Doherty amplifier and power amplifier

1. A power amplifier comprising:

a plurality of Doherty amplifiers for use in a Z-ohm system, and

a power combining circuit that includes a balun circuit and that combines output power of the plurality of Doherty amplifiers,

wherein each of the respective Doherty amplifiers comprises:

a carrier amplifier;

a peak amplifier; and

an impedance transformer line that converts a load of the carrier amplifier when an input signal is small,

wherein the impedance transformer line has a characteristic impedance smaller than Z ohms and equivalent to an optimum load impedance of the carrier amplifier,

wherein a load of the respective Doherty amplifier is smaller than Z ohms, and

wherein a load connected to a combined output terminal of the power combining circuit is Z ohms, and the balun circuit converts an impedance of the load of Z ohms so that an input impedance of an input terminal to which the respective Doherty amplifier is connected is an impedance smaller than Z ohms.

2. The power amplifier according to claim 1 ,

wherein: the Z ohms is 50 ohms, and

the optimum load impedance is five ohms or less.

3. The power amplifier according to claim 1 ,

wherein: the impedance transformer line is formed of a plurality of quarter-wavelength transformers in a cascade connection, and

the balun circuit is formed of a plurality of coupled circuits in a cascade connection.

4. The power amplifier according to claim 3 ,

wherein the plurality of coupled circuits of the balun circuit includes coupled lines with different characteristics.

5. The power amplifier according to claim 4 ,

wherein the balun circuit sets even-mode impedances in the plurality of coupled lines in a cascade connection so that levels of impedances are alternately changed in order of the cascade connection.

6. The power amplifier according to claim 5 ,

wherein a characteristic of the balun circuit is a Chebyshev response.

7. A power amplifier comprising:

a plurality of amplifiers; and

a power combining circuit that combines outputs of the plurality of amplifiers,

wherein: the plurality of amplifiers each includes

a distribution circuit that distributes an input signal inputted to an input signal terminal,

a carrier amplifier that is operated since the input signal is small,

a peak amplifier that starts operation when the input signal is high,

a phase compensation circuit that adjusts phases of an output of the carrier amplifier and an output of the peak amplifier,

a quarter-wavelength transmission line connected between the output of the carrier amplifier and the output of the peak amplifier, and

an impedance transformer connected to a node coupling the quarter-wavelength transmission line to the output of peak amplifier;

the carrier amplifier and the peak amplifier are each configured of a transistor having an optimum load impedance of five ohms or less;

the quarter-wavelength transmission line has a characteristic impedance equivalent to the optimum load impedance of the carrier amplifier smaller than five ohms, and the quarter-wavelength transmission line is directly connected to an output terminal of the carrier amplifier;

an input impedance of the power combining circuit is smaller than 50 ohms;

a load of 50 ohms is connected to an output terminal of the power combining circuit;

the quarter-wavelength transmission line and the impedance transformer are formed as a microstrip line or strip line on a first substrate, and

the power combining circuit is formed on a second substrate having a dielectric constant lower than a dielectric constant of the first substrate or having a thickness greater than a thickness of the first substrate.

8. The power amplifier according to claim 7 ,

wherein: the power amplifier is targeted to amplify a signal having an upper limit frequency almost two times a lower limit frequency or less, and

the power combining circuit includes a balun circuit that combines outputs of the plurality of amplifiers two each and that cancels a second-order harmonic produced in the plurality of amplifiers.