Directional coupler and dual-band transmitter using the same

A dual-band transmitter for a dual-band mobile communication terminal, comprising:

• a first power amplifier for amplifying power of a first band signal by an amplification factor determined depending on a bias voltage applied thereto, said first band signal being a high-band signal;
• a second power amplifier for amplifying power of a second band signal by an amplification factor determined depending on a bias voltage applied thereto, said second band signal being a low-band signal;
• a directional coupler for coupling a part of said first band signal power-amplified by said first power amplifier and a part of said second band signal power-amplified by said second power amplifier, respectively, said direction coupler including:
• a first transmission device for transmitting said first band signal power-amplified by said first power amplifier;
• a first directional coupling device including a first terminal and a second terminal and spaced apart from said first transmission device by a predetermined distance, said first directional coupling device coupling a part of said first band signal from said first transmission device and generating the coupled signal at said first terminal thereof, said second terminal of said first directional coupling device being connected to a ground terminal;
• a first inter-digital capacitor having its one side connected to said first transmission device and its other side connected to said first directional coupling device;
• a second transmission device for transmitting said second band signal power-amplified by said second power amplifier;
• a second directional coupling device including a first terminal and a second terminal and spaced apart from said second transmission device by a predetermined distance, said second directional coupling device coupling a part of said second band signal from said second transmission device and generating the coupled signal at said first terminal thereof, said second terminal of said second directional coupling device being connected to said ground terminal;
• a second inter-digital capacitor having its one side connected to said second transmission device and its other side connected to said second directional coupling device;
• a first filter connected to said first terminal of said first directional coupling device for high pass filtering the coupled signal from said first directional coupling device;
• and
• a second filter connected to said first terminal of said second directional coupling device for low pass filtering the coupled signal from said second directional coupling device;
• and
• a power amplifier controller for comparing the level of a signal coupled by said directional coupler with a predetermined reference value and regulating said bias voltage to said first power amplifier or second power amplifier according to a difference therebetween to control said amplification factor of said first power amplifier or second power amplifier.