Pulse width modulation circuit and switching amplifier using the same

A pulse width modulation circuit, comprising:

• a charging unit;
• a constant bias current source;
• a first voltage control unit that changes a voltage of the charging unit as a function of an input signal voltage during a first period which is in synchronization with a predetermined clock signal;
• a second voltage control unit that changes, during a second period following the first period, the voltage of the charging unit towards a predetermined voltage in an opposite direction to a direction in which the voltage is changed during the first period, by a constant bias current from the constant bias current source;
• a detecting unit that detects when the second period ends by detecting when the voltage of the charging unit reaches the predetermined voltage and stops the changing of the voltage of the charging unit by the second voltage control unit as a function of detecting when the voltage of the charging unit reaches the predetermined voltage, and that detects a time starting from when the second period starts to when the second period ends;
• and
• a pulse signal generating unit that generates, based on the detected time which is repeatedly output from the detecting unit for each clock signal, a pulse signal having a pulse width of the time.