Amplifiers – With semiconductor amplifying device – Including temperature compensation means
Reexamination Certificate
1999-11-10
2001-12-11
Shingleton, Michael B (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including temperature compensation means
C330S133000, C330S134000, C330S310000
Reexamination Certificate
active
06329879
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a high frequency power amplifier system and a mobile communication system with the high frequency power amplifier system built therein such as a cellular phone, and particularly to a technique effective for application to a high frequency power amplifier system (high frequency power amplifier module) which is used in a phase modulation (PM) system or the like and requires low distortion.
A high frequency power amplifier system has been used in a transmitter of a mobile communication system such as a car phone, a handyphone system or the like.
In digital handyphone systems such as PDC (Personal Digital Cellular), PHS (Personal Handyphone System), N-CDMA (code division multiple access), W-CDMA, etc., a phase modulation system has been adopted as a modulation scheme.
An improvement in distortion characteristic of a power amplifier with respect to a change in temperature is of importance in the phase modulation system. In order to achieve the stabilization of the distortion characteristic, the stabilization (fixing) of a drain current has heretofore been carried out.
As techniques for achieving the stabilization of the drain current, (1) a system for adding parts such as diodes, a thermistor, etc. to a circuit, (2) a system for changing the direction of a crystal axis of an FET (Field-Effect Transistor) to thereby control a change in impedance due to internal stress (piezo) (“Report ED97-182 on Technical Investigations of the Institute of Electronics, Information and Communication Engineers” issued by the Institute of Electronics, Information and Communication Engineers, 1998-01, P9-P14), and (3) a system (“Report MW97-33 on Technical Investigations of the Institute of Electronics, Information and Communication Engineers” issued by the Institute of Electronics, Information and Communication Engineers, 1997-06, P37-P42) for matching an operating point with a bias point (cross point: point Q) at which no drain current changes even if the temperature changes in a voltage (Vg) vs. current (Ids/IdssO) characteristic of each FET, etc. have been adopted.
SUMMARY OF THE INVENTION
A mobile communication system (cellular phone system) needs to reduce adjacent channel leakage power (ACP) at idle and make a call at an adjacent channel satisfactory.
A gate bias circuit of a conventional high frequency power amplifier is constructed as shown in FIG.
25
. The high frequency power amplifier
1
has an input terminal (Pin)
2
, an output terminal (Pout)
3
, a first reference potential terminal, e.g., a voltage terminal (Vdd)
4
, a control terminal (Vgg)
5
, and a second reference potential terminal, e.g., a ground terminal (GND)
6
.
A matching circuit
10
is provided between a gate terminal G (control terminal) of a transistor (FET)
7
and the input terminal (Pin)
2
. A drain terminal D (first terminal) of the FET
7
is electrically connected to the voltage terminal (Vdd)
4
through a source or power line
11
and connected to the output terminal (Pout)
3
through a matching circuit
12
. A source terminal S (second terminal) of the FET
7
is electrically connected to the ground terminal (GND)
6
. The FET
7
is an HEMT (High Electron Mobility Transistor) composed of, for example, a GaAs compound semiconductor.
Further, two resistors
15
and
16
are electrically connected to the gate terminal G of the FET
7
. The resistor R
1
(
15
) on the high potential side is electrically connected to the ground terminal (GND)
6
, and the resistor R
2
(
16
) on the low potential side is electrically connected to the control terminal (Vgg)
5
, both of which constitute a resistance type potential divider circuit (bleeder circuit). The resistors
15
and
16
are chip resistors respectively. Since a resistance portion of each chip resistor is made up of a thick film resistance, the chip resistor has hardly a temperature characteristic.
Since a portable terminal such as a portable telephone is used outdoors, the power amplifier needs a temperature characteristic stable over a range of from −20° C. to 85° C., for example. An amplifier (having, for example, a multi-stage configuration in which a single FET or FET are successively cascade-connected in plural form) deteriorates a distortion characteristic because it has a positive temperature coefficient and an idle current changes according to the temperature.
Since the gate bias (Vgs) circuit is comprised of the chip resistors having hardly the temperature characteristic as described above, the change in idle current has heretofore exerted a bad influence directly on the distortion.
The above-described respective systems have been adopted to make constant the idle current, in other words, drain current of the high frequency power amplifier system.
However, the above means (1) for adding the parts such as the diodes, thermistor, etc. makes use of outboard parts. Thus, the means increases not only the number of parts but also the scale of a circuit for matching each FET with a temperature coefficient.
Further, the above means (2) for changing the direction of the crystal axis of the FET to thereby control the change in the impedance due to the internal stress (piezo) changes stress according to a substrate equipped with an FET and is inferior in general versatility.
Furthermore, the above means (3) using the point Q is inferior in general versatility because it is shifted in position according to FET characteristics.
In such a high frequency power amplifier
1
on the other hand, the two resistors constituting the above-described bleeder circuit comprise the chip resistors and their resistance values are kept constant even if the temperature varies. In contrast to this, the internal resistance of the FET changes with an increase in temperature and the idle current of the FET
7
increases (it has a positive temperature characteristic at about 0.14%/° C., for example). As a result, the operating point varies and thereby the distortion characteristic and efficiency greatly change.
On the other hand, there has been a demand for an improvement in efficiency of the portable terminal. The efficiency and ACP are placed in a so-called trade-off relationship in that the other of them becomes worse if one thereof becomes better and vice versa.
Therefore, the portable terminal has heretofore been designed at some penalties in efficiency and with an increased margin of ACP at ordinary temperatures.
The present inventors have measured idle currents at temperatures of −20° C., 25° C. and 100° C. to obtain the optimum idle current value at the time that ACP is at detuning of ±50 KHz. The result of measurements is indicated by a graph shown in FIG.
26
. It is understood from the same graph that there is a tendency that when the temperature is low, the idle current increases, whereas as the temperature increases, the idle current decreases.
Further, a change in temperature with respect to the idle current is already known to date in the sense that a change in channel resistance and a change in parasitic capacitance influence the impedance due to a piezo effect produced by thermal stress which acts between each FET and the substrate equipped with it.
As a result of basic investigations of a method for changing Vgs to cancel such a change in impedance to thereby vary the input impedance of a gate, thus improving distortion, the present inventors have traced the fact that the distortion could be held constant by allowing the idle current of an amplifier to have a negative temperature characteristic.
Therefore, the present inventors have found out that owing to the setting of the resistor R
1
on the high potential side, of the bleeder circuit as a temperature compensating resistor having a positive temperature coefficient and an increase in the resistance value of the resistor R
1
on the high potential side with a rise in temperature, the potential at the gate terminal G could be reduced to thereby restrain the idle current from increasing with the rise in temperat
Akamine Hitoshi
Kobori Tsutomu
Maruyama Masashi
Moriyama Shinji
Hitachi , Ltd.
Mattingly Stanger & Malur, P.C.
Shingleton Michael B
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