RF bandpass filter design using capacitive degeneration

This work proposes the use of a capacitively degenerated transconductor to construct an RF bandpass filter. The fundamental theoretical elements for the application of this technique are provided. First, the impedance seen from the source/emitter of a transistor with capacitive degeneration and an LC tank connected at its gate/base is analyzed. The conditions under which this impedance exhibits a large value in a narrow bandwidth are obtained showing good agreement between theoretical and simulated results. A general RF filter structure based on the analyzed active resonant cell is introduced and described with a macromodel. A sample 5.2 GHz bandpass filter is designed for a SiGe BiCMOS 0.25 mum process. Simulation results for the designed RF filter show S21 > 6dB, Q > 20 consuming 1 mA from a 2.5 V supply