Till input voltage Reaches 4V No zener is in Breakdown Region So V0=Vi Then Now when Vi changes between 4V to 6V One Zener with 4V will Breakdown are P.D. across This zener will become constant and Remaining Potential will drop. acro Resistance in series with 4V Zener. Now current in circuit increases Abruptly and source must have an internal resistance due to which. Some potential will get drop across the source also so correct graph between V0 and t. will be
We have to Assume some resistance in series with source.