Negative feedback amplifier and method of controlling loop gain thereof

A negative feedback amplifier comprising;

• input portions for receiving an in-phase component and a quadrature component of a base-band signal;
• adder circuits coupled to said input portions, respectively;
• a quadrature modulator coupled to said adder circuits through loop filters, for performing a quadrature modulation on outputs of said adder circuits;
• a first signal level variable unit coupled to said quadrature modulator
• an amplifier unit coupled to said first signal level variable unit;
• a feedback circuit for negative feedbacking in-phase component and a quadrature component signals of an output of said amplifier unit to said adder circuits, said feedback circuit including a second signal level variable unit and a first control unit, said second signal level variable unit being controlled by said first control unit;
• and
• a second control unit coupled to one of said loop filters, for controlling an output of said first signal level variable unit so that said output of said adder unit becomes a predetermined output level.

A negative feedback amplifier for a transmitter having a quadrature modulator, comprising:

• input portions for receiving an in-phase component and quadrature component of a base-band signal;
• a feedback circuit for generating negative feedback signals of said in-phase component and quadrature component of said base-band signal on the basis of an output signal from said negative feedback amplifier;
• adder circuits for generating an output of addition of said in-phase component (I) of said base-band signal to said input portion of said negative feedback amplifier and said negative feedback signal of said in-phase component, and an output of addition of said quadrature component of said base-band signal to said input portion of said negative feedback amplifier and said negative feedback signal of said quadrature component;
• an amplifier circuit that receives said outputs from said adder circuits and amplifies said outputs to produce an amplified output;
• a detector circuit for detecting the output signal level from said amplifier circuit;
• a first level adjustment circuit for adjusting said signal level in said feedback circuit according to a first control signal;
• and
• a control circuit for generating a control signal by which said first level adjustment circuit is controlled according to the signal level detected by said detector circuit so that the output from said amplifier circuit can be maintained to be a certain value,
• said amplifier circuit including:
• loop filters for producing an in-phase signal component and a quadrature signal component resulting from limiting the frequency bands of said in-phase component and quadrature component of said base-band signal produced from said adder circuits;
• a quadrature modulator for orthogonally modulating a carrier signal with said in-phase signal component and quadrature signal component produced from said loop filters to produce a modulated signal;
• a comparator circuit for comparing the signal level of either one of said in-phase signal component and quadrature signal component produced from said loop filters, as a first input signal, and a predetermined reference signal as a second input signal, and producing the result of the comparison;
• and
• a second level adjustment circuit for adjusting the signal level of aid modulated signal according to a second control signal, and
• said control circuit generating said second control signal for controlling said second level adjustment circuit so that the loop gain of said negative feedback amplifier can be maintained to be a predetermined value in accordance with the result from said comparator circuit when a certain DC voltage is applied to said input portion as either one of said in-phase component and quadrature component of said base band signal with the other made zero level.

A gain control method of a negative feedback amplifier comprising the steps of:

• supplying an input signal to said negative feedback amplifier;
• inputting said input signal to an adder unit;
• performing a quadrature modulation in a quadrature modulator on an output of said adder unit;
• controlling a signal level of a negative feedback circuit for feedbacking said output of said negative feedback amplifier to said adder unit so that said output of said negative feedback amplifier becomes a predetermined level;
• and
• controlling a signal level of an output of said quadrature modulator based on the output of said adder unit so that said output of said adder unit becomes a predetermined output level.