Theory and performance of parabolic true logarithmic amplifier

A novel design technique for the true logarithmic amplifier scheme is presented. The methodology is based on a parabolic approximation of the ideal logarithmic characteristic yielding a major reduction of the logarithmic error. The basic theory is developed and presented in this paper. A sample design in GaAs 0.5 μm MESFET monolithic low-noise technology has been performed. The realised amplifier features ultrabroadband performance from 0.3 GHz to 5 GHz, together with a good input-output match throughout the operating bandwidth. The complete amplifier has been assembled utilising a modular strategy, allowing the fulfilment of design goals with a simple increase in the number of stages. The design steps and the measured results both for the single stage and for the cascade of three stages are presented in the following