when the diode is turned on in the forward direction, when the current is greater than 1ma, the voltage at both ends is about .7V, which is a fixed value; Just like when a current flows across a wire, we think that the resistance of the wire is zero and the voltage is V. When the diode is turned on in the forward direction, you can regard it as a voltage source with .7V, and the voltage source of .7V plus the voltage source of Uref=3.v What is the voltage at both ends? I don't need to talk about it, just think about the experiment of lighting a small light bulb by stringing several batteries together in an hour.