Telecommunications – Transmitter – Power control – power supply – or bias voltage supply
Reexamination Certificate
2000-10-26
2004-08-10
Urban, Edward F. (Department: 2685)
Telecommunications
Transmitter
Power control, power supply, or bias voltage supply
C455S127100, C455S253200
Reexamination Certificate
active
06775525
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to radio communication apparatus such as cellular phones, and semiconductor devices incorporated in the radio communication apparatus, and particularly to a radio communication apparatus in which a high-frequency power amplifier module of a single-stage arrangement using a single high-power amplifier that has a plurality of transistors connected in parallel or of a multi-stage arrangement using a plurality of such high-power amplifiers of cascade connection is provided at the transmitter-side output stage, and in which an antenna is provided to be connected to this high-frequency power amplifier module, so that prevention of thermal runaway, improvement of efficiency and prevention of oscillation can be achieved.
The mobile communication apparatus (radio communication apparatus) such as car phone and cell phone has a high-frequency power amplifier module (high-frequency power amplifier circuits) built in its transmitter-side output stage. The output (transmission power) of this high-frequency power amplifier module is automatically controlled by a bias controlling circuit (APC: Automatic Power Control).
In general, a system for portable telephone is constructed in which the output from a cellular phone is changed to conform to the surrounding environment according to a power level indicating signal transmitted from a base station, thereby preventing the cross talk to or from other cellular phones. In the cellular phone, its bias control circuit supplies a certain signal (control signal) according to the received power level indicating signal to the high-frequency power amplifier module, thereby making the output from the radio communication apparatus be adjusted.
The high-frequency power amplifier module is of a single stage arrangement using a single semiconductor amplifier element (transistor) or of a multi-stage arrangement using a plurality of such transistors connected in cascade. The transistors used are bipolar transistors, MOS FET (Metal Oxide Semiconductor Field-Effect-Transistor), GaAs-MES (Metal-Semiconductor) FET, HEMT (High Electron Mobility Transistor), and HBT (Heterojunction Bipolar transistor).
U.S. Pat. No. 5,629,648 (issued May 13, 1997) discloses a high power amplifier circuit using a heterojunction bipolar transistor. This document describes that a high power bipolar transistor circuit can suppress the thermal runaway by providing a resistor for DC bias and a capacitor for AC signal at the base of each of a plurality of transistors connected in parallel, and by the DC voltage drop across the resistor.
In JP-A-7-7014 laid open Jan. 10, 1995 (corresponding to U.S. Pat. No. 5,321,279 issued Jun. 14, 1994) as another example of the prior art, it is described that a power HBT can suppress the thermal runaway (current hogging: hot-spot) by connecting a ballast high impedance to the base fingers of a Si bipolar transistor of multi-finger structure having emitter fingers (emitter terminals), base fingers (base terminals) and collector fingers (collector terminals) in order that the apparatus can be operated with high reliability. That is, the hot-spot formation due to current concentration can be suppressed by providing a ballast impedance (resistor) at each base finger.
In this document, a capacitor (bypass capacitor) is also connected in parallel to the ballast impedance (resistor) in order to assure the minimum gain loss of transistor. It is also described that the power HBT mentioned above can make a high power amplifier and can be used for cellular phone.
In addition, JP-A-7-94975 (laid open Apr. 7, 1995) discloses a high-frequency HIC module (high-frequency power amplifier module) of three-stage configuration having cascade-connected MOS FETs. This high-frequency HIC module is constructed to have a first bias circuit that biases the gate of a certain one of a plurality of MOS FETs according to the output control voltage, a second bias circuit that biases the gates of the other MOS FETs than the certain one on the basis of a fixed power source, and switching means that switches the paths of the fixed power source and the second biasing circuit on the basis of the output control voltage. This module thus increases the controllability of the output, and improves the efficiency. In addition, each biasing circuit is formed of three resistors and one capacitor.
SUMMARY OF THE INVENTION
The HBT is being used as a semiconductor amplifying element of a high-frequency power amplifier module incorporated in a radio communication apparatus such as a cellular phone because it has excellent characteristics of high speed and low power consumption.
The inventors examined the means for compressing both thermal runaway and oscillation phenomenon of HBT element and at the same time for improving the efficiency while they were developing a HBT power amplifier module (high-frequency power amplifier module) for cellular phone. From the examination, it was confirmed that the radio communication apparatus employing the well-known circuits caused the following drawbacks.
FIG. 18
is a schematic circuit diagram of part of a radio communication apparatus having incorporated therein a single-stage amplifier (high-frequency power amplifier module) using a single HBT of multi-finger structure. This module is the same as the circuit arrangement (conventional example 1) described in U.S. Pat. No. 5,629,648. Here, for the sake of explanation, a portion formed of an emitter terminal (emitter finger), a base terminal (base finger) and a collector terminal (collector finger) is called a transistor (unit cell), and a group of such transistors connected in parallel as a multi-finger structure transistor or multi-finger transistor. Thus, the above-mentioned HBT is one multi-finger transistor.
A high-frequency power amplifier module
1
has, as external terminals, an input terminal (RF in), an output terminal (RF out), a first voltage terminal (Vcc) serving as the collector terminal, too, a second voltage terminal (ground: GND) serving as the emitter terminal, too, and a biasing terminal (control terminal: Vapc) serving as the base terminal, too. The output terminal (RF out) is connected to an antenna
2
through a filter or the like not shown.
The HBT is a multi-finger transistor that has N transistors connected in parallel. The transistors, Q
1A
~Q
1N
each has emitter terminal
5
, base terminal
6
and collector terminal
7
.
In this circuit arrangement, the input terminal and the control terminal are separated. Junction capacitors C
2A
~C
2N
are connected between the input terminal and the base terminals
6
of the transistors Q
1A
~Q
1N
, and ballast resistors R
2A
~R
2N
between the control terminal and the base terminals
6
of the transistors Q
1A
~Q
1N
.
In addition, the first voltage terminal (Vcc) and the collector terminals
7
are connected to a single inductor Lc, and the emitter terminals
5
to ground (GND). The high-frequency power amplifier module
1
also has a matching circuit
9
provided at the output side in order to match with the impedance of the antenna
2
.
In this radio communication apparatus, since an AC signal is supplied through only capacitors, when the efficiency of the apparatus is raised to about 60% (corresponding to about 70% of the efficiency of the high-frequency power amplifier module), the element stability becomes poor, and oscillation phenomenon is apt to occur because of no loss in the signal paths, with the result that stable communication might be lost. In other words, the impedance viewing the external circuit at high frequencies is small, and the stabilization coefficient K becomes noticeably small, thus the transistors being unstable.
FIG. 6
shows a graph of the correlation between the stability coefficient K and collector current. In this graph, the curve indicates the stability coefficient of conventional example 1. From the curve, it will be seen that the stability coefficient K is about 0.15 relative to a collector current of 0.01 A and further decreases with the increase o
Tanoue Tomonori
Yamashita Kiichi
Chow C.
Kenyon & Kenyon
Renesas Technology Corporation
Urban Edward F.
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