Restrict a state value in an Extended Kalman Filter

I am trying to use an EKF to filter/predict a noisy sinusoidal waveform using the states of phase, rate of change of phase and amplitude. I am finding, however, that sometimes the EKF amplitude state estimate is a negative value which, in the context of my problem does not make any sense, although a negative amplitude is mathematically plausible as it can represent a phase shift that flips or inverts the waveform.

Is there any way to limit the state evaluations within the EKF such that the amplitude will always be a positive value? This will, of course, also have implications for the estimation of the phase state?