Abstract: A variable gain amplifying circuit comprises two stages, a first stage and a second stage with a common voltage-to-current converter. Each stage comprises two BJTs, two voltage controlled current sources, a variable resistor with a variable resistance, and the common voltage-to-current converter. The two BJTs construct a differential amplifier for amplifying a pair of differential signals. The variable resistor with the variable resistance is connected between the emitters of the two BJTs wherein the variable resistance of the variable resistor is represented as RE. The variable resistance RE is controlled by a control voltage Vctrl. The two voltage controlled current sources are respectively connected between the corresponding emitter of BJTs and ground. The currents respectively through the voltage controlled current sources are equal to each other and represented as IE. The current IE is controlled by the common voltage-to-current converter according the control voltage Vctrl.

Abstract: A variable gain amplifying circuit comprises two stages, a first stage and a second stage with a common voltage-to-current converter. Each stage comprises two BJTs, two voltage controlled current sources, a variable resistor with a variable resistance, and the common voltage-to-current converter. The two BJTs construct a differential amplifier for amplifying a pair of differential signals. The variable resistor with the variable resistance is connected between the emitters of the two BJTs wherein the variable resistance of the variable resistor is represented as RE. The variable resistance RE is controlled by a control voltage Vctrl. The two voltage controlled current sources are respectively connected between the corresponding emitter of BJTs and ground. The currents respectively through the voltage controlled current sources are equal to each other and represented as IE. The current IE is controlled by the common voltage-to-current converter according the control voltage Vctrl.

Abstract: An image rejection mixer includes first and second mixers, a phase shift circuit and a summer. The first mixer receives an RF input signal and a first local oscillator (LO) signal and generates a first intermediate frequency (IF) output signal. The second mixer receives the RF input signal and a second LO signal and generates a second IF output signal. The first LO signal is a phase lag signal (sin ?lot) and the second LO signal is a phase lead signal (?cos ?lot), where ?lot is a frequency signal generated by a local oscillator. The phase shift circuit is connected between the summer and the first mixer, receives the first IF output signal and generates a phase lag signal. The phase shift circuit causes the phase lag signal to lag the first IF output signal by about 90 degrees. The summer is connected to the phase shift circuit and the second mixer, receives the second IF output signal and the phase lag signal, and generates a combined IF output signal.

Abstract: An image rejection mixer includes first and second mixers, a phase shift circuit and a summer. The first mixer receives an RF input signal and a first local oscillator (LO) signal and generates a first intermediate frequency (IF) output signal. The second mixer receives the RF input signal and a second LO signal and generates a second IF output signal. The first LO signal is a phase lag signal (sin &ohgr;lot) and the second LO signal is a phase lead signal (−cos &ohgr;lot), where &ohgr;lot is a frequency signal generated by a local oscillator. The phase shift circuit is connected between the summer and the first mixer, receives the first IF output signal and generates a phase lag signal. The phase shift circuit causes the phase lag signal to lag the first IF output signal by about 90 degrees. The summer is connected to the phase shift circuit and the second mixer, receives the second IF output signal and the phase lag signal, and generates a combined IF output signal.