Class-E Silicon Carbide

VHF Power Amplifier

For Space Applications

Marc J. Franco

N2UO

Presented by: Bob McGwier, N4HY

Outline

• High efficiency, class-E VHF power amplifiers

• Active device requirements and available

technologies

• Restrictions imposed by space applications

• Amplifier design with silicon carbide (SiC)

MESFET

• Simulated and experimental results

Motivation

• Switch-mode operation (i.e. class-E) involves high

peak drain/collector voltage

• GaAs transistors are inherently low voltage devices

• LDMOS devices are not suitable for flight (oxide layer

susceptible to total radiation dose)

• Silicon carbide (SiC) and gallium nitride (GaN) are

ideal for high efficiency space applications

Class-E amplifiers

• Very high dc to RF efficiency by operating the

active device as a switch

• High peak drain or collector voltage (almost

4x class AB)

• High breakdown voltage devices are ideal

candidates

• Linearity can be restored by amplitude

modulation of the drain voltage (HELAPS)

Flight hardware issues

• Radiation hardness becomes a concern

• GaAs is the dominant semiconductor in space

applications

• GaAs devices have a low breakdown voltage – hard

to operate in class-E

• LDMOS transistors have higher breakdown voltage,

but generally cannot be used in high radiation

environments (oxide layer)

SiC and GaN in space?

• No oxide layer

• SiC solar cells have been flown

• SiC has excellent temperature properties

• Both SiC and GaN are ideal for class-E operation

• Weatherford and Anderson, “Historical perspective

on radiation effects in III-V devices,” IEEE

Transactions on Nuclear Science

SiC Class-E amplifier design

• Classic class-E schematic (Sokal)

• SiC MESFET SiC60pr by Cree

• Q=2.27, Vds=30V, RL=18 ohms

1000 pf

V= -13.5 V

SiC60pr3 24 pf

470 pf 0 - 30 V

12 pf 56 pf

68 pf 47 pf 22 pf

47 nH

90 nH

90 nH

82 pf 47 nH 56 nH220 nH

47 nH OUT

IN

• Output low pass filter

• Decoupling allows drain modulation

• No tuning elements

Input match

Waveform shaping Impedance matching

Low pass filter

Output power vs. drain voltage

MEASURED

SIMULATED

0

5

10

15

20

25

2 6 10 14 18 22 26 30

Vds [V]

Po

[W

]

Drain efficiency vs. drain voltage

70

75

80

85

90

95

100

2 6 10 14 18 22 26 30

Vds [V]

Eff

[%

]

SIMULATED

MEASURED

Power added efficiency vs. drain voltage

SIMULATED

MEASURED

20

30

40

50

60

70

80

90

2 6 10 14 18 22 26 30

Vds [V]

PA

E [

%]

SIMULATED

MEASURED

RF output voltage vs. drain voltage

0

10

20

30

40

50

2 6 10 14 18 22 26 30

Vds [V]

Vo

[V

]

FUNDAMENTAL

2nd

HARMONIC

Output spectrum

3rd

HARMONIC

Output power vs. frequency

15

17

19

21

23

25

140 142 144 146 148 150

Freq. [MHz]

Po

[W

]

SIMULATED

MEASURED

Gate waveform

Summary of results

Parameter Simulated Measured

Maximum Output Power 21.5 W 20.5 W

Nominal Input Power 27 dBm 27 dBm

Maximum Drain Efficiency 88 % 86.8 %

Maximum Power Added Efficiency 84.8 % 83.5 %

Maximum spurs and harmonics -58 dBc -58 dBc

Gain 16.3 dB 16.11 dB

Conclusion

• SiC devices can be successfully used in class-E VHF operation

• This amplifier can be used on AMSAT Eagle’s 2 meter downlink

phase array

• Very high efficiency can be achieved (~87%)

• Greater break-down voltage than GaAs allows for high power

amplifier designs

• No oxide layer – advantage over LDMOS for space applications

• Very high operating temperature (120 ºC case, 255 ºC junction)

Thank you for your attention