LTSpice Log Amplifier Circuit problem

Thread Starter

5O5mm

Joined May 15, 2021
15
Hi,

The circuit works fine for natural log, base 2 and 3 but it didn't work for any log base greater than 4. For any log base I adjust the differential gain with the feedback resistors which are dependent to the value of thermal voltage (and since every BJT model has a slight different thermal voltage, I didn't want to take it as a constant 26mV instead used a behavioral source to adjust the resistor value) according to the equations attached.
What I'd like to see at the output is when we use behavioral source as input such as pow(n, n) and step the n parameter which is the log base, the output voltage should be the value of n but it isn't working for values of n greater than 4.

Adsız.png
logbase_n.pnglogbase_n-2.pnglogbase_n-3.png
 

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Last edited:

eetech00

Joined Jun 8, 2013
3,087
Hi,

The circuit works fine for natural log, base 2 and 3 but it didn't work for any log base greater than 4. For any log base I adjust the differential gain with the feedback resistors which are dependent to the value of thermal voltage (and since every BJT model has a slight different thermal voltage, I didn't want to take it as a constant 26mV instead used a behavioral source to adjust the resistor value) according to the equations attached.
What I'd like to see at the output is when we use behavioral source as input such as pow(n, n) and step the n parameter which is the log base, the output voltage should be the value of n but it isn't working for values of n greater than 4.

View attachment 257051
View attachment 257018View attachment 257019View attachment 257020
Why do you think its not working the way you want it to?
 
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