# Strange behaviour of real log amp

I'm facing some problems with my log amp circuit. My configuration is this: The "gain" function in case of ideal op-amp is: $$V_{out} = -V_t \cdot log\frac{V_{in}}{R1 \cdot I_{es}}$$ with $I_{es}$ saturation current at the emitter.

First plot: (red fit excludes the first measures)

1) The log behaviour is good, but it's not similar to the predictions (cyan color) where the range for $V_{out}$ is of hundreds of millivolts.

2) Despite there's a good log behaviour, there's an exaggerated offset. I tried to consider the input bias current but it should just give me an error of +/- 1mv in $V_{in}$. And, if I'd like just to put to my data another offset to correct the first, how I should choose it?

Second plot:

3) With greater R1 = 470 kOhm, I've a like a linear increasing trend for low V. The reason could be that the transistor is not working in that range? How I can predict this?

• Why isn't the output going negative as Vin rises from zero? – Andy aka Oct 29 '15 at 12:53
• Because as I said I suppose there's some kind of offset like V_out = -V_t log(V_in/R1 I_es) + V_off :| – Brontolo Oct 29 '15 at 13:07
• I notice that your schematic does not ground the mid-point of the two 15-volt supplies. Is that an error in the schematic, or are you really running the circuit that way? If so, you need to correct that. – WhatRoughBeast Oct 29 '15 at 14:44
• @WhatRoughBeast it's an error in the schematic. And all the grounds are referring to a same one. – Brontolo Oct 29 '15 at 15:01
• Have you tried replacing your transistor with, say, a 100k resistor, and checking to see that the op amp is good? – WhatRoughBeast Oct 29 '15 at 15:12