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Low Quiescent Current HBT Power Amplifiers
For Cellular Phones
Kazutomi MORI, Shintaro SHINJO and Tadashi TAKAGI
Mitsubishi Electric Corporation
5-1-1 Ofuna, Kamakura, Kanagawa, 247-8501 Japan
Tel: +81-467-41-2544, Fax: +81-467-41-2519, [email protected]
Abstract
This paper describes two types of low quiescent HBT power amplifiers for cellular
phones. The first one is a CV(Constant Voltage)/CC(Constant Current) parallel
operation HBT PA which achieves low quiescent current by connecting CV mode HBT
and CC mode HBT in parallel, that can compensate the AM-AM distortion each other.
The other is a SiGe HBT driver amplifier having self base bias control circuit which can
enhance P1dB without the increase of the quiescent current.
1. Introduction
In many years, high efficiency are required for
power amplifiers used in cellular phones and a lot
of techniques were introduced in this technical area.
In early years with the first generation analog
cellular phones, the optimum load techniques for
the fundamental and harmonic frequencies [1]-[4],
and low operation voltage amplifiers [5],[6] were
introduced. According with the use of digital
modulated signal in the second generation digital
cellular phones such as PDC and PHS systems, high
efficiency with low distortion became key technical
issues and still be important in the third generation
W-CDMA or Cdma2000 systems.
Then the
optimum load techniques were enhanced to realize
high efficiency with low distortion including
optimization of the fundamental and the harmonic
impedances of the final stage transistors [7]-[9] and
interstage impedances [10]. As external additional
components, distortion compensation techniques
including adaptive digital predistortors [11],[12]
and analog miniaturized size predistortion
linearizers [13]-[17] were also investigated.
In recent years, with the use of code-division
multiple-access (CDMA) scheme including
wide-band CDMA (W-CDMA), wide dynamic
transmit power range is required. High efficiency
not only at the maximum output power region but
also in large backoff region are key issues for the
power amplifiers used in these applications. In
order to achieve high efficiency, some techniques
were introduced in resent years. There are two
approaches. The first is focusing to only reducing
power consumption in large backoff region, for
examples, bias voltage controlled amplifier
techniques [18]-[20] and low quiescent amplifiers
techniques [21]-[23] including Doherty amplifier
[23]. The other approach achieves high efficiency
by compensating distortion of PAs, that includes
dynamic envelop tracking technique [24]-[25],
adaptive digital predistortion techniques [25]-[27],
re-investigated with the technical progress of digital
signal processing.
The active devices used in power amplifiers
were MESFETs in the analog cellular phones and
were shifted to PHEMTs in the second generation
because of their good efficiency and linearity.
Now in the third generation, GaAs HBT devices are
employed because of their low quiescent current
characteristics [28],[29]. As another trend, SiGe
HBTs are becoming to be used in RF-IC for cellular
phones because of low cost and easy
implementation of integrated Ics.
Power
amplifiers employing SiGe HBT devices were also
introduced in recent years [30],[31].
In addition to these technical issues, power
amplifiers used in cellular phones must be
implemented in low cost and small size. From this
viewpoint, the power amplifier used to be selected
in cellular phones practically. In such a viewpoint,
low quiescent current PA without any external
additional component and the the smart amplifier
that has good performance with internal additional
intelligent circuit, for example, bias control circuit
based on Si or SiGe BiCMOS technologies, are
considered to be good candidates in near future.
In this paper, two low quiescent HBT power
amplifiers are introduced. The first PA is a CV
(Constant Voltage) / CC(Constant Current) parallel
operation HBT PA [32]. The PA can achieve low
quiescent current by employing parallel connected
CV and CC mode HBT to compensate the AM/AM
distortion each other. The other is a SiGe HBT
driver amplifier having self base bias control circuit
[33] which can enhance P1dB without the increase
of the quiescent current.
2.2 Fabrication and Measured Results
The CV/CC parallel operation HBT PA is
fabricated to confirm its low distortion and high
efficiency characteristics with low quiescent current.
The schematic diagram of the fabricated CV/CC
parallel operation HBT PA has been already shown
in Fig.1. The CV-mode HBT PA is also fabricated
as the reference. In CV-mode operation, 4 x 20
mm2 x 40fingers HBT with the quiescent current of
20mA is optimal emitter size and bias condition to
obtain output power of around 26.0dBm and high
efficiency with ACPR of –40dBc. The total
emitter size of CV/CC HBT PA is determined to be
4 x 20mm2 x 40fingers. The finger size ratio of
CV and CC-mode HBTs, and the difference of bias
conditions between CV and CC-mode HBTs have to
be optimized to achieve high efficiency with the
specified ACPR. The bias condition difference can
be adjusted by bias feed resistance Rs.
P1dB difference (DP1dB) is calculated for
CV/CC HBTs that have finger size ratio (CV mode
HBT : CC mode HBT = X : Y) of 30:10, 20:20 and
10:30, and has base bias resistor Rs of from 100ohm
to 10kohm. The DP1dB is defined as the P1dB
degradation of the CV/CC parallel operation HBT
PA from the CV mode HBT PA. DP1dB = 0
means that CV/CC HBT PA has the same P1dB as
the CV mode HBT PA.
Figure 2 shows the calculated DP1dB as
functions of the finger size ratio and Rs. In the
case of the finger size ratio of 30:10, the DP1dB is
quite small and is less than -0.2dB for all Rs values.
Therefore, the finger size ratio is desided to be
30:10. The maximum DP1dB is obtained at Rs of
1100ohm. Then Rs is determined to be 1100ohm.
Figure 3 shows the photograph of the
fabricated CV/CC HBT PA. The chip area is
0.8mm x 0.8mm wherein a pair of HBT’s and two
different bias circuits are included.
Constant Voltage
Source
Ls
Vcc
Q1
RFout
RFin
M.S.
M.S.
Q2
Rs
Constant Voltage
Source
Fig. 1
Schematic diagram of the CV/CC parallel
operation HBT PA.
1.0
Area of ⊿P1dB <-0.2 in case of 30:10
0.5
0.0
⊿ P 1dB [dB]
2. CV/CC Parallel Operation InGaP/GaAs
HBT PA
2.1 Circuit Configuration
The CV/CC pararell operation InGaP/GaAs
HBT PA is introduced as an example of the low
quiescent current PA in this section. Fig.1 shows
the block diagram of the CV/CC parallel operation
HBT PA. The PA is comprised of two HBT’s (Q1
and Q2) connected in parallel. Q1 and Q2 have
separated CC and CV base bias circuits,
respectively. The base bias of Q1 and Q2 are
supplied through an inductor (Ls) and an resistor
(Rs), respectively. The collector bias circuit and
the matching circuits for both input and output are
common.
In near class-B operation, the gain of CV mode
HBT is expanded with the increase of input power.
On the other hand, the gain of CC mode HBT is
decreased. Therefore, the gain deviations can be
compensated each other and the linearity will be
improved by connecting CV and CC mode HBT in
parallel as shown in Fig. 1.
-0.2dB
-0.5
Area of ⊿P1dB <-0.2
in case of 20:20
-1.0
-1.5
-2.0
CV mode HBT : CC mode HBT = X : Y
X : Y =30 : 10
X : Y =20 : 20
X : Y =10 : 30
-2.5
-3.0
10
100
1000
10000
100000
Rs [ohm]
Fig. 2
Calculated DP1dB as functions of Rs
and transistor finger ratio.
RFout
(30fingers)
Vbe
(30fingers)
RFin
(30fingers)
RFout
(10fingers)
RFin
(10fingers)
Vbe
(10fingers)
0.8mm
0.8mm
Fig. 3 Photograph of the fabricated CV/CC PA.
-20
50
42.2%
-30
40
40.0%
ACPR
-40.0dBc
-40
30
-50
20
PAE
-60
PAE [%]
ACPR, NACPR [dBc]
CV/CC (Idq=17mA)
CV (Idq=20mA)
10
NACPR
26.4dBm
-70
5
Fig. 4
10
15
26.8dBm
20
25
Pout [dBm]
30
0
35
Measured ACPR, NACPR and PAE for
the fabricated CV/CC and CV HBT PA.
Table 1 Measured temperature dependence of
Pout and PAE @ ACPR=-40.0dBc
for the CV/CC and CV HBT PA.
CV/CC
CV
Temprature
Maximum PAE [%] Maximum PAE [%]
Icq [mA]
[℃]
Pout [dBm] @Max.Pout Pout [dBm] @Max.Pout
40
26.6
41.6
24.0
32.0
ACPR>-40.0dBc with Pout of 15dBm
30
26.6
42.1
-20
ACPR>-40.0dBc with Pout of 15dBm ACPR>-40.0dBc with Pout of 15dBm
20
ACPR>-40.0dBc with Pout of 15dBm ACPR>-40.0dBc with Pout of 15dBm
17
40
26.6
40.8
26.7
39.3
30
26.6
41.1
26.4
39.5
25
20
26.7
41.8
26.4
40.0
ACPR>-40.0dBc with Pout of 15dBm
17
26.8
42.3
40
26.3
42.8
26.1
37.7
30
26.3
43.2
26.1
38.3
85
20
26.2
43.2
26.1
38.7
17
26.2
43.5
26.0
39.1
-30
CV/CC
CV
-35
ACPR@Pout=15dBm [dBc]
ACPR (5-MHz offset), NACPR (10-MHz
offset) and PAE for the CV/CC parallel operation
HBT PA and CV mode HBT PA are measured at
1.95GHz. An HPSK modulated signal with a chip
rate of 3.84Mcps is used. The quiescent current
Icq is set to 17mA for the CV/CC parallel operation
HBT PA, and is set to 20mA for the CV mode HBT
mode HBT PA at the room temperature of 25 degC.
The measured results of ACPR and NACPR and
PAE are shown in Fig. 4. The CV/CC parallel
operation HBT PA achieves Pout of 26.8dBm and
PAE of 42.2% with ACPR of -40.0dBc. On the
other hand, the CV mode HBT PA achieves Pout of
26.4dBm and PAE of 40.0% with ACPR of
-40.0dBc. At the maximum output power region,
the CV/CC PA achieves the 0.4dB higher output
power and 2.2% higher PAE compared with the
CV-mode PA. Also, it is confirmed that the
quiescent current of the CV/CC parallel operation
HBT PA is 3mA lower than that of the conventional
CV-mode PA at the room temperature of 25degC.
Table 1 shows the measured temperature
dependence of Pout and PAE with ACPR of
-40.0dBc for the CV/CC and CV-mode HBT PAs.
These characteristics are measured at the –20degC,
25degC and 85degC. Icq is set to 17, 20, 30 and
40mA at 25 ℃ , respectively. If ACPR exceed
-40.0dBc at the large back-off region (Pout of 15
dBm), the measured results are not described in this
table.
Particularly at –20degC condition, the
CV/CC PA has more advantage compared with the
CV-mode PA. For the CV/CC parallel operation
HBT PA, ACPR exceeds -40.0dBc at Pout of 15.0
dBm in the conditions of only Icq = 17 and 20mA.
On the other hand, for the CV mode HBT PA,
ACPR exceeds -40.0dBc with Pout of 15.0dBm in
the conditions of Icq = 17, 20 and 30mA. This is
because temperature dependence of CV-mode and
CC-mode HBT can be compensated each other in
the case of CV/CC PA. Fig.5 shows measured
ACPR with output power of 15dBm for the CV/CC
parallel operation and the CV HBT PA at –20, 25,
85degC. In Fig.5, it is shown that the CV/CC
parallel operation HBT PA achieves the minimum
quiescent current of 23mA, and the CV mode HBT
PA achieves that of 32mA at –20degC, respectively.
Therefore the total quiescent current of CV/CC PA
can be reduced by 9mA compared with CV-mode
PA over the temperature range of –20℃ to 85℃.
-40dBc
-40
-20degC
-45
25degC
-50
85degC
-55
-60
10
23mA
20
32mA
30
40
50
Icq [mA]
Fig. 5 Measured ACPR with output power of
15dBm for the CV/CC parallel operation
and the CV HBT PA.
Bandgap
reference
current
Iref2
Ipc
Chip area
Self base bias
control circuit
M : 1
Q5 Q4
RFin
Iref1
Icdc2
Q3
Iref
RFout
M.S.
M.S.
Rfb Cfb
Ice
Q2
1
Ibe
Rb Vbe N
Q1
Fig. 6 Schematic diagram of an amplifier
with a self base bias control circuit.
As an example of the smart PA, the driver
amplifier with self base bias control circuit is
introduced in this section. If the base voltage Vbe
is controlled to increase with the increase of RF
input power, more saturated output power and P1dB
are expected to obtain. In other words, higher
P1dB can be automatically obtained with low
quiescent current if such kind of base bias control
circuit would be employed. Fig. 6 shows the
schematic diagram of the driver amplifier with the
proposed self base bias control circuit. In Fig.6,
Q1 is referred to the RF transistor of a driver
amplifier, Q2 is referred to the bias transistor that
composes current mirror with Q1, Q3 is referred to
the transistor for compensation of base current.
Q4 and Q5 are p-MOSFET’s for the current mirror
which derives the self base bias control. The base
current Ibe of RF HBT Q1 increases with input RF
power, particularly when the output power is
saturated. The increased base current Ibe are fed
back by p-MOS current mirror circuit and positively
added to the reference current Iref of the bias
transistor Q2, which composed current mirror with
Q1. Therefore, the base voltage Vbe increase with
the increase of RF input power automatically.
Ibe, and Vbe are represented by the following
equations,

Ibe ( final )  Ibe ()  Ibe   Ibe (n)!
n 1
MN n
 (
)  Ibe

M
n0

Vbe ( final ) 
(2)
where q is electron charge, k is Boltzman’s constant,
T is temperature and Is is saturation current. N
and M are the current mirror ratios between Q1 and
Q2, and between Q4 and Q5, respectively.
Type-A
Type-B
15.2dBm
15
60
50
12.8dBm
10
40
Pout
5
32.8mA
0
Ice
-5
(1)

q
1
MN n
 ln(1 
(
)  Ibe )
qVbe
nkT
nkT n  0   M
Is  e
20
f=1.95GHz,
Vcc=Vpc=3.0V, Icq=15.0mA
-10
-20
Fig. 7
-15
Ice [mA]
Vpc
3.2 Fabrication and Measured Results
In order to confirm the advantage of the
proposed self base bias control circuit, the driver
amplifiers with the proposed self base bias control
circuit and the conventional current mirror type
base bias circuit are fabricated. The amplifiers
employ SiGe HBT technology because bias circuits
and power amplifiers can be easily integrated on a
single chip. The emitter size of the SiGe HBT
driver amplifier is 100mm2. The bias condition is
the quiescent current of 15.3mA, and Vcc, Vpc of
3.0V.
Fig.7 shows the simulated Pout and
collector current Ice of the driver amplifiers with
the conventional and the self base bias control
circuits. The driver amplifier with the self control
bias circuit achieves P1dB of 15.2dBm. P1dB
improvement of 2.4dB is achieved by using the self
base bias control circuit. The collector current Ice
of the driver amplifier with the self bias control
circuit is 32mA, that is higher than that of the driver
amplifier with the conventional bias circuit at P1dB
output power. Fig.8 shows simulated base current
Ibe and base voltage Vbe of the driver amplifiers.
As predicted by eqs.(1) and (2), it is shown in Fig.8
that Ibe and Vbe increases with the increase of input
power. The calculated results used by eqs.(1) and
(2) are also shown in Fig.8 as the dotted lines. The
simulated and calculated Ibe and Vbe are in good
agreement. A photograph of the fabricated driver
amplifier with the proposed self base bias control
circuit is shown in Fig.9. Chip size is 0.8mm x
1.1mm.
Figure 10 shows the measured output power
and collector current of the fabricated driver
amplifier with the self control bias circuit. The
quiescent current is set to be 15.3mA. It is shown
Pout [dBm]
3. Driver SiGe HBT PA with Self Base Bias
Control Circuit
3.1 Circuit Configuration
30
24.9mA
20
10
0
0.8dBm 2.6dBm
-10
-5
0
5
Pin [dBm]
Simulated Pout and Ice of the driver
amplifiers with the conventional and
the self control bias circuit.
0.7
0.89
f=1.95GHz,Icq=15.0mA
Vcc=Vpc=3.0V
0.6
0.87
Vbe
0.4
0.83
0.3
0.81
Ibe
0.2
Vbe [mV]
0.85
Ibe [mA]
0.5
0.79
Conventional bias circuit
S elf control bias circuit (sim.)
S elf control bias circuit (cal.)
0.1
0.77
0.0
0.75
-20
-15
-10
-5
Pin [dBm]
0
5
Fig. 8 Simulated Ibe and Vbe of the driver
amplifiers.
1.1mm
Self base bias
control circuit
RF transistor
RF_out
RF_in 0.8mm
Fig. 9 Photograph of the fabricated driver
amplifier with self base bias control circuit.
20
60
15.0dBm
50
f=1.95GHz,
Vcc=Vpc=3.0V
Icq=15.3mA
10
40
5
30
32.6mA
0
20
-5
10
Pout
Ice
-10
-20
-15
-10
-5
Pin [dBm]
Ice [mA]
Pout [dBm]
15
2.2dBm
0
5
0
Fig. 10 Measured output power and collector
current of the fabricated Type-B driver amplifier.
3.0
Iref [mA]
2.5
S elf control bias circuit
f=1.95GHz,Icq=15.0mA
Vcc=Vpc=3.0V
Measured
Simulated
2.1mA
2.0
1.5
1.0
-20
-15
-10
-5
Pin [dBm]
2.2dBm
0
5
Fig. 11 Measured Iref of the fabricated driver
amplifier with the self control bias circuit.
in Fig. 10 that P1dB of 15.0dBm is achieved and
that the collector current at P1dB is 32.6mA as the
measured results. These measured results agree
well with the simulated results shown in Fig. 7. In
order to confirm that Ibe and Vbe increase with the
increase of input power, Iref is measured instead of
Ibe or Vbe. Fig.11 shows measured Iref of the
fabricated driver amplifier. It is shown in Fig.11
that the measured Iref is well agreed with the
simulated one. Therefore, it is verified that the
self base bias control circuit can generates
additional base current automatically and that Ibe
and Vbe increase with the increase of input power.
4. Conclusion
Two low quiescent current HBT PAs were
introduced. The first one is CV(Constant Voltage)/
CC(Constant Current) parallel operation HBT PA
that is an example of the low quiescent HBT power
amplifiers with any additional circuit. The PA is
comprised with the parallel connected CV and CC
mode HBTs. By combining CV and CC HBT’s in
parallel, gain deviations are compensated each other,
and adequate ACPR can be achieved over the wide
output power range. The total quiescent current of
the fabricated CV/CC parallel operation HBT PA
can be 9mA lower than that of the conventional
CV-mode PA. At the maximum output power
region, the PA achieved Pout of 26.8dBm and PAE
of 42.0% with ACPR of -40.0dBc. The other is
the SiGe HBT driver amplifier with the self base
bias control circuit using the p-MOSFET current
mirror, that is a kind of smart PA with the internal
intelligent bias circuit integrated on a single chip.
The proposed bias circuit automatically controls the
base voltage of RF transistor according to the output
power level, and can achieve high output power and
P1dB with low quiescent current. The driver
amplifier having the proposed bias circuit realized
P1dB improvement of 2.4dB compared with the
driver amplifier having the conventional constant
base voltage circuit under the same quiescent
current of 15.0mA as the simulated results. The
fabricated driver amplifier having the proposed bias
circuit achieved high P1dB of 15.0dBm with low
quiescent current of 15.3mA.
High efficiency and low distortion in wide
output power dynamic range have to be realized in
small size and low cost in the power amplifiers used
in cellular phone terminals,
The techniques
described in this paper considered to be useful
because they do not need any additional
components out of the amplifiers and easy to be
integrated within the current used PAs in addition to
their good performances.
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