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 cascode current mirror architecture. 
The cascode current mirror is aso designed using skywater 130nm technology. 

This circuit which is designed for 1.2V gives an output voltage of 1.2V 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 40 to 140 degree celsius.
3. check 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 desinged 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 1.2V constant voltage over a particular range of temperature.

The challenges faced were
ngSPICE is a new open source tool to me. So it was difficult in the beginning to cope with the new software.
It was difficult for me to attach skywater 130nm PDK files everytime when i change the circuit nets.

In the begininng of the marathon I started researching some good papers on band gap reference. Then I tried to simulate it using eSim 180nm technology which was a pre installed library.
Then I started to simulate the circuit by attaching the skywater 130nm PDK files.