exponential curve to linear curve circuit implementation

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aliraza786

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hello is there any method to convert to exponential output to linear output ... like i have a ir sensor whose output is exponential but i want a linear response is there any method to convert this exponential curve in to linear curve...kindly help me ... thnkas in advance
 

Re: exponential curve to linear cuurve circuit implementation

Hello my friend
Yes there is a simple method available . you can use Anti log taker circuits with op amps . those are for your purpose .
Best Wishes
Goldsmith
 

Re: exponential curve to linear cuurve circuit implementation

thnku so much for your suggestion... i tried to simulate log and anti log circuit in simulator but it doesnt work there...... kindly suggest me any ic which take log anf anti log of any signal input...i tried to use opamp and transistor ...loke 2n3904 and opamp lm741 to make anti log circuit.. but it doestn work......
 

Re: exponential curve to linear cuurve circuit implementation

Why it doesn't work? it is a popular circuit and very useful and simple? what is your simulator? can you attach the schematic that you have simulated , please?
Best Wishes
Goldsmith
 

Re: exponential curve to linear cuurve circuit implementation

BJT&OP log circuits are easy and straightforward. I guess, you have problems related to signal voltage range and correct scaling. What's the actual sensor characteristic?
 

Re: exponential curve to linear cuurve circuit implementation

sir Fvm and sir Goldsmith... i agree with u... i am posting actual sensor output curve and the circuit which i use for log and antilog this is basiclayy mutiplier circuit by using log and antilog circuits.. here i dnt know the resistance value so i pick it randomly... but i know there is some proper method for this so kindly guid me...

 

Re: exponential curve to linear cuurve circuit implementation

Dear aliraza
Hi
About the value of R ( at log taker ): it has two effect, at first , it is the input resistance of your log taker and at the other step , it has relationship with the collector current . Ic=Is(e^(Vbe/n*vt) -1 ) . thus the Ic = vi / R . and for selecting that R , you should know the out put impedance of your last stage.
Good luck
Goldsmith
 

Re: exponential curve to linear cuurve circuit implementation

The log/antilog circuit doesn't work as shown and the transfer characteristic isn't calculated correctly. It would work for sqrt(x*y). But to calculate x*y, the adder must have a gain factor of 1 for both inputs. So the output has an unwanted offset of 1*Vbe. For a temperature indepent log/antilog multiplier, you need a third log stage with a constant reference input current and it's output substracted from log(a)+log(b). Or at least a constant offset voltage, if temperature dependency doesn't matter.

See figure 5 in **broken link removed**
 
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