Output resistance of emitter follower voltage regulator
Like all Emitter Followers (aka Common Collector) in the linear range, Ze=Zb/hFE

Dz has specifications for Zzk at knee threshold and Zzt at terminal current (max)

The variables are Uin,hFE Vce (min), Uout, Rv, Vz, Zzt,Zzk and RL.

The lowest fixed parameter Zout=Zz/hFE.
The highest is Zout= Zzt/hFE.
  - where Iz = min(Uin-Uout) / Rv
  - and/or hFE is very low (e.g. 10)      -   due to Vce(sat) @If as guaranteed in transistor spec.

These variations are the limitation of 3 simple parts.

But specs often get added for better efficiency, or wider operating range, lower Vce drop or smaller step load regulation error or lower costs.

e.g. MM3Z13VB Vz=13V(2%) @ 5mA, Zzk=160Ω @Iz=1mA, Zzt= 28Ω @Iz=5mA
thus given Vce>>Vce(sat)and hFE=100 min,    


  Zout= ranges from 1.6Ω @Iz=1mA to 0.28Ω @Iz=1mA 


This may be suitable for 4mA base drive variation or 400mA variation in loads and making compromises to all your limits and tolerances in your design specs.

However reactive step loads will cause more step load errors, as this regulator cannot sink current, only source. The choice of transistor must be able to dissipate worst case heat loss and have base-emitter ESR or bulk resistance &lt;&lt; Zzt.

note

At {Iz=5mA * Vz=13V}= 65mW but max rated power is 200mW @25'C ( using an infinite heatsink) and since package thermal resistance is ~600'C/W without heatsink, 5mA means 65mW*800'C/W= 52'C junction rise or absolute Tj=80'C @ 28'C. This is why you never use absolute maximums.