Transistor as current amplifier
Yes, generally the power supply has to be capable of supplying all the current. There are exceptions we'll look at later.

When we say "a transistor amplifies current" what we mean is that we can use a transistor to allow a small current to control the flow of a larger current. This is analogous to, for example, power steering where a weak turning effort on the wheel of the car or truck is amplified into a high force at the wheels. The power obviously has to come from somewhere and in our electronic amplifier it comes from the battery or power supply.

The exceptions I mentioned earlier include cases where a transistor is used as an interface between a low power circuit and another high power circuit. In this case the amplified current is coming from a different power supply.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A common emmiter transistor switch. A couple of milliamps flows from the 3.3 V micro into the base of the transistor which turns on and allows about 500 mA to flow through a 6 W lamp powered by another supply. Note the common ground connection between the two circuits.