A STUDY ON DRAIN EFFICIENCY OF EDSM/EPWM
TRANSMITTER USING CLASS-E AMPLIFIER
Makoto Taromaru
Faculty of Engineering, Fukuoka University, Nanakuma 8, Johnan-ku, Fukuoka, Japan
taromaru@fukuoka-u.ac.jp
Keywords:
drain efficiency, power amplifiers, amplitude modulation, EER, pulse width modulation, delta-sigma modula-
tion
Abstract:
Drain efficiency of EPWM (envelope pulse width modulation) transmitter composed of a class-E amplifier
is evaluated by circuit simulation, and the pulse modulation method to the constant envelope RF signal by
PWM pulses is studied. The class-E amplifier is designed as normal for continuous wave (CW) and constant
envelope signals. Each transistor is modeled with an ideal switch and an on-resistor, where the drain-source
capacitance is modeled as a linear one together with the shunt capacitor. Simulation results show that the
conventional PWM switching method gating the input RF signal produces unnecessary transient responses
making the drain voltage rise and the output signal distorted; the efficiency is degraded. Another method that
gates the DC supply voltage by the PWM pulses works as expected and intended, and the result shows the
drain efficiency is as high as that of a class-E amplifier driven with a CW signal.
1 INTRODUCTION
Linear modulation schemes, band-limited PSKs
and multilevel QAM combined with OFDM (Or-
thogonal Frequency Division Multiplex) are widely
used for recent m obile radio communications such as
for wireless L ANs, and terrestrial digital television
broadcasting due to high spectrum efficiency and tol-
erance to multipath distortion, The modulated radio
signal, however, has dynamically varying envelope
and high peak power to average power ratio (PAPR).
Although power amplifiers of class-B/C/D/E/F can
operate with high efficiency especially in saturated
region, these nonlinear amplifiers cannot be used for
those linear modulation signal due to degradation of
modulation accuracy and spectral regrowth at the ad-
jacent channels.
Several techniques using nonlinear amplifiers for
linear modulations have been proposed, Doherty (Do-
herty, 1936), Cartesian loop, feed-forward, and so on
(Raab et al., 2002). Among these techniques, Kahn
Envelope Elimination and Restoration (EER) (Kahn,
1952) is recently being studied well and its applicabil-
ity is shown for microwave transmitters (Raab et al.,
2002) and the IEEE 802.11a/g wireless LAN systems
using QAM with OFDM (Diet et al., 2004). Fur-
thermore for b etter linearity and efficiency, direct RF
modulation techniques by the PWM pulses digitized
from the envelope signal have been studied for these
ten years (Adachi et al., 2002; Wang, 2003; Taro-
maru et al., 2007; Yokozawa and Yamao, 2011). This
transmitter architecture is called envelope delta-sigma
modulation (EDSM)(Dupuy and Wang, 2004), or en-
velope pulse width modulation (EPWM)(Yokozawa
and Yamao, 2011; Takahashi and Yamao, 2010).
With the EDSM/EPWM architecture, the direct pulse
modulation causes considerable switching spurious
or quantization noise at the output of the power am-
plifier, which should be eliminated with a band-pass
filter (BPF). Some experiments or circuit simulation
studies have been made for EDSM/EPWM transmit-
ter using class-E (Wang, 2003; Dupuy and Wang,
2004) and class-F (Takahashi and Yamao, 2010; Choi
et al., 2007) amplifiers. However, the burst RF mode
operation and the transient response due to the pulse
modulation are not well studied although the transient
response directly affects the drain efficiency.
In this paper, drain efficiency of EPWM transmit-
112
Taromaru M.
A STUDY ON DRAIN EFFICIENCY OF EDSM/EPWM TRANSMITTER USING CLASS-E AMPLIFIER.
DOI: 10.5220/0005414801120117
In Proceedings of the First International Conference on Telecommunications and Remote Sensing (ICTRS 2012), pages 112-117
ISBN: 978-989-8565-28-0
Copyright
c
2012 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
Modulation
Processing
22
QI
'6 modulation
Limiter
Class-B/C/D/E/F saturation
or switchin
g
am
p
lifier
PWM
BPF
On-Off
Keying
Transmission
output
Figure 1: EDSM/EPWM transmitter
ter composed of a class-E amplifier is evaluated by
circuit simulation, and the method of pulse modula-
tion by PWM pulses is studied.
2 EDSM/EPWM Transmitter Using
Class-E Power Amplifier
2.1 EDSM/EPWM Architecture
Figure 1 shows the block diagram of the
EDSM/EPWM transmitter architectur e. The left
half portion, which is common to conventional
EER transmitters, generates digitized PWM pulses
of the envelope signal A(t)=
I
2
(t)+Q
2
(t) and
the phase-modulated constant envelope RF signal
expressed as Re
[{I(t)+ jQ(t)}/A(t)] exp( j2π f
c
t)
,
where I(t) and Q(t) are the in-phase and quadrature
component of the desired modulation respectively,
Re z is the real part of complex number z,and f
c
is the
carrier frequency. In this architecture, the PWM pulse
directly drives the RF switch, which is implemented
as Gilbert cell mixer or DC bias switching of the
amplifier circuit to make the RF signal in burst shape.
The burst RF signal becomes amplitude modulated
signal whose envelope is proportional to the duty
ratio of the PWM pulse through the BPF. With
this architecture, all RF amplifiers can be nonlinear
ones and free from AM-PM conversion because the
amplifiers are driven by a constant envelope signal or
zero. Additionally, since the amplitude is controlled
by the pulse width, near 100% deep AM can be
performed, so long as the isolation of the RF switch
is enough.
2.2 Class-E Amplifier
Figure 2 shows the circuit topology of the class-E am-
plifier. The MOS transistor works as a switch and
driven with the RF input signal. The series resonant
tank circuit is tuned approximately on but slightly
lower than the carrier frequency depending on the res-
onance Q factor (Albulet, 2001). The shunt capac-
itance is composed of the external capacitor C
S
and
the d rain-source capacitance C
DS
, which is nonlinear.
3 Circuit Simulation
3.1 Circuit Design and Simulation
Model
The class-E amplifier is designed normally for con-
tinuous wave, for this study. The designed schematics
are shown in Figs. 3 and 4. Each transistor is mod-
eled with an ideal switch and an on-resistor where
the drain-source capacitance C
DS
is modeled as a lin-
ear one together with the shunt capacitor as C
S
=
Figure 2: Class-E amplifier.
A Study on Drain Efficiency of Edsm/Epwm Transmitter Using Class-E Amplifier
113
Figure 3: Simulation schematic of RF switching case.
PWM pulse modulation is implemented at the RF inputs.
C
S
+C
DS
.TheQ factor of the series LC resonator is
set to 10, and the carrier frequency is 100MHz. The
load impedance R = 50[Ω], and the output power is
27 dBm for CW. All the inductance and capacitance
values are designed in accordance with the way de-
scribed in (Albulet, 2001). The on-resistance of the
transistors are assumed 1 Ω.InthecaseofFig.3,the
envelope PWM modulation, making the burst switch-
ing of RF input, is implemented at the RF inputs as as-
sumed or designed in (Wang, 2003; Dupuy and Wang,
2004), while it is done at the drain DC bias circuits in
the case of Fig. 4 as described in (Choi et al., 2007).
In this paper, the former and the latter configurations
are hereafter called “RF switching” and “DC switch-
ing” respectively. Note that in the former case, the
RF switching operation of both the transistors stops
and they are kept open during each off duration of the
PWM pulses, while they continue RF switching all
the time. The diode in Fig. 4 at the right side of the
PWM switch driven by the PWM pulse v
PW M
work
as a “free wheel diode” and they are turned-on by the
current charged in the choke inductor L
C
; the DC bias
current of drain is somewhat filtered, and the pulsa-
tion due to PWM is smoothed.
3.2 Simulation Parameters and
Condition
For PSPICE simulation, the input RF signal Dr is de-
fined as rectangular pulses of 100MHz without phase
modulation (CW). The PWM pulse v
pwm
is assumed
to be a couple of the fixed period T
PW M
= 0 .1, 1 [µs],
and the drain efficiency is evaluated varying the duty
ratio of the PWM pulse. The drain efficiency is cal-
culated from the averaged power consumption at the
load R
L
and the supplied DC power from the DC
sources. It must be mentioned that the output power
gradually rises where the tim e constant is determined
by the choke inductance, the on-resistance and etc, so
Figure 4: Simulation schematic of DC switching case.
PWM pulse modulation is implemented at the drain DC bias
circuits.
the power is averaged during the 5 µs after elapsing
5 µs.
3.3 Simulation Results
Figure 5 shows the waveforms in the RF switching
case, where the duty ratio of PWM is 50%. The upper
waveform is the drain voltage of the main amplifier
v
S
, the lower ones are PWM pulse v
PW M
, RF input
Dr, which is rectangular waveform to drive the tran-
sistor modeled as a switch and a on-resistor, and the
output signal v
o
. Since the CW output is designed
0.5 W and the PWM duty is 0.5, the output power is
expected 0.5 0.5
2
= 0.125 W; however, It is seen that
only 50 mW power is gained and not a few distortion
is observed. Therefore, this configuration is unsuit-
able.
Figure 6 the wave forms in the DC switching case,
where the duty ratio of PWM is 20%. The upper
waveform is the drain voltage v
D
, and the lower ones
are PWM pulse v
PW M
, RF input Dr defined as rect-
angular wave, and the output signal v
o
. In this case,
the output power is gained as expected since the root
mean square of v
o
is seen to be around 2.6V. It can
also be seen that there are little distortion and rip-
ples of the envelope about v
o
. The output spectrum
is shown in Fig. 7. The spurious due to PWM switch-
ing, to apper 100 ± 10 MHz, is observed to be sup-
pressed by 28 dB to the fundamental component of
the carrier. Drain efficiency due to the duty ratio of
the PWM pulse is shown in Fig. 8. The efficiency a
little degrades in the lower du ty region because of the
switching loss.
First International Conference on Telecommunications and Remote Sensing
114
Y
R
Y
'
'
U
Y
3:0
Figure 5: Simulated waveforms of RF switching case.
Y
'
Y
3:0
Y
R
'U
Figure 6: Simulated wav eforms of DC switching case.
A Study on Drain Efficiency of Edsm/Epwm Transmitter Using Class-E Amplifier
115
+] 0+] 0+] 0+] 0+] 0+] 0+] 0+] 0+] 0+] 0+]
9
9
9
9
9
Figure 7: Output spectrum of DC switching case.
3.4 Discussions
Above-mentioned simulation results show that an
EDSM/EPWM transmitter using a class-E amplifier
should be configured as the DC switching for enve-
lope pulse modulation. However, the PWM signal
power to drive the switching device must be higher
than the RF switching case and it might not be ig-
nored. The degradation in the RF switching case is
clearly caused by transient response after turn-off of
every PWM pulse due to the charged current in the
drain choke inductors. Note that the choke induc-
tors operate as a low-pass filter to smooth the pulsa-
tion of PWM and reduce the PWM spurious or the
quantization noise at the output more than the se-
ries resonator tank as the BPF where Q is only 10.
Therefore, the undesired transient response and loss
could be mitigated if the choke components are re-
placed by LC-resonators or quarter-wavelength stubs
as the class-F amplifier case in (Takahashi and Ya-
mao, 2010); however, the filtering effect of the choke
inductor cannot be used any more, so the requirement
of BPF characteristics becomes severe, sharper cutoff
with low insertion loss. Combination of an inductor
and a quarter-wavelength stub might be good.












'UDLQ(IIHFHQF\>@
'XW\>@
7SZP >QV@
7SZP >˩V@
Figure 8: Drain efficiency due to PWM duty (DC switching
case).
4 Conclusion
Drain efficiency of EDSM/EPWM transmitter
composed of a class-E amplifier is evaluated by cir-
cuit simulation, especially on envelope pulse modu-
lation method. Simulation results show that the RF
switching method produces unnecessary transient re-
sponses and loss, and the output signal distorted, if the
drain choke circuits are inductors. Another method
that gates the drain DC bias b y the PWM pulses works
expectedly and the drain efficiency as high as ex-
pected; however, it must be noted that the DC switch-
First International Conference on Telecommunications and Remote Sensing
116
ing is done by a transistor and the driving power may
not be ignored. This transmitter architecture has to be
carefully designed with above-mentioned trade-off.
ACKNOWLEDGEMENTS
The authors wish to thank Prof. T. Suetsugu, Fukuoka
University and Prof. H. Sekiya, Chiba University, for
useful discussions, and Mr. N. Oyama, a graduate stu-
dent of School of Engineering, Fukuoka University,
for his simulation work.
REFERENCES
Adachi, T., Iida, M., and Asakura, H. (2002). Transmitting
circuit apparatus and method. United States Patent,
US 2002/0186440 A1.
Albulet, M. (2001). RF power amplifiers. Noble Publishing,
Atlanta.
Choi, J., Yim, J., Yang, J., Kim, J., Cha, J., Kang, D.,
Kim, D., and Kim, B. (2007). A ΔΣ-digitized polar
rf transmitter. IEEE Trans. Microwave Theory Tech.,
55(12):2679–2689.
Diet, A., C. Berland, M. V., and Baudoin, G. (2004). EER
architecture specifications for ofdm transmitter using
a class E amplifier. IEEE Microwave Wireless Com-
pon. Lett., 14:389–391.
Doherty, W. H. (1936). A new high efficiency power am-
plifier for modulated waves. Proc. IRE, 24(9):1163–
1182.
Dupuy, A. and Wang, Y. E. (2004). High efficiency power
transmitter based on envelope delta-sigma modulation
(edsm). In Proc. 60th IEEE Vehicular Technology
Conf. (VTC2004 Fall), pages 2092–2095.
Kahn, L. R. (1952). Single-sideband transmission by
envelope elimination and restoration. Proc. IRE,
40(7):803–806.
Raab, F. H., P. Asbeck, S. C., Kenington, P. B., Popovi´c,
Z. B., Pothecary, N., Sevic, J. F., and Sokal, N. O.
(2002). Power amplifier and transmitters for RF and
microwave. IEEE Trans. Microwave Theory Tech.,
50(3):814–826.
Takahashi, S. and Yamao, Y. (2010). Burst RF signal am-
plification for EPWM transmitter. IEICE Technical
Report, 110(307, MW2010-118):75–80.
Taromaru, M., Ando, N., Kodera, T., and Yano, K. (2007).
An EER transmitter architecture with burst-width en-
velope modulation based on triangle-wave compari-
son PWM. In Proc. 18th Int. Symp. Personal, Indoor
and Mobile Radio Commun. (PIMRC 2007),page
819.
Wang, Y. (2003). An improved kahn transmitter architec-
ture based on delta-sigma modulation. IEEE MTT-S
Microwave Symposium Digest 2003, 2:1327–1330.
Yokozawa, S. and Yamao, Y. (2011). Suppression of quan-
tization noise for EPWM transmitter with 2nd-order
Δ-Σ modulator. In Proc. 73rd IEEE Vehicular Tech-
nology Conf. (VTC2011 Spring).
A Study on Drain Efficiency of Edsm/Epwm Transmitter Using Class-E Amplifier
117