Switching between the external power source and the primary battery current source

A switching circuit (30) in a portable telephone connects a gate of a field effect transistor (Q1) at a voltage lower than the threshold voltage of the gate of the transistor when the voltage at a first terminal (B ) current source falls below a threshold voltage, thereby disconnecting the first terminal (B) of a current source terminal (B +) to mobile current input and connecting a second terminal (a) current source to the terminal (B +) current input. The threshold voltage is set to a voltage higher than the minimum supply voltage of the mobile phone. The first terminal of current source can be connected to a main battery and the second terminal of current source can be connected to such an adapter a battery removal adapter, a handsfree adapter or a transceiver adapter mobile. This switching circuit prevents a current call to be disconnected when the current sources are exchanged during a GSM burst.