the present patent application describes an approach to the protection of voltage sensitive instruments using a protection circuit that may be easily integrated in a three - terminal device , and which overcomes the drawbacks of previous approaches . the protection circuit of the invention performs very fast against overvoltages , automatically switching to a low impedance conduction state as soon as the overvoltage situation ends . it also minimizes insertion losses , shows a very low level of thermal noise and allows for obtaining broad bandwidths due to its low serial impedance in the conduction state . furthermore , it does not require any power source or control signal .