Logarithmic amplifier

Claim 1

A D.C. stabilized logarithmic amplifier which in response to an input video signal V in having blanking and picture intervals provides a temperature compensated logarithmic signal V out (comp), compensated for a thermal voltage V t and a saturation leakage current I S , comprising: a. a logarithmic amplifier cell having first and second matched bipolar transistors connected to compensate for I S and a third transistor for providing a constant collector current I REF , through the second transistor, an output voltage V out being produced at the junction of the second and third transistors and; b. feedback means including: i. a voltage to current convertor responsive to V in and an error signal to provide the collector current I c for the first transistor during picture information intervals; ii. means for producing a black reference current I BLK and for summing such I BLK current with the output of said voltage to current converter during the blanking interval to provide a collector current I c during blanking for the first transistor; and c. a clamp feedback circuit producing the error signal and during the blanking interval being responsive to V out to adjust the error signal so that I c equals I REF thereby providing temperature compensated D.C. stabilization for V out ; and d. a 1/Vt amplifier responsive to V out for correcting the thermal voltage Vt to provide a compensated voltage V out (comp) during the picture interval.