# Parallel Diode with AC voltage and DC voltage

Hello, I have some question about the circuit i posted as photo file. I have there a AC source 4V amplitude. R= 100k ohm. Uv=1.5v. RB=680k ohm. I know that diode flows current at one direction. And when u have AC source you have either a only positive voltage through a diode or a only a negetive voltage. But I dont understand the circuit when you have a DC source of 1.5V for first part and then 4.5 V for second part.

I did the simulation of the two parts: The green one is AC signal and the blue one is de voltage through de Resistor 680K ohm. But I dont understand the working of this circuit? Can someone please explain me this circuit.

The diode only conducts when Anode is more positive then the Cathode. Since the Anode is fixed at 1.5V, the Cathode could not be lower than 0.8V during AC swing of the voltage source. Therefore Vx varies between ~0.8V and $\frac{R_B}{R_B+R}$.4V =3.4V.