An add-on card for a personal digital assistant (PDA) for wireless communication includes a connector for connecting to the PDA, a power managing circuit connected to the connector for storing a first current supplied from the connector, and a radio-frequency (RF) circuit for transmitting an RF signal corresponding to a signal from the connector or transmitting a signal to the PDA via the connector according to a received RF signal. The power managing circuit is electrically connected to the RF circuit. When the RF circuit transmits the RF signal, the power managing circuit will provide a second current to the RF circuit.