This is my first time using Teensy (or any kind of microcontroller by myself) and I am wondering if it is possible to add a backup power supply such that:

- if the USB cable is plugged in, the Teensy will be using the USB power

- else (the USB cable is not plugged in), the Teensy will be using the external power


I have read the link below, but seems like the methods are only eliminating the USB power??
http://www.pjrc.com/teensy/external_power.html

How would the circuitry look like (using transistor as a switch?)?

Thank you.