buckboost

Can anyone tell me how can i add a rectifier to this circuit so i can get a fixed output voltage?

It has a rectfier and the output should be fixed already.

well this is a buck boost circuit and i am asked to design it to take an ac input of 90-240V and a fixed dc output of 5V

i asked my proffessor this is what he said :
There are several thing you need to include. You are starting with an AC signal, how do you get a DC from that? You can get use a PWM source to control different duty cycles. I am not asking you to build a control circuit, so just have different simulations for different duty cycles.
I hope this makes it a little clearer,

so i thought that we could use a rectifier to get it to change from AC to dc

Add a bridge rectifier and a large capacitor filter at the output of the function generator on the left.