The designed circuit is Bandgap reference circuit.
This circuit in short called BGR is used to provide a constant reference voltage for other circuits like DC DC converters.
BGR produces a constant voltage of around 1.1V to 1.3 volts with respect to any change in temperature and input voltage.

This circuit is being simulated using eSim open source simulator. And the technology used here is Skywater 130nm. 

The architecture used in BGR is opamp architecture. 
The opamp is also designed using skywater 130nm technology. The opamp here refers to the 7-pack p-differential amplifer
which provides a gain of around 73dB.
The purpose of using this opamp is to match the inverting and non inverting terminal as it is made to work in negative feedback.
As the opamp is in negative feedback virtual short concept comes into picture.

This circuit which is designed for 1.1V gives an output voltage of 1.1953V to 1.1543V across the temperature sweep of minus 30 to 80 degree celsius.


Steps to simulate 
1. Give a command which is used to run DC analysis.
2. sweep the temperature from minus 30 to plus 80 degree celsius.
3. check the output waveform.
4. Give a command which is used to run transient analysis.
5. start the analysis from 0.1ms to 1000ms.
6. checck the output waveform.
  

The model files used here are
For nMOS sky130_fd_pr__nfet_01v8
For pMOS sky130_fd_pr__Pfet_01v8
For BJT sky130_fd_pr__pnp_05v5_W3p40L3p40 

As the BGR is a self biased circuit. It needs some external circuit to turn the mosfets on at the transient.
So the startup circuit is designed to turn the mosfets on and drive them into saturation region. As the biasing is done the startup circuit turn off.

The input supply voltage of 3.3V is given and is being dropped down to 1.1V constant voltage over a particular range of temperature.

The challenges faced were:
1.ngSPICE is a new open source tool.So it was difficult in the beginning to cope with the new software.
2.Difficulty faced during attachment of skywater 130nm PDK files everytime circuit nets are changed.

 Begininng of the marathon started by researching some good papers on band gap reference. Then tried simulation using eSim 180nm technology which was a pre installed library.
Then started with simulation of the circuit by attaching the skywater 130nm PDK files.