A Wide-Band UHF Logarithmic Amplifier

Logarithmic amplifiers can be made using three basic techniques: nonlinear feedback, nonlinear loads, and successive detection. For very high-frequency, wide-band applications, the successive detection method is the most practical. This paper describes a logarithmic amplifier at UHF frequencies which uses this successive detection method. The design was made with simplicity, cost, and the minimum use of special instruction techniques as prime considerations. Development of a linear amplifier with an amplitude response flat within 3 dB from 380 to 1020 MHz is presented first. The method used to convert the linear amplifier to a logarithmic one is then shown. The result is a logarithmic amplifier with a transfer characteristic which is within /spl plusmn/ 1 dB of logarithmic over an input dynamic range of 40 dB. This amplifier is capable of responding to a 2.5 ns pulse-width RF pulse with a center frequency of 800 MHz.