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WE2B-1 A Fully-Integrated Highly-Efficient RF Class E SiGe Power



Abstract This paper reports on the results of a highly
efficient monolithically fully-integrated SiGe Class E power
amplifier using envelope tracking techniques for EDGE
applications. The Envelope-tracking (ET) system includes a
discrete linear op-amp and a switching power converter. The RF
Class E amplifier was fabricated in a 0.18
�m BiCMOS SiGe
technology. The RF Class E power amplifier achieved a collector
efficiency (CE) of 62.7% and the overall power added efficiency
(PAE) of the ET system is 44.4% at an output power of 20.4dBm
for an 881MHz EDGE modulated signal. A discrete envelope
switching amplifier achieved 82.8% efficiency while driving the
Class E PA voltage supply. The linearized SiGe PA passed the
stringent EDGE transmit spectrum mask

Index Terms power amplifier, switching amplifiers,
heterojunction bipolar transistors
I. I
NTRODUCTION

The SiGe heterojunction bipolar transistor (HBT)
technology has been instrumental in increasing performance
and reducing cost for wireless consumer products and
handsets. A key benefit is the co-availability of high ft and
high breakdown device options in most commercial SiGe
process technologies. This makes the SiGe BiCMOS
technology very attractive for integrating the power amplifier
(PA) with other transmit and receive microcircuitry, especially
for cellular handsets and WLAN products. Achieving this
integration allows decreased bill of materials, reduced
packaging requirements and form factors, and potentially
decreased unit cost.

The GSM 900 wireless handset specification includes the
constant amplitude envelope GMSK waveform, and the newer
non-constant envelope EDGE (Enhanced Data Rate for GSM
Evolution) 8-PSK waveform. Both modulation formats are
modulated at 270k symbols/s and occupy 200kHz transmit
channels from 880MHz to 910MHz. The EDGE signal
provides 3 bits of information for every symbol (i.e. better
spectral efficiency) for a maximum bit throughput of 810kB/s.
The disadvantage of the EDGE signal is that it has a moderate
peak-to-average power ratio (PAR) of 3.3dB. The spectral
mask requirements are -54dBc at 400kHz and -60dBc at
600kHz, and worst case rms EVM requirement is 10%. The
EDGE linearity requirement is achieved using traditional
current mode PA typologies (Class A/AB) by operating the
amplifier several dB below the P
1dB
compression point. This
causes significant degradation in RF PA power efficiency.
Switched mode PA typologies, particularly the Class E PA,
can afford dramatically increased power added efficiency
(PAE) over current mode PA typologies. The distinct feature
of switched mode power amplifiers, including the Class E PA,
is that the active power devices are used as switches as
opposed to transconductors. The appeal of the Class E PA is
that it can be monolithically integrated in silicon technologies
and have good PAE [1]. Four requirements characterize ideal
Class E PA operation: (a) the switch voltage is zero when the
switch is ON; (b) the voltage slope is zero when the switch if
first turned ON;(c) all the current during the OFF state is
integrated onto the shunt capacitor; and (d) the output tank
delivers in phase current and voltage at the fundamental
frequency to the load. This guarantees that the voltage and
current across the switch are never simultaneously non-zero,
and charge is not shunted by the switch during each RF cycle.


Therefore, power is dissipated across the switch is
minimized and the power at the fundamental is transferred to
the load. These requirements can be optimized with proper
sizing of the switch device, shunt capacitance, choke inductor,
and the high Q-factor output resonant tank [1,2,3].


WE2B-1
A Fully-Integrated Highly-Efficient RF Class E SiGe Power
Amplifier With an Envelope-Tracking Technique for EDGE
Applications
Jeremy Popp
#
, Donald Y. C. Lie, Feipeng Wang, Donald Kimball, Lawrence Larson
#
Space and Naval Warfare System Center, San Diego, CA , Dept. of Electrical and Computer
Engineering, University of California San Diego, La Jolla, CA
Q
Drive
V
DD
Lchoke
C1
Co
Lo
R
L

Fig. 1 Class E Amplifier with Collector I/V Waveforms

Circuit non-idealities that limit the practical RF Class E PA
power efficiency to <70-80% includes the finite switching
speed, switch loss, passive component loss, device breakdown,
and voltage rail limitations. The optimum Class E PA switch
device has attributes of a fast digital device and high
breakdown power device. The higher device breakdown
becomes important because the Class E PA switch voltages
can peak as much as 3.57 times the voltage supply rail even
for ideal matching at the output load [2,3].
The saturated nature of the switched mode PAs causes
signal distortion and loss of power efficiency for non-constant
envelope driving signals. The classic Class E PA has a linear
response of output amplitude peak to the supply voltage [3].
This feature makes the usage of Envelope Elimination
Restoration (EER) and/or Envelope tracking (ET) techniques
with Class E PAs very applicable. The EER system provides
constant amplitude phase-only signals to the Class E PA input,
and the amplitude envelope modulation to the Class E PA
voltage supply. The ET system adds the amplitude envelope
modulation to the Class E PA input as well. These techniques
allow the PA to operate in a high efficiency saturated (or
nearly saturated) mode, and the amplitude envelope of
modulation signals, such as EDGE, can be implemented.

II. RF C
LASS
E PA D
ESIGN AND
R
ESULTS

This work focuses on the development of RF Class E PAs
because of their ability to be monolithically integrated in
silicon based technologies, while still achieving good
performance. The Class E design equations in open literature
have been found to provide sub-optimal results at RF
frequencies [1,4]. Extensive large signal simulations were run
in Cadence Spectre/SpectreRF to optimize components values
across temperature, process, and voltage supply.

The quality factor of the choke inductor plays a crucial
role in overall power efficiency, due to the fact that all the
average current (RF and DC) must flow through the choke
inductor. The low achievable quality factors at 900MHz
available in the SiGe technology (Q~10-12) required the
choke inductor to be implemented off chip. All the rest of the
components are fully-integrated on-chip. Fig. 2 shows the
schematics of this monolithic PA. The fabricated SiGe
amplifier die shown in Fig. 3 is 1.1mm x 1.7mm.

The SiGe PA dies were bonded onto RF PC boards for testing.
Single tone testing was completed on the RF Class E PA. Fig.
3 shows a micrograph of the fabricated SiGe Class E PA.





Fig. 4 shows that the best Class E PA measured
performance was 72.5% CE, 65.6% PAE, at 22.5dBm output
power with single-tone input. Increases in the input power
show the saturating nature of the Class E above Pin~5dBm.

Figure 5 shows a linear in dB change in output power of 2.6dB
per 1 volt supply change.

III. E
NVELOPE
T
RACKING AND
GSM EDGE
The wide band envelope tracking (WBET) system block
diagram used in this work is shown in Fig. 6.
SiGe Class E Measurements
(Vcc=3.3v, Vbb=0.65v, Fin=881MHz)
-50
-40
-30
-20
-10
0
10
20
30
-10
-5
0
5
10
Pin (dBm)
Pou
t
(
d
B
m
)
0
20
40
60
80
100
CE
/
P
AE

%
Pout
CE%
PAE%

Fig. 4 Measured Class E Output Power/PAE vs. input power

SiGe Class E Measurements
(Vbb=0.65v, Fin=881MHz, Pin=13dBm)
20
21
22
23
24
2.5
2.7
2.9
3.1
3.3
Vsupply (V)
Po
u
t
(d
B
m
)
0
20
40
60
80
100
CE
/P
A
E

%
Pout
CE%
PAE%
Fig. 5 Measured Class E Output Power/PAE vs. Vsupply

HBT1
Vbb
200pF
5.39nH
RFin
V
DD
33nH
3.39nH
12pF
RFout

Fig. 2 SiGe Class E Amplifier Design Schematic


Fig. 3 Fabricated SiGe Class E Amplifier Micrograph


The time delay mismatch between the RF input signal and
voltage supply amplitude modulation in dynamic power
supply schemes is a significant concern for EVM distortion.
As concluded in [5], the ET technique applied to hard limiting
PAs provides less sensitivity to delay mismatch than the EER
technique. Envelope-Tracking also relaxes the bandwidth
requirement for the envelope amplifier and the RF path vs.
EER. In addition, ET system provides higher gain at low
output power than EER since the device is nearly saturated at
low output power (while saturated at high output power). No
amplitude or phase predistortion was applied to the ET Class
E operated with EDGE. This is due to the smaller 3.3dB PAR
of EDGE and the very linear V
supply
amplitude response of the
Class E wh