Amplifiers – With semiconductor amplifying device – Including plural amplifier channels
Reexamination Certificate
2001-03-27
2002-09-10
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including plural amplifier channels
C330S296000
Reexamination Certificate
active
06448859
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims benefits of priority under 35 U.S.C.119 to Japanese Patent Application No. P2000-89060 filed Mar. 28, 2000, the entire contents of which are incorporated by reference herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high frequency power amplifier using a bipolar transistor, more particularly to a high frequency power amplifier having high efficiency and low distortion, which uses a heterojunction bipolar transistor.
2. Description of the Background
For recent mobile telephones and mobile information terminals, transistors efficiently performing power amplification at a frequency band of 1 GHz or more have become indispensable constituent components. Among these transistors, a heterojunction bipolar transistor formed on a gallium arsenide (hereinafter referred to as GaAs) substrate is excellent in a high frequency characteristic and operates at a low voltage with high efficiency. Accordingly, the heterojunction bipolar transistor meets social demands for reducing the number of cells to lighten the telephones and the terminals, and attracts social attention. In addition, the heterojunction bipolar transistor shows less three-dimensional distortion, and has a characteristic suitable for digital modulation for which high linearity of operation is required.
Although the heterojunction bipolar transistor using a material of the GaAs group has the principally excellent characteristic, this transistor sometimes makes its characteristic deteriorated when it is intended to obtain large output power. This originates from the fact that heat conductivity of the GaAs substrate is as comparatively low as about 0.4 W/K/cm (about ⅓ of silicon), and a rise of the device temperature becomes large with an elevation of an output level. When the bipolar transistor is driven while keeping the base-emitter voltage thereof constant, it has been known that a collector current increases due to the temperature rise. Accordingly, a positive feedback of a current increase, a power increase, a temperature rise and a current increase is produced, in which the current increase creates the consumption power to rise the device temperature, thus further increasing the current. There is a drawback that unevenness of current distribution occurs in the high frequency power amplifier having a plurality of emitter fingers and a large area, and a thermal runaway state may be brought about at the worst, resulting in breakdown of the transistor.
To cope with such a problem, the most familiar method from way back is a ballast resistance method (G. Gao et al. IEEE Trans. Electronic Dev., 1991, pp. 185-196) for providing a ballast resistance which increases either an emitter resistance or a base resistance to apply a negative feedback to a current increase and a voltage relation between a base and an emitter, thus canceling a positive feedback due to a temperature rise. An example of a high frequency power amplifier by heterojunction bipolar transistors, which use the ballast resistance method, is shown in
FIG. 1
, and a high frequency power amplifier using the conventional bipolar transistors will be described.
In
FIG. 1
, an output voltage of a reference voltage generation circuit
12
for generating a reference voltage as a base bias is distributed to bipolar transistors
1
a
,
1
b
,
1
c
and
1
d
serving as fingers of a transistor circuit
10
via a bias generation circuit
2
for performing an impedance conversion by a transistor
11
. The reference voltage as the base bias is adjusted in accordance with the temperature of a diode
6
. The bias circuit having such constitution shall be called a diode bias circuit in the following descriptions.
An emitter of each transistor
1
a
,
1
b
,
1
c
and
1
d
is connected to an earthed electrode via corresponding one of ballast resistances
5
a
,
5
b
,
5
c
and
5
d
. A high frequency power is connected to a base of each of the transistors
1
a
,
1
b
,
1
c
and
1
d
of the transistor circuit
10
via a metal insulator metal (hereinafter referred to as MIM) capacitor device
4
. To prevent the high frequency power from leaking to the base bias circuit, a resistance
3
is connected between an emitter of the impedance conversion transistor
11
and the high frequency power transistor
1
. Accordingly the bias generation circuit
2
shown in
FIG. 1
comprises a bipolar transistor
11
for impedance conversion, a resistance
3
for blocking a high frequency, and a resistance
FIG. 2
shows a pattern layout in a circuit constitution of the high frequency power amplifier using the conventional heterojunction bipolar transistor shown in FIG.
1
. This pattern layout will be described in detail in the description of a first embodiment of the present invention while comparing with a pattern layout of a high frequency power amplifier as the first embodiment of the present invention. In the conventional power amplifier, 32 emitter fingers, each having a size of 4×30 &mgr;m, are arranged in a chip of 1 mm×2 mm as shown in
FIG. 2
, and a linear output of 30 dBmW is obtained. Here, reference numerals
1
a
,
1
b
,
1
c
and
1
d
denote transistors, each having eight emitter fingers connected in parallel. The bias circuit
2
composed of a diode bias circuit is arranged in the position shown in
FIG. 2
, and a DC potential is supplied to the base of each of the four transistor blocks
1
a
to
1
d
. The resistance
3
is provided for blocking the high frequency. A high frequency signal is connected to the base of each of the four transistor blocks
1
a
,
1
b
,
1
c
and
1
d
via the MIM capacitor
4
.
So called a MMIC (Monolithic Microwave Integrated Circuit) is constituted by forming the transistor circuit having such constitution generally on a GaAs chip integratedly. In this circuit constitution, a change in temperature of the chip is detected by the diode
6
, and a bias voltage in accordance with the temperature of the chip is supplied to the high frequency power transistor. However, when a high frequency power density becomes large, a temperature difference among the finger transistors of the high frequency power transistor circuit
10
occurs, thus making the current distribution uneven.
Particularly, temperature is apt to rise at the central portion of the high frequency transistor circuit
10
, and in the example shown in
FIG. 1
, a sum of currents flowing in the finger transistors
1
b
and
1
c
is larger than that of currents flowing in the finger transistors
1
a
and
1
d
. In
FIG. 3
, the position of the transistor block in the conventional bipolar transistor circuit shown in FIG.
1
and the value of the collector current thereof are illustrated. As shown in
FIG. 3
, it is proved that the value of the collector current of the transistor positioned at the center of the bipolar transistor circuit varies more when the ballast resistance is 2 &OHgr; than when the ballast resistance is 3.5 &OHgr;.
Generally, when the ballast resistances
5
a
to
5
d
are made to be larger, a resistance to thermal runaway increases, and uniformity of the current distribution can be improved. However, when the ballast resistances are made to be too large, a drawback occurs in which a voltage of the transistor at a saturated region increases, thus deteriorating efficiency and lowering a gain.
Even if the ballast resistances
5
a
to
5
d
are made to be larger in the conventional bipolar transistor circuit shown in FIG.
1
and the resistance to the thermal runaway of the high output transistor
10
can be increased, resistance to breakdown of the bias circuit may be a problem. This means a problems that when a large amount of the collector current flows through the transistor circuit
10
compared to a normal use because of fluctuation of an external additional resistance connected to the collector of the high output transistor circuit
10
, the transistor
11
of the base bias circuit
2
is broken.
Specifically, when the collector current increases
Nguyen Khanh Van
Pascal Robert
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