U.S. patent application number 10/424251 was filed with the patent office on 2004-10-28 for power amplifier device and method thereof.
This patent application is currently assigned to Motorola, Inc.. Invention is credited to Gilsdorf, Benjamin R., Newman, David A..
Application Number | 20040212433 10/424251 |
Document ID | / |
Family ID | 33159429 |
Filed Date | 2004-10-28 |
United States Patent
Application |
20040212433 |
Kind Code |
A1 |
Newman, David A. ; et
al. |
October 28, 2004 |
POWER AMPLIFIER DEVICE AND METHOD THEREOF
Abstract
A power amplifier's base current is biased by a control circuit
that produces a linear relationship across varying temperatures and
processes. A voltage to current converter controls a voltage
follower configured operational amplifier in response to a
reference device to drive the voltage and current of the power
amplifier. A slope control circuit is coupled to the reference
device to limit a maximum power control slope.
Inventors: |
Newman, David A.; (Tempe,
AZ) ; Gilsdorf, Benjamin R.; (Phoenix, AZ) |
Correspondence
Address: |
TOLER & LARSON & ABEL L.L.P.
5000 PLAZA ON THE LAKE STE 265
AUSTIN
TX
78746
US
|
Assignee: |
Motorola, Inc.
|
Family ID: |
33159429 |
Appl. No.: |
10/424251 |
Filed: |
April 28, 2003 |
Current U.S.
Class: |
330/285 |
Current CPC
Class: |
H03F 3/211 20130101;
H03F 2200/504 20130101; H03F 1/302 20130101 |
Class at
Publication: |
330/285 |
International
Class: |
H03G 003/10 |
Claims
What is claimed is:
1. A radio frequency power amplifier device comprising: a current
source to provide a total current; a control circuit coupled to the
current source, the control circuit responsive to a first portion
of the total current; a reference device responsive to the control
circuit, the reference device having an input responsive to a
second portion of the total current and having a reference output;
and an amplifier responsive to the reference device, the amplifier
comprising a control input, a radio frequency input, and a radio
frequency output.
2. The radio frequency power amplifier of claim 1, wherein the
control input is responsive to the reference output.
3. The radio frequency power amplifier device of claim 1, wherein
the RF output is to provide a signal proportional to a signal at
the reference output.
4. The radio frequency power amplifier device of claim 1 further
comprising a buffer coupled between the reference device and the
amplifier.
5. The radio frequency power amplifier device of claim 1, wherein
the control circuit is slope control circuit to control a maximum
slope of the radio frequency power amplifier.
6. The radio frequency power amplifier device of claim 5, wherein
the slope control circuit comprises a resistive element.
7. The radio frequency power amplifier device of claim 6, wherein
the slope control circuit comprises a voltage reference.
8. The radio frequency power amplifier device of claim 6, wherein
the slope control circuit comprises a transistor.
9. The radio frequency power amplifier device of claim 1, wherein
the reference device comprises a current rectifying device.
10. The radio frequency power amplifier device of claim 9, wherein
the current rectifying device is a transistor having its control
electrode tied to one of its current electrodes.
11. The radio frequency power amplifier device of claim 1, wherein
the total current of the current source is responsive to a voltage
input.
12. An integrated circuit device comprising: a RF power amplifier
comprising a RF input, a bias control terminal, and a RF output; a
slope control circuit comprising a slope shaping terminal coupled
to the bias control terminal, the slope control circuit to provide
a signal at the slope shaping terminal to bias the RF power
amplifier to facilitate generation of a smoothed signal at the RF
output.
13. The integrated circuit device of claim 12, wherein the RF power
amplifier further comprising: a RF power transistor having a first
current electrode, a second current electrode, and a control
electrode; a bias control circuit coupled to the control electrode;
a reference device coupled to the control electrode; and wherein
the RF input is coupled to the control electrode.
14. The integrated circuit device of claim 13, wherein the RF slope
control circuit comprising: a resistive element comprising a first
node coupled to the control electrode, and a second node; and a
voltage reference device comprising a first node, coupled to the
second node of the resistive element, and a second node.
15. The integrated circuit device of claim 14, wherein the voltage
reference device is to facilitate providing a voltage less than a
threshold voltage of the reference device.
16. The integrated circuit of claim 15, further comprising a buffer
comprising an input coupled to the bias control circuit and an
output coupled to the control electrode of the RF power transistor,
wherein the reference device is connected to the input of the
buffer, and the RF input is connected to the control electrode.
17. The integrated circuit of claim 16, wherein the reference
device has a ratio relative to the power amplifier of 1:N.
18. The method of claim 17, wherein the bias control circuit
comprises a voltage to current converter.
19. The method of claim 18, wherein the voltage to current
converter and the first electrode of the power amplifier are
coupled to a common power supply.
20. A method of operating an RF power amplifier device, the method
comprising: receiving a bias current at a conductive element of the
RF power amplifier; providing a first portion of the bias current
received at the conductive element to the reference device, wherein
the portion of the bias current is less than the bias current; and
amplifying a RF signal based on the portion of the bias
current.
21. The method of claim 20 further comprising: providing a second
portion of the bias current received at the conductive element to a
slope control circuit.
22. The method of claim 21 wherein providing the current comprises
providing the current to the reference device where the reference
device comprises a transistor.
23. The method of claim 22, wherein providing the portion of the
current to the slope control circuit comprises the slope control
circuit comprising a resistive element.
24. The method of claim 21, wherein providing the portion of the
current to the slope control circuit comprises the slope control
circuit comprising a resistor.
Description
FIELD OF THE INVENTION
[0001] This invention relates to power amplifiers and, more
specifically, to a device and method for controlling the bias of a
power amplifier.
BACKGROUND OF THE INVENTION
[0002] Radio Frequency (RF) power amplifiers are used as components
in many communication devices, including many wireless
communication devices, such as base stations and mobile devices
such as cell phones. Hetero-junction bipolar transistor (HBT) power
amplifiers are a specific type of power amplifier used for cellular
applications due to their high power density and reduction in die
size. Unfortunately, biasing these transistors with a constant
current poses some difficulty. The voltage supply limitation
typical to mobile applications combined with a relatively high Vbe
of HBT devices make traditional integrated methods unusable.
[0003] FIG. 1 shows a typical diode based biasing control of an HBT
transistor. A power amplifier Qpa HBT 100 is biased by a diode
configured transistor 110 where the base and collector are shorted
together and receive a current through a resistor 120 and supply
voltage V.sub.REF 130. This configuration requires that a separate
voltage V.sub.REF 130 (different from the battery voltage V.sub.BAT
140 supplied to the collector of the power amplifier 110) be
applied to the diode transistor and the biased base of the power
amplifier in order to tightly control the biasing current. This
configuration leads to several problems for power amplifier
applications in mobile communications. Typically, the power
amplifier 100 is N times larger than the diode transistor 110
leading to current stealing. Additionally, R.sub.REF 120 needs to
be large to provide stability over variations in temperature and
process, but needs to be small to provide enough current to
properly bias the power amplifier, resulting in a circuit that
would require a stable reference which supplies a prohibitively
large amount of current and is not a viable circuit for power
amplifiers in mobile communications applications.
[0004] Another solution, shown in FIG. 2, solves the problem of
current stealing by using a current mirror with an emitter follower
to bias the current supplied to the power amplifier's base. The
base of a power amplifier transistor 200 is connected to a base of
mirrored transistor 210 and the emitter of a emitter follower
transistor 250. The collector of the mirrored transistor 210 is
connected to the base of the emitter follower transistor 250 and is
connected to a reference voltage 230 through a reference resistor
220 while the collector of the emitter follower transistor 250 is
connected to the battery voltage 240 which is also connected to the
collector of the power amplifier transistor 200 through some
impedance 270. However, this type of circuit is not viable because
gallium arsenide (GAS) HBT power amplifiers as now used have Vbe's
in the order of 1.4 volts while battery voltage supplies are
required to be in the range of 2.7 volts. To control the voltage at
the base of the power amplifier, the voltage supply, V.sub.REF 230,
would need to be greater than is desirable for mobile communication
applications and the solution is therefore not viable.
[0005] In certain applications, RF power amplifiers are placed
within feedback control loops to provide for power control. A
measurement of the output RF power delivered by the RF power
amplifier vs. the input voltage will often indicate a steep slope
condition where the RF power amplifier output changes very rapidly
with respect to changes in the input voltage. When an RF power
amplifier presents the steep slope condition instability in the
power control loop and other undesirable overall RF power amplifier
breakdown conditions may result. Thus, it would be desirable to
provide an RF power amplifier device that addresses the steep slope
condition while maintaining high performance operation.
[0006] Accordingly, there is a need for an improved RF power
amplifier device and method of operation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a simplified electrical schematic of a prior art
HBT diode based biasing circuit;
[0008] FIG. 2 is a simplified electrical schematic of another prior
art HBT biasing circuit;
[0009] FIG. 3 is a simplified electrical schematic of an HBT power
amplifier bias controller according to an embodiment of the
invention;
[0010] FIG. 4 is a block diagram of a power amplifier according to
an embodiment of the disclosure;
[0011] FIG. 5, is a graph illustrating a set of transfer curves;
and
[0012] FIGS. 6 and 7 are simplified schematics of a slope control
circuits.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0013] Referring to FIG. 3, one model of an embodiment of a bias
control for a hetero-junction bipolar transistor (HBT) power
amplifier is shown. Though the circuit was designed for HBT
technology, it is not limited to this technology and could be used
in technologies such as enhancement mode metal semiconductor
field-effect transistors (MESFETS). Similar reference numerals are
used throughout the figures to represent similar features when
possible.
[0014] An HBT power amplifier 300 is biased based on the voltage
measured on reference HBT transistor 310 by way of a CMOS chip 355.
Although the depiction shows the reference device and power
amplifier device to be an HBT transistor, other reference devices
and power amplifier devices are contemplated.
[0015] The collector of the HBT power amplifier 300 is tapped for
an RF output 385 and is supplied voltage from a battery source 340
and some impedance 374 while the emitter is connected to ground.
The base of the HBT power amplifier 300 is connected through some
impedance 370 to a first input 352 of the operational amplifier
360. The connection of the first input 352 is coupled to ground
through a capacitor 365. Additionally, an RF input signal 390 is
injected into the base of the HBT power amplifier 300 through some
capacitor 380. Although the RF input and output signals are shown,
they are not necessary to the discussion of the operation of the
bias control of the power amplifier and are shown only for
completeness.
[0016] The output 353 of the operational amplifier is fed back and
connected to the first input 352 of the operational amplifier in
order to cause the operational amplifier to function as a voltage
follower where the voltage appearing on a second input 351 of the
operational amplifier 360 appears some minimal time later on the
output 353 of the operational amplifier 360. The second input 351
of the operational amplifier 360 is connected to an output of a
voltage-to-current converter 368 as well as to the base through
some impedance 372 and to the collector of the reference device
310, in this case another HBT transistor. A control 366 of the
voltage-to-current converter is connected to a voltage control
signal 350 and the battery supply 340 is used to supply voltage to
the voltage-to-current converter 368 through another input 367.
[0017] In operation, the present disclosure can use an external
CMOS chip and bias control 355, consisting of an operational
amplifier 360 and a voltage-to-current converter 368 to bias the
HBT power amplifier 300. An analog voltage, V.sub.CONTROL 350,
adjusts the reference current, I.sub.REF 330, through the reference
device 310. The VBE of this reference device is measured by the
operational amplifier 360 and applied to the base of the HBT power
amplifier 300. The HBT power amplifier's collector current I.sub.C
342 reflects the reference current I.sub.REF 330 times the ratio of
the size difference between the power amplifier 300 and the
reference device 310.
[0018] This configuration of biasing a power amplifier transistor
maintains several advantages over traditional methods. The voltage
requirements are only 1 V.sub.BE plus the overhead of the current
source that typically is only a few hundred millivolts. Also,
current through the reference device 310 is significantly less
temperature dependent due to the high output impedance of the
current source compared to a resistor. Additionally, the reference
device 310 can be sourced from the normal battery source operating
the power amplifier rather than having to create an independent
stable reference. Other advantages are that I.sub.REF is not a
function of the battery voltage or of process leading to more
stabilized control and linearity of the bias control. Additionally,
the control voltage Vcontrol can operate the bias as low as
Vcontrol=0 volts.
[0019] FIG. 4 illustrates, in block diagram form a specific
embodiment of the present disclosure that illustrates a biased
power amplifier module 400, such as that illustrated in FIG. 3, and
a slope control circuit 405.
[0020] To assure appropriate resolution at their outputs and
adequate stability under all conditions, power amplifiers are often
specified to have a maximum power control slope. This maximum power
control slope is the slope of the transfer function of output power
as a function of control voltage. However, the use of power
amplifiers with control voltages described herein results in a
transfer curve having very steep transfer functions at specific
certain control voltages. To decrease the power control slope, a
slope smoothing circuit is used in the circuit of FIG. 4 to remove
bias current from the biased power amplifier 400. The amount of
current that is removed is based on the voltage on the control
electrode, e.g., the base-collector node of Q.sub.REF 310. The
amount of bias current that is removed is roughly proportional to
the control voltage until Q.sub.REF 310 is turned completely on.
After Q.sub.REF 310 is turned on, the amount removed is fairly
constant. This removal of bias current in this manner results in
the power amplifier turning on more slowly, resulting in a smoother
power control slope, i.e., a smaller maximum power control slope.
This can be better understood with reference to FIGS. 3-7.
[0021] In operation, a control voltage is applied to the biased
power amplifier module 400, at an input labeled BIAS CTL. The
biased power amplifier 400 receives an RF INPUT signal, at an input
labeled RFIN, that is amplified to produce the signal RF OUTPUT at
the output labeled RFOUT. The slope control circuit 405 receives a
sink current I from an output of the biased power amplifier module
400 labeled Slope CTL C. The current I affects the output of the
power amplifier and bias circuit 400 such that the transfer
function from the control voltage to the RF OUTPUT will be
smoother, as compared with the power amplifier and bias circuit
without the slope control circuit. For example, Curve 410 of FIG. 5
represents the V.sub.CONTROL to power output transfer function of a
power amplifier device without the slope smoothing circuitry, while
the curve 415 represents the transfer function of a power amplifier
device with the slope smoothing circuitry. The transform function
observed with the slope smoothing circuitry is a much smoother
curve and a slope of approximately one-tenth the magnitude.
[0022] FIG. 6 illustrates a specific embodiment of a slope control
circuit 405 coupled to the power amplifier of FIG. 3. The slope
control circuit of FIG. 6 comprises resistive element 431 coupled
in series with a voltage reference source 435, labeled
V.sub.SLOPE,CTL. By selecting the value of V.sub.SLOPE,CTL to be
less than the threshold voltage, e.g. the reference voltage, of the
reference device Q.sub.REF 310 a portion of the current supplied by
the bias circuit 355 to the conductive element coupled to the
collector of Q.sub.REF is provided to the resistive element 431.
This results in less current being provided to the reference device
Q.sub.REF. In one embodiment the value of V.sub.SLOPE,CTL can be
zero (0) volts. In other words, only a resistor 431 is needed in
one embodiment.
[0023] FIG. 7 illustrates another specific embodiment of the slope
control circuit of FIG. 6, where the voltage supply device 435 has
been implemented using a transistor 445 and an amplifier 446 as the
voltage supply 435. Specifically, the transistor 445 has a first
current electrode coupled to the resistive element 431, a second
current electrode tied to a reference, such as ground, and a
control electrode coupled to the first current electrode output of
amplifier 446. The amplifier 446 is a differential amplifier having
a positive input coupled to the first electrode of the transistor
445, and a negative electrode coupled to the voltage reference
source V.sub.SLOPE,CTL.
[0024] While the invention has been particularly shown and
described with reference to preferred embodiments thereof, it will
be understood by those skilled in the art that the foregoing and
other changes in form and details may be made therein without
departing from the spirit and scope of the invention. For example,
the slope smoothing techniques can be used with various power
amplifiers and power transistors.
* * * * *