This paper provides exponential stability results for a family of nonlinear ODE systems which involves sampled-data states and a time-varying gain. Sufficient conditions ensuring global exponential stability are established in terms of Linear Matrix Inequalities (LMIs) derived on the basis of Lyapunov-Krasvoskii functionals. The established stability results prove to be useful in designing exponentially convergent observers based on the sampled-data measurements. It is shown throughout simple examples from the literature that the introduction of time-varying gains is quite beneficial for the enlargement of sampling intervals while preserving the stability of the system.