log amplifier using op amp???

[skarthikshines]

hi all am trying to design a log amplifier using op amp.. here am attaching my circuit.. and this is the formula for log amp
vout= Vt ln(vin/Is*R1)..
where vt = thermal voltage and
Is = saturation current.. here i need to know how to calculate the Vt and Is value..

 

[FvM]

Is is a constant for a given transistor, vt is a temperatur dependent material const, for Si it's about 26 mV at room temperature.

see Bipolar junction transistor - Wikipedia, the free encyclopedia

The Is value for your transistor can be derived from datasheet values.

 

[skarthikshines]

Is is a constant for a given transistor, vt is a temperatur dependent material const, for Si it's about 26 mV at room temperature.

see Bipolar junction transistor - Wikipedia, the free encyclopedia

The Is value for your transistor can be derived from datasheet values.

can you say me the formula for saturation current Is..