Amplifiers – With semiconductor amplifying device – Including gain control means
Reexamination Certificate
2002-05-01
2004-04-27
Nguyen, Khanh Van (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including gain control means
C330S129000
Reexamination Certificate
active
06727760
ABSTRACT:
THE FIELD OF THE INVENTION
The present invention relates to an output power control system, and more particularly, to an output power control system for controlling output power from a power amplifier in a radio-frequency transmitter.
BACKGROUND OF THE INVENTION
Radio-frequency transmitters are used to transmit radio-frequency signals in a variety of applications. Typically the transmitters are designed to meet standards that specify output power levels and output signal rise and fall times. In many applications, output power control circuits or systems control the output power from power amplifiers employed in the radio-frequency transmitters. One type of output power control circuit includes a feedback loop that feeds back a portion of the output signal to an input of the power amplifier to control the output power. One standard that is used is the EIA Interim Standard, IS-19-B, 3.1.3.3, January 1988, USA, for the 800 MHz cellular subscriber units of the EIA system. For example, this standard specifies that the rise and fall times of the transmitted output power signal be less than two milliseconds.
A conventional output power control circuit
10
for controlling the output power of a power amplifier
12
is illustrated in FIG.
1
. Output power control circuit
10
is a voltage-controlled output power control circuit which couples a percentage of the transmitted output power signal through a detector diode circuit. Power amplifier
12
has a voltage supply connection at
18
and a ground connection at
20
. Power amplifier
12
transmits an input signal received at an input
14
and provides an output signal at an output
22
. The output signal at output
22
is routed through a coupler
24
to provide an output signal at
26
. A portion of the output power sent through coupler
24
is routed to an input
28
of a signal detector
30
. Signal detector
30
responds to the peak level of the output signal at
26
. Signal detector
30
includes a detecting diode and produces a detection signal at
34
. A comparator
32
compares a reference voltage at
36
with the detection signal at
34
. Comparator
32
provides an automatic power control voltage (Vapc) to an input
16
of power amplifier
12
to bring the detection signal at
34
in conformity with the reference voltage at
36
.
One disadvantage of the conventional output power control approach employed by output power control circuit
10
are losses which reduce available output power and system efficiency. The couplers typically available for radio-frequency amplification systems are large, expensive, and have poor directivity characteristics. The poor directivity of the couplers results in poor output power accuracy, especially when the radio-frequency load impedance varies. Since the peak current loads of the power amplifiers are not monitored with this approach, the currents can be driven well above nominal levels under certain operating conditions, thereby further reducing system efficiency. For battery applications such as for mobile telephones, the result is a reduction in available talk time.
Another conventional approach used to control the output power of a power amplifier
42
is illustrated in FIG.
2
.
FIG. 2
illustrates a conventional current-controlled output power control circuit
40
which uses a sense resistor
54
in series with the voltage supply connection at
56
to develop a voltage proportional to the power amplifier
42
current. Power amplifier
42
has an input signal at
44
and an output signal at
52
. Power amplifier
42
has a voltage supply connection at
48
which is coupled to one end of a sense resistor
54
, and has a ground connection at
50
. Sense resistor
54
is coupled at the other end to the voltage supply at
56
. Capacitor
57
is coupled across sense resistor
54
between the voltage supply at
56
and the connection at
48
of power amplifier
42
to shunt instantaneous currents around sense resistor
54
so that sense resistor
54
conducts an average current.
The approach illustrated in
FIG. 2
uses a feedback loop to convert a current through sense resistor
54
to a power control voltage at
46
. A comparator
58
provides a voltage at
50
which corresponds to the current through sense resistor
54
. Comparator
62
compares the voltage at
50
to a reference voltage at
64
and provides an automatic power control voltage (Vapc) at
46
to control the gain of power amplifier
42
.
One disadvantage of this second approach are losses which reduce available output power and system efficiency. The sense resistors can be expensive and can be physically large. The sense resistors can also cause significant voltage drops which reduce the output power availability and efficiency of the power amplifier. This approach also has the disadvantage of having the total power amplifier current being fed back within the control loop. This can result in a loss of power control accuracy because the total current does not necessarily vary in direct proportion to the output power.
In view of the above, there is a need for an improved output power control system such as employed for controlling output power from a power amplifier in a radio-frequency transmitter.
SUMMARY OF THE INVENTION
One aspect of present invention provides an output power control system which includes an amplifier configured to supply a first current, a reference source configured to supply a second current which is proportional to and less than the first current, and a feedback converter responsive to the second current to control a gain of the amplifier.
REFERENCES:
patent: 4849712 (1989-07-01), Jarrett
patent: 5146180 (1992-09-01), Hyakutake
patent: 5337006 (1994-08-01), Miyazaki
patent: 6008698 (1999-12-01), Dacus et al.
patent: 6047167 (2000-04-01), Yamashita
patent: 6194967 (2001-02-01), Johnson et al.
patent: 6297698 (2001-10-01), Callahan, Jr.
Jansen Bartholomeus H. W. E.
Parkhurst, Jr. Ray Myron
Agilent Technologie,s Inc.
Nguyen Khanh Van
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