We provide sufficient conditions for the existence and stability of periodic solutions of the second-order non-autonomous differential equation of the Nathanson's model x + x + aẋ − b(v 0 + δv(ωt)) 2 (1 − x) 2 = 0, and of the comb-drive finger model x + x + aẋ − 4b(v 0 + δv(ωt)) 2 x (1 − x 2) 2 = 0, where x ∈ R, c, β, v 0 and δ are positive parameters, v(ωt) is a 2π/ω-periodic function. The results are obtained using the averaging theory.