Hi,
(The picture is a bit small, hard to recognize the part values)
This is not a technical term. Some may mis-interprete "no gain" with gain = 0.
But in your case "no gain" means gain = 1.
--> please in future use the term: "gain = 1"
****
Analyzing your circuit:
The first an mayb ebiggest problem is: You use an OPAMP supply of 9V, whil the OPAMP supply is spicified with 2.6V ... 6.0V.
--> You are beyond specification! This may kill the OPAMP.
Now look at the signal path of the left diode (non iverting path).
* diode anode at +5V
* from cathode there is a 10k to GND
* also from cathode there are 2x 1k (in series) to GND. (you may ignore the influence of the OPAMP, because the input current should be negligible)
--> the total diode load is: 10k in parallel to 2k = 1.6666k (this may be good or not, you need to know)
*****
Now let´s calculate the voltage at the noninverting OPAMP input (this determines the common mode voltage)
The voltage at the cathode (referenced to GND) is: diode_current x 1.666k
Because of the 1k/1k voltage divider --> the voltage at the OPAMP input if half of the cathode_voltage: IN+_voltage = diode_current / ( 2 x 1.6666k)
Now you see with no diode current there is 0V. This is critical because it is close to the OPAMP_supply_rail.
But according datasheet the OPAMP can handle common_mode_input_voltage down to -0.3V. So everything is OK so far.
Now assume the left diode carries some current.
--> the voltage at IN+ rises
--> the voltage at OPAMP_output rises.
Everything OK so far.
Now assume no diode carries current.
--> the OPAMP_output should be 0.000V
This is critical, because it is at the OPAMP supply rail.
According datasheet: "Linear Output Voltage Swing" is only down to 100mV, then it becomes unlinear and will saturate at about 20mV.
--> Trouble. You estimate 0mV, but there will be something between 20mV and 100mV
Now assume left diode carries no current, but right one carries some current. .
--> the OPAMP_output should be negative
This is critical, because it is below OPAMP supply rail.
According datasheet: "Linear Output Voltage Swing" is only down to 100mV, then it becomes unlinear and will saturate at about 20mV.
--> Trouble. You estimate negative voltage, but there will be something between around 20mV positive.
*****
Conclusion:
There are two major problems to solve now:
* Power supply for the OPAMP: Either use a suitable OPAMP or modify the supply voltage for the OPAMP.
* You need to define the expected output_voltage vs the "differential_input_current".
Next steps will follow...
Klaus