High Efficiency Frequency Tunable Inverse Class-E ... · High Efficiency Frequency Tunable Inverse Class-E ... and since the circuit has a varactor diode, ... High Efficiency Frequency

High Efficiency Frequency Tunable Inverse Class-E

Amplifier in VHF Band

Young Kim

Kumoh National Institute of Technology, 1 Yangho-Dong, Gumi, Gyungbuk, 730-701, Korea

[email protected]

Abstract This paper proposes the use of an inverse class-E amplifier with a tunable parallel resonator at the

output port in order to obtain a high power-added efficiency (PAE) and output power in a wide

frequency range. The tunable resonator circuit has a constant Q factor in the operating frequency

range, and since the circuit has a varactor diode, the inductor and capacitor values of the resonator

can be changed. Further, the inductance value for zero-current switching (ZCS) is implemented a

lumped element and the capacitance value is made a distributed element for phase compensation. The

inverse class-E amplifier can deliver an output power of 25dBm and can achieve a maximum a PAE of

75% in the frequency range 65-120MHz.

Keywords: Inverse Class-E Amplifier, High Efficiency, Tunable Amplifier.

1. Introduction

In wireless communication systems, the linearity and efficiency of power amplifier used in

transmitter are important factors that decide system performance. To improve the efficiency of the

amplifier, many techniques, including Envelope Elimination and Restoration (EER), Envelope

Tracking (ET), and hybrid ET method [1]-[3], have been developed. Further, the high-efficiency power

amplifier may be a class-D, class-E, or class-F amplifier [4]-[7].

Although the class-D amplifier consists of two active elements and has high efficiency, its large

circuit size is a major drawback. The class-F amplifier has high efficiency and can therefore control

higher harmonics; however, the circuit is very complex and cannot eliminate harmonic signals. The

class-E amplifier with a shunt capacitance has found widespread application due to the simplicity of its

design and high operational efficiency. Further, the inverse class-E amplifier with a series inductance

has a lower peaking switching voltage, lower inductance values, and greater tolerance to variations in

the circuit component values as compared to the classical topology [8].

In this study, we investigate a tunable inverse class-E amplifier with zero-current switching to

obtain a high power-added efficiency (PAE) and a constant output power in VHF range. For stable

operation in the presence of frequency variation, the tunable resonator includes a varactor diode to

control the capacitance and inductance values. Experimental results show that the tunable inverse class-

E amplifier can be operated in the frequency range 65 MHz~ 120 MHz.

2. Tunable inverse class E amplifier

2.1. Operation theory

Despite the conventional class-E amplifier being operated in the zero-voltage switching (ZVS), the

inverse class-E amplifier is operated a zero-current switching (ZCS) mode, and the output matching

networks consist of a parallel LC resonator and a series inductor to induce current. The block-diagram

of inverse class-E amplifier is shown in Fig. 1.

High Efficiency Frequency Tunable Inverse Class-E Amplifier in VHF Band Young Kim

International Journal of Engineering and Industries(IJEI) Volume2, Number3, September 2011 doi : 10.4156/ijei.vol2.issue3.4

31

RFin Rload

LS

ΔCCp Lp

+

-

vs

is

+ -vL

RFC

Vcc

Figure 1. Block Diagram of Inverse Class E Amplifier.

(a) Class E (b) Inverse Class E

Figure 2. Waveforms of Switching Voltage and Current.

The inverse class-E amplifier circuit consists of an active component, a parallel resonator to cancel

higher harmonics, and a parallel capacitor to compensate for the current and voltage phase difference.

The active component has an on-off characteristic by reason of current of inductance. When the active

component is in the on state, the active component has zero internal resistance and zero voltage drop

and current flows through it. Further, if the active component operates in the off state, it has infinite

resistance and there is a voltage drop across it because of breaking current flow.

The switching voltage and current waveforms of class-E and inverse class-E amplifiers are

presented in Fig. 2. It can be observed that the class-E amplifier operates in the zero-voltage switching

(ZVS) mode, while the inverse class-E amplifier operates in the zero-current switching (ZCS) mode.

In ZCS waveform, if the inverse class-E amplifier has a 50% switching period and 100% efficiency,

the component values are calculated by using the following equations [9].

22 4

8

DC

o

VR

P

(1)

2

22

DCs

o o

VL

f P (2)

2

2 2

4

4( 4)

o

o DC

PC

f V

(3)

2p

o

RL

f Q (4)

0 144 288 432 576 720

wt

-1

0

1

2

3

4

5

No

rm

ali

zed

is,

vs

0 144 288 432 576 720

wt

-1

0

1

2

3

4

5

ZVS

vs

is

ZCS

vs

is


32

2p

o

QC

f R (5)

where VDC is the collector or drain voltage, fo is the center frequency, Po is the output power and

Q is the quality factor of parallel resonator.

Using (1)-(5) equations, we design an inverse class-E amplifier with constant Q that is capable of

stable operation in the presence of frequency variations. Figure 3 shows a schematic of the proposed

amplifier.

The parallel resonator components have a variable parameter, which is controlled by the bias

voltage. The Lp-Cp parallel resonator circuit operates at the fundamental frequency in the open circuit

and at other frequencies in the short-circuit configuration. Its function is identical to the classical series

resonator of a class-E amplifier.

Figure 3. Tunable Inverse Class E Amplifier Schematic.

2.2 Variable parallel resonator design

In parallel resonator shown in Fig. 3, the tunable capacitor circuit contains a varactor diode to

control the capacitance, which is carried out by adjusting the reverse bias voltage in Fig. 4(a). Further,

the tunable inductor circuit consists of a varactor diode and λ/4 transmission line in Fig. 4(b). These

connection operate an inductance and its value is calculated the below equation.

22

1

oin o

ZZ j CZ

j C

(6)

The λ/4 transmission line, which is used to implement the inductance, can help the π-type lumped

equivalent circuit compensate for longer transmission lines at low frequency. The equivalent circuit is

depicted in Fig. 5. Since the resonator consists of the tunable capacitor and tunable inductor, the

tunable characteristic of resonator is satisfied to operate in 65 ~ 120 MHz tunable frequency by the bias

voltage in case of constant Q.

(a) (b)

Figure 4. (a) Variable capacitor circuit (b) Variable inductor circuit.

RFin

Tunable Parallel Resonator @ ƒo

LS

ΔCCp Lp

IMN

Cbypass

DCB

Vlow

Cbypass Vhigh

RFCRFout

DCB

RFin

VCRFC

DCB

θRFin

VLRFC

DCBZo


33

Figure 5. π-type equivalent circuit of λ /4 transmission line.

3. Simulation and experiment results

3.1. Design and Simulation

In order to confirm the variable characteristics of the parallel resonator, we simulate the tunable

capacitor and inductor using a 1T362 varactor diode manufactured by Sony Inc. The simulation results

are presented in Fig. 6; the simulation is carried out by using the ADS software developed by Agilent

and by setting the center frequency to be 75 MHz. The graph gives the impedance values of tunable

capacitor and tunable inductor circuits. The operating range of each component corresponds to that for

an equivalent capacitance is 2.5 ~ 27 pF and an equivalent inductance of 180 ~ 680 nH when the

reverse voltage of the diode is in the range 0 ~ 25 Volts at 75 MHz.

For Q=1.3, the variation in the values of the elements of the inverse class-E amplifier with

frequency change is shown in Fig. 7. These data show that the range of values of the tunable

capacitance and inductance in the parallel resonator corresponds to that observed in Fig. 6. In Fig. 7,

the change of Ls and ΔC values cause a little effect in the circuit operation.

Figure 6. Impedance values of variable capacitor and inductor circuit

θ

Zo

Cπ/2 Cπ/2

Lπ


34

Figure 7. Characteristics of element value with frequency variation in case of Q=1.3.

Figure 8 shows the simulated output power and PAE for the inverse class-E amplifier with a tunable

parallel resonator. Despite variations in the frequency, the output power and PAE of the amplifier has a

small variation because the resonator facilitates frequency adaptive operation.

The switching voltage and current waveforms at a center frequency of 75 MHz and an output power

of 23 dBm are shown in Fig. 9. It is apparent that the zero current and zero voltage points do not

coincide. This indicated that the actual active components are not operated as ideal switches. The

internal capacitance of active component affects the on-off normal operation.

Figure 8. Characteristics of output power and PAE


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Figure 9. Waveforms of switching voltage and current

3.2 Fabrication and Measurement Results

A measurement data of variable capacitor circuit is shown in Fig. 10. It is presented that the

simulation and measurement data is almost coincide. Compared the simulation with the measurement

results, because the internal resistance of varactor diode contribute the insertion loss of tunable circuit,

the result is some different. Also, compare to the simulation results, the variable inductor operation is

almost the same. The characteristic of tunable resonator is shown in Fig. 11 in the frequency range 65 ~

120 MHz. Because of resistance of varactor diode, the tunable resonator has a loss.

A photograph of the implemented tunable inverse class-E amplifier is shown in Fig. 12. The

amplifier was implemented using microstrip technology on a Teflon substrate with εr=3.55 and height

(h) of 0.815 mm; the design frequency was fo=75 MHz.

The active component used was an NE85634 transistor of NEC Corp., with 3.5V bias voltages. The

amplifier requires a large-sized PCB since the controlling circuit of parallel resonator, consisting of a

tunable inductor and capacitor, is to be accommodate. The fabricated ΔC has a large electrical length,

which is the capacitance value of 4.7pF. Also, the value of inductor Ls is 210nH.

Fig. 13 shows a plot of the output power versus PAE for the tunable inverse class-E amplifier at a

center frequency of 75 MHz. A PAE of 72% is obtained at an output power of 22.5 dBm. Since the

internal resistance of diode contributed the loss of output power, the PAE of measurement data is

reduced at high power region especially.

Fig. 14 shows the measurement data in the frequency range 65 MHz ~ 120 MHz. This graph shows

that the output power is 20.5 ~ 26.5 dBm at a PAE of 42 ~ 75 %. The maximum power and PAE of the

proposed amplifier are 25 dBm and 75%. At 120 MHz, the performance is degraded because of

abnormal operation of the parallel tunable resonator.


36

Figure 10. Characteristics of simulated and measured capacitance impedance values.

Figure 11. Tunable characteristic of the resonator in case of constant Q.

Figure 12. Fabricated tunable inverse class E amplifier

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Frequency [MHz]

0

5

10

15

20

25

[dB

]


37

Figure 13. Characteristics of output power verse PAE of the proposed amplifier in case of 75 MHz.

Figure 14. Measured data of output power and PAE

4. Conclusion

This paper presents experimental validation of the operation of a tunable inverse class-E amplifier

with a tunable parallel resonator; the amplifier can provide a constant high power and power-added

efficiency. The proposed amplifier has a 40 ~ 75% power-added efficiency and a 25 dBm output power

in the frequency range 65 MHz ~ 120 MHz.

In the presence of frequency variations, the proposed amplifier shows high efficiency in a wide

operating frequency range. The size of the proposed amplifier can be reduced by using MMIC

technology.

5. Reference

[1] Kahn, L.R., "Single Sideband Transmission by Envelope Elimination and Restoration," Proc. IRE, vol.40, pp.803-806, July 1952.

[2] J. Jeong, D. F. Kimball, M. Kwak, P. Draxler, and P. M. Asbeck, "Envelope Tracking Power Amplifiers with Reduced Peak-to-Average Power Ratio RF Input Signals," IEEE Radio and Wireless Symposium, pp.112-115, January 2010.


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[3] D. Kim, D. Kim, J. Choi, and B. Kim, "LTE Power Amplifier for Envelope Tracking Polar Transmitters," Proceedings of the 40the European Microwave Conference, pp.628- 631, September 2010.

[4] Steve C. Cripps, “RF Power Amplifiers for Wireless Communications,” Norwood, MA, New York, 1999.

[5] Sokal, N.O., and A.D.Socal, "Class E-A New Class of High Efficiency Tuned Single-Ended Power Amplifiers", IEEE J. Solid State Circuits, SC-10, no.3, pp.168-176, June 1975.

[6] F. H. Raab, "Class-F Power Amplifiers with Maximally Flat Waveforms," IEEE Trans. Microw. Theory Tech., vol.45, no.11, pp.2007-2012, November 1997.

[7] R. Miyahara, H. Sekiya, M.K. Kazimierczuk, "Novel Design Procedure for Class-E Power Amplifiers," IEEE Trans. Microwave Theory Tech., vol.58, no.12, pp.3607-3616,Dec, 2010.

[8] Thian Mury, Vincent F. Fusco, "Inverse Class-E Amplifier with Transmission-Line Harmonic Suppression," IEEE Trans. on Circuits and Systems, vol.54, no.7, pp.1555-1561, July 2007.


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High Efficiency Frequency Tunable Inverse Class-E ... · High Efficiency Frequency Tunable Inverse Class-E ... and since the circuit has a varactor diode, ... High Efficiency Frequency