Amplifiers – With semiconductor amplifying device – Including plural amplifier channels
Reexamination Certificate
2002-07-23
2004-02-03
Choe, Henry (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including plural amplifier channels
C330S286000, C330S307000
Reexamination Certificate
active
06686801
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates to a power amplifier integrated circuit employed in wired or wireless system application, and more specifically, to a power amplifier integrated circuit having a distributed capacitor.
2. Description of the Prior Art
Power amplifier integrated circuits have been widely used in different kinds of wired or wireless system applications. Power amplifier integrated circuits such as those employing heterojunction bipolar transistors operate with elevated junction temperatures. High junction temperatures degrade device reliability and limit maximum current density, and thus maximum power, of the power amplifier.
When operated at high power, power amplifier integrated circuits can suffer thermal runaway where, because of emitter current non-uniformity or temperature profile non-uniformity, an emitter of the power amplifier integrated circuit conducts an increasing amount of current until catastrophic device failure. Operating at high power also reduces the device life and mean time to failure (MTTF). Moreover, a larger device is mandated for a given application when a power amplifier integrated circuit cannot operate at a required power.
Please refer to FIG.
1
.
FIG. 1
shows a prior art heterojunction bipolar transistor power amplifier
10
. Typically, the power amplifier
10
includes a plurality of emitter fingers and base fingers. Accordingly, the power amplifier
10
is an equivalent circuit comprising transistors
12
a
-
12
c
. The power amplifier
10
further includes ballasting resistors
14
a
-
14
c
to stabilize the transistors
12
a
-
12
c
when operating with a high current density, and bypass capacitors
16
a
-
16
c
in parallel with the ballasting resistors
14
a
-
14
c
. An RF input signal and DC voltage are supplied through a common input node
18
and output is taken from the collector nodes
20
. The power amplifier
10
is further described by Khatibzadeh et al. in U.S. Pat. No. 5,321,279, which is included herein by reference. While, the ballasting resistors
14
a
-
14
c
provide device stability, they also undesirably reduce the overall gain of the power amplifier
10
.
FIG. 2
shows a second prior art power amplifier as described by Pratt in U.S. Pat. No. 5,629,648, which is included herein by reference. Similar to the power amplifier
10
, the power amplifier
30
comprises transistors
32
a
-
32
c
and ballasting resistors
34
a
-
34
c
. Bypass capacitors
36
a
-
36
c
are also provided. An RF signal can be applied at a node
38
, a DC voltage can be applied at a node
39
, and a corresponding output can be taken from collector nodes
40
. However, use of capacitors
36
a
-
36
c
is inefficient and mandates an overly complicated layout.
Generally, the prior art power amplifier circuits provide power amplifier stability at the cost of gain, and do so in an inefficient way. Moreover, layout of the prior art power amplifier integrated circuits is inefficient.
SUMMARY OF INVENTION
It is therefore a primary objective of the present invention to provide a power amplifier having a distributed capacitor to solve the problems of the prior art.
According to one preferred embodiment of the present invention, the power amplifier includes a plurality of transistors. Each transistor has a base and a ballast resistor having a first terminal and a second terminal. The embodiment further comprises a DC node, an RF node, and a capacitor having a third terminal and a fourth terminal. The first terminal of each ballast resistor is connected to a base of the plurality of bases, and the second terminal of each ballast resistor is connected to the DC node. The third terminal of the capacitor is connected to the RF node and the fourth terminal of the capacitor is connected to the plurality of bases of the transistor.
According to the preferred embodiment, the transistor is a heterojunction bipolar transistor.
According to the preferred embodiment, the ballast resistors are set to maximize uniformity of temperature of a plurality of emitters of the transistor.
According to the preferred embodiment, the capacitor has a substantially high capacitance so that an RF input signal applied at the RF node suffers low signal loss.
It is an advantage of the present invention that the capacitor provides a distinct path for the RF input signal, such that the RF input signal does not suffer significant signal loss.
It is a further advantage of the present invention that the capacitor supplies capacitance that is distributed to each base of the plurality of bases.
REFERENCES:
patent: 4081759 (1978-03-01), Yen
patent: 5321279 (1994-06-01), Khatibzadeh et al.
patent: 5629648 (1997-05-01), Pratt
patent: 6201445 (2001-03-01), Morimoto et al.
patent: 6448858 (2002-09-01), Helms et al.
Cho Jin Wook
Xue Hongxi
Choe Henry
Hsu Winston
MediaTek Inc.
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