A very linear BiCMOS transconductor for high-frequency filtering applications

A differential transconductance stage implemented in BiCMOS technology is described. The key features of the new stage are: (1) a total harmonic distortion (THD) less than 0.15% up to a 3 V_pp differential input signal, assuming 2% mismatch of the input devices, with a 5-V supply; (2) a second pole frequency typically higher than 2 GHz; and (3) a gain of more than 50 dB. All of these features are obtained from simulations performed using SPICE and correspond to a BiCMOS process featuring 2-μm minimum channel length and 7-GHz bipolar f_T. The structure of the transconductance is described and its operation explained. The nonidealities of the stage, like distortion, finite gain, parasitic poles, noise, and offset, are discussed. The complete implementation is presented. The simulated performance of a bandpass filter based on the new transconductor is reported. The results demonstrate that using the new circuit a filter centered around 5 MHz with a Q of 22 should result in a Q precision better than 12% without any Q tuning