Hello , There is a very intresting circuit taken from the attached article.
I am trying to understand its mathemtical logic.
Could you help me logickally dissamble it into different parts so I will se the functionally of each part and how they play together?

https://www.edn.com/a-temperature-compensated-calibration-free-anti-log-amplifier/

 

The circuit is drawn in a strange arrangement, which I perceive as being intended to confuse us. I am not experienced in anti-log circuits, BUT it certainly looks like Q1a has a reversed biased base-emitter junction, not likely to conduct very much

 

It all depends on the power supplies to the op-amps. These must be bipolar else Q1a and U5 cannot function.

 

It’s pretty much out of the Nat Semi application note:
http://www.bitsavers.org/components/national/_appNotes/AN-0311.pdf

 

The difference from the AN-0311 app note is that here Vo is where a +15V source is in the app note.
AND EXCELLENT searching, Ian0!!

 

Hello,I learned several things about this circuit that i want to share:
1.What power supply should I use in this case ?
2."The input signal is inverted by U5 and has a fixed gain that is TC compensated by
{ptc} resistor"
How can I know the needed value of PTC resistor in ltspice simulation when input signal enters U5?

3.the collector of Q1b is connected to the negative port of U2.
I am having trouble to understand the sentence bellow.
How can I test the U2 Q1b relation to see what is needed from this part of the circuit?
"U5 output (VS scaled version of Vi) drives the base of Q1b which has it's collector forced to 0 volts by negative feedback in U2 and the collector current is fixed at Vref/Ref at 10MΩ or 250na."

 

UPDATE:

I am trying to start simulating parts of the structure according to the original circuit as shown in the photo and LTspice.
few question:
1.what kind of signal I need to put inside the input signal to see the regulating in action?
2.after the opamp stage we are basicly ampliying the input signal and puting it into the base of q1.
what state the Q1 needs to operate in so this half of the circuit will function properly?
Thanks.

 

I do not see the defined output for the circuit. I could guess, but it would probably be wrong.