Amplifiers – With semiconductor amplifying device – Including particular biasing arrangement
Reexamination Certificate
2000-07-18
2002-08-13
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including particular biasing arrangement
C330S149000
Reexamination Certificate
active
06433641
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to power amplifiers and communication devices. More particularly, the present invention relates to a power amplifier for transmission which is used in a radio communication device or the like requiring amplification with low distortion, and a communication device which includes the power amplifier.
2. Description of the Background Art
As in mobile communication systems for mobile phones or the like, radio communication systems using quasi-microwave and microwave bands have rapidly spread in recent years. That greatly contributes to attaining portable terminals of lighter weight and lower power consumption.
In order to achieve lighter weight of portable terminals, it is effective to use lighter batteries of a small capacity type. However, employment of the small capacity type generally shortens the time till the battery runs out. Therefore, lower power consumption, that is, improvement in the power efficiency is strongly required for power amplifiers for transmission which lead to most power consumption of terminals at the time of transmission.
In constant amplitude analog modulation/demodulation systems using conventional FM modulation/demodulation methods, power amplifiers can operate in a saturated state and thus they can be made more efficient relatively easily. Recently, however, communication systems which employ digital modulation/demodulation using QPSK (quadrature phase shift keying) modulation or the like with high frequency utilization efficiency have become the mainstream.
In the digital modulation/demodulation methods, information is transmitted by both the amplitudes and phases of signals, and thus power amplifiers are required to linearly amplify input signals. Generally, as an increase in output power due to the increased input power approaches saturation, the power amplifiers come to have higher distortion and power efficiency. Therefore, high power efficiency and low distortion are in a tradeoff relationship. Attempts to improve the power efficiency are often made by adding a distortion compensation circuit so that the operation is performed with low distortion even at high input power. One example is described in Japanese Patent Laying-Open No. 9-260964 entitled “High Frequency Amplification Circuit” (hereinafter, referred to as Document 1).
FIG. 15
shows a circuit configuration of a power amplifier disclosed in Document
1
. Referring to
FIG. 15
, a power amplifier
900
includes a bipolar transistor Tr
90
for power amplification, a diode D
90
, a capacitor C
90
, and bias resistors R
91
, R
92
. Diode D
90
and capacitor C
90
form a distortion compensation circuit. When a bias voltage Vb is supplied, the base bias condition of bipolar transistor Tr
90
is determined by the direct current characteristics of bias resistors R
91
, R
92
and diode D
90
.
Capacitor C
90
has capacitance which is regarded as a grounded state regarding a high frequency at the operating frequency of power amplifier
900
. Impedance measured from the base end of bipolar transistor Tr
90
toward diode
90
is only the resistance and capacitance components of diode D
90
with regard to a high frequency. The impedance is equivalent to parallel connection between the base and emitter of bipolar transistor Tr
90
with regard to a high frequency.
An input signal causes the instantaneous voltage between the base and emitter of bipolar transistor Tr
90
to fluctuate with time. However, since a diode characteristic is observed between the base and the emitter, higher voltage fluctuation and lower voltage fluctuation of the instantaneous voltage are asymmetrical, with the center being a voltage when a signal is not applied. Thus, the average voltage varies with input power. Specifically, because of the diode characteristics, when the voltage across the diode increases and the current increases, then the impedance is lowered. Since the voltage amplitude at the higher voltage side is small, the average voltage is shifted toward the lower voltage side because of the input signal. The shift amount increases with an increase in the input power.
The capacitance component of a diode depends on a voltage across the both ends of the diode (both end voltage dependence). Therefore, the voltage shift due to an increase in input power changes the capacitance between the base and emitter of bipolar transistor Tr
90
. Thus, the reactance component of bipolar transistor Tr
90
from the base end is changed, which changes the transmit of a signal. This is “amplitude-phase distortion” as a cause of distortion for a power amplifier.
In power amplifier
900
shown in
FIG. 15
, phase distortion caused by the non-linearity of capacitance between the base and emitter of bipolar transistor Tr
90
is compensated for by adding a distortion compensation circuit formed of diode D
90
and capacitor C
90
.
In other words, an increase in input power decreases the average voltage of the diode portion (between the base and emitter) of bipolar transistor Tr
90
and, at the same time, increases the average voltage across the both ends of diode D
90
which is connected in parallel, with regard to a high frequency, with the base and emitter of bipolar transistor Tr
90
. Therefore, the change in the diode capacitance value between the base and emitter of bipolar transistor Tr
90
and the change in the capacitance value of diode D
90
, which are caused by an increased or decreased input power, offset each other, mitigating the dependence of the transmit on the input power in the power amplifier. Thus, bipolar transistor Tr
90
can effectively maintain linearity even at input power closer to saturation. As a result, the power efficiency is improved.
If bias power supply Vb and the base of bipolar transistor Tr
90
are connected only through fixed resistance, the higher the base power caused by an increase in input power, the higher the effect of suppressing an increase in the base current becomes, which is caused by a reduction in the voltage at the fixed resistance portion. Therefore, the collector current is prevented from increasing, which reduces a gain due to an increase in the input power, that is, causes amplitude-amplitude distortion. In power amplifier
900
, as the base current flowing in diode D
90
is larger, the resistance component of diode D
90
is lowered and the voltage drop is mitigated. It is therefore possible to reduce amplitude-amplitude distortion.
As described above, the amplitude-phase distortion characteristic and the amplitude-amplitude distortion characteristic have to be compensated for to lower distortion of the power amplifier. In the above described distortion compensation method in power amplifier
900
, however, only the non-linearity of the resistance and capacitance components of diode D
90
is used to provide compensation. Thus, once diode
90
to be used is determined, the non-linearity of the resistance and capacitance components are fixed at the same time. The non-linearity of them cannot be set to an optimum value separately.
Consequently, in some cases, only one of amplitude-phase distortion and amplitude-amplitude distortion can be compensated for or distortion compensation of one of them can exacerbate the other distortion. When the amplitude-phase distortion and amplitude-amplitude distortion are to be compensated for at the same time, compensation for both may be insufficient.
Such a power amplifier has to be operated at a higher bias current to maintain low distortion. If low distortion is realized, the power efficiency decreases, that is, the power consumption increases. Therefore, when the power amplifier is used for a battery-driven communication terminal, the communication time till the battery runs out is shortened.
SUMMARY OF THE INVENTION
Therefore, the present invention provides a power amplifier of low distortion and high power efficiency, and a low power consumption communication device including the power amplifier.
A power amplifier according to one aspect of the present invention inclu
Nguyen Khanh Van
Sharp Kabushiki Kaisha
LandOfFree
Power amplifier and communication device including power... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Power amplifier and communication device including power..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Power amplifier and communication device including power... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2890122