AbstractThe effect of alternating‐current amplitude (ACA) on the giant magnetoimpedance (GMI) output stability of Co‐based amorphous wires is studied for highly sensitive sensor applications using a precision impedance analyzer placed in a magnetically shielded space. Experimental results indicated that the “spike” phenomenon is related to the inhomogeneous distribution of local critical magnetic fields at a lower ACA, and the GMI output stability of wires can be significantly enhanced by increasing the ACA applied by more than 15 mA. The dependencies of the maximum GMI ratio [ΔZ/Z0]max and magnetic field sensitivity ξmax on the frequency at IAC = 15 mA have been observed: [ΔZ/Z0]max has a maximum value of 117.32% at 15 MHz, and the corresponding GMI peak position Hp tends to be the saturation of 1 Oe until 12 MHz, and ξmax has the maximum of 248.34%/Oe at 12 MHz. An applied ACA of 15 mA and a working frequency of 12 MHz are therefore recommended for the circuit design of a high‐resolution magnetic‐field sensor.