Circulator is used to transmit the incident wave which entry from any port of its body to the next port, according to a certain direction confirmed by Static bias magnetic field. It is a nonreciprocal devices with sevral ports. For example, input signal from port 1, the signal can just output from port 2, the same princple, input signal from port 2, the signal can just output from port 3. By parity of reasoning, it is called circulator. Circulator is also called Isolator for the outstanding feature that it can  transmit high frequency signal energy in one way. It controls the electromagnetic wave transmission along a circular direction. The characteristics that transmitting high frequency signal energy in one way is often used between output and load of the high frequency power amplifier,  playing a part in isolating them from each other.  Load impedance will not influence the amplifier when it is changing or even open-circuited or short-circuited.