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High Linearity Class B Power Amplifiers
in GaN HEMT Technology
S. Xie, V. Paidi, R. Coffie, S. Keller, S. Heikman, A. Chini,
U. Mishra, S. Long, M. Rodwell
Department of Electrical and Computer Engineering,
University of California, Santa Barbara
Email: [email protected], [email protected]
Phone: (805)893-8044 Fax: (805)893-5705
Slide 1
2002 Topical Workshop on Power Amplifiers
September 2002, UCSD
9/3/2002
Outline
UCSB
S. Xie, V. Paidi
• Introduction
How does Class B PA work?
Why single-ended Class B?
• Highly linear single-ended Class B PA design and
simulation
• Measurement results
• Conclusions
Slide 2
9/3/2002
How does push-pull Class B PA work? UCSB
V. Paidi, S. Xie
VDS
1
+Vin
Vin
0
180
180
•
•
•
•
0
-Vin
Vout =
VDS1 – VDS2
VDS
2
Two identical devices working in 50% duty cycle with 180° phase shift.
Half sinusoidal drain current on each device, but full sinusoidal drain voltage.
Even harmonics are suppressed by symmetry => wide bandwidth (limited
by the power combiner).
Class B: Ideal PAE 78.6%; feasible PAE 40-50% (typical GaN HEMT at Xband); Class A: Ideal PAE 50%, feasible PAE 20-30%.
Slide 3
9/3/2002
Why not push-pull for RF MMIC
UCSB
V. Paidi, S. Xie
ID+
VDS+
Vin
Vin+
VD
Cbias
RL
Vout
VG
Cbias
1:1
IDVDS-
1:1
Vin-
To obtain high efficiency (78%), a half-sinusoidal current is
needed at each drain. This requires an even-harmonic short.
This can be achieved at HF/VHF frequencies with transformers
or bandpass filters. However,
1. Most wideband microwave baluns can not provide effective
short for even-mode. Efficiency is then poor.
2. They occupy a lot of expensive die area on MMIC.
Slide 4
9/3/2002
Single-ended Class B = push-pull
Push-pull Class B
ID1
+Vin
Vin
+
0
-ID2
180
180
-Vin
V. Paidi, S. Xie
I D1 a1Vin a2Vin2 a3Vin3
ID1
0
UCSB
I D 2 (Vin ) a1Vin a2Vin2 a3Vin3
-Vin
ID2
I D I D1 I D 2
= ID
Vin
Even harmonics suppressed by symmetry
2a1Vin a2Vin2 2a3Vin3
Single-ended Class B with Bandpass filter
2
3
V
I
R
R
[
a
V
a
V
a
V
ID
out
D L
L 1 in
2 in
3 in ]
vout Zero Z at 2fo
vi
RL
band pass
filter @ fo
Even harmonics suppressed by filter
Conclusion: From linearity point of view,
push-pull and single-ended Class B with
bandpass filter B are equivalent – same
transfer function.
Bandwidth restriction < 2:1
Slide 5
9/3/2002
Why is biasing critical for Class B?
ID1
ID1
Vin
+
Vp
ID2
Vp
ID2
Vin
Vin
= ID
Vin
= ID
Vin
Ideal Class B
Vin
+
Vp
ID2
= ID
V. Paidi, S. Xie
ID1
Vin
+
UCSB
Vin
Bias too low: Class C
Slide 6
Vin
Bias too high: Class AB
9/3/2002
Voltage Transfer Function as a
Function of Bias Voltage
UCSB
V. Paidi, S. Xie
Output Voltage, V
20
Class AB, nonlinear
15
10
Class B, linear
5
0
-5
Class C, nonlinear
-10
-15
-20
-4
-3
-2
-1
0
1
2
3
4
Input Voltage, V
Class B is linear given that the current transfer function is linear
Slide 7
9/3/2002
A source of IM3 distortion:
Transconductance distortion
UCSB
V. Paidi, S. Xie
Highly linear characteristics of GaN HEMT on SiC
0.8
0.7
Drain Current (A/mm)
0.6
Experimental (A/mm)
Modeled (A/mm)
The third order term in
the Taylor expansion is
very small when biased
at pinch off.
0.5
0.4
0.3
Bias point
0.2
0.1
The distortion will be
0
minimum when the
-5
-4
-3
-2
-1
0
amplifier is biased at
Gate Bias (V)
Class B by using GaN
2
3
I I 0 I1 *Vgs I 2 *Vgs I 3 *Vgs ......... HEMT on SiC.
Slide 8
9/3/2002
Single-ended Class B Power Amplifier
vout
vi
RL
UCSB
S. Xie, V. Paidi
•
Lossy input matching network
to increase the bandwidth
•
Cds is absorbed into the section output lowpass filter
band pass
filter @ fo
Gate 2
RF IN
L2
TLIN
Cds
C1
Output
matching
network
(short at 2fo, 3fo...)
Gate 1
BIAS
TEE
Input
matching
network
R1
L1
Vg
Lossy input matching
BIAS
TEE
RL
Vd
- section lowpass filter
Slide 9
9/3/2002
Simulation performance of Class B
Drain Voltage, V
50
30
40
30
25
20
20
Saturated PAE ~48%
10
0
15
0
5
10
15
20
25
30
40
35
30
25
20
15
10
0
50
100
150
200
150
200
time, psec.
1000
800
Drain Current, mA
35
45
PAE (%)
Output power, dBm
50
Saturated output power ~3770dBm
60
S. Xie, V. Paidi
Waveforms of drain voltage
and current
Simulation of class B amplifier @10GHz
40
UCSB
600
400
200
0
0
50
100
time, psec.
Input power, dBm
Class B bias: Saturated output power ~ 37 dBm,
Saturated PAE ~ 48%
Slide 10
9/3/2002
Simulation of Intermodulation
Suppression and PAE @10GHz
Class B bias:
C/IMD3~44dBc
PAE ~ 48%
50
Saturated PAE (%)
S. Xie, V. Paidi
Class A bias
C/IMD3~42dBc
PAE ~ 35%
46
50
45
40
44
35
42
30
40
38
36
-5.5
Class AB bias
25
Class C bias
-5
-4.5
-4
-3.5
-3
-2.5
-2
IMD3 @6dB back_off (dBc)
48
UCSB
20
-1.5
Gate Bias, V
Best IM3 suppression is achieved at Class B and Class A
Slide 11
9/3/2002
Device Performance of GaN HEMTs
UCSB
V. Paidi, S. Xie
Performance of 12 fingers (1.2mm) device:
•
Lg ~ 0.25um
•
ft ~ 55 GHz (~ 50 GHz for dual gate)
•
Idss ~ 1A/mm
•
Vbr ~ 40V (~ 55V for dual gate)
DC I_V curve
RF Performance
35
~1.2 A
30
UPG
25
H21
20
15
10
Ft~55 GHz
5
0
1.00E+00
Slide 12
1.00E+01
1.00E+02
1.00E+03
9/3/2002
Chip photograph of Class B power amplifier
UCSB
V. Paidi, S. Xie
Gate 2
Gate 1
Source
Air bridges
Drain
(Approximately 6mmX1.5mm)
Slide 13
9/3/2002
UCSB
Measurement setup
Bias T
Signal
generator_1
Bias T
Power
amplifier_1
Power
combiner
Signal
generator_2
DUT
S. Xie, V. Paidi
Power
amplifier_2
Coupler
CH_A
Power
meter
- 20 dB
CH_B
Coupler
- 20 dB
- 20 dB
Coupler
50 Ohm
Load
- 20 dB
- 20 dB
Spectrum
Analyzer
Measurements:
•
Single tone from 4 GHz to 12 GHz;
•
Two-tone measurement at f1 = 8 GHz, f2 = 8.001 GHz;
•
Bias sweep: Class A (Vgs = -3.1V), Class B (Vgs = -5.1V,
Class C (Vgs = - 5.5 V) and AB (Vgs = -4.5 V).
Slide 14
9/3/2002
Class B power amplifier measurement result
UCSB
S. Xie, V. Paidi
Gain vs. frequency
20
Class AB
Gain, dB
15
10
5
Class B
0
-5
2
4
6
8
10
12
14
16
Frequency, GHz
3 dB bandwidth: 7GHz - 10GHz
Slide 15
9/3/2002
Class B bias @Vgs = - 5.1V
UCSB
S. Xie, V. Paidi
40
Single tone performance @ f0 = 8GHz:
35
0.4
30
0.3
25
0.2
20
0.1
15
0
0
5
10
15
20
25
PAE
Output power, dbm
0.5
Saturated output power 36 dBm
PAE (maximum) ~ 34%
30
Input power, dbm
0.25
f1,f2
20
Two tone performance @
f1=8GHz, f2=8.001GHz :
0.2
10
0
0.15
2f1-f2, 2f2-f1
-10
0.1
-20
-30
PAE
Output power, dBm
30
0.05
-40
0
-50
-15
-10
-5
0
5
10
15
20
Input power, dBm
Slide 16
Good IM3 performance:
• 40dBc at Pin = 15 dBm, and
• > 35 dBc for Pin < 17.5 dBm
9/3/2002
Class A bias @Vgs = - 3.1V
S. Xie, V. Paidi
0.4
40
Single tone performance @ f0 = 8GHz:
0.35
30
0.3
0.25
20
PAE
Output power, dBm
UCSB
0.2
10
Saturated output power 36 dBm
0.15
0.1
PAE (maximum) ~ 34%
0
0.05
0
-10
-20
-10
0
10
20
30
Input power, dBm
0.35
40
f1,f2
0.3
0.25
20
0.2
10
0.15
0
2f1-f2, 2f2-f1
-10
0.05
0
-30
-15
-10
-5
0
5
Saturated output power each tone ~ 33dBm
0.1
-20
-20
PAE
Output power, dBm
30
Two tone performance @ f1=8GHz,
and f2=8.001GHz :
10
Input power, dBm
15
20
Good IM3 performance at low
power level but becomes bad
rapidly at high power levels
Slide 17
9/3/2002
UCSB
Summary of IM3 suppressions
S. Xie, V. Paidi
IM3 compression, dBc
60
Class B
50
Class A
40
30
Class AB
Psat
20
Class C
10
0
5
10
15
20
25
30
35
Pout, dBm
•
•
•
Low output power levels (Pout < 24 dBm), Class A and Class B both
exhibit good linearity (Class B > 36 dBc, Class A > 45 dBc).
Higher output power levels, Class A behaves almost the same as
Class B.
Class AB and C exhibit more distortion compared to Class A and B.
Slide 18
9/3/2002
UCSB
Class B vs. Class A
PAE of single tone
S. Xie, V. Paidi
IM3 suppression and PAE of two-tone
0.35
0.2
Class B
0.15
Class A
0.1
0.25
0.2
40
0.15
30
Class B
0.1
20
PAE, twotone
0.25
IM3 suppresion, dBc
50
0.3
PAE, single
0.3
Class A
0.05
0.05
0
0
10
10
15
20
25
30
35
40
-5
Output power, dBm
0
5
10
15
20
25
30
35
Output power, dBm
While maintains the same IM3 suppression as Class A, Class B
can get more than 10% of PAE.
Slide 19
9/3/2002
Conclusions
UCSB
S. Xie, V. Paidi
• For a less than octave bandwidth, a push-pull Class
B amplifier can be replaced by a single-ended Class
B amplifier with bandpass or lowpass filter.
• A single-ended Class B MMIC power amplifier in
GaN HEMT is designed and 36dBm of saturated
power and 35dBc of IM3 suppression are obtained.
• Class B is better than Class A because it can get
good IM3 performance comparable to that of Class
A, while providing PAE ~10% higher than that of
Class A.
Slide 20
9/3/2002
Acknowledgements
UCSB
S. Xie, V. Paidi
This work was supported by the ONR under grant
(N00014-00-1-0653)
Special thanks to Dr. Walter Curtice, who provide us the
C_FET3 model for simulation;
Thanks to L.-Y. (Vicky) Chen and Likun Shen, who helped
us for the measurement.
Slide 21
9/3/2002