We present a physically meaningful locally active device model which is suitable for modeling locally active memristors or threshold switching devices. The proposed model is derived through a 2-transistor-1-resistor (2T1R) circuit composed of 2 BJTs and a linear bias resistor and is exhibiting negative differential resistance characteristics. We introduce an external capacitor into the 2T1R circuit such that it physically governs respective dynamics. Adopting the Ebers-Moll model for the BJTs, we precisely derive the differential algebraic set of equations for the 2T1R circuit. The numerical simulation results match very well with the Spice simulation results of the 2T1R circuit.