Audio signal amplifier circuit and portable telephone set and portable electronic device using the same audio signal amplifier circuit

A portable telephone set formed as an IC and including an audio signal amplifier circuit formed and having an input stage including a differential amplifier and an output stage including a single-ended power amplifier circuit, said portable telephone set comprising:

• an external output terminal of said IC connected to an output line of said single-ended power amplifier circuit;
• a first resistor connected between a certain terminal of said IC other than said external output terminal and a feedback input of said differential amplifier circuit;
• a first capacitor connected between said external output terminal and a loud speaker;
• a second capacitor between said certain terminal of said IC and a wiring line between said external output terminal and said loud speaker;
• a filter circuit provided on a signal input side of said differential amplifier circuit and including a second resistor and a third capacitor for attenuating signal components having frequencies in a middle and high frequency ranges;
• and voltage follower means provided between said input stage and said output stage, wherein said first capacitor is a small capacitor having a capacitance value in the order of 30 μF or smaller and an attenuation characteristics of said first capacitor and an impedance of said loud speaker in a low frequency range is compensated for by a relative amount of increase of amplification gain in the low frequency range determined by a time constant of said second capacitor and said first resistor and a time constant of said third capacitor and said second resistor.

A portable electronic device formed as an IC and including an audio signal amplifier circuit formed and having an input stage including a differential amplifier and an output stage including a single-ended power amplifier circuit, said portable electronic device comprising:

• an external output terminal of said IC connected to an output line of said single-ended power amplifier circuit;
• a first resistor connected between a certain terminal of said IC other than said external output terminal and a feedback input of said differential amplifier circuit;
• a first capacitor connected between said external output terminal and a loud speaker;
• a second capacitor between said certain terminal of said IC and a wiring line between said external output terminal and said loud speaker;
• a filter circuit provided on a signal input side of said differential amplifier circuit and including a second resistor and a third capacitor for attenuating signal components having frequencies in a middle and high frequency ranges;
• and voltage follower means provided between said input stage and said output stage, wherein said first capacitor is a small capacitor having a capacitance value in the order of 30 μF. or smaller and an attenuation characteristics of said first capacitor and an impedance of said loud speaker in a low frequency range is compensated for by a relative amount of increase of amplification gain in the low frequency range determined by a time constant of said second capacitor and said first resistor and a time constant of said third capacitor and said second resistor.