Design considerations for a 30 GHz differential Colpitts VCO with high fosc/fT ratio in 0.35µm SiGe BiCMOS

A fully integrated 30 GHz differential Colpitts VCO is presented in 0.35 μm SiGe BiCMOS with an f_t of 60 GHz. The topology of the differential Colpitts architecture and the design considerations are discussed in detail. The discussion encompasses critical portions of the circuit such as the LC tank, varactors and choice of transistors. The use of a common-collector Colpitts VCO topology precludes the need for a buffer, which lowers overall power consumption. The differential operation improves common-mode noise rejection which is especially important when building circuits on silicon. The symmetric inductor that is used for differential operation also results in a higher Q factor which improves the phase noise. Additionally, after employing a parasitic cancellation technique, the tank inductor can be made larger for operation at 30 GHz, which leads to an improved voltage swing and hence better phase noise. The in-depth discussion of the design considerations will benefit designers of BiCMOS Colpitts VCOs operating at high f_osc/f_T ratios.