if connect a diode to an AC signal can it be kept forward bias with increasing voltage at n-side?
Vi will become blocked by the diode D1 above 4.5V, while D2 conducts and thus Vx cannot exceed ~0.7 + 4.5 so this acts as a peak limiter at Vx.

Diode drop depends on current rating relative to 100mA source and Vi is assumed ideal ESR=0.


Note D1 conducts only below blocking voltage and D2 conducts above blocking voltage of battery.

If the signal on left was another variable battery and current source is a generator, we call this a "battery isolator" circuit so that the lowest battery gets charged yet neither discharges the other.