Very high-frequency log-domain bandpass filters

A transistor-level all n-p-n differential log-domain integrator geared toward low-distortion very high-frequency (VHF) applications is proposed. Systematic design steps are suggested and used to realize second and higher order balanced bandpass filters, based on the method of operational simulation of doubly-terminated LC ladders. We discuss the special sensitivity of log-domain integrators to small de biasing offsets, practical issues in the design of the input and output interface stages, and the effect of parasitic resistors and capacitors on high-frequency log-domain filters. Experimental results from a 0.8-mu m BiCMOS implementation of a single-ended 220-MHz biquad and a balanced 130-MHz fourth-order bandpass filter are reported, with emphasis on their distortion characteristics.