Interference and Noise Signal
The PWM signal is given to the servo by the control wire. The noise or interference signals from the surrounding electronics or other servos can cause positional errors. To eliminate this problem the control signals are supplied after amplification. This will suppress the noise and interference signals.
Servo Modification for full Rotation
One may want to use the servo for his robot applications and desire to move the servo continually. This is possible by a little modification. The servo gear box has a mechanical stop which avoids the servo to make full rotation. File off the mechanical stop(s) so that the gear box is free to make a complete rotation.
But this is not the only sufficient thing. The servo works on a feedback mechanism. So the pot of the servo must be first moved to the centre position. This can be done by sending medium pulses to the servo by a microcontroller. Then fix the gears attached to the pot shaft with glue. This will keep an impression to the control electronics of the servo that the current position is the middle point. So the servo would then move with respect to the middle position and not to the current position.