[![Level shifter with emitter follower and voltage divider][1]][1]


As we can see that input fed to this amplifier is the output of final stage of a multistage amplifier and if we apply KVL to base-emitter loop then
[![KVL of Base-emitter loop][2]][2]

**Vbb** is the DC level acquired from the previous stage. 
Now, It is clear that the output of **Q1 (emitter-follower)** is less than (Vbb-Vbe1) here.**I want to know** What the importance and function of the *Q2 constant current source* are here. **Further**, it is also not clear to me that how the DC level of the previous stage gets shifted upwards and why we are trying to pull that down with the help of this level shifter circuit.

Please help me with the functioning of DC level shifters.

  [1]: https://i.sstatic.net/ZBmSn.jpg
  [2]: https://i.sstatic.net/M2B3y.jpg