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Piezoelectric elastic metamaterials offer the ability to overcome the fixed, narrow bandwidth characteristics of passive elastic metamaterials. Interesting ultrasonic band gaps exist in piezoelectric plate metamaterials with periodic electrodes connected to shunted circuits. These band gaps result from an avoided crossing between electrical and mechanical bands, and can arise at lower frequencies than Bloch wave band gaps. Current analytical modeling techniques for these systems are numerically cumbersome, and assume an infinitely periodic plate. We present an approximate two-dimensional analytical model that can be used to directly calculate scattering coefficients for finite length plates. This model is shown to predict a band diagram that compares well with diagrams obtained from finite element analysis (FEA). Lower than 10% difference in the estimation of the location of the band gap was found for a plate thickness of $2$ mm, electrode width of $1$ mm, and gap between electrodes greater than $1.2$ mm. We calculate effective impedances and effective wavenumbers from global scattering coefficients. The calculated effective normalized wavenumber swings from positive values ($0<k_{\mathrm{eff}}\leq 1$) to negative values ($0>k_{\mathrm{eff}}\geq -1$) at the low-frequency band gap, resembling wavenumbers for negative stiffness Helmholtz resonator metamaterials. This presents a new perspective on periodic shunted circuit piezoelectric plates as electrically tunable, negative stiffness metamaterials analogous to Helmholtz resonator lined acoustic waveguides.