Increasing 78xx Voltage Regulator Current

Shown in the figure is an IC voltage regulator (also known as stabilizer), designed to increase its current output with the use of an outboard pass transistor. Voltage regulators are designed to produce positive inputs, such as 78xx series, and negative inputs, such as 79xx series. Using this circuit will help increase the current output from a 78xx series regulator. Alternatively, 79xx series can also be used with an NPN type of transistor.

Using a power transistor enables additional current to be loaded while sustaining a steady voltage. But keeping in mind for the limit of input voltage as it should be a few volts above the output voltage. Regulators like 7812, having a 12V output can be set to produce 20V output.  Some 78xx series can surpass up to 36 volts input. Having a high power difference could lead to an overheat and would require sufficient heat sink. Without the heat sink, the transistor might collapse. Lower input could also cause failure due to the decrease in temperature. Power dissipation can be computed as the product of the voltage and the current  P = V * I.
Source:www.zen22142.zen.co.uk/Circuits/Power/boosti.htm