Download Improved Hybrid Amplifier Efficiency Using SiGe

IEEE BCTM 7.2
Improved Hybrid SiGe HBT Class-AB Power
Amplifier Efficiency Using
Varactor-Based Tunable Matching Networks
W.C.E. Neo X. Liui, Y. Lini, L.C.N. de Vreedel, L.E. Larson,
M. Spirito, A. Akhnoukh', A. de Graauw3, and L.K. Nanver'
1DIMES, Delft University of Technology, Feldmannweg 17, 2628 CT, Delft, The Netherlands,
2University of California at San Diego UCSD, La Jolla, CA 92093, USA
3Philips Semiconductors, Gerstweg 2,6534 AE, Nijmegen, The Netherlands.
Abstract: This paper presents a 1.8GHz prototype
class-AB power amplifier using a QUBIC4G
(SiGe, ft=40GHz) handset device with adaptive inand output matching networks. The realized
amplifier provides: 13dB gain, 28 dBm output
power, with an efficiency greater than 33-51%
over a 10dB output power control range.
I. INTRODUCTION
The power amplifier stage in a mobile handset is
considered to be one of the most power hungry
components. As a result, the talk-time of a typical
handset is restricted to several hours by the limited
power amplifier (PA) efficiency and battery
performance. There are two basic constraints for a
mobile system that are responsible for this limitation:
- The maximum output power required, related to
transmitted power, when the handset is operated
at a far distance from the base-station.
- The high linearity requirement of a modern
wireless communication system, which translates
to a power back-off condition of several dB's for
the output stage.
The high linearity requirement of modem
communication standards typically requires the use of
class-AB operation for the output stage [1], which
provides a workable compromise between linearity
and efficiency. When considering the linearity, the
class-AB output stage must be dimensioned so that it
provides its peak-output power without saturation. As
a result, for a given peak-output power (Ppeak) and
battery voltage (VYC), the load impedance at the
fundamental frequency is fixed: RL= 0.5.
Unfortunately, class-AB operation provides only its
highest efficiency under maximum drive conditions.
When operated at the required back-off level for
linearity reasons, a dramatic loss in efficiency occurs.
Furthermore, the efficiency of the Class-AB amplifier
is even lower when the amplifier is operated at
"average" output power levels in a CDMA
environment [1]. It is for these reasons that improving
amplifier efficiency, while maintaining linearity, is a
currently a major research topic in wireless
communications.
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Most of these efforts are focused on improving
the device or amplifier linearity up to compression, in
this way the output stage can be operated closer to the
compression point yielding a higher efficiency.
Examples include the use of harmonic or out-of-band
terminations [2] or digital pre-distortion [3]. Although
effective, these methods are less effective when the
handset PA is used in low-output power mode.
Techniques that address this mode are dynamic
biasing [4] or regulation of the supply voltage of the
output stage [5]. Dynamic biasing provides only
modest improvements in efficiency, and supply
voltage regulation requires an efficient dc-to-dc
conversion, increasing system cost and complexity.
An alternative for improved class-AB efficiency is
load-line adjustment as function of output power
using an adaptive or reconfigurable output matching
network. Typically, such a network is based on PINdiode or PHEMT switching of matching elements like
inductors, transmission-lines or capacitors. In
addition, MEMS capacitors are currently being
considered for this application although they still have
manufacturing and reliability constraints.
An altemative approach is to use varactor diodes
to continuously tune the impedance of a matching
network [6]. Recent work [7], demonstrated ultra-low
loss (Q>100@2GHz) ultra-low distortion anti-series
configured varactor diodes which can act as nearideal tuning elements [8]. By using a customized
silicon-on-glass technology, these varactor topologies
were used to implement two continuous-tunable
matching networks, which provide low-loss and lowdistortion and a 1:10 impedance transformation range
[9]. In this paper we present for the first time, a
complete amplifier with reconfigurable input and
output matching networks based on high performance
varactors. The resulting amplifier provides an
efficiency of greater than 31-55% over a 10dB output
power control range, the total size of the amplifier is
less then 10mm2.
II. OPTIMUM LOADING CONDITONS
For an ideal Class-B amplifier, when both V, and
RL are fixed, the collector efficiency decreases
linearly with the square root of the RF power [10]. As
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IEEE BCTM 7.2
a result the ideal class-B efficiency will drop from
78.5% to 24.8% for a 10dB power back-off condition.
However, the efficiency can be dramatically
improved by applying adaptive matching at the
output. For example, the load-line can be adjusted in
such a way that the voltage amplitude at the collector
always equals the bias voltage (Vcc), which will yield
the maximum class-B efficiency of 78.5% for a single
tone excitation, even under reduced output power
conditions. If the loading impedance is changed
dynamically with the envelope of the modulated
signal, high efficiency can be also achieved for
modulated signals with a high peak-to-average signal
power ratio.
Fig. 1. Block diagram schematic ofimplemented
power amplifier circuit
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III. PA and RECONFIGURABLE
MATCHING NETWORKS
The principle schematic is given in Fig. 1., which
consists of an active device embedded in a tunable
input- and output matching network.
At the input a simple tunable LC configuration is
used as pre-match for the active device. A two-stage
LC-ladder topology has been utilized for the outputmatching network. All the tunable capacitors are
composed out of an anti-series configuration of two
varactors (Fig. 3), which form a so-called varactor
stack (VS). A high ohmic resistor and two diodes in
anti-parallel configuration have been used to realize
sufficiently high impedance for the VS center tap, in
order to avoid linearity degradation for narrow tone
spacings [7]. Each VS is independently controlled
through its center-tap voltage to achieve the desired
capacitance variation and impedance transformation.
Since uniformly doped varactors have been used for
the VS, the capacitance tuning range is limited in this
case to Cma,/Cmin<2.5 for the applied RF signal
conditions. The optimized values of the inductances
and the zero-bias capacitance of the varactors are
given in Table 1. The related matching network
achieves an impedance transformation ratio > 10 with
varactor control voltages less than 15 volts.
Input Matching Network
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performed, where the QUBIC4G device was modeled
with Mextram. For each input power level we obtain
the load impedance that provides the maximum
efficiency and give the related output power (Fig 2).
As can be observed, a 10dB power output range can
be achieved using only a relative limited change in
loading impedance.
Output Matching Network
30
Fig. 2. a) Optimum loading trajectory for maximum
efficiency as function of output power. b) efficiency
and gain for the optimum loading trajectory.
When considering the PA configuration of Fig.1
the maximum output power delivered to the load
network is set by the supply voltage V,, and loading
condition FL. For an ideal PA (without output
capacitance and resistance) the optimum loading is
purely resistive and decreases with increasing output
power (Fig. 2.). For a real device, however, the
presence of output parasitics complicates the
determination of the optimal load for a given power
level. To solve this, load-pull simulations were
(b)
Fig. 3. (a) Schematic of input matching network (b)
Schematic of output matching network
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Table 1. Component valuesfor the output matching
network.
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IEEE BCTM 7.2
IV. IMPLEMENTATION
A microphotograph of the hybrid amplifier is shown
in Fig. 4. For the active device we have used a twoWatt SiGe, QUBIC4G transistor with a fT of 40 GHz
[11]. Both the input and output matching network
(Fig. 2) are implemented using the TU Delft siliconon-glass technology, which has been customized for
the implementation of ultra low-loss high
performance varactors (Q>100@2GHz). By using
these varactors in an anti-series configuration, a
tunable capacitor with extremely low distortion is
realized [7]. The inductors were implemented using
bond-wires to facilitate some additional adjustment of
the inductances values as well.
To have an estimate of the degradation in the
overall power efficiency due to the losses in the
output matching network, we have performed a
simulation using an ideal class-B device and the
measured s-parameter values of the output matching
network. In Fig. 6a the efficiency traces of the ideal
class-B amplifier for different output loading
conditions as function of output power are given
assuming a loss-less matching network. In Fig. 6b this
simulation has been repeated but now taking in
account the measured losses of the adaptive outputmatching network. As consequence of the losses in
the output matching network the efficiency decrease
approximately 12% at high output power levels
(30dBm) and 30% at 10dB back-off (20dBm).
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V. MEASUREMENTS
- Ideak Matching
02-
Adaptive matching network: First we have separately
characterized the adaptive output matching network
using s-parameter measurements. Fig. 5(a) gives the
measured rL provided by the output matching
network. As can be concluded from Fig. 5(a), the
optimum loading conditions for the QUBIC device
for various output power levels (as plotted in Fig.
2(a)) are sufficiently covered by the tuning range of
the adaptive output network. Note that the matching
network has a relatively low-loss (0.7 dB) at small
input impedances; at higher input impedances the
losses tend to increases (Fig. 5b). This behavior is a
result of the optimization procedure used for the
tuning range of the matching network, which was
focused on achieving low-Q conditions (low-losses)
in the ladder network for high signal powers (low-
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Fig. 6. Computed power efficiency of an ideal ClassB amplifier with (a) ideal matching network and (b)
with our matching network
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Fig. 7. Measured gain and efficiency of the power
amplifier circuit in the high-power mode.
Amplifier measurements: Swept power single-tone
measurements have been performed at various
settings of the varactor stack control voltages. Fig. 7
shows both the gain and collector efficiency for the
high output power mode, with 28 dBm maximum
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IEEE BCTM 7.2
VI. CONCLUSION
In this work we have designed and implemented a
RFPA operating at 1.8GHz using low-loss lowdistortion reconfigurable in- and output matching
networks, which facilitates high efficiency over a
large power back-off range. Although not reported in
this paper, future versions of the presented amplifier
are in principle capable to handle antenna miss-match
conditions, band-switching or provide even a good
starting point for dynamic loadline concepts [12],
where the envelop of the modulated signal is used to
modify the load impedance, yielding high efficiency
even for signals with a large peak-to-average power
ratio. Consequently, its compact size, low cost,
flexibility, and high performance making it an
interesting option for application in future handsets.
output power with
a maximum efficiency of 51%.
Using the previously measured s-parameters of the
output-matching network we were able to reasonably
predict the required control voltages for providing the
optimum efficiency loading conditions for lower
output power levels (Figure 8). The device was biased
in class-AB operation while the input network was
tuned for a conjugate match. It can be noted from Fig.
8 that an efficiency greater than 33-51% over a 10dB
output power control range is achieved.
We observe that just as predicted from
simulations the efficiency traces move left from
maximum power setting (O trace) to a low power
setting (+ trace). Consequently, if we link all peak
efficiency points for the different settings of the
output matching network together; we are able to
visualize the efficiency improvement of our power
amplifier circuit over classical static matching
solutions (Figure 9). Note the more than 15%
improvement, which is basically a doubling of the
initial power efficiency in back-off.
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VII. REFERENCES
[1] J. Deng et al, "A High Average-Efficiency SiGe HBT
Power Amplifier for WCDMA Handset Applications, "
IEEE Trans. on Microwave Theory and Techniques, Vol.
53, Issue: 2, Feb. 2005 pp.529-537
[2] M. van der Heijden et al, "On the optimum biasing and
input out-of-band terminations of linear and power efficient
class-AB bipolar RF amplifiers," IEEE Bipolar and
BiCMOS Circuits and Technology Meeting, pp.44-47, Sept.
2004.
[3] R. Marsalek et al,"From post-distortion to pre-distortion
for power amplifiers linearization," IEEE Communications
Letters, Vol. 7, Issue: 7, pp. 308-3 10, July 2003
[4] A. Khanifar, "Enhancement of power amplifier
efficiency through dynamic bias switching," IEEE MIT-S
International Symposium Digest, Vol.3, pp.2047-2050,
June2004.
[5] G. Grillo et al, "Adaptive biasing for UMTS power
amplifiers," in Proc. IEEE Bipolar and BiCMOS Circuits
and Technology Meeting, pp. 1 88-191, Sept. 2004.
[6] Yumin Lu et al, "A MEMS reconfigurable matching
network for a class AB amplifier," IEEE Microwave and
Wireless Components Letters, Vol. 13, Issue: 10, pp. 437439, Oct. 2003.
[7] K. Buisman, et al., "'Distortion Free' varactor diode
topologies for RF adaptivity", To be published in IMS
2005.
[8] R.G. Meyer et al., "Distortion in Variable Capacitance
Diodes," IEEE JSSC, vol.Sc-10, no. 1 Feb 1975
[9] K. Buisman et al, "Low-Distortion, Low-Loss Varactorbased Adaptive Matching Networks, Implemented in a
Silicon-on-Glass Technology," To be published in RFIC
2005
[10] S. Cripps, RF Power Amplifier for Wireless
Communication. Boston, MA: Artech House, 1999.
[ 1] http://www.semiconductors.philips.com/
[12] F.H. Raab, "High-efficiency linear amplification by
dynamic load modulation," 2003 IEEE MTT-S International
Microwave Symposium Digest, Vol.3, pp. 1717-1720, June
2003.
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Amplifier Circuit at various output matching network
settings. Legend shows the control voltages of the
varactor at the device and at the load-side
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Fig. 9. Collector efficiency improvement with the
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