed of two 90° and two 270° transmission-line sections, is fabricated on a substrate of εr= 3.4 and h=0.76 ㎜ and measured. In this case, 276.7 Ω characteristic impedance is fabricated using the suggested high impedance transmission-line sections. The measured coupling power is ?14.5 ㏈, isolation and matching is almost perfect at a design center frequency of 2 ㎓, showing good agreement with the prediction.