Amplifiers – With plural amplifier channels – Redundant amplifier circuits
Reexamination Certificate
2001-10-31
2004-10-19
Nguyen, Patricia (Department: 2817)
Amplifiers
With plural amplifier channels
Redundant amplifier circuits
C330S051000
Reexamination Certificate
active
06806768
ABSTRACT:
BACKGROUND INFORMATION
I. Field of the Invention
The present invention relates generally to a balanced power amplifier circuit using improved power amplifier bypassing techniques. It is particularly useful in a wireless communication device, such as a CDMA wireless phone, but has other utilities as well.
II. Description of the Related Art
In various communication systems, including most hand-held wireless devices such as code-division-multiple-access (CDMA) cellular phones or any form of time-division-multiple access (TDMA) technology, RF power output from a mobile unit varies in large dynamic ranges. In a CDMA wireless system, multiple message signals are transmitted simultaneously on the same frequencies (spread spectrum). The signals are spread with different digital codes, thus allowing detection of the desired signal while the unintended signals appear as noise or interference to the receiver. Spread spectrum systems can tolerate some interference, and the interference added by each wireless transmitter increases the overall interference in each cell site. Each wireless transmitter introduces a unique level of interference, which depends on its received power level at the cell site.
The CDMA system uses power control to minimize mutual interference. Precise power control is critical to avoid excessive transmitter signal power that is responsible for contributing to the overall level of interference. The power transmitted by a particular wireless device is a function of its distance to the base station with which it is communicating and the number of other subscriber wireless devices in use talking to the same base station.
In a typical hand-held wireless unit, the power amplifier (“PA”) is biased class AB to reduce power consumption during periods of low transmit power, but power continues to be consumed. One arrangement to avoid continuous battery drain is to employ a switched bypass of a power amplifier, and then remove the DC power or power down the bypass amplifier. This arrangement is illustrated in
FIG. 5. A
single PA circuit
100
comprises a PA
102
and a circulator
104
. Typically an isolator/circulator is used to isolate the PA from the effects of load impedance in subsequent stages. An RF-signal is inserted to a pole of a first switch
106
. When the PA
102
is on, the first switch
106
connects the RF-input, via path
108
, through a band-pass filter
110
to an input of PA
102
. A second switch
114
connects the circulator
104
to ground through a terminating resistor
116
. A biasing and control circuit
118
operates to bias the PA
102
and control the operation of the first and second switches
106
,
114
. The RF-signal is amplified and output to the circulator
104
, and then transmitted to the RF-output port of the PA circuit
100
.
When the PA
102
is powered down, its input and output appear as reflective impedances. Therefore the RF-signal must be routed around the PA
102
. To accomplish bypassing, the first switch
106
connects the RF-input to a bypass path
112
and the second switch
114
routes the signal to the circulator
104
. The RF-signal enters the circulator
104
from the second switch
114
and is routed to the output of the PA
102
. The PA
102
, appearing as a reflective impedance, reflects the RF-signal back to the circulator
104
, which routes the signal to the RF-output port. This technique has drawbacks, however. When switches are used in the RF-output path, the switching loss must be overcome by the PA. Using the circulator
104
in the RF-output path removes the additional loss. However, using a circulator in the circuit requires another separate component from the PA, which consumes to circuit board space and cost. This can tend to cancel the benefits of bypassing. Furthermore, using a switch and a circulator requires more power to operate and is more costly to build.
FIG. 6
illustrates a prior art single PA circuit
130
. An analog signal is fed from a driver amplifier
132
through a band pass filter
134
to a first switch
106
. The first switch
106
alternately routes the signal between a bypass path
112
and an amplifier path
136
. In the amplifier path
136
, a PA
102
amplifies the signal. A circulator
104
is connected between the output of the PA
102
and the RF-output port. A second switch
114
connects the circulator to either the bypass path
112
or ground through a terminating resistor
34
. A control circuit controls the first switch
106
and the second switch
114
. When bypassing the PA
102
, the first switch
106
and the second switch
34
route the signal through the bypass path
112
. The signal then enters the circulator
104
and routes to the output of the powered-down PA
102
. The signal reflects off the PA
102
and back to the circulator
104
, and routes to the RF-output port. While this approach provides the benefits of bypassing, it exhibits the same drawbacks as discussed above with regard to
FIG. 5
, wherein the circulator burdens the circuit with the need for a separate external component from the PA package. The configuration of
FIG. 5
is also costlier to build when using the separate switch and circulator components. Added costs exist when the switches are external to the PA package, as well as other components. The need for external switches from the PA package also adds to the complexity of the circuit because a purchaser of the PA package must implement the bypass switches discretely.
Between FIG.
5
and
FIG. 6
, the circuit of
FIG. 5
is preferable because the position of the band-pass filter permits more power to get to the output port and permits the circuit to bypass the loss associated with that band-pass filter. When you are in bypass mode, more of the available power from the driver amplifier (not shown in
FIG. 5
) is inserted into the bypass path and losses from the band-pass filter will not be experienced The need for the band-pass filter is alleviated in the bypass mode because in this mode less power is required and out-of-band spurious emissions due to the up-converter (not shown), which are normally filtered by the band pass filter, are not large enough to be detrimental.
The advantages of the circulator method PA bypass circuit include (1) saving current by turning the PA off when the high-power levels are not needed, (2) not requiring a switch directly in the large signal path, (3) avoiding loss, (4) reducing the size and cost by removing a switch, and (5) having a stepped gain which reduces the dynamic range requirement of a variable gain amplifier which precedes the PA.
The circulator-type power amplifiers as shown in
FIGS. 5 and 6
have several drawbacks, however. The circulator requires more space in the circuitry of the mobile station because it is not integrated into the PA package. The circulator method introduces undesirable variability over many of the operating conditions of the PA. Finally, the circulator method is costly to manufacture because of the extra components required.
Some benefits are experienced by replacing the circulator with hybrid circuits which split the RF-signals into in-phase and out-of-phase signals, which are independently amplified in balanced amplifier circuits. Many of the benefits of using circulators or isolators are also achieved by using balanced amplifiers. Balanced amplifiers provide a high degree of stability, are useful in broadband applications, and provide protection from load mismatch. Balanced amplifiers also ensure that the amplifier will be linear in the event of a load mismatch. Another function of the circulator which is also achieved by balanced amplifier is to provide a good 50• termination to the duplexer which follows. The duplexer is a passive filter and is designed with specific terminations. The amplifier is matched not conjugately but for maximum linear power and when you look into the output of a power amplifier, it is not 50•. Thus you cannot directly connect an amplifier to a filter which is designed to work in a 50• system. Each function and advantage discussed ab
Chan Paul L.
Klaren Jonathan
Persico Charles J.
Walter Scott
Brown Charles D.
Nguyen Patricia
Qualcomm Incorporated
Seo Howard H.
Wadsworth Philip
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