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A SiGe PA With Dual Dynamic Bias Control and Handset Applications

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A SiGe PA With Dual Dynamic Bias Control and Handset Applications
1210
IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 41, NO. 5, MAY 2006
A SiGe PA With Dual Dynamic Bias Control and
Memoryless Digital Predistortion for WCDMA
Handset Applications
Junxiong Deng, Member, IEEE, Prasad S. Gudem, Member, IEEE, Lawrence E. Larson, Fellow, IEEE,
Donald F. Kimball, Member, IEEE, and Peter M. Asbeck, Fellow, IEEE
Abstract—This paper demonstrates a two-stage 1.95-GHz
WCDMA handset RFIC power amplifier (PA) implemented in a
0.25- m SiGe BiCMOS process. With an integrated dual dynamic
bias control of the collector current and collector voltage, the average power efficiency of the two-stage PA is improved from 1.9%
to 5.0%. The measured power gain is 18.5 dB. The gain variation
with dynamic biasing is less than 1.8 dB. An off-chip memoryless
digital predistortion linearizer is also adopted, satisfying the
3GPP wideband code division multiple access (WCDMA) linearity
specification by a 10 dB improvement of adjacent channel power
ratio (ACPR) at 26 dBm average channel output power.
TABLE I
3GPP WCDMA CLASS 3 HANDSET POWER AMPLIFIER SPECIFICATIONS [3]
+
Index Terms—ACPR, average power efficiency, digital predistortion, dynamic bias control, linearity, memoryless systems, power
amplifiers, Silicon Germanium, WCDMA.
I. INTRODUCTION
ILICON GERMANIUM (SiGe) is an attractive candidate
for the development of handset power amplifiers (PAs),
because of its competitive performance in efficiency, linearity,
cost, and integration with BiCMOS technology [1], [2]. In
wireless communications systems, PAs are key components
that consume a significant portion of the DC power budget
in the transmitter. For third-generation cellular systems, like
wideband code division multiple access (WCDMA), more
stringent linearity is required in terms of adjacent channel
power ratio (ACPR) for higher spectral efficiency. Therefore,
linearity and efficiency are the most critical parameters in the
WCDMA PA design.
In WCDMA systems [3], different power classes are required
for the maximum transmit power at the output of the transmitter.
Also, there are three operation bands (band I: 1920–1980 MHz;
band II: 1850–1910 MHz; band III: 1710–1780 MHz) under
different scenarios. Power Class 3 is the highest power class
applicable to all three operation bands and the most common
power class in practice, and hence the design objective in this
S
Manuscript received September 9, 2005; revised December 17, 2005. This
work was supported by the UCSD Center for Wireless Communications and its
Member Companies and by a UC Discovery Grant.
J. Deng was with the Department of Electrical and Computer Engineering,
University of California at San Diego, La Jolla, CA 92093 USA. He is now with
Qualcomm Inc., San Diego, CA 92121 USA (e-mail: [email protected]).
P. S. Gudem is with Qualcomm Inc., San Diego, CA 92121 USA.
L. E. Larson, D. F. Kimball, and P. M. Asbeck are with the Department of
Electrical and Computer Engineering, University of California at San Diego, La
Jolla, CA 92093 USA.
Digital Object Identifier 10.1109/JSSC.2006.872735
paper. Correspondingly, the design specifications for WCDMA
Class 3 PAs are summarized in Table I.
“Average” power efficiency, which is the ratio of the energy
delivered to the antenna to the energy derived from the battery,
is the key factor determining the talk time and battery life for
portable wireless applications [4]. Linear PAs are typically operated in class AB mode and are often backed off from the maximum output power to achieve the desired linearity. Even though
peak power efficiency may be high at high output powers, it
drops quickly with the power back-off, resulting in a poor average power efficiency.
To increase power efficiencies in the low-power region,
different dynamic biasing techniques [5]–[8] have been developed. Altering the DC bias current in response to changing
power requirements—also known as dynamic current biasing
(DCB)—often results in significant amplifier gain variation.
Although this gain-variation issue may be alleviated through
the power control loop of the CDMA handset, it adds an extra
burden to the algorithm. Furthermore, considering the power
amplifier as a standalone block, this gain-variation can dramatically degrade the error vector magnitude (EVM), a system-level
figure of merit for the accuracy of digitally modulated signals.
Altering the DC bias collector voltage in response to changing
power requirements—also known as dynamic voltage biasing
(DVB)—typically requires the use of DC-DC converters with
their associated large off-chip components and extra chip area
and cost. Therefore, it is not attractive for small feature size
handset PAs.
A dual dynamic bias (DDB) control scheme [9] that does
not require any additional off-chip components is proposed
in this paper to provide an integrated solution with improved
0018-9200/$20.00 © 2006 IEEE
DENG et al.: A SIGE PA WITH DUAL DYNAMIC BIAS CONTROL AND MEMORYLESS DIGITAL PREDISTORTION FOR WCDMA HANDSET APPLICATIONS
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Fig. 2. BJT current versus voltage, demonstrating different dynamic biasing
strategies (DC/VB: dynamic current/voltage biasing).
Fig. 1. DC current consumption of a two-stage class AB PA with fixed bias and
a representative CDMA probability distribution function (PDF) [15].
average power efficiency and near-constant power gain for
WCDMA Class 3 handset power amplifiers. In addition, an
off-chip memoryless digital predistortion (DP) [10]–[14] based
on the measured AM-AM and AM-PM characteristics is also
employed to substantially improve the linearity of the PA. The
resulting performance meets the WCDMA linearity specifications with significantly improved efficiency. The result is a
low-cost WCDMA power amplifier.
This paper is organized as follows. In Section II, we propose the dual dynamic bias technique to enhance the average
power efficiency of a prototype two-stage power amplifier, and
design considerations will be illustrated in detail. In Section III,
we introduce memoryless digital predistortion as the linearization technique for the power amplifier. Predistortion algorithms
and implementation details are also presented. In Section IV,
the validity of the dual dynamic bias and digital predistortion
is verified with experimental results. Conclusions are given in
Section V.
II. DUAL DYNAMIC BIAS CONTROL
FOR EFFICIENCY ENHANCEMENT
To improve the average power efficiency, it is necessary to
reduce the dc current when the PA delivers a low output power.
Fig. 1 shows the DC current consumption of a two-stage class
AB power amplifier and a representative CDMA probability distribution function (PDF) as a function of output power [15]. At
low output powers, the power amplifier is operated at a fixed
bias current, and therefore, the efficiency is degraded; more
than 90% of the output power occurs between 15 dBm and
15 dBm, where the efficiency is low. The average power efficiency is defined as
(1)
is the output power,
the probability of
where
and
the efficiency at output power
output power
. For example, using the PDF of Fig. 1, the average power
efficiency of a Class AB amplifier is very low—below 2%.
One of the main design objectives in this work is to achieve an
average power efficiency far better than 2%.
Fig. 3. Simplified schematic of a dynamic current bias.
A. Principle of Dual Dynamic Biasing
As shown in Fig. 2, we propose an integrated DDB control
that reduces both the DC bias current and voltage without the
use of external DC-DC converters. The DDB adopts a one-step
approach with current re-use in the “low-power” group to reduce
the DC voltage across the transistors.
The dual dynamic biasing control includes dynamic current
biasing (DCB) and dynamic voltage biasing (DVB). DCB is
achieved by varying the bias current in response to the output
power requirements. As shown in Fig. 3, there are two groups
of SiGe HBT transistors: a high-power group (of 100 devices)
and a low-power group (of 20 devices). The high-power group is
biased at a current of 110 mA, whereas the low-power group is
biased at 22 mA. The switching between different power groups
is controlled by low-loss NFET switches on the bases of the
HBT transistors. When the PA enters the low-power region, the
high-power group is switched off and the low-power group is
switched on. Therefore, the total bias current is reduced.
On the other hand, DVB is realized by reusing the bias current. As shown in Fig. 4, the high-power group is the same as
that in Fig. 3. The low-power group has two sub-groups and
each sub-group has 20 devices. In the high-power group, each
; whereas
transistor is connected in parallel and biased at
in the low-power group, two sub-groups are series-connected to
reuse the common bias current. Each transistor in the sub-group
, and hence the DC bias voltage is lowered.
is biased at
Combining DCB and DVB, the DC power consumption of the
power amplifier is significantly reduced.
The DDB technique can be expanded to more than the one
power control step used here. Several factors influence the op-
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IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 41, NO. 5, MAY 2006
• A constant collector current density is essential for
achieving constant gain [16].
• The connection between two power groups adds parasitics
to each individual group, which degrades the overall
gain. However, this connection decreases the difference
between the gains in the two power modes since the input
impedance of the output stage changes little between
high-power and low-power operation.
from the output of the
• The routing line inductance
low-power group to that of the high-power group is optimized to boost the gain in the low-power mode, by partially resonating the output capacitance of the high-power
group. Since the output impedance of the low-power device is higher than that of the high-power device, the parasitic capacitance at the output has a larger degrading effect
on the low-power gain. By partially resonating the parasitic
capacitance in low-power operation, the gain difference between the two power modes is further reduced.
Fig. 4. Simplified schematic of a dynamic voltage bias.
Fig. 5. Comparison of average power efficiencies versus the number of power
control steps for a SiGe power amplifier.
timum number of power control steps, including the added complexity and parasitics associated with additional steps, the expected gain variation, and the reduced power consumption at
the lowest output power levels. For example, based on the simulation results for a SiGe PA shown in Fig. 5, a single output
stage with a two-step DDB achieves better average power efficiency (8.9%) than its one-step counterpart (7.7%); however,
the corresponding circuit complexity increases dramatically and
the achievable power gain also drops due to the added parasitics.
More steps result in further diminishing returns, so the one-step
topology was finally adopted in our prototype PA.
B. Gain Variation With Power Control
The gain of the PA should remain roughly constant as it
switches from low-power to high-power operation.1 The following design methodologies are employed to achieve this
goal.
1Besides the gain variation, the phase change is another important parameter
for EVM. There is a phase jump when the amplifier is switched from the highpower mode to the low-power mode in the DDB operation. Since this phase
change is fixed, it is readily compensated during the handset calibration period.
C. Circuit Design Considerations
1) Two-Stage Design: A simplified schematic of the prototype two-stage PA with the DDB control is shown in Fig. 6.
The driver stage, low-power group, and high-power group are
all biased in class AB mode, to achieve the best tradeoff of efficiency and linearity. The driver stage has 20 HBTs and the
low-power group has 20 switches and 2 20 HBTs in series.
The high-power group uses 100 HBTs and 100 NFET switches.
m
m and each NFET switch
Each HBT emitter is
is 45 m/0.25 m. The optimal design of the NFET switches
for power gain and 1 dB compression point has been analyzed
in [16].
2) Bias Network: To achieve better linearity [17], a low DC
base impedance is employed for the driver stage and the highpower group in the output stage. Since the DC current behavior
in the low-power group is determined by the lower 20 devices,
a low DC impedance is provided at the bases of these devices.
In this way, when the input power increases, the DC current in
the low-power group also rises. Consequently, the linearity of
the low-power group improves. On the other hand, the top 20
devices are biased with a fixed bias voltage through a resistor to
the base, eliminating the need for an extra bias inductor, since
the linearity requirement is readily met at low powers. As shown
in Fig. 7, the bias network is composed of a helper and a
low impedance buffer, to provide a constant voltage bias and to
terminate the low-frequency components for improved linearity
[18]. The DC impedance is approximately
(2)
for
MHz
Based on our simulations,
(i.e., the channel bandwidth for WCDMA handset PAs). Fig. 8
shows the simulated third-order intermodulation ratio IMR3
(i.e., the ratio between the third-order intermodulation product
and the fundamental tone) at 12 dBm output power and 10-MHz
offset frequency. Clearly, the low-frequency termination improves the linearity of the power amplifier.
DENG et al.: A SIGE PA WITH DUAL DYNAMIC BIAS CONTROL AND MEMORYLESS DIGITAL PREDISTORTION FOR WCDMA HANDSET APPLICATIONS
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Fig. 6. Simplified schematic of a two-stage prototype power amplifier with the dual dynamic biasing in the output stage.
Fig. 7. Simplified schematic of the constant voltage bias network.
3) Miscellaneous Details: Ballasting resistors are inserted in
the emitter of each transistor to prevent thermal runaway. From
to prevent thermal runaway is given
[19], a lower bound on
by
(3)
and
are the DC collector bias current and voltage,
where
is the thermal resistance.
respectively, and
[20],
mA and
Assuming that
, we find
to insure thermal stability.
The output matching network (OMN) is primarily designed
for the high-power group to achieve the maximum output
power, i.e., 26 dBm. The OMN transfers the load impedance
(50 ) into the load-line impedance (6 ) of the high-power
group while resonating the output susceptance. To achieve
better power gain and efficiency, the OMN is implemented
with high-Q off-chip discrete components. The routing line
from the output of the low-power group to
inductance
that of the high-power group acts as one part of the OMN in
the low-power mode to boost the power gain, as depicted in
Section II-B.
Fig. 8. Simulated third-order intermodulation ratio versus the bias network
impedance.
III. MEMORYLESS DIGITAL PREDISTORTION
FOR LINEARITY IMPROVEMENT
Linearization techniques improve the linearity of power amplifiers as well as their maximum output power and efficiency.
Memoryless digital predistortion (DP) techniques [10] are attractive for a number of reasons. First, they can be more economical than feed-forward techniques, which are widely applied
in base-station PAs [11]. Even though the cost to implement
DP in commercial handsets is still high, the demand for wireless high-data-rate applications should keep lowering the cost.
Second, DP does not result in the gain loss that is common in
analog predistortion techniques [21]. Third, memory effects are
not strong in handset PAs compared to base-station PAs, so only
memoryless distortion, including gain compression (AM/AM
distortion) and phase deviation (AM/PM distortion), needs to
be considered and the corresponding predistortion algorithm
is straightforward. Finally, since predistortion is implemented
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IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 41, NO. 5, MAY 2006
Fig. 9. Block diagram of memoryless digital predistortion.
with digital signal processing (DSP), it can be performed accurately and flexibly. Therefore, DP is employed to improve the
linearity of the prototype WCDMA PA.
A. System Topology
A block diagram of the DP system is shown in Fig. 9. The DP
system downconverts a portion of the 1.95-GHz output signal to
an analog IF of 140 MHz, which is then converted to a 12-bit
digital second IF of 26.88 MHz at a sample rate of 107.52 MHz
and fed to the DSP. After comparison with the corresponding
input signal during a “training” period, the resulting error amplitude and phase signal are used to adaptively predistort subsequent input I/Q signals. The predistorted digital signal is converted to a 14-bit analog signal at an IF of 140 MHz, and then
upconverted to the final 1.95-GHz output. This digital IF approach was recently demonstrated in a low-power WCDMA upconverter IC [22].
B. Predistortion Algorithm
The amount of predistortion is controlled by a look-up table
(LUT) in the DSP that characterizes the nonlinearities of the RF
PA. For the LUT implementation, a number of predistortion algorithms have been proposed [12]–[14]. A straightforward solution is mapping predistortion. As shown in Fig. 10(a), a two-dimensional table in a random access memory (RAM) contains
complex predistorting signals. The mapping table accepts the
input signal in Cartesian form as the table address and outto predistort the
puts the corresponding correction value
signal . The predistorted signal is obtained with the sum of
and
. The major drawback of this approach is the size of
the two-dimensional table (e.g., 2 Mwords in [12]).
To overcome the size issue of the mapping predistortion,
Faulkner proposed a polar predistortion [13], which contains
two one-dimensional tables controlling gain and phase compensations, respectively. As shown in Fig. 10(b), the input
signal is predistorted after the gain factor multiplication and
phase rotation. In order to find the gain factor in the gain table,
the Cartesian input signals need to be converted into the one-dimensional amplitude signal as the address in the gain table.
The address of the phase table is obtained by multiplying the
amplitude signal with the gain factor. The adaptive correction
terms for the gain and phase tables are obtained by comparing
the gain and phase differences, respectively, between the input
signal and the sampled output signal. Compared with the
mapping counterpart, the polar digital predistortion reduces
Fig. 10. Predistortion algorithm comparison [23]. (a) Mapping predistortion.
(b) Polar predistortion. (c) Complex-gain predistortion.
the table size considerably. The size of the polar table is three
orders of magnitude less than that of the mapping counterpart
[12], as shown in Fig. 11.
Cavers [14] presented another approach using a complex-gain
table instead of two separate gain and phase tables. As shown in
Fig. 10(c), the input signal is predistorted by a complex multiplication between the input signal and the complex gain factor.
The address of the complex gain factor is obtained by calculating the squared magnitude of the input signal. The complexgain approach requires more than three orders of magnitude less
memory than its mapping counterpart [12] and reduces convergence time. Because of its low memory and fast adaptation, the
complex-gain predistortion is adopted as the predistortion algorithm in this paper.
DENG et al.: A SIGE PA WITH DUAL DYNAMIC BIAS CONTROL AND MEMORYLESS DIGITAL PREDISTORTION FOR WCDMA HANDSET APPLICATIONS
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Fig. 12. Conceptual implementation diagram of complex-gain predistortion
[24].
Fig. 11. Look-up table (LUT) comparison. (a) Mapping LUT. (b) Polar LUT.
Fig. 13. Block diagram of memoryless digital predistortion.
To simplify the above expression, let
C. Complex-Gain Digital Predistortion
The operating principle of the complex-gain digital predistortion is illustrated in the conceptual implementation diagram
shown in Fig. 12. Note that the quadrature up- and down-converters and the reconstruction and anti-aliasing filters are ignored for simplicity. It is straightforward to derive the output
as
signal
(4)
is the input signal,
denotes the transfer function
where
of the nonlinear PA, and
represents the predistortion cor, is
rection function. At time instant , the amplifier output
. Hence,
equal to the desired output
(5)
For the ideal amplification with a constant gain
. Then
(9)
The magnitude correction of the predistortion function
can be found as
(10)
Consequently, the way to calculate the magnitude correction of the predistortion function is a procedure of rescaling
the input magnitude. The principle of magnitude rescaling is
illustrated in Fig. 13. For any input power , the corresponding
. The actual amplifier is
ideal output power is equal to
capable of yielding the same output power, but at the scaled
. Therefore,
. This inverse
input power
scaling is readily described in (10).
is simply the inverse of the phase
The phase correction
error at the rescaled magnitude:
(11)
(6)
Combining (5) and (6), we have
D. Practical Issues
(7)
Assume that
components of
yields
and
are the magnitude and phase
. Taking the magnitude of both sides of (7)
(8)
From the output of the DSP to the input of the PA, the predistorted signal passes through the transmitter IC (TxIC), which
consists of a D/A converter, a reconstruction filter, and a modulator. To maximize the overall system performance, it is critical
to analyze and correct the nonidealities of the TxIC, including
phase deviation of the reconstruction filter [25] and I/Q imbalance of the modulator [24].
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IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 41, NO. 5, MAY 2006
TABLE II
EFFECTS OF DIFFERENT RECONSTRUCTION FILTERS WITH f
Fig. 14. Visualization of a two-tone test in a transmitter. (a) Without digital
predistortion and with an ideal TxIC. (b) With digital predistortion and an ideal
TxIC. (c) With digital predistortion and a nonlinear TxIC.
Fig. 15. Magnitude and phase responses of an ideal analog filter, where is
the phase deviation of the IMD3 relative to the fundamental tone.
1) Phase Deviation: To simplify the analysis, a two-tone test
in a transmitter will be analyzed here. Fig. 14(a) shows the case
without digital predistortion and with an ideal TxIC, where a
third-order intermodulation product (IMD3) occurs at the output
of the nonlinear PA. Fig. 14(b) demonstrates the case with digital predistortion and an ideal TxIC. Two pre-generated IMD3
and
) together with two fundaproducts (
mental tones ( and ) are transmitted to the input of the
nonlinear PA. Without any dispersion by the ideal TxIC, the resultant IMD3 becomes zero at the output of the nonlinear power
amplifier, due to the cancellation of the digitally generated products and the nonlinear products of the amplifier.
In the case with a nonideal TxIC, there is a phase deviation
) relative to the funof the pre-generated IMD3 (e.g.,
damental tone (e.g., ). As shown in Fig. 14(c), the resultant
IMD3 will be greater than zero and hence the linearization performance is degraded.
The reconstruction filter yields the major phase deviation in
the TxIC. The magnitude and phase responses of an ideal analog
the
filter is shown in Fig. 15, where is the filter order,
3 dB frequency,
(e.g., ) and
(e.g.,
)
= 40 MHz
Fig. 16. Vector analysis of IMD3 cancellation.
the fundamental and third-intermodulation tones, respectively,
relative to
. Generally,
and the phase deviation of
there is negligible amplitude deviation, since the filter magni. The phase shift of
tude response remains flat when
relative to the DC frequency will be neglected here for
simplicity, since it is typically small.
As an example, different types of seventh-order2 filters are
,
compared with the phase deviation in Table II, where
, and
are assumed to be 40 MHz,3
2 MHz, and 4 MHz, respectively. The resulting IMD3 degradations are also listed. As expected, elliptic filters achieve the
best performance of the three types of filters, due to their flatter
phase response within the passband.
A simple vector analysis can be used to illustrate this cancellation degradation problem [26]. In Fig. 16, vectors A1 and
A2 are the IMD3 and anti-IMD3 sidebands generated by the PA
and the PD, respectively, for the case without a filter. The initial
phase of A1 is and the phase difference of A1 and A2 is .
The amplitude of the IMD3 after cancellation is A3:
(12)
The filter can alter both the amplitude and phase of the fundamental and predistorted sideband components. For simplicity,
we make several assumptions. First, the baseband amplitude
variation is ignored, which is reasonable since most filters for
this application will have small passband ripple—usually less
than 0.5 dB. Second, there are no PM to AM and PM to PM distortions in the PA, which is also reasonable, if the PA is represented by a quasi-memoryless model. With these assumptions,
and
have the
the filtered IMD3 and anti-IMD3 vectors
same amplitude as A1 and A2, but the phase difference between
instead of due to the different
them is changed to
2Here,
3The
a seventh-order filter is chosen as a representative realistic order [25].
predistorted input signal after the DSP has wider bandwidth than that
of an ideal input signal without the digital predistortion. Consequently, the D/A
converter and the reconstruction filter need to be very wideband to accommodate
the predistorted signal. In our experimental predistortion system, the bandwidth
of the reconstruction filter is selected to be 40 MHz.
DENG et al.: A SIGE PA WITH DUAL DYNAMIC BIAS CONTROL AND MEMORYLESS DIGITAL PREDISTORTION FOR WCDMA HANDSET APPLICATIONS
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phase shift within the passband of the filter shown in Fig. 15.
:
The amplitude of the new IMD3 after cancellation is
(13)
Using (12) and (13), the cancellation degeneration due to the
phase dispersion can be quantified.
To maximize the predistortion system performance, it is necessary to correct the phase deviation due to the reconstruction
filter in the predistortion algorithm. In [27], this “memory effect” has been effectively compensated with an iterative technique of successive approximations. The effect can be corrected
as follows:
• Memoryless distortion extraction. By applying a single
tone signal to the nonlinear amplifier, the AM/AM and
AM/PM distortions are extracted. Accordingly, a memoryless predistortion table is generated in the DSP.
• Memory effect extraction. Based on the memoryless predistortion table, a two-tone signal with varying tone spacing is
predistorted. After letting the resulting predistorted signal
pass through the transmitter and sampling the amplifier
output signal, the memory effect due to the phase deviation of the reconstruction filter is characterized at each frequency offset with the AM and PM tables.
• Frequency domain correction. The predistorted time domain input signal is transformed into the frequency domain through the fast Fourier transform (FFT). Based on
the PM table generated in the previous step, a phase equalizer is employed in the frequency domain to adjust individual phases at different frequencies. Therefore, the phase
deviation due to the reconstruction filter will be corrected
at the IMD3 frequency. Then the resultant signal is transformed back into a time-domain signal through the inverse
fast Fourier transform (IFFT). This time domain signal will
be transmitted to the A/D converter.
With the above procedure, the phase deviation due to the reconstruction filter is compensated.
2) I/Q Imbalance: It is important to maintain equal gains
and exactly 90 degree phase difference between the in-phase (I)
and quadrature (Q) paths of the upconversion chain. Any disparity in gain and phase will respectively yield amplitude- and
phase-dependent amplitude (AM-AM and PM-AM) distortion.
Since an amplitude-based predistortion algorithm will not correct phase-dependent errors, I/Q imbalance needs to be separately corrected in the DSP.
I/Q imbalance can be measured by transmitting a sine wave
on the I path
and a cosine wave
on the Q path
,
where the I path LO signal is assumed to be perfect for simand
are the amplitude and phase errors
plicity, and
of the Q path LO signal. Therefore, at the output the upconverter, we have
(14)
(15)
in which the output amplitude
is found to be
at the desired frequency
(16)
where the output amplitude error
is
(17)
which includes PM-AM distortion as well as AM-AM distortion.
is given by
Furthermore, the output phase
(18)
Consequently, the output phase error is nonlinear and comprises both AM-PM and PM-PM distortions.
In general, I/Q imbalance is compensated digitally [28] with
less expense and higher precision. A simple criterion for the
compensation algorithm is employed by observing the output
at the image frequency
, which is
amplitude
readily found to be
(19)
and
As shown in Fig. 17, there are two tones at
, respectively, on the resulting output spectrum, with
the first being much larger than the second. Assuming that I/Q
imbalance has been compensated perfectly (i.e., both
and
are zero) and there are no other modulator imperfecwill be completely suptions, the second tone at
pressed (i.e., (19) is zero).
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Fig. 17. Output spectrum showing imperfectly and perfectly balanced IQ modulations.
Fig. 20. Photograph of the packaged die of the prototype power amplifier. The
die area is 1.8 mm .
Fig. 18. Measured input/output behavior without digital predistortion—measured data (dots), ideal performance (dashed line).
Fig. 19. Measured input/output behavior with digital predistortion—measured
data (dots), ideal performance (dashed line).
E. Experimental Validation
The effectiveness of digital predistortion is checked by comparing the AM/AM relationship between input and output envelope amplitudes. Fig. 18 shows the AM/AM curve without digital predistortion. With digital predistortion, the output amplitude has been thoroughly linearized, as shown in Fig. 19. Based
on the above discussions, digital predistortion is an effective linearization technique for handset PAs.
IV. MEASUREMENT RESULTS
The prototype two-stage PA with the dual dynamic bias control was fabricated in a 0.25- m SiGe BiCMOS process [29].
The die area is 1 mm 1.8 mm. A packaged die with bonding
wires is shown in Fig. 20. The Micro Lead Frame (MLF20)
package is used due to its small body size and short bond-wires
along with excellent electrical and thermal performance.
Fig. 21 compares the measured DC currents for different
biasing approaches for the two-stage WCDMA PA, superimposed on a representative probability distribution function for
the output power [15]. These approaches include constant base
voltage (CV) biasing with a fixed number of parallel transistors
(the traditional Class-AB approach) and the dual dynamic
bias (DDB) proposed here. Their corresponding power-added
efficiencies (PAEs) are compared in Fig. 22, where the DDB
PA achieves much better PAE in the low-power region.
As shown in Fig. 23, average power efficiencies are calculated
from (1) to be: 1.9% for CV biasing, 3.8% for the DCB approach
proposed earlier [16]4 and 5.0% for DDB, all in two-stage PAs.
This verifies that DDB does achieve dramatically improved average efficiency for WCDMA applications.
With a WCDMA reverse-link signal, the linearity of the
power amplifier is measured in terms of ACPR for two cases:
with and without digital predistortion (DP). The measured
results at the output power of 22.4 dBm are shown in Fig. 24,
where the ACPR performance of the power amplifier is improved by 10 dB with digital predistortion.
The ACPR measurement at 5 MHz is shown in Fig. 25. Note
that it is only necessary to utilize the predistortion in the highpower mode, so DP is not applied in the low-power mode; this
is the reason for the large discontinuity in ACPR in the “after
DP” curve. The ACPR is improved by at least 8 dB with digital predistortion, and the maximum output power satisfying the
WCDMA linearity specification is improved from 22.4 dBm
to 26 dBm. This satisfies the WCDMA Class 3 requirement of
maximum output power. Correspondingly, the peak PAE is improved by 60% (from 17% to 27%).
Fig. 26 shows the measured gain of the DDB PA with DP.
The gain change for DDB is less than 1.8 dB, which is much
4Note that the average power efficiency reported in [16] is based on a single
output stage; therefore it is higher than what are reported here based on a twostage power amplifier, where the driver stage adopts the classical CV bias considering the gain degradation and circuit complexity.
DENG et al.: A SIGE PA WITH DUAL DYNAMIC BIAS CONTROL AND MEMORYLESS DIGITAL PREDISTORTION FOR WCDMA HANDSET APPLICATIONS
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Fig. 24. Comparison of measured spectrum with DP (dark line) and without
DP (light line).
Fig. 21. Output power probability distribution function Pe and measured DC
current comparison for different biasing techniques (CV: constant voltage
biasing; DDB: dual dynamic biasing). The switch point from high-power mode
to low-power mode is 16 dBm.
Fig. 25. Measured ACPRs of the DDB power amplifier with digital predistortion (DP) (before DP and after DP).
Fig. 22. Measured power-added efficiency (PAE) of CV and DDB power amplifiers.
Fig. 26. Measured gain of the DDB power amplifier with DP.
Fig. 23. Average power efficiency comparison (CV: constant base voltage;
DCB: dynamic current biasing; DDB: dual dynamic bias) in a two-stage power
amplifier.
in a SiGe HBT BiCMOS technology. With the proposed dual
dynamic bias technique, the average power efficiency of the
power amplifier is improved from 1.9% to 5.0%. With the
complex-gain digital predistortion, the ACPR and the peak
PAE of the power amplifier are improved by 8 dB and 60%,
respectively. The measured maximum output power is 26 dBm,
and the peak PAE is 27%.
ACKNOWLEDGMENT
more constant than the case if the power amplifier is operated
with dynamic bias without changing the device size.
V. CONCLUSION
An integrated two-stage power amplifier chip with dual
dynamic bias control and memoryless digital predistortion for
WCDMA handset applications was fabricated and measured
The authors would like to acknowledge useful discussions
with Mr. M. Li and Mr. H. Khatri, both of the University of
California at San Diego.
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Junxiong Deng (S’02–M’05) received the B.S.
degree in electrical engineering from Southeast University, Nanjing, China, in 1997, and the M.S. and
Ph.D. degrees in electrical engineering from the University of California at San Diego (UCSD), La Jolla,
in 2003 and 2005, respectively. His doctoral research
concerned high-efficiency and high-linearity power
amplifiers for advanced wireless communication
systems.
He is currently a Senior Design Engineer with
Qualcomm, San Diego, CA, where he has worked
on RF/analog design for next-generation wireless handset applications.
Prasad S. Gudem (M’96) received the Ph.D. degree
in electrical and computer engineering from the
University of Waterloo, Waterloo, ON, Canada, in
1996. His doctoral research concerned the modeling
and numerical simulation of amorphous silicon
devices.
After a brief time with Mitel Semiconductors,
Kanata, ON, Canada, he joined Cadence Design Systems, San Diego, CA, where he was involved with
semiconductor-device modeling for analog and RF
applications. In August 2000, he joined the Watson
Research Center, IBM, where he was involved with RF integrated-circuit
design for WCDMA applications using IBM’s SiGe technology. He was also
appointed a Lecturer with the University of California at San Diego, La Jolla,
where he taught a three-quarter course on communication circuit design. He
recently joined Qualcomm Inc., San Diego, CA, where he continues to work
on RF circuit design for wireless applications.
Lawrence E. Larson (S’82–M’86–SM’90–F’00) received the B.S. and M.Eng. degrees in electrical engineering from Cornell University, Ithaca, NY, in 1979
and 1980, respectively, and the Ph.D. degree in electrical engineering from the University of California
at Los Angeles (UCLA) in 1986.
From 1980 to 1996, he was with Hughes Research
Laboratories, Malibu, CA, where he directed the
development of high-frequency microelectronics in
GaAs, InP, and Si/SiGe and MEMS technologies.
In 1996, he joined the faculty of the University of
California at San Diego (UCSD), La Jolla, where he is Inaugural Holder of the
Communications Industry Chair. He is currently Director of the UCSD Center
for Wireless Communications. During the 2000–2001 academic year, he was on
leave with IBM Research, San Diego, CA, where he directed the development
of RF integrated circuits (RFICs) for third-generation (3G) applications. He
was a Visiting Professor at TU Delft in the Netherlands during the 2004–2005
academic year. He has authored or coauthored over 200 papers. He holds 31
U.S. patents.
Dr. Larson was the recipient of the 1995 Hughes Electronics Sector Patent
Award for his work on RF MEMS technology. He was co-recipient of the 1996
Lawrence A. Hyland Patent Award of Hughes Electronics for his work on lownoise millimeter-wave high electron-mobility transistors (HEMTs), the 1999
IBM Microelectronics Excellence Award for his work in Si/SiGe HBT technology and the 2003 Custom Integrated Circuits Best Invited Paper Award.
DENG et al.: A SIGE PA WITH DUAL DYNAMIC BIAS CONTROL AND MEMORYLESS DIGITAL PREDISTORTION FOR WCDMA HANDSET APPLICATIONS
Donald F. Kimball (S’82–M’83) was born in
Cleveland, OH, in 1959. He received the B.S.E.E.
(summa cum laude) and M.S.E.E. degrees from The
Ohio State University, Columbus, in 1982 and 1983,
respectively.
From 1983 to 1986, he was a TEMPEST Engineer
with the Data General Corporation. From 1986 to
1994, he was an Electromagnetic Compatibility Engineer/Manager with Data Products New England.
From 1994 to 1999, he was a Regulatory Product
Approval Engineer/Manager with Qualcomm, Inc.
From 1999 to 2002, he was a Research and Technology Engineer/Manager
with Ericsson, Inc. Since 2003, he has been a Principal Development Engineer
with Cal(IT)2 with the University of California at San Diego, La Jolla. He
holds four U.S. patents with two patents pending associated with high-power
RF amplifiers (HPAs). His research interests include HPA EER techniques,
switching HPAs, adaptive digital predistortion, memory effect inversion, mobile
and portable wireless device battery management, and small electric-powered
radio-controlled autonomous aircraft.
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Peter M. Asbeck (M’75–SM’97–F’00) received the
B.S. and Ph.D. degrees from the Massachusetts Institute of Technology (MIT), Cambridge, in 1969 and
1975, respectively.
He is the Skyworks Chair Professor in the Department of Electrical and Computer Engineering,
University of California at San Diego (UCSD), La
Jolla. He worked at the Sarnoff Research Center,
Princeton, NJ, and at Philips Laboratory, Briarcliff
Manor, NY, in the areas of quantum electronics and
GaAlAs-GaAs laser physics and applications. In
1978, he joined Rockwell International Science Center, where he was involved
in the development of high-speed devices and circuits using III-V compounds
and heterojunctions. He pioneered the effort to develop heterojunction bipolar
transistors based on GaAlAs-GaAs and InAlAs-InGaAs materials and has
contributed widely in the areas of physics, fabrication and applications of these
devices. In 1991, he joined the UCSD. His research interests are in development of high-speed heterojunction transistors and their circuit applications. His
research has led to more than 250 publications.
Dr. Asbeck is a Distinguished Lecturer for the IEEE Electron Devices Society
and the Microwave Theory and Techniques societies. He received the 2003 IEEE
David Sarnoff Award for his work on heterojunction bipolar transistors.
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