In this paper, design and analysis of two modified Colpitts voltage-controlled oscillators (VCOs) with or without transformer feedback are presented. The loaded quality factors of these two VCOs are analyzed, and the result is the use of transformer feedback in the Colpitts oscillator decreases the loaded quality factor. Two K-band VCOs with and without transformer feedback using 0.5-μm GaAs enhancement/depletion pseudomorphic high electron mobility transistor (E/D-PHEMT) process are presented for the phase noise evaluation. Operating at a dc supply voltage of 2 V, the former consumes a dc power of 68 mW, exhibits a tuning range of 0.3 GHz and a phase noise of -128 dBc/Hz at 10-MHz offset. While the latter consumes a dc power of 32 mW, exhibits a tuning range of 1.2 GHz and a phase noise of -130 dBc/Hz at 10-MHz offset. Both of them occupy a chip size of 1 × 1 mm 2 including the RF and DC pads.