Amplifiers – With semiconductor amplifying device – Including particular biasing arrangement
Reexamination Certificate
2003-03-11
2004-06-29
Choe, Henry (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including particular biasing arrangement
C330S285000
Reexamination Certificate
active
06756852
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates to a method for reducing output noise of a power amplifier, and more particularly to a method for reducing output power of an output signal of the power amplifier in a frequency band and keeping the output power of the output signal in another frequency band larger than a predetermined value.
2. Description of the Prior Art
In this modern information based society, wireless mobile communication has become an important channel for users to communicate or interchange data with others. For example, users use mobile phones to transmit audio signals so as to communicate or interchange knowledge with other users. Because the communication signals are transmitted with high frequency electromagnetic waves, in order to accurately receive the information carried by the communication signals, the mobile phones must have specific power amplifier to amplify the power of the communication signals. Moreover, the use of a digital signal application instead of an analog signal in wireless telephony technology has been developed, but there are still some limitations due to interference between channels. A Digital Enhanced Cordless Telecommunication (DECT) system, which digitizes the signal and utilizes a Time Division Multiple Access (TDMA) protocol, was defined by the European Telecommunications Standards Institute in 1992 in order to satisfy the increasing service density and quantity requirements.
For example, the Global System for Mobile communication 900 (GSM 900) is based on TDMA, and two frequency bands of 25 MHz each are reserved for it in a mobile unit: 890-915 MHz for transmission and 935-960 MHz for reception. These frequency bands are divided into 124 frequency channels with a spacing of 200 kHz. Moreover, according to the TDMA specification, each frequency channel is divided into 8 time slots. Each mobile phone is given one time slot for transmission and reception, so that each frequency channel can simultaneously carry eight calls and interference between the eight calls in the same frequency channel occurs rarely. However, because the prior art method for controlling the power amplifier is improper, interference between these frequency channels usually occurs.
Please refer to
FIG. 1
, which is a circuit diagram of a power amplifier
10
according to the prior art. The power amplifier
10
is installed in a mobile phone and is electrically connected to a communication module of the mobile phone. The communication module is used to modulate radio signals into a baseband signal according to the TDMA specification so as to transmit an input signal V
RF
to the power amplifier
10
. The power amplifier
10
is used to amplify the input signal V
RF
to generate an output signal V
OUT
. The power amplifier
10
comprises an input terminal
20
, an input match circuit
30
, a first order circuit
40
, a match circuit
50
, a second order circuit
60
, and an output match circuit
70
. The input terminal
20
is electrically connected to the output terminal of the communication module to receive the input signal V
RF
. The input match circuit
30
is used to match the impedance of the communication module and first order circuit
40
. The match circuit
50
is used to match the impedance of the first order circuit
40
and the second order circuit
60
, and the output match circuit
70
is used to match the impedance of the second order circuit
60
and an antenna. The first order circuit
40
comprises a first bipolar junction transistor (BJT)
42
, which has a base electrically connected to the input terminal
20
via the input match circuit
30
and to a first bias terminal B
1
via a first bias resistance
44
, a collector electrically connected to the match circuit
50
and to a power supply terminal Vcc via a first collector resistance
48
, and an emitter electrically connected to the ground via a first grounded resistance
46
. Similarly, the second order circuit
60
comprises a second BJT
62
, which has a base electrically connected to the collector of the first BJT
42
via the match circuit
50
and to a second bias terminal B
2
via a second bias resistance
64
, a collector electrically connected to the output match circuit
70
and to the power supply terminal Vcc via a second collector resistance
68
, and an emitter electrically connected to the ground via a second grounded resistance
66
.
A bias signal V
R
is applied to the first bias terminal B
1
and the second bias terminal B
2
to activate the power amplifier
10
to amplify the input signal V
RF
. Please refer to
FIG. 2
, which is a timing diagram of relative signals according to the prior art. The bias signal V
B
and the output signal V
OUT
have the same period T. The communication module periodically modulates digital data into the baseband within a given time slot T
1
. Within each time slot T
1
, the bias signal V
B
is pulled up from low to high so that the two BJTs
42
and
62
are turned on. When the two BJTs
42
and
62
are turned on, the power amplifier
10
begins to amplify the input signal V
RF
to output the amplified output signal V
OUT
If the power that the input signal V
RF
provides to the power amplifier
10
is defined as an input power P
RF
, and the power that the output signal V
OUT
provides is defined as an output power P
OUT
, the output power P
OUT
is capable of being represented as
∑
i
=
0
n
Ai
(
P
RF
)
i
,
and is expressed as the equation below:
P
OUT
=
∑
i
=
0
n
A
i
(
P
RF
)
i
=
A
0
+
A
1
P
RF
1
+
A
2
P
RF
2
+
…
+
A
n
P
RF
n
(
1
)
where the variable n is an integer greater than 2, and each of the variables A
0
-A
n
is defined as a power coefficient. One of the power coefficients A
j
is defined as a jth power coefficient, where the variable j is an integer. For example, the power coefficient A
3
is defined as a third power coefficient. In theory, the output power P
OUT
is absolutely equal to
∑
i
=
0
n
Ai
(
P
RF
)
i
only when the variable n approaches infinity. However, the power coefficients A
0
-A
n
are arrangedindescending order, so the output power P
OUT
is usually represented as an approximation, such as
∑
i
=
0
3
Ai
(
P
RF
)
i
or
∑
i
=
0
5
Ai
(
P
RF
)
i
.
Moreover the power coefficients A
0
-An are not unchanging. The operations of the inner circuit of the power amplifier
10
, such as the two BJTs
42
and
62
, may influence the power coefficients A
0
-A
n
.
According to the prior art, when the bias voltage V
B
is pulled up from low to high, the two BJTs
42
and
62
are turned into an active forward operating mode. However, because of the intermodulation distortion and the non-linearity of the power amplifier
10
, the output signal V
OUT
has many unnecessary noises, which result in an output power P′
OUT
provided by the output signal V
OUT
within an unexpected frequency band larger than a predetermined value. Therefore, the communication within other frequency channels is interfered. Please refer to
FIG. 3
, which is a spectrum diagram of the output power P′
OUT
. A first frequency band I, a second frequency band II, and a third frequency band III are shown in FIG.
3
. Each of the three frequency bands has a bandwidth 200 kHz and respectively corresponds to a corresponding frequency channel in the TDMA system. The second frequency band II is used by the communication module, which connects to the input terminal
20
, to transmit signals. Both the input signal V
RF
and the output signal V
OUT
could be represented as sums of a plurality of sine waves, and the spectrum of the input signal V
RF
is located within the second frequency band II. If the power amplifier
10
operates ideally, the spectrum of the output signal V
OUT
should be limited within the second frequency band II. However, because of the intermodulation distortion and the non-linearity of the power amplifier
10
, the output signal V
OUT
has some noises that make the spectrum of the output signal V
OUT
Choe Henry
Gatax Technology Co., Ltd.
Hsu Winston
LandOfFree
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