Tunable MMIC Negative Group Delay Transversal Filter-Based Negative Capacitor in 0.1-μm GaAs pHEMT Technology

A tunable negative capacitor using monolithic microwave integrated circuit (MMIC) technology is presented. The proposed on-chip tunable non-Foster element is synthesized with a reconfigurable negative group delay (NGD) circuit based on an active transversal filter topology. The gain cells are implemented in 0.1-um GaAs pseudomorphic high electron mobility transistor (pHEMT) technology. A low-loss forward-biased PIN diode as a phase shifter is used to significantly improve the tuning range of the negative capacitance (NC). Two designs with different drain biasing schemes, i.e., inductive bias (Device 1) and resistive bias (Device 2), are fabricated and measured. By optimizing the bias voltages of the gain cells and phase shifter, a tunable NC ranging from -0.05 pF to -0.25 pF for Device 1 and -0.05 pF to -0.22 pF for Device 2 are achieved, respectively. In addition, Device 2 exhibits a low-frequency gain cut-off of 100 MHz when intended as an amplifier, further extending the useful bandwidth for low-frequency operation. Both devices are unconditionally stable while achieving a fractional NC bandwidth of 10.4% at $f$o of 5.75 GHz with a compact footprint of 2.75 mm x 1.85 mm. To the best of our knowledge, the proposed tunable NC exhibits the smallest footprint with the largest capacitance tuning ratio (>4) and highest reported operating frequency among the prior published NGD-based negative capacitors.