Semiconductor device

Details Semiconductor device Semiconductor device

2000-03-20

2002-02-05

Mottola, Steven J. (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including current mirror amplifier

C330S289000, C330S307000

Reexamination Certificate

active

06344775

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor high frequency power amplifier and more particularly to a bipolar transistor high-frequency power amplifier having high efficiency and excellent linearity.
With rapid widespread of portable radio terminals, a linear modulation is adopted as a recent standard of communication terminals to improve efficient use of radio waves and to satisfy needs for high-speed data communication. In the linear modulation, the magnitude of transmission power varies with a signal. Band width of a spectrum of the transmission power spreads wider than that of an input power if a power amplifier has a non-linear amplification characteristic. This wide spectrum interferes with a signal in the channel existing in an adjacent frequency band. A strict restriction is thus imposed to control the spreading of the spectrum within a certain threshold value. Therefore, it is essential that power amplifiers used for recent portable radio terminals must be sufficiently linear.
Generally, a bias point of a power amplifier transistor is so set as to operate as a Class B or Class AB amplifier to ensure linear amplification. However, a power efficiency of the Class A amplifier is as low as 50% at maximum in the theoretical limit and the efficiency of an actual amplifier is generally only about 30% at maximum. Since the power efficiency of a power amplifier is a great factor for deciding the battery life in a portable terminal, the efficiency of the Class A amplifier is not sufficient. Among the non-linear distortions, only the odd-degree distortion affects frequencies around an input signal band, so that the even-degree distortions are permitted. From such a viewpoint, a Class B amplifier is now adopted as a circuit which is more efficient than a Class A amplifier and satisfies the distortion standard required for the portable terminal. In an actual portable terminal, however, the distortion produced by the power amplifier in which a theoretical Class B bias point is set, is likely to increase when the input signal level is small. Therefore, the bias point is often set at a point called as the Class AB which is an intermediate point of the Class A and the Class B. Hereinafter, for simplicity of discussion, the problems of the prior arts will be explained using an example of a Class B amplifier.
Firstly, a bipolar transistor amplifier as shown in
FIG. 1
will be considered. A collector output characteristic curve of a bipolar transistor
120
and a load line are shown in FIG.
2
. As shown in FIG.
2
(
b
), an output current of the Class B amplifier shows a waveform rectified in a half a wave. With the rectified wave form, no odd-degree distortions are generated. With increases of an output power, however, the rectified half waveform cannot be maintained and odd-degree distortions are generated when an instantaneous collector voltage lowers to the saturation voltage of the transistor. Therefore, the Class B amplifier can be used with a high linearity required for a portable terminal up to the critical point where the instantaneous collector voltage is lowered to the saturation voltage and the collector power efficiency at this time is expressed as follows:
&eegr;=&pgr;/4×(1−Vsat/Vcc)
where Vsat indicates a saturation voltage of the bipolar transistor which is typically about 0.2 V. Assuming Vcc as 3.5 V, the efficiency becomes 74%, which is a higher efficiency than that of a Class A amplifier.
However, the experiment of the inventors shows that the high efficiency and the high linearity are incompatible with each other in an element having particularly large output power. The reason is described below. As shown in FIG.
2
(
b
), the DC current supplied from a collector bias power source (
130
) to the transistor
120
varies depending on the output power level in the Class B amplifier. Therefore, power consumption of the transistor element, which is very small when there is no input signal, increases as the input signal level increases and the junction temperature of the output-transistor also increases. Input and output characteristic (hereinafter referred to as I/O characteristic) of the bipolar transistor is shown in
FIG. 3
using the junction temperature as a parameter. In a bipolar transistor, when the junction temperature rises, the diffusion potential lowers. Accordingly, as shown in
FIG. 2
, a collector current increases as the temperature of the transistor element rises when a base is biased by a constant voltage source (
140
). The reason is that the I/O characteristic curve
150
with no temperature rise in the transistor element shifts to a characteristic curve
160
when the junction temperature increases by 20 K. The curve also shifts to a curve
170
when the junction temperature increases by 40 K. Therefore, as shown in
FIG. 4
, a load line
180
with no temperature rise of the transistor element shifted to a line
190
when the element temperature rises by 20 K and to a line
200
when the element temperature rises by 40 K. With the shifts of the load line, DC input power increases correspondingly, and the power efficiency deteriorates.
FIG. 5
shows a result of an experiment by the inventors and of a theoretical analysis. In the experiment, a heterojunction bipolar transistor (hereinafter referred to as HBT) is used in which a base and a collector are made of GaAs and an emitter is made of InGaP. Also in the experiment, a bias point of Class AB is set. As shown in
FIG. 5
, the power efficiency at the point (
210
,
220
), where the distortion starts to increase is expected to be as high as 58% when the thermal resistance of the element is 0 K/W. However, the power efficiency of the transistor element having an actual thermal resistance of 30 K/W deteriorated to 45% with decrease of 13%. Since the power consumption of the element at the point
220
is 1.2 W, the element temperature rose by 36 K compared with the temperature at the time when there is no signal input. Since a temperature coefficient of a potential between the base and the emitter of the InGaP/GaAs-HBT is about −1.1, mV/K, a DC voltage between the base and the emitter substantially increased by 40 mV, resulting in an increase in the collector current and in deterioration in the efficiency.
SUMMARY OF THE INVENTION
An object of the present invention is to solve the aforementioned problems of the prior arts and to provide a highly efficient and linear amplifier for suppressing the increase in the collector current caused by the heat generation of the power amplifying transistor element.
Another object of the present invention is to provide a highly efficient and linear transistor amplifier for power amplification, which is biased in a Class B or a Class AB.
Still another object of the present invention is to provide a transistor amplifier for high-frequency power amplification suited to such a small and lightweight communication device as a portable telephone operated by a battery.
According to the present invention, a high-frequency power-amplifying bipolar transistor is provided which is so biased as to operate as the Class B or Class AB amplifier. A current mirror circuit including a bipolar transistor supplies a base potential for the power-amplifying transistor. The transistor included in the current mirror circuit and the power-amplifying transistors are so arranged as to establish a thermal linkage between them and to reduce a difference between their junction temperatures.
According to an embodiment of the present invention, a metallic electrode layer is provided for establishing the thermal linkage.
According to another embodiment of the present invention, the transistor in the current mirror circuit is provided between fingers of the power-amplifying transistor for establishing the thermal linkage.
According to yet another embodiment of the present invention, a distance between the transistor in the current mirror circuit and one of the fingers of the power amplifying transistor is made smaller than the thickness of a sem

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