Baseband circuit incorporated in direct conversion receiver free from direct-current offset voltage without change of cut-off frequency

A base band circuit for producing an output base band signal from an input base band signal, comprising:

• a first low pass filter circuit supplied at an input port thereof with said input base band signal for restricting a frequency band of said input base band signal, and outputting a filtrate signal from an output port thereof;
• a series of variable gain amplifiers forming a series circuit together with said first low pass filter, and responsive to an internal gain control signal indicating a gain to be achieved so as to produce said output base band signal from said filtrate signal at an output port of said series circuit through an amplification at said gain;
• and
• a feedback loop connected between said output port of said series circuit and an input port of said series circuit, and including
• a direct current component extracting circuit attenuating said output base band signal with an attenuation factor corresponding to said gain and extracting a direct current component from said output base band signal, and
• an eliminating circuit eliminating said direct current component from said input base band signal, said direct current component extracting circuit including,
• an attenuation circuit connected to said output port of said series circuit and attenuating said output base band signal at said attenuation factor for producing an attenuated signal from said output base band signal, and
• an inverting integrating circuit connected between said attenuation circuit and said input port of said series circuit and extracting said direct current component from said attenuated signal for producing a direct current component signal representative of a value of said direct current component, and
• wherein said inverting integrating circuit includes,
• a resistor connected to said output port of said series circuit,
• an operational amplifier having an inverted input node connected to said resistor, a non-inverted input node connected to a constant voltage source and an output node for outputting said direct current component signal and
• a condenser connected between said inverted input node and said output node of said operational amplifier.

A base band circuit for producing an output base band signal from an input base band signal, comprising:

• a first low pass filter circuit supplied at an input port thereof with said input base band signal for restricting a frequency band of said input base band signal, and outputting a filtrate signal from an output port thereof;
• a series of variable gain amplifiers forming a series circuit together with said first low pass filter, and responsive to an internal gain control signal indicating a gain to be achieved so as to produce said output base band signal from said filtrate signal at an output port of said series circuit through an amplification at said gain;
• and
• a feedback loop connected between said output port of said series circuit and an input port of said series circuit, and including
• a direct current component extracting circuit attenuating said output base band signal with an attenuation factor corresponding to said gain and extracting a direct current component from said output base band signal and
• an eliminating circuit eliminating said direct current component from said input base band signal, in which said direct current component extracting circuit is an inverting integrating circuit including
• a variable resistor connected at an output node thereof to said output port of said series circuit and producing a variable resistance against said output base band signal,
• an operational amplifier having an inverted input node connected to an output node of said variable resistor, a non-inverted input node connected to a constant voltage source and an output node for outputting a direct current component signal representative of a value of said direct current component, and
• a condenser connected between said inverted input node and said output node of said operational amplifier.