Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2000-11-17
2002-06-18
Zweizig, Jeffrey (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
Reexamination Certificate
active
06407617
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a bias circuit and a method of fabricating a bipolar integrated circuit in which elements of the bias circuit are integrated.
Recently, a field effect transistor formed from GaAs with small power consumption (MESFET) is widely used as a transistor of a transmitting power amplifier used in mobile communication equipment such as a portable telephone. A negative power source is generally used for bias for a gate electrode of a MESFET. Accordingly, in using a MESFET in a transmitting power amplifier, two power sources, namely, a positive power source and a negative power source, are required. This is a disadvantage to downsizing of the amplifier, and hence, a transistor operated by a positive power source alone is earnestly desired.
Furthermore, in recent communication systems such as CDMA (code division multi-channel access), an output current of a transmitting power amplifier is required to have small distortion (namely, to be linear). As a transistor meeting these requirements, a heterojunction bipolar transistor (HBT) including the emitter formed from a semiconductor having a larger band gap than a semiconductor forming the base is practically used.
In a conventional power amplifier using HBTs, a bias circuit is generally constructed on the same chip for supplying a current necessary for the base of an HBT used as a power transistor. An HBT has, however, a characteristic that the on state voltage decreases as the temperature increases as is shown in
FIG. 10
(which characteristic is hereinafter referred to as the temperature characteristic of an HBT). Therefore, when a given voltage is applied between the base and the emitter, a collector current (hereinafter referred to as the idle current) of the HBT is largely increased as the temperature increases. Accordingly, the bias circuit is required to reduce change with temperature of the idle current of the HBT serving as the power transistor.
A bias circuit for overcoming the problem will now be described with reference to
FIG. 11
, which shows a bias circuit
100
used in a conventional power amplifier.
The base terminal of a bipolar transistor Tro
101
serving as a power transistor is connected through a resistor R
103
of 4 &OHgr; to a bipolar transistor Tr
102
so as to compose an emitter-follower circuit. Also, the base terminal of the transistor Tr
102
is grounded through transistors Tr
103
and Tr
104
in each of which the base and the collector are short-circuited. The transistors Tr
103
and Tr
104
are PN diodes having the same on state voltage as the transistors Tr
101
and Tr
102
. When the temperature is increased in this circuit, the idle current C of the transistor Tr
101
, that is, the HBT, is increased owing to the temperature characteristic. On the other hand, a current flowing through the transistors Tr
103
and Tr
104
is also increased owing to the same temperature characteristic. Accordingly, a current flowing through a resistor R
101
connected to the transistors Tr
103
and Tr
104
in series is increased. Since the resistance of the resistor R
101
is constant (530 &OHgr;), a voltage applied to the resistor R
101
is increased as the current increases. In other words, a potential at a point P
5
of
FIG. 11
is lowered. Accordingly, the base potential of the transistor Tr
102
connected to the resistor R
101
is lowered. As a result, the emitter current of the transistor Tr
102
is decreased, so as to lower the base potential of the power transistor Tr
101
. In this manner, the idle current C of the power transistor Tr
101
can be suppressed from increasing.
The bias circuit too of
FIG. 11
thus suppresses the idle current C of the power transistor Tr
101
from increasing in accordance with the temperature increase.
In the conventional bias circuit
100
, however, the suppression of the change of the idle current is disadvantageously insufficient.
SUMMARY OF THE INVENTION
The present invention was devised to overcome the aforementioned disadvantage, and an object is providing a bias circuit in which change with temperature of an idle current of a power transistor is suppressed and a method of fabricating a semiconductor device including the bias circuit.
The bias circuit of this invention comprises a first bipolar transistor having an emitter, a base and a collector; and at least one Schottky diode connected to the base of the first bipolar transistor, and the at least one Schottky diode is disposed for supplying a base potential for suppressing a collector current of the first bipolar transistor from changing in accordance with temperature change.
In a bipolar transistor, the collector current is changed in accordance with temperature change when a given voltage is applied between the base and the emitter. Since the Schottky diode is provided so as to supply a base potential for suppressing the change, a substantially constant collector current can be obtained regardless of the temperature change.
The bias circuit preferably further comprises a second bipolar transistor having an emitter, a base and a collector; a PN diode connected to the base of the second bipolar transistor; and two or more Schottky diodes connected to the base of the second bipolar transistor, and the emitter of the second bipolar transistor is preferably connected to the base of the first bipolar transistor, and the two or more Schottky diodes are preferably connected to the PN diode in series.
When the PN diode and the two or more Schottky diodes are connected to the base of the second bipolar transistor, a current flowing through the PN diode and the two or more Schottky diodes is increased as the temperature increases with a given voltage applied to the bias circuit, and hence, the base potential of the second bipolar transistor can be lowered. As a result, the emitter current of the second bipolar transistor is decreased, so as to lower the base potential of the first bipolar transistor. Accordingly, the collector current of the first bipolar transistor can be suppressed from increasing. When the temperature decreases, the collector current of the first bipolar transistor can be suppressed from decreasing in accordance with the temperature decrease through a mechanism completely reverse to that described above. In particular, a Schottky diode has a smaller on state voltage than a PN diode used in the conventional bias circuit. Also, the change with temperature of the on state voltage of the Schottky diode is substantially the same as change with temperature of the on state voltage of the PN diode. Accordingly, in the two or more serially connected Schottky diodes, the change of a current caused by changing the temperature under application of the same voltage as in the PN diode can be larger than in the PN diode. As a result, the base potential of the first bipolar transistor can be more largely changed so as to more sufficiently suppress the change of the collector current of the first bipolar transistor.
The bias circuit preferably further comprises a second bipolar. transistor having an emitter, a base and a collector; and three or more Schottky diodes connected to the base of the second bipolar transistor in series, and the emitter of the second bipolar transistor is preferably connected to the base of the first bipolar transistor.
In the three or more serially connected Schottky diodes, the change of a current caused by changing the temperature under application of the same voltage as in the PN diode can be larger than in the PN diode. Accordingly, the base potential of the first bipolar transistor can be more largely changed so as to more sufficiently suppress the change of the collector current of the first bipolar transistor.
The bias circuit preferably further comprises at least one Schottky diode connected to be branched from connection between the emitter of the second bipolar transistor and the base of the first bipolar transistor.
In the Schottky diode connected to be branched from the connection of the base of the first bipolar transistor, the change
Sugimura Akihisa
Tanaka Tsuyoshi
Yanagihara Manabu
Matsushita Electric - Industrial Co., Ltd.
Nixon & Peabody LLP
Robinson Eric J.
Zweizig Jeffrey
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