A circuit of an operational amplifier includes an operational main circuit, a plurality of current sources, and at least one clamp circuit. The current sources are configured to connect the operational main circuit to a high voltage source or a ground voltage source. The clamp circuit is connected between the operational main circuit and at least one of the current sources. Here, a transistor device connected to the clamp circuit has a crossing-voltage endurance level which is lower than a preset crossing-voltage endurance level of the operational main circuit.