Analog multiplication can be carried out with a basic circuit like that of Fig. 9-7. Essential to the operation of the logarithmic amplifier is the use of a feedback-loop device that has an exponential terminal characteristic curve; one such device is the semiconductor diode of Chapter 2, which is characterized by
iD = I0 (e VD / hVT - 1 ) @ I0 e VD / hVT
A grounded-base BJT can also be utilized, since its emitter current and base-to-emitter voltage are
related by