Effects of transistor nonidealities on log-domain filters

Log-domain filtering makes explicit use of the diode nature of bipolar transistors to realize log-domain integrators, resulting in coupled translinear circuits. In this paper, the departure from ideal integration due to the major transistor nonidealities is studied, and practical filter deviation equations are derived. The device nonidealities to be tackled include: parasitic emitter resistance, finite beta, base resistance and Early effect. SPICE simulations, both large and small-signal analysis, are performed to verify the results. Compensation schemes are also proposed.