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

Modulation

Processing

QI 

'6 modulation

Limiter

Class-B/C/D/E/F saturation

or switchin

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)=



(t)+Q

(t) and

the phase-modulated constant envelope RF signal

expressed as Re



[{I(t)+ jQ(t)}/A(t)] exp( j2π f



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

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



and

the d rain-source capacitance C

, 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

is modeled as a lin-

ear one together with the shunt capacitor as C

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.



.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

; 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

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

, 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

. 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

= 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

, and the lower ones

are PWM pulse v

PW M

, RF input Dr deﬁned as rect-

angular wave, and the output signal v

. In this case,

the output power is gained as expected since the root

mean square of v

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

. 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

3:0

Figure 5: Simulated waveforms of RF switching case.

3:0

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).

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