A circuit which can receive input signal Xr and Xi and send out output signal Yr and Yi in pre-set frequency band domain comprises the first differential unit, the second differential unit, the firstfeedback unit, the second feed bank unit, the third feedback unit and the fourth feedback unit. The coefficient of each unit can be derivated out after real number low pass filter is frequency-displaced and feedback coupling is executed only at series desiring feedback couple by resistance, capacity or both so it can be realized in great flexibility.