Enclosed is a simulation. I'm having trouble understanding how the volatge (vout) is generated from the input voltages (vin+ and vin-).
I do not see under which function the op amp is operating (comparator, integrator differential etc) , none of the formulas work for compting the output voltage from the inputs. Any help greatly appreciated.

 

Sorry, I do not do .asc simulations. Can you post the circuit diagram in .jpg or .png?

 

ntetlow...if you want help you should us enable to see your circuit...otherwise....

 

hi ntetlow,
This is what I see in LTS, increased the .tran to 50mS.
What is your question.?
E
Added 2nd image, shows all 5 Vpoints, also tidied the layout.

 

hi ntetlow,
This is what I see in LTS, increased the .tran to 50mS.
What is your question.?
E
Added 2nd image, shows all 5 Vpoints, also tidied the layout.

Afternoon Eric,
nice to find you replying on a Sunday. My question is this:- Vin- and Vout are almost the same, so the difference must be accounted for by vin+. They are not added together aljebraically or subtracted aljebraically, however. Can you explain how vout is derived?

 

This circuit won't work with most 'real' opamps. The inverting input is at -14.3V which is 2.3V negative of the amplifier negative supply which would be outside the common mode range.

 

hi,
I measure the Vin+ as: -14.7V and you are using a -12V supply to the OPA.?

Vin- is close to 0V.!

Check the 741 d/s for the CMV and Voltage input limits when using +/-12V OPA supplies.
E

 

Vin- and Vout are almost the same

You have trouble understanding the circuit operation because the circuit is not a correct design, and the op amp is saturating at the negative rail so the circuit is not operating properly.
There's a 3.3V difference between Vin+ and Vin- (below) and that difference has to be very near zero for the op amp to not be saturated due to the high open loop gain of the op amp (over 10,000).

At a minimum, add a 1k resistor in series with the Vin- node and connect Vin+ to ground.

What is that odd circuit supposed to do?

 

The 741 opamp is over 50 years old so maybe that entire circuit is also over 50 years old and should be buried.

 

The basic OpAQmp model looks something like this -


So you can add feedback around it, do KVL and KCL analysis, and take the
limit of the equations when G becomes very large. This of course is the DC
model of a basic OpAmp.

Regards, Dana.

 

The circuit comes from http://www.ecircuitcenter.com/Circuits_Audio_Amp/Diff_Amp_w_Mirror/Diff_Amp_w_Mirror.htm.
It looks fairly good to me, it attemps to explain various amp circuits.