[SOLVED] Temp compensated opamp based logarithmic amplifier

[maktoomi]

Hi,
I a little confused about loggers.
In the classic logger shown in the following


the output is given by


Now, in order to make the logger temperature insensitive they make R4 temco to cancel that of VT(=KT/q).
What about R3? won't it vary with temperature in real life, too? I mean to ask, even R4 (<<R3) cancels impact of VT , why logger should not be temp sensitive due to variation of R3 with temp?

Thanks.

 

[FvM]

As you can easily calculate, the required gain TC is about 0.34 %/K (or 3400 ppm/K) at ambient temperatures. Please relate this number to the usual TC of precision resistors and answer the question yourself.

 

[crutschow]

.........................................
What about R3? won't it vary with temperature in real life, too? I mean to ask, even R4 (<<R3) cancels impact of VT , why logger should not be temp sensitive due to variation of R3 with temp?

Of course. Temperature variation of any of the resistors will affect the output. That's why you select low temperature coefficient resistors for all of them except R4, which has a temco to match that of the diode-connected transistor.

 

[maktoomi]

Thank you FvM and crutschow for your helpful comments.