Opam circuit where Vout =(V11/3 + V25/2)1/3
Anti-Log (exponential) op amp circuits are insanely tricky. Look up an exponential converter on rene schmitz website for a detailed tutorial that uses an op-amp. You need to have a non-linear element (diode or transistor) in the feedback path, however those active components have nasty temperature dependencies. The simplest log circuit has a single diode in the feedback path and an input resistor into the inverting summing node.