SiGe 30GHz active inductor based on cascode gyrator

Abstract

IntroductionOn Si MMICs spiral inductors require large amounts of substrate area, high series resistance, crosstalk problems and don't allow inductance adjustments. At lower frequencies inductances has been synthesized with GICs realized with Opamps. At microwave the gyrators are obtained connecting at least two transistors in a feedback loop [1], [2]. In this paper, active inductors fabricated in IHP SG25H3 SiGe BiCMOS process are discussed [3]. The goal of the study presented is to obtain active inductors to be used on VCO and tuned active filters at 30GHz. At such high frequency, only active inductors on high cost GaAs based technologies were reported [4]. Active Inductor TopologyThe active inductor studied in this paper is based on a simple gyrator topology consisting of a feedback cascode with two HBTs [2,5,6]. In figure 1 is presented the circuit schematics of the proposed active inductor, based on the simple gyrator. A cascode current mirror (T4+T5) is used to bias the T1 to T3 HBTs. Resistors of 3.46kΩ, were used for base bias, except for T3 (204Ω). DC decoupling capacitors of 425fF are introduced in the bases of both T1 and T3 transistors. All transistors are one finger, npnSHP1 type, with a maximum current of 2mA. The figure 1 circuit components with only numerical values don't change significantly the active inductor value L and its Q factor as it will be shown on next section. Capacitor C1 in parallel with T1 base-emitter junction and output capacitor C2, are important to define the active inductor frequency behaviour. Voltage Vmirror controls Q value. The L and Q factor are also dependent on the T2 base resistor Rb1. The values indicated in figure 2 are those of the fabricated prototype (C1=15fF, C2=10fF and Rb1=10Ω). Simulation ResultsThe nominal collector DC bias voltage is V CC =3V. At the schematic level an inductance with a maximum Q at 31.7GHz (f Qmax ) was obtained. At 28.5GHz the inductance was L=122pH with a Q=28. These values were obtained with V mirror =2.72V. For these conditions, the current mirror collector bias current was I mirror =I CC ≈1.6mA. When the layout was extracted with Diva the f Qmax was shifted to a lower frequency. An adjustment of C 1 was necessary to increase f Qmax . The voltage V mirror was adjusted in agreement with to have a high Q with stability. The following values were obtained reducing C 1 from 20fF to 15fF and with V mirror =2.81V (I mirror =1.77mA): L=230pH and Q=160.4 at 28.5GHz, and Q max at 28.3GHz. Due to the parasitic capacitances the active inductor presents now a resonance frequency, Im(Z out )=0, at f res =38.67GHz. The V CC bias currents is I CC =1.719mA (figure 2). Capacitor C 2 changes the resonance frequency but the frequency for maximum Q is almost constant. Reducing C 2 , f res increases and inductance L is slightly reduced. Capacitor C 1 changes the f Qmax , the centre of active inductor frequency bandwidth. Reducing C 1 , both f Qmax and f res increase and the inductance L decreases. However, to have a high Q stable active inductor the V mirro...