Variable-gain amplifier circuit, offset control method in variable-gain amplifier circuit, radio receiver with variable-gain amplifier circuit, and radio receiving method in radio receiver with variable-gain amplifier

An offset control method in a variable-gain amplifier circuit which comprises signal amplifying means for amplifying an input signal, gain control means for controlling a gain of said signal amplifying means, mode selection means for selecting any one of a signal amplifying mode in which said input signal is amplified by a predetermined gain and an offset detection mode in which an offset of said signal amplifying means is detected, offset retaining means for retaining the detected offset, and mode switching means for allowing an input offset of said signal amplifying means to be inputted to said offset retaining means and for shutting off said input signal in said offset detection mode and for allowing a signal derived by subtracting an output of said offset retaining means to be inputted to said signal amplifying means in said signal amplifying mode, so as to produce a control signal for correcting said offset, said offset control method comprising the steps of:

• controlling a gain of said gain control means so that the detection sensitivity of the quantity of offset of said signal amplifying means is optimum after a power supply is turned on and in the case of the offset detection mode;
• detecting the quantity of offset of said signal amplifying means while being controlled so that said detection sensitivity of the quantity of offset is optimum, and retaining the quantity of offset by said offset retaining means;
• controlling said gain of said gain control means in accordance with the amplitude of an input signal after said offset detection mode is switched to said signal amplifying mode by said mode selection means;
• and
• amplifying and outputting said input signal in accordance with said controlled gain by said signal amplifying means.

A radio receiver with a variable-gain amplifier circuit, comprising:

• a radio frequency amplifier circuit for amplifying an input signal of a radio frequency;
• a signal supply selecting circuit for determining whether said input signal is supplied;
• a frequency mixer circuit for mixing frequencies of said input signal;
• and
• a variable-gain amplifier circuit which comprises signal amplifying means for amplifying an input signal, gain control means for controlling a gain of said signal amplifying means, mode selection means for selecting any one of a signal amplifying mode in which said input signal is amplified by a predetermined gain and an offset detection mode in which an offset of said signal amplifying means is detected, offset retaining means for retaining the detected offset, and mode switching means for allowing an input offset of said signal amplifying means to be inputted to said offset retaining means in said offset detection mode and for allowing a signal derived by subtracting an output of said offset retaining means to be inputted to said signal amplifying means in said signal amplifying mode, so as to produce a control signal for correcting said offset,
• wherein said signal supply selecting circuit interrupts said input signal inputted to said frequency mixer circuit in said offset detection mode, and said offset retaining means retains a DC offset produced in a signal line extending from said frequency mixer circuit, in which the signal input is interrupted, to said variable-gain amplifier circuit.

A radio receiving method in a radio receiver which comprises:

• a radio frequency amplifier circuit for amplifying a radio frequency input signal;
• a signal supply selecting circuit for determining whether said input signal is supplied;
• a frequency mixer circuit for mixing frequencies of said input signal;
• and a variable-gain amplifier circuit which comprises signal amplifying means for amplifying an input signal, gain control means for controlling a gain of said signal amplifying means, mode selection means for selecting any one of a signal amplifying mode in which amplifying said input signal is amplified by a predetermined gain and an offset detection mode in which an offset of said signal amplifying means is detected, offset retaining means for retaining the detected offset, and mode switching means for allowing an input offset of said signal amplifying means to be inputted to said offset retaining means in said offset detection mode and for allowing a signal derived by subtracting an output of said offset retaining means to be inputted to said signal amplifying means in said signal amplifying mode, so as to produce a control signal for correcting said offset, wherein a signal interrupting circuit interrupts said input signal inputted to said frequency mixer circuit in said offset detection mode, and said offset retaining means retains a DC offset produced in a signal line extending from said frequency mixer circuit, in which the signal input is interrupted, to said variable-gain amplifier circuit, said radio receiving method comprising the steps of:
• selecting said offset detection mode by said mode selection means when said radio receiver does not carry out communication, and retaining a DC offset produced in said signal line by said offset retaining means;
• and
• selecting said amplifying mode by said mode selection means when said radio receiver carries out communication, and outputting an offset-corrected and amplified signal by said signal amplifying means.