I work on cellular radios and we always use a rectifier to convert the AC to DC. It seems like we should be able to use AC to run the equipment. What is the specific reason we can't?
we can but we don't want, it may cause instability and unwanted performance. for example if a chip needs source voltage 5 volt and it is ac then in some voltages (if it is AC) it won't work cause it goes from -5 to 5 with a stanadar frequency but you need all the time 5V. Also the behaviour of the inductors and the capacitors will be uncertain. But the reason that the AC is used in the jack is that it's transported more easily in big distances in unlike the DC
As far as the radio is concerned, the circuitry is designed for DC. There are various circuits like diodes and transistors that can only accept one-way current, and if you push current the other way you can damage them. The primary reason for using AC is simply because electricity can travel much, much farther than with DC.
Audio and RF circuits require DC to operate without buzz that is caused by the AC 60 cycle frequency. I believe that if you could see the various waveforms in your equipment on an oscilloscope with the 60 Hz AC added to it, you might understand better the why of DC versus AC to power your equipment. The AC would modulate everything which would make communications, and the digital control of the devices extremely difficult. For digital control, the AC could cause false information pulses that would make the equipment unreliable, if it worked at all.
You can use AC motors, but often you want to vary the motor speed and AC motors basically are governed by the frequency of the AC signal. While varying frequency is not impossible, it generally involves much more sophisticated and expensive circuitry than just varying the magnitude of the current as is done with DC motors.
