Problem logarithmic amplifier based on ua741

[Electronicboard]

I'm involved in an educational project and I want to design a logarithmic amplifier based on ua741. Following capture shows my circuit:

http://upload.wikimedia.org/wikipedia/commons/0/0e/Op-Amp_Logarithmic_Amplifier.svg

Diode is 1N4148
Resistor Value is 33Kohms

And the results in lab and simulating don't match the results using the following formula:

http://upload.wikimedia.org/wikipedia/commons/math/0/7/5/075cdba904044102061caee6eab71d2f.png

Vin= 6v
Vdd+=15v
Vdd-=-15v
Vt= 0.026 v
Is (Saturation Current = Reverse current) = 25 nA (I took from diode datasheet)
Even simulating results and lab results don't match.

Vin Simulating (Using multisim) ____Lab _____Formula
6v=> -0.552 _________________-0.523____ -0.231
5v=> -0.547 _________________ -0.516 ___ -0.226
4v=> -0.541 _________________-0.505_____-0.221

Is there a way to check which parameters use Multisim to simulate ua741 or diode 1N4148

Thank you very much in advance,

[_user32]

Electronicboard,

I had trouble opening the svg type file so I wasn't able to look at the circuit directly.  

I would recommend recreating the circuit in a Multisim schematic (and posting it if you'd like some additional help) and then you'll have control over the SPICE parameters in which you are interested in.   It might be that the assumptions in the equation (like the resistor values being the same, offset voltage/current in the opamp to be 0, or diode parameters matching identically) will allow you to experiment show you the results you are seeing in the lab.

In this way you'll be able to experiment with the Multisim circuit in the ideal case and then in the non ideal case:  A) what if the resistor values mismatch by 5%, B) what if the offset voltage of the opamp is 1 mV not 0V, etc...

Please post the Multisim circuit and the forums users will be able to look into more details.

Regards,

Pat N

[i AM ALSO SEARCHING SAME dr_parjit@hotma]

find here a attached file dr_parjit@hotmail.com