AoE Figure 2.16 Putting an emitter follower in front of a switch makes it easy for a low-current control signal to switch a high-current load

On the left: the 6.7mA is needed to drive the transistor to switch on the 500mA power LED On the right: only 0.083mA is needed to drive the emitter follower which drives the transistor to switch on the 500mA power LED