As with all electronic oscillation, motorboating occurs when some of the output energy from an amplifying device like a transistor or vacuum tube gets coupled back into the input circuit of the device (or possibly into an earlier stage of the amplifier circuit) with the proper phase for positive feedback. This indicates there is an unwanted feedback path through the circuit from output to input of an amplifying stage. The technical conditions for oscillation, given by the Barkhausen stability criterion, are that the total gain around the feedback loop (comprising the amplifying device and the feedback path) at the oscillation frequency must be one (0 dB), and that the phase shift must be a multiple of 360° (2π radians). Since most amplifying devices, transistors and tubes, are inverting, with the output signal 180° opposite in phase from the input, the feedback path must contribute the other 180° of shift.