A true i.f. logarithmic amplifier using twin-gain stages

A true i.f. logarithmic amplifier is described which avoids the successive detection principle and its associated video delay line. Each stage comprises a pair of long-tailed pairs in a complementary role. Transistors are used throughout but `bottoming¿ is avoided, and bandwidth variations are negligible. A dynamic range of about 100 dB is possible. A video output may be derived if desired from a single detector after the last stage, but when an output at i.f. is taken, phase variations with amplitude may be of interest, and have been investigated. A technique for correcting such phase variations is described, and a development of the basic amplifier circuit is given which should minimize such phase variations at their source.