An active inductor (AI) based on a cascade gyrator for 30 GHz applications implemented with a 0.25 μm in SiGe
technology is presented. The gyrator converts not only a key capacitor into an inductor, but also an added
resistor, into a negative resistor. This gyrator-RC has its losses compensated by the negative resistor improving
the active inductor Q factor. Changing the bias voltage and current allows to obtain a variable AI. A study of a
cascade gyrator AI topology is done to understand the circuit behavior and key elements. For this purpose, an AI
impedance model is introduced and discussed. An improved AI with the added resistor replaced by a voltage
controlled mosfet resistor is proposed. This extra control voltage allows the variable AI quality factor fie tuning.
Schematic and circuit extracted from layout simulations are presented, and compared with the measured results
of two prototypes of the AIs (one with a fied resistor and other with a voltage controlled resistor). A prototype of
a high pass fiter using the AI with fie Q control was fabricated. Non-linear simulations for different input signal
levels were performed and compared with measurements. Also discussion on the non-linear models accuracy is
performed.