Power amplifier

1. Field of the Invention

The present invention relates to a power amplifier for use in wireless devices such as mobile terminals.

2. Background Art

Power amplifiers typically have a function to switch between two or more power amplification paths therein so as to achieve high power load efficiency over the entire power range. A high frequency switch is inserted in series with the output side of each power amplification path in order to isolate the transistors in one path from those in other paths and thereby prevent each transistor from being subjected to unwanted voltages when the transistor is turned off.

Further, some power amplifiers are configured such a manner that a high frequency switch is connected between the output side of one of the power amplification paths and ground (a shunt configuration), and that power amplification path is fully short-circuited to ground by the high frequency switch when the high frequency switch is turned on, thereby controlling the impedance of the power amplification path (see, e.g., Japanese Laid-Open Patent Publication No. 2002-185270).

In the case of a power amplifier in which a high frequency switch is inserted in series with the output side of a power amplification path, the loss in the high frequency switch serves to reduce the power load efficiency of the power amplifier. This means that since high frequency switches formed of Si material have relatively high power loss, it is difficult to design a power amplifier of Si material in which a high frequency switch is inserted in series with a power amplification path. Further, reducing the power loss of a high frequency switch (e.g., of Si material) requires increasing its size, making it difficult to reduce the size of the power amplifier.

On the other hand, power amplifiers in which a high frequency switch is connected between the output side of a power amplification path and ground are disadvantageous in that it is necessary to prevent a DC current from flowing through the high frequency switch when the high frequency switch is turned on. Further, the matching circuits of the power amplifier must be configured so as to prevent high frequency signals in the transmission frequency range of the power amplifier from entering the short circuit formed by the high frequency switch. This may put a restriction on the design of the matching circuit for the power amplification path to which the high frequency switch is connected.

The present invention has been made to solve the above problems. It is, therefore, an object of the present invention to provide a power amplifier which exhibits a reduced circuit loss while maintaining isolation between two power amplification paths therein.

According to the present invention, a power amplifier comprises: an input terminal; an output terminal; a first amplifier connected between the input terminal and the output terminal; a first junction point located between the input terminal and an input of the first amplifier; a second junction point located between the output terminal and an output of the first amplifier; a second amplifier connected between the first junction point and the second junction point and connected in parallel with the first amplifier; a third junction point located between an output of the second amplifier and the second junction point; a first capacitor connected between the third junction point and ground; and a switch connected between the third junction point and ground and connected in series with the first capacitor, wherein the second junction point is selected to be the lowest impedance point along a power amplification path that includes the input terminal, the first amplifier, and the output terminal, the switch is turned off when the second amplifier is turned on, and the switch is turned on when the first amplifier is turned on.

The present invention makes it possible to provide a power amplifier which exhibits a reduced circuit loss while maintaining isolation between two power amplification paths therein.

Other and further objects, features and advantages of the invention will appear more fully from the following description.

FIG. 1 is a diagram showing a power amplifier in accordance with a first embodiment of the present invention.

FIG. 2 is a diagram showing a power amplifier in accordance with a second embodiment of the present invention.

FIG. 3 is a diagram showing a power amplifier in accordance with a third embodiment of the present invention.

FIG. 4 is a diagram showing a power amplifier in accordance with a fourth embodiment of the present invention.

FIG. 5 is a diagram showing a power amplifier in accordance with a fifth embodiment of the present invention.

FIG. 6 is a diagram showing a power amplifier in accordance with a sixth embodiment of the present invention.

FIG. 7 is a diagram showing a power amplifier in accordance with a seventh embodiment of the present invention.

A power amplifier according to the embodiments of the present invention will be described with reference to the drawings. The same components will be denoted by the same symbols, and the repeated description thereof may be omitted.

First Embodiment

FIG. 1 is a diagram showing a power amplifier in accordance with a first embodiment of the present invention. Amplifiers A 1 and A 2 are connected in series between an input terminal IN and an output terminal OUT. A junction point P 1 is located between the input terminal IN and the input of the amplifier A 1 . A junction point P 2 is located between the output terminal OUT and the output of the amplifier A 2 . The junction point P 2 is selected to be the lowest impedance point along the power amplification path that includes the input terminal IN, the amplifiers A 1 and A 2 , and the output terminal OUT, connected in series in that order. (The lowest impedance point may be, e.g., the collector terminal, or drain terminal, of the amplifier A 2 .)

An amplifier A 3 is connected between the junction points P 1 and P 2 ; that is, the amplifier A 3 is connected in parallel with the series connection of the amplifiers A 1 and A 2 . The output power of the amplifier A 3 is lower than those of the amplifiers A 1 and A 2 . A junction point P 3 is located between the output of the amplifier A 3 and the junction point P 2 . A switch SW is connected at one end to the junction point P 3 and at the other end to one end of a capacitor C 1 . The other end of the capacitor C 1 is connected to ground.

The capacitor C 1 and the switch SW together form a shunt high frequency switch circuit, one end of which is connected to junction point P 3 and the other end of which is connected to ground. The switch SW is turned off when the amplifier A 3 is turned on, and the switch SW is turned on when the amplifiers A 1 and A 2 are turned on. Thus, either the power amplification path including the amplifiers A 1 and A 2 or the power amplification path including the amplifier A 3 is used depending on the required output power level, and the switch SW is turned on when the power amplification path including the amplifier A 3 is not used.

The capacitance value of the capacitor C 1 is selected to be sufficiently high relative to the transmission frequency range and such that the reflection coefficient on the Smith chart which corresponds to the impedance of the power amplification path including the amplifier A 3 , as seen from the junction point P 2 , has a phase angle of −90 to 90 degrees when the switch SW is turned on. As a result, the impedance of that power amplification path, as seen from the junction point P 2 , is substantially infinite (as if the power amplification path were disconnected from the junction point P 2 ) when the switch SW is turned on.

Since the shunt high frequency switch SW is connected between the junction point P 3 and ground, this power amplifier exhibits a lower circuit loss than power amplifiers in which a high frequency switch is inserted in series with a power amplification path. Further, since the power amplification path including the amplifier A 3 is connected to the lowest impedance point along the power amplification path including the amplifiers A 1 and A 2 , high frequency signals in the transmission frequency range of the power amplifier are prevented from entering the shunt high frequency switch circuit even when the high frequency switch SW is turned on and hence the junction point P 3 is short-circuited to ground through the capacitor C 1 . This results in reduced loss in the entire circuit of the power amplifier. The capacitor C 1 prevents a DC current from flowing through the shunt high frequency switch circuit to ground. Further, since the capacitance value of the capacitor C 1 is sufficiently high relative to the transmission frequency range, the shunt high frequency switch circuit can provide isolation between the power amplification paths which is comparable to that provided by a series high frequency switch circuit inserted in series with a power amplification path.

Second Embodiment

FIG. 2 is a diagram showing a power amplifier in accordance with a second embodiment of the present invention. An inductor L is connected between the output of the amplifier A 3 and the junction point P 2 located on the power amplification path including the amplifiers A 1 and A 2 . This series inductor L serves to increase the Q value of the shunt high frequency switch circuit including the capacitor C 1 . This makes it possible to reduce the capacitance value of the capacitor C 1 and thereby reduce the layout area. Further, the increased Q value means that the impedance of the power amplification path to which the shunt high frequency switch circuit is connected, as seen from the junction point P 2 , is substantially infinite (as if the power amplification path were disconnected from the junction point P 2 ), making it possible to enhance isolation between the power amplification paths of the power amplifier.

Third Embodiment

FIG. 3 is a diagram showing a power amplifier in accordance with a third embodiment of the present invention. A junction point P 4 is located between the output of the amplifier A 3 and the junction point P 2 . A capacitor C 2 is connected between the junction point P 4 and ground. Other than this feature, the power amplifier of the third embodiment is identical in construction to the power amplifier of the second embodiment. The amplifier A 3 has a different optimum output impedance than the amplifiers A 1 and A 2 . Therefore, for example, when the output of the amplifier A 3 is connected to the same load to which the amplifier A 2 is connected, the output impedance of the amplifier A 3 must be converted to match that of the amplifier A 2 . Such conversion is provided by the LC circuit consisting of the series inductor L and the shunt capacitor C 2 and inserted between the junction points P 2 and P 3 . A plurality of conversion circuits (or LC circuits) may be connected in series to reduce impedance conversion loss and thereby further reduce circuit loss.

Fourth Embodiment

FIG. 4 is a diagram showing a power amplifier in accordance with a fourth embodiment of the present invention. This power amplifier differs from that of the third embodiment in that the series inductor L and the shunt capacitor C 2 are connected between the output of the amplifier A 3 and the shunt high frequency switch circuit, as shown in FIG. 4 . This LC circuit also provides impedance conversion, meaning that the fourth embodiment has the same advantages as described above in connection with the third embodiment.

Fifth Embodiment

FIG. 5 is a diagram showing a power amplifier in accordance with a fifth embodiment of the present invention. This power amplifier differs from that of the first embodiment in that the capacitor C 1 is replaced by a plurality of parallel connected capacitors C 1 a to C 1 d disposed around the amplifier A 3 . This configuration prevents the high frequency signal amplified by the amplifiers A 1 and A 2 from entering the amplifier A 3 , as well as preventing the high frequency signal amplified by the amplifier A 3 from entering the power amplification path including the amplifiers A 1 and A 2 , thereby enhancing isolation between the power amplification path including the amplifiers A 1 and A 2 and the power amplification path including the amplifier A 3 .

Sixth Embodiment

FIG. 6 is a diagram showing a power amplifier in accordance with a sixth embodiment of the present invention. It should be noted that two spaced apart grounding conductors for high frequency signals may not be at equal ground potentials due to a slight inductive component, causing high frequency signals to leak into unintended paths through grounding conductors. The power amplifier of the sixth embodiment is configured to avoid this problem and differs from the power amplifier of the fifth embodiment in that the capacitors C 1 a to C 1 d of the shunt high frequency switch circuit and the amplifier A 3 are grounded to a common ground. This reduces high frequency signals leaking into other paths through grounding conductors, making it possible to enhance isolation between the power amplification path including the amplifiers A 1 and A 2 and the power amplification path including the amplifier A 3 .

Seventh Embodiment

FIG. 7 is a diagram showing a power amplifier in accordance with a seventh embodiment of the present invention. This power amplifier is similar to that of the first embodiment, except that the shunt high frequency switch circuit has a different configuration. Capacitors C 1 and C 2 are connected in series between the output of the amplifier A 3 and the junction point P 2 . A junction point P 3 is located between the capacitors C 1 and C 2 . A switch SW is connected at one end to the junction point P 3 and at the other end to one end of an inductor L. The other end of the inductor L is connected to ground.

Whereas the shunt high frequency switch circuit of the first embodiment includes the switch SW and one capacitor, the shunt high frequency switch circuit of the present embodiment includes the switch SW, two series connected capacitors, and an inductor connected to the junction between these capacitors. The shunt high frequency switch circuit of the present embodiment can also provide the same advantages as described above in connection with the first embodiment. However, the inductance value of the inductor L and the capacitance values of the series connected capacitors C 1 and C 2 must be such that the reflection coefficient on the Smith chart which corresponds to the impedance of the power amplification path including the amplifier A 3 , as seen from the junction point P 2 , has a phase angle of −90 to 90 degrees when the shunt high frequency switch circuit is turned on.

It should be noted that the high frequency switch SW may be configured using an FET/HEMT, MEMS, or diode, etc. Further, although the present invention has been described in connection with power amplifiers having two power amplification paths, it is to be understood that the present invention may be applied to power amplifiers having three or more power amplification paths. It should be further noted that in the power amplifier of each of the above embodiments a high frequency switch is connected between one of the two power amplification paths and ground so as to isolate these power amplification paths from each other in terms of high frequency signals. This means that a DC current can still flow between the power amplification paths. Therefore, it is not necessary to separate the power supply path even if a capacitor is not connected in series between the power source and active devices such as transistors.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

The entire disclosure of Japanese Patent Application No. 2012-277202, filed on Dec. 19, 2012, including specification, claims, drawings, and summary, on which the Convention priority of the present application is based, is incorporated herein by reference in its entirety.