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Full text of "DTIC AD1041342: Linear Distributed GaN MMIC Power Amplifier with Improved Power-added Efficiency"

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Linear Distributed GaN MMIC Power Amplifier with Improved Power- 

added Efficiency 


Jeong-sun Moon, Jongchan Kang, Dave Brown, Robert Grabar, Danny Wong, Helen Fung, 
Peter Chen, Dustin Le, Hwa Y. Tai, and Chuck McGuire 

1) HRL Laboratories, 3011 Malibu Canyon Road, Malibu, CA 90265 


Abstract: We report on a multi-octave (100 MHz - 8 
GHz), linear nonuniform distributed amplifier (NDPA) in 
a MMIC architecture using scaled 120-nm short-gate- 
length GaN HEMTs. The linear NDPAs were built with six 
sections in a nonuniform distributed amplifier approach, 
where each cell consists of main and g m 3 cells. The small 
signal gain was >10 dB over the band, with saturated CW 
output power of -35 dBm at Vdd = 17 V. The PAE 
improved by 7% - 10% within the band compared to the 
previous NDPA with 150-nm gate-length GaN FETs. 
Based on two-tone testing, the linear NDPA showed 
improved OIP3 of -50 dBm, compared to OIP3 of 42 dBm 
for the NDPA without linearization. Under QPSK LTE 
waveform, the ACPR1 improved by -10 dBc at average 
output power of 23 dBm, without digital pre-distortion. 

Keywords: GaN, linear amplifiers, wideband amplifier, 
OIP3, LTE 

Introduction 

RF communications with spectral efficiency utilizes 
complex modulation schemes that require amplifier 
linearity [1]. GaN HEMTs have a high breakdown voltage 
that offers high output impedance and power density per 
input capacitance over GaAs PHEMTs. Over the last 
decade, various wideband GaN HEMT MMIC power 
amplifiers [2-7] and low-noise amplifiers [8-9] have 
demonstrated high dynamic range with an excellent output 
third-order intercept point (OIP3) of 40 to 52 dBm. 
However, for the high OIP3, these wideband GaN 
amplifiers exhibit an OIP3/Pdc ratio <5, and consume 
large amounts of DC power. 

Especially in linear transmitters, several linearization 
techniques, such as feed-forward and pre-distortion, have 
been used to remove intermodulation products. For 
wideband and multi-octave linearization, analog pre¬ 
distortion has been introduced [10]. In general, these 
approaches increase the complexity of the RF system. 

Current linearity/power-added-efficiency (PAE)/output 
power trade-offs impose a significant increase in an RF 
system’s size, weight, and power (SWaP). In 2015, Moon 
et al., reported the first multi-octave (100 MHz - 8 GHz) 
GaN MMIC nonuniform distributed amplifier (NDPA) 
with built-in linearization and a g m 3 cancellation method in 
class A and class C architectures [11]. Based on two-tone 
testing, the linear NDPA showed improved OIP3 of -50 

@2017 HRL Laboratories, LLC. All Rights Reserved. 


dBm, compared to OIP3 of 42 dBm for a NDPA without 
linearization. The resulting OIP3/Pdc was 16:1, which is 
the highest reported amongst GaN-based distributed 
amplifiers. The amplifier gain, however, was rather low. 
With P sat of -35 dBm, PAE of the linear NDPA was 40% 
- 12% over 100 MHz - 8 GHz bands. Challenges to 
improving PAE versus linearity in wideband amplifiers 
remain. 

In this paper, we report on the measured CW performance 
of a multi-octave (100 MHz - 8 GHz) GaN MMIC NDPA 
fabricated with improved GaN HEMTs with a shorter gate 
length of 120 nm and peak RF gm of 480 mS/mm. The 
linear DP As demonstrated improved gain of 10 - 12 dB. 
With P sat of -35 dBm, the PAE improved by 7% - 10% 
over 100 MHz - 8 GHz bands. The adjacent channel 
power ratio (ACPR1) under quadrature phase shift key 
(QPSK) long-term evolution (LTE) waveforms also 
improved by -10 dBc at an average linear power of 23 
dBm. 


FP gate (Lg = 120 nm) 



Source AlGaN/GaN/AlGaN Drain 


Ohmic 

regiwfh 

(b) 


GaN HEMT 

Prior Art 

This work 

Peak Ft (GHz) 

52 

60 

Peak RF gm (mS/mm) 

380 

480 

Cgs (pF/mm) 

0.96 

1.13 

Cgd (pF/mm) 

0.2 

0.24 

Pinch-off Voltage (V) 

-3.2 

-2.3 


* Extracted from S-parameters at Vdd =10V, and peak gm bias 


Figure 1. (a) A schematic drawing of -120 nm gate 
length field-plate (FP) GaN HEMT is shown, (b) 
Modeled device parameters based on S-parameter 
measurements are summarized. 


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120nm Gate length field-plated GaN HFET 

Figure 1(a) shows a schematic of a scaled 120-nm gate 
length field-plate (FP) GaN HEMT, where the FP GaN 
HFET was processed with ohmic regrowth instead of n+ 
GaN source contact ledge demonstrated in previous work 
[12]. The ohmic source-drain spacing was 3 pm. The on- 
state resistance was 1.6 Q-mm. The maximum source- 
drain current (I max ) was 1.0 A/mm at Vds = 10 V. The 
table in Figure 1 summarizes the GaN HEMT 
performance compared to prior 140-nm gate length GaN 
HEMTs used in previous linear distributed amplifiers. The 
MMICs shown in Figures 2(a) and 3(a) were fabricated in 
a microstrip layout with 2-mm-thick SiC substrate and via 
source contacts, SiN x and Hf0 2 MIM capacitors. TaN 
resistors were used in MMIC fabrication. 

Measured GaN MMIC Performance 

To maximize PAE over the frequency band, the GaN 
NDPAs were built in six sections with different transistor 
sizes, ranging from 6 x 85 pm to 2 x 65 pm in a 
nonuniform distributed amplifier approach. 

Figure 2(a) shows a photograph of the fabricated GaN 
linear NDPA, consisting of main and auxiliary cells in 
each section, where the auxiliary cells were biased in 
class-C to cancel in-band intermodulation products such as 
IM3. Figure 2(b) shows measured on-wafer S-parameters 
at Vds = 20 V with a small signal gain of >10 dB over 0.1 
- 8 GHz, a more than 6 - 9 dB improvement over the prior 

( a \ Linear wideband DPA 



(b) Linear Wideband DPA: Vds = 20 V, Ids= 220 mA 



Frequency (GHz) 

Figure 2. (a) Photograph of the fabricated GaN 
NDPA, (b) Measured on-wafer S-parameters at Vds 
= 20 V and Ids = 220 mA 



Frequency (GHz) 

Figure 3. Measured on-wafer large-signal CW 
performance of GaN linear NDPA at Vds = 17 V at 
(a) 1 GHz and (b) as a function of frequencies, in 
comparison to the prior linear NDPA large-signal 
oerformance 

linear NDPA [10]. 

Figure 3 shows the measured on-wafer, large-signal CW 
performance at Vds = 17 V at 1 GHz compared to prior 
linear NDPA large-signal performance. Peak PAE 
improved to 38% with drain efficiency (DE) of 45%, 
compared to a prior PAE of 29% and DE of 35%. Output 
power was similar, at ~35 dBm, and associated power gain 
was 7.8 dB. 

Linearity Improvement 

We measured small-signal, two-tone spectra of the GaN 
NDPA and linear NDPA at Vd = 17 V versus frequency 
range of 100 MHz to 8 GHz. As shown in Figure 4, the 
linear NDPA showed OIP3 improved to 45 - 50 dBm with 
an average of 47 dBm within 100 MHz to 6 GHz, 
compared to OIP3 of 42 dBm for the conventional NDPA 
without linearization. The resulting maximum OIP3/Pdc 
was 14:1. Green et al., reported GaN NDPAs with OIP3 of 
43 dBm [2]. Kobayashi reported excellent OIP3 of 51 
dBm over a 3-GHz bandwidth, where a cascode feedback 
design was used with high-voltage operation of 40 V [9]. 
The OIP3/Pdc was 5.2:1, which consumed about 30 W. 


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Figure 4. Measured 2-tone 0IP3 performance of 
linear GaN DPA and a conventional GaN DPA 
over frequency range of 100 MHz to 8 GHz 



Figure 5. 5MHz QPSK LTE spectra of NDPA and 
linear NDPA at 17 V with average output power of 
23 dBm 


Figure 5 shows the 5 MHz QPSK LTE spectra of the 
conventional NDPA and linear NDPA at Vdd = 17 V. The 
spectral regrowh was evaluated with ACPR1. At an 
average output power of 23 dBm, the linear NDPA offers 
-10 dB improvement in ACPR1 at 5 MHz offset, 
compared to the conventional NDPA. 

Conclusion 

Utilizing a 120-nm gate length field-plate GaN HEMT 
MMICs process, we demonstrated 0.1-8 GHz linear 
distributed amplifiers at the MMIC level with improved 
PAE. Two-tone testing showed excellent OIP3/Pdc ratio 
of -14 among GaN distributed amplifiers. For the first 


time, a ACPR1 of -40 dBc was demonstrated at an average 
output power of 28 dBm over a wide frequency range. 

Acknowledgement 

This material is based upon work supported by the 
Office of Naval Research (ONR) under contract number 
N00014-14-C-0140, which was monitored by Dr. Paul 
Maki. The views expressed are those of the author and do 
not reflect the official policy or position of the Office of 
Naval Research (ONR) or the U.S. Government. 

References 

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[12] J. S. Moon et al., “>70% power-added-efficiency dual-gate, 
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