Following is the circuit I have been analysing.

Here the output voltage Vout-A, observed,

I have been trying to solve the this result with mathematical equation.

If D1, R5 and R8 were not there calculation would have been as follows,
As per kirchhoff's voltage law,
Net drop across R3 will reflect across R12.
Current through R3 = 12 V /(10 kΩ + 10) = 1.19 mA
Drop across R3 = 1.19 *10 = 11.9 mV.
This would be the drop at R12. from that collector current, output voltage would have been calculated.

But with current circuit, voltage across the R12 is shown as 21.3 mV. (from simulation)
I am stuck to start with this output Vout-A calculation due to this situation.

Any lead do the output-voltage calculation for the current circuit will be appreciated.
Could someone help me with this.
Thank you for your consideration

 

That circuit is way beyond me, but I'm intrigued. Where did it come from, and what is its purpose? If there are no other inputs or outputs, and assuming none of those resistors are variable, it seems awfully complicated for just basic voltage or current control.