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In this paper, a portable subcircuit for BJT is developed that incorporates self heating as well as thermal coupling effects. Our approach is to use built-in controlled sources in the simulator to model inter dependent thermal as well as electrical behavior. The great advantage of this model is the portability to a large number of general purpose electronic circuit simulators. It also eliminate need for separate thermal simulator and/or need to modify any model equation. Thermal parameters are modeled using thermal-electrical analogy. Finite difference approach is used to model the thermal grid network. This facilitates simultaneous and accurate thermal as well as electrical simulation. BJT Current mirror circuit is simulated to show the significance of the model. For discrete BJT current mirror, standard circuit simulation produces error up to 84% versus 3.8% for our model when compared to experimental results. The results obtained for integrated circuit current mirror shows the utility of electrothermal model for simulation of BJT integrated circuits where thermal coupling effect plays important role.