SIM300 GSM modem works for supply voltages from 3.4V to 4.5V. This voltage is difficult to generate as we dont get a standard regulator at this voltage. Do not give 5V directly to the modem as it is above its absolute maximum ratings .
Here is a simple circuit which we can build using standard 5V regulators which are easily available. I use these circuits regularly in my projects as they are easy and require less external components to function and no design work. The 5V regulators are given a input of 5V which reduce the voltage to around 3.6 to 4V (due their own internal dropout voltages) which is the required voltage for the SIM300 modem . The 1000uF electrolytic capacitor is very necessary as it gives stability to the GSM MODEM power supply when it consumes large currents during Calling or sending SMS. If the capacitor is not added and your 5V power supply is not capable of delivering required currents during Call and sending SMS then the modem will keep getting reset . I recommend using a 5V 2A power adaptor for the 5V input.
The regulator used is a NCP1117ST50T3 SMD 5V 800mA SOT223 regulator which is easily available. You can also use a standard 7805 regulator also if you don't get the NCP1117. But the dropout voltage of the 7805 is large which is around 3V. Due to this sometimes the 7805 circuit may give lesser voltage than 3.5V if there are any other heavy loads connected to the 5V power supply. But normally it should work.
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