Square root and log circuits

[Moneer81]

hey guys:

I am trying to design a circuit that finds the square root of a number. The way to do that is by building 3 circuits: one to find the log of a number, the other one to find the anti log, and the third one is a simple voltage divider.

you can easily prove that the output voltage will be the square root of your input voltage when these three circuits are combined. I am just wondering if any of you guys have tried this before, because I am facing some difficulties in building the log circuit and I am not sure how to get it to work.

Thanks.

Moneer.

 

[Nigel Goodwin]

hey guys:

I am trying to design a circuit that finds the square root of a number. The way to do that is by building 3 circuits: one to find the log of a number, the other one to find the anti log, and the third one is a simple voltage divider.

you can easily prove that the output voltage will be the square root of your input voltage when these three circuits are combined. I am just wondering if any of you guys have tried this before, because I am facing some difficulties in building the log circuit and I am not sure how to get it to work.

Is there any reason for wanting to do this?.

The obvious easy way is to use a microprocessor and calculate it, otherwise you're talking an analogue computer.

 

[Moneer81]

hey:
yeah I am doing as a part of project on op amps for a class. we have to use op amps.

 

[Nigel Goodwin]

hey:
yeah I am doing as a part of project on op amps for a class. we have to use op amps.

You need to build an analogue computer then, opamps were originally designed for that - but tend to be used for other things these days.

Try googling for 'analogue computer' and see what you can find.

 

[Oznog]

Very strange project. Op amps are always applied in a basically linear or saturated mode as far as I've seen.

I could see trying to do an exponential by charging a cap with an RC circuit which gets switched off with a separate circuit with a linear voltage rise feeding a comparator against the voltage input. But it's a switched circuit, not an immediate response thing. I'd suspect you'd need a component with an exponential DC I/V curve. Diodes, varistors, etc have sort of exponential responses but they're far from clean.

 

[Roff]

There are myriad log-antilog converters in the literature. Just going to **broken link removed**, I found these:
https://www.electro-tech-online.com/custompdfs/2004/05/AN-301.pdf
https://www.electro-tech-online.com/custompdfs/2004/05/AN-30.pdf
https://www.electro-tech-online.com/custompdfs/2004/05/AN-31.pdf
https://www.electro-tech-online.com/custompdfs/2004/05/AN-20.pdf
https://www.electro-tech-online.com/custompdfs/2004/05/LB-25.pdf
There may be others there.
I believe all of them contain examples of circuitry that should help you. You might want to try other mfr's app notes: Analog Devices, Linear Technology, Texas Instruments, etc.