A new structure of an active quasi-circulator(QC) is presented to separate the weak receive signal(RX) from the transmit signal (TX) in 77-GHz monostatic FMCW radars. Process variations, antenna mismatch as well as parasitics in the circuit degrade the TX/RX isolation. A control unit in form of a W-band impedance tuner is used for compensating these effects. The QC with impedance tuner is fabricated in a 200-GHz fT Silicon-Germanium (SiGe) HBT technology. Based on on-wafer measurements, a TX/RX isolation of 50 dB at 77GHz is demonstrated. Over a bandwidth of 1 GHz the isolation remains more than 30 dB. The insertion loss for the TX-path and the RX-path is around 0 dB and 1 dB, respectively. Now the tuner facilitates the change of the QC performance during one frequency ramp. With respect to isolation, satisfying results for automotive FMCW radars can be shown from 65-85 GHz.