An Amplitude-Balanced Current-Reused CMOS VCO Using Spontaneous Transconductance Match Technique

A 3.5 GHz, 0.18 mum CMOS current-reused voltage-controlled oscillator (VCO) with very high amplitude balance is presented. While the current-reused VCO can dramatically save dc power consumption, it has the drawback of output amplitude imbalance resulting from the asymmetric circuit structure. A spontaneous transconductance match (STM) technique is proposed to balance the transconductance of nMOS and pMOS transistors by the imbalance-induced voltage at the center-tapped point of the tank inductor. This transconductance match takes place spontaneously with the occurrence of signal imbalance such that imbalances can be instantly eliminated. The measured amplitude imbalance ratio is less than 0.7% over the entire tuning range of 2.93 to 3.62 GHz, significantly reduced from 3% of the VCO without STM. The power consumption is as low as 1.65 mW from a 1.5 V supply. The phase noise is -122 dBc/Hz at 1 MHz offset. A very high FOM of -195.7 dBc/Hz is achieved.