Download A protection circuit for HBT RF power amplifier under load

A protection circuit for HBT RF power amplifier under
load mismatch conditions
Walid Karoui
Thierry Parra
Freescale Semiconductors
Avenue du General Eisenhower, BP 1029,
31023 Toulouse, France
[email protected]
LAAS-CNRS and Toulouse University, UPS
7 Avenue du Colonel Roche,
31077 Toulouse, France
[email protected]
Abstract—This paper investigates the radiofrequency failure
mechanisms of Power Amplifier (PA) integrated in an HBT
technology, and proposes a circuit solution for PA protection
against impedance mismatches. It exposes the failure
mechanisms that occur when a PA under extreme conditions
(high battery voltage and high input power) is exposed to
impedance mismatches at its output. Protection against high
voltage operation is addressed by integrating a parallel base
resistor which increases significantly emitter collector
breakdown. A current sensor is then associated to a feedback
loop on the PA biasing circuit that operates when the PA is in
high dissipated power conditions by limiting the collector
current. These protections are easily implemented on the PA die
without any extra area. Experiments confirm the effectiveness of
these principles: the protection is indexed on the collector supply
voltage and acts for all output loads leading to VSWR up to 1
0:1, whereas output power and power efficiency on a 50Ω load
are not affected.
I.
PA failures. In a second step we point out that the PA failure
is mainly related to a high dissipated power at the PA’s final
stage.
Based on these investigations, we propose a new
monolithically integrated current limiter, indexed to supply
voltage, which is able to improve HBT amplifier’s
ruggedness, and which avoids any isolator implementation.
The effectiveness of this protection circuit is also proved as it
can prevent the failure of the PA under severe mismatch
conditions and high supply voltage, while maintaining RF
performances on a 50Ω load.
II.
vswr'
RF IN
INTRODUCTION
In a phone handset, the Power Amplifier Module (PAM) is
the last active component of the transmission path (Figure1),
and thus is directly exposed to impedance mismatches induced
by user motion and environment variations of the handset
antenna (disconnected antenna, proximity of a metallic plane,
user touching the antenna…). The PA must withstand these
impedance mismatches even it operates at extreme conditions
such as high battery voltage and high input power. Thereby,
the ruggedness of PA is a key specification for PAM in
wireless applications. In order to meet the ruggedness
specification, it is necessary to understand the root cause of
radiofrequency failure in PA and then design an effective
protection circuit.
The static base collector breakdown voltage at open base
(BVCEO) was used, for a long time, as a metric to qualify the
robustness of Hetero-junction Bipolar Transistor (HBT). In
this paper we demonstrate, in a first step, that this metric is too
conservative and that the collector voltage swing of the PA
can go beyond the static breakdown voltage without causing
978-1-4244-2332-3/08/$25.00 © 2008 IEEE
IMPEDANCE MISMATCHES
PAM
vswr
VC
IC
Isolator
Antenna
RFOUT
R load
Figure 1. Impedance mismatches in the transmission path
In previous generations of handsets, an isolator was
usually implemented in the transmission path between the
PAM and the handset antenna, in order to isolate the amplifier
from any impedance variation of the antenna terminal
(Figure1). The design of modern handsets attempts the
suppression of this isolator because this component makes the
PAM integration incompatible with the constraints of
downsizing and cost reduction. Moreover, as it inserts losses,
the isolator degrades the system available output power, as
well as its power added efficiency which affects directly the
handset autonomy (talk time). However, the isolator
241
suppression increases the PAM sensitivity to load impedances
mismatches that can translate into PAM oscillations and/or
lead to its failure. This is particularly true for SiGe HBT
power amplifier because the technologies remain fragile.
As the reflected power at the antenna terminal can create,
under certain conditions, constructive interferences at the
collector port of the PAM, the mismatches can lead to high
collector voltage, if the phase of the load is around zero degree
(high impedance mismatch), or high collector current, if the
phase of the load is around 180 degree (low impedance
mismatch) (Figure2). From current specifications, the high
impedance mismatch is close to the open circuit (OC),
whereas the low impedance mismatch is close to the short
circuit (SC). In intermediate situations, the imaginary part of
the load can resonate with parasitic capacitances and
inductances of the PAM transistors and can lead to oscillations
[1].
Low Impedance
mismatch
(High Current)
High Impedance
mismatch
(High Voltage)
B
A
of the PAM transistors, we can expect that the main failure
source under RF operations becomes the dissipated power that
runs over the limit.
In order to verify this statement, we performed
experiments on a three stage GaAs PA integrating a parallel
base resistor on RF transistors. The PA is supplied with high
battery voltage (5V). The level of the RF input signal is
chosen to put the PA into saturation on a 50Ω load. Then, an
impedance mismatch with a VSWR (Voltage Standing Wave
Ratio) of 10:1 is set at the PA output with a VSWR tuner, and
the phase of the load is varied by the tuner slide.
BVCER
Figure 3. Collector emitter breakdown voltage versus base paralell
resistance (example of SiGe HBT)
This 10:1 VSWR at the PA output corresponds to a 20:1
VSWR at the antenna terminal, assuming that the front-end
between the PA and the antenna (filter and antenna switch)
introduces 1 dB of losses. A spectrum analyzer is used to
measure the PA output power and detect potential oscillations.
Finally, the collector current of each stage of the PA is
measured for each phase of the load. The test bench is
depicted on Figure4.
10:1 VSWR
Figure 2. Cases of impedance mismatches for a 10:1 VSWR
III.
FAILURE MECHANISMS IN HBT RF POWER AMPLIFIER
Two issues can affect the ruggedness of HBTs: breakdown
voltage and thermal issue due to high dissipated power [2].
The open base static collector emitter breakdown voltage
(BVCEO) was commonly considered as the voltage limit of the
RF output signal swing. However, it has been demonstrated
that the collector voltage can run over BVCEO without causing
PAM failure [3], [4]. This can be explained by the fact that the
collector emitter breakdown voltage depends strongly on the
impedance which is presented on the base-emitter junction of
the transistor, for example by the bias circuit. By providing a
low impedance path for the avalanche current, resulting from
high voltage at the collector-base junction, an amount of this
avalanche current flows through this impedance instead of
flowing through the emitter-base junction, delaying on that
way the bipolar positive feedback that causes HBT failure. So,
for RF HBT PA applications, the collector emitter breakdown
voltage with the base grounded through a resistance (BVCER)
appears more relevant (Figure 3).
Because the high value reached by the breakdown voltage
BVCER when a 5 kΩ resistor is implemented on the HBT base
Tektronics Scope
HP 6626A Supply
1
2
3
4
Function Generator Yokogawa
1
2
FSIQ7 Spectrum Analyzer
3
R & S SMIQ
VSWR Tuner
Vreg
Vbat
-3dB
RFin_HB
RFout_HB
RFin_LB
RFout_LB
-0.5dB
Coupler
Amplifier Under Test
Pin
VSWR
Figure 4. Ruggedness test bench
The measurement results are given in Figure5. When the
load impedance phase varies from 0 to 360 degrees, the
collector current of the final stage (IC3) varies whereas the
current of drivers (IC12) remains constant. Thus, in the case of
a multistage PA, only the final stage need to be protected
against impedances mismatches. The PA failure occurs for
242
load phases near 180 degrees (low impedance mismatch zone)
when the collector current of the final stage exceeds 2.7A. No
failure or damage is observed for phases around 80 degrees
where the PA collector voltage is very high and exceeds
significantly the BVCEO.
1.3V (the turn on voltage of a GaAs HBT). Consequently the
transistor Qfb turns on and decreases the final stage bias
current. This feedback on the current mirror of the PA biasing
circuit hence reduces the amplifier gain. Finally, the low pass
filter R2C2 reduces the Qs collector voltage swing and
prevents the voltage saturation of Qs.
4000
Failure
Zone
3500
3000
IC (mA)
2500
2000
1500
Ic12 (mA)
1000
Ic3 (mA)
500
0
0
40
80
120
160
200 240
Phase (degree)
280
320
360
Current limiter
Figure 7. Layout of the protected PA final Stage
Figure 5. PA collector current versus load phase at VSWR of 10:1
IV.
PROTECTION CIRCUIT DESIGN
As we previously demonstrated, when a parallel base
resistor is implemented on the HBT, the main failure reason
becomes the dissipated power which runs over the limit when
the RF PA is subject to load impedance mismatch. Moreover,
only the final stage of a multistage PA needs to be protected.
So, the protection circuit we propose is a current limiter
implemented on the final stage of the PA. It is presented on
figure 6 [5].
Vbat
RFOUT
Vreg
Vbat
QRF3
RFIN
R2
Ibias
Mirror
C2
C
R
This protection circuit has been implemented on the final
stage of a GaInP/GaAs HBT RF power amplifier. The layout
of the die is presented in figure 7, where the protection circuit
is highlighted. As it can be picked up, the protection circuit
doesn’t increase the die area.
V.
VSWR 10:1
Qs
2
Vdet
Rstab
R1
EXPERIMENTAL RESULTS
In a first step, the detection voltage (Vdet) is measured in
an open loop configuration (base of Qfb not connected).
Results are given in figure 8. Performed for two values of
supply voltage, these measurements show that Vdet value is
higher than 1.3V when the final stage collector current reaches
2.5A. This voltage is able to turn on the bias feedback
transistor Qfb. Moreover, Vdet value increases when the PA
supply voltage is higher, which insures an efficient dissipated
power limitation which threshold is indexed on bias.
1,8
C1
Vdet (V)
1,6
Qfb
1,4
1,2
1
Figure 6. Schematic of the protected PA final Stage
0,8
The amplifier is biased using the current mirror biasing
scheme which is providing temperature compensation [6]. The
voltage Vreg sets the amplifier gain. The final stage collector
current (Ic) is sensed by a small transistor Qs (100 µm2) in
parallel with the final stage QRF3. Then, the sensed current,
which is proportional to the collector current, flows through
resistor R1. The voltage across R1 is filtered by the capacitor
C1 and then applied to the base of the transistor Qfb. As Ic
increases, the detection voltage (Vdet) applied to the base of
the transistor Qfb increases. When the collector current
exceeds a predefined threshold, Vdet becomes superior to
Vdet
Vdet @3.2V
@3.2V
Vdet @4V
Vdet
@4V
0,6
1
1,5
2
2,5
3
Ic (A)
Figure 8. Sensed voltage versus final stage collector current, indexed to
supply voltage
Then the robustness of the protection is tested on a PA
operating in CEL/EGSM frequency bands and under GSM
conditions. Measurements are carried out at the maximum
supply voltage (5V) and when the final stage of the PA is in
243
saturation on a 50Ω load. A VSWR of 10:1 is then set at the
PA output and the load phase is varied from 0 to 360 degrees.
Figure 9 shows the final stage collector current versus load
phase. Without the protection circuit, this current can increase
up to 3.1A before the PA fails. With the protection, this
current does not exceed the set threshold of 2.7A and no
damage is observed.
These experimental results confirm the effectiveness of
the protection to prevent PA failure under severe mismatch
conditions and high supply voltage. Moreover, because the
current sensor is implemented from small components (1mA
current consumption, low parasites) we verify that this
protection is not affecting the output power and the power
added efficiency when the PA is loaded on 50Ω.
VSWR=10:1
base. In these conditions, the main cause of failure becomes
the high dissipated power (high collector current) at the
collector terminal of the transistor. For a multistage PA, only
the final stage is sensitive to impedance mismatches and
should be protected.
We proposed an efficient protection circuit based on a
current sensor associated to a feedback loop on the PA biasing
circuit. It operates when the PA is in high dissipated power
conditions by limiting the collector current of the final stage.
Because it eliminates the need for band-gap reference circuit,
operational amplifier and coupler, this protection circuit is
easily implemented on a monolithic PA die without any extra
area. Experimental results confirm the protection is indexed
on the supply voltage and acts for all output loads leading to
VSWR up to 10:1, whereas output power and power
efficiency on a 50Ω load are not affected.
IPC mode : F=915 MHz, Pin=6 dBm, Vbat=5V)
4.5
REFERENCES
Failure
4
[1]
3.5
IC (A)
3
2.5
[2]
2
1.5
1
IC
without protection
IC_SL
IC with protection
IC_AL
0.5
[3]
0
0
60
120
180
Phase ( °)
240
300
360
[4]
Figure 9. PA collector current comparison versus load phase
VI.
CONCLUSION
[5]
We demonstrated that the collector voltage of a power
amplifier can go significantly beyond the static collector
emitter breakdown voltage at open base (BVCEO), without any
failure of transistors, when a parallel resistor is added on the
[6]
244
J.F. Imbornone, M. Murphy, R.S. Donahue and E. Heaney "New
insight into subharmonic oscillation mode of GaAs power amplifiers
Under Severe Output Mismatch Condition", IEEE Journal of SolideState Circuits, Vol. 32, pp. 1319-1325, September 1997.
A. Inoue, S. Nakatsuka, S. Suzuki, K. Yamamoto, and T. Shimura,
“Direct measurement of the maximum operating region in GaAs HBTs
for RF power amplifiers,” IEEE MTT-S Digest, pp. 1687-1690, June
2001.
S. Heckmann, J.M. Nébus, R. Quéré, J.C. Jacquet, D. Floriot and P.
Auxemery "Measurement and modelling of Static and Dynamic
Breakdowns of Power GaInP/GaAs HBT’s", IEEE MTT-S Digest,
pp1001-1004, June 2002.
A.J. Joseph, J. Dunn, G. Freeman, and D.L. Harame "Product
Applications and technology Directions With SiGe BiCMOS", IEEE
Journal of Solid State Circuits, Vol. 38, N0 9, pp 1471-1478.
September 2003.
W.Karoui , P.Riondet , G.Montoriol , T.Parra : "An adaptive protection
circuit for a power amplifier", Freescale worldwide patent
PCT/EP2005/005210.
E. Jarvinen, S. Kalajo, and M. Matilainen, “Bias circuits for GaAs HBT
power amplifier,” IEEE MTT-S Digest, pp 507-510. June 2001.