BACKGROUND

The present disclosure relates to a bias circuit. A power amplifier that amplifies a radio frequency (RF) signal to be transmitted to a base station is used in a mobile communication terminal such as a cellular phone. The power amplifier includes a transistor that amplifies the RF signal and a bias circuit that controls a bias point of the transistor. As this type of bias circuit, a bias circuit using a current mirror circuit is known as described in, for example, U.S. Patent Application Publication No. 2015/0349715. In addition, as this type of transistor, a heterojunction bipolar transistor is known.

However, a heterojunction bipolar transistor has a large thermal resistance, and thus generates heat as an RF signal output increases, thereby resulting in a lower transconductance. As the transconductance decreases, the gain of the power amplifier decreases. When the RF signal output further increases, the bias point of the heterojunction bipolar transistor increases, and thus the gain of the power amplifier turns to an increase pattern from a decrease pattern. In this manner, the linearity of the power amplifier deteriorates due to changes in the gain of the power amplifier in its low and middle output region. Such linearity deterioration distorts the RF signal, and thus it is not desirable especially for a communication system using a linear modulation scheme.

BRIEF SUMMARY

The present disclosure proposes a bias circuit that improves a nonlinear operation of an amplifier.

According to embodiments of the present disclosure, a bias circuit includes (i) a first bipolar transistor that has a first collector terminal, a first emitter terminal, and a first base terminal, the first collector terminal being connected to a power source terminal through a first current path, the first emitter terminal being grounded, (ii) a second bipolar transistor that has a second collector terminal, a second emitter terminal, and a second base terminal, the second collector terminal being connected to a constant voltage source or a constant current source through a second current path, the second emitter terminal being grounded, the second base terminal being connected to the first base terminal, (iii) a third bipolar transistor that has a third base terminal, a third collector terminal, and a third emitter terminal, the third collector terminal being connected to the power source terminal through the first current path, the third emitter terminal being connected to the first collector terminal through the first current path and supplying a bias signal to an amplifier, (iv) a fourth bipolar transistor that has a fourth base terminal, a fourth collector terminal, and a fourth emitter terminal, the fourth base terminal being connected to the third base terminal, the fourth collector terminal being connected to the constant voltage source or the constant current source through the second current path, the fourth emitter terminal being connected to the second collector terminal through the second current path, the fourth collector terminal being connected to the fourth base terminal, and (v) a filter circuit that is connected between the first collector terminal and the ground through the first base terminal and that has a frequency characteristic for attenuating a high frequency component of an RF signal to be input to the amplifier.

According to the bias circuit according to the embodiments of the present disclosure, the nonlinear operation of the amplifier may be improved.

Other features, elements, and characteristics of the present disclosure will become more apparent from the following detailed description of embodiments of the present disclosure (with reference to the attached drawings).

