Darlington pair question

[likephysics]

Why is the first transistor in the D-pair connected to one side of the load and not directly to the Vcc rail.
As I understand, the purpose of the first transistor is to supply enough base current, so that the second transistor has adequate collector current to turn on the load.

Fig here:
http://www.sciencelobby.com/junction-transistors/images/darlington-pair.gif

 

[Bob S]

The first transistor needs ~ 3 volts V_ce to perform well, so you cannot drive the Darlington second transistor into saturation. Even with the first transistor fully saturated, the second transistor has a V_ce of over a volt (min). Darlington is ok if V_ce >=4 volts in my opinion. Darlington is not a good switching application because V_ce voltage drop is too high in ON state.

Bob S

 

[waht]

Tapping from the collector rail instead of Vcc provides some negative feedback which makes the amplification more stable.

Also, darlington pairs are self contained units. Adding extra resistance to tap from Vcc instead of a collector is a waste of power, especially when they are used in high current applications.