Selection of emitter ballast resistance using Ic-VBE fly-back characteristic in bipolar transistors

In this paper, the I_C-V_BE characteristic of bipolar transistors are described by novel analytical formulations, which for the first time present the expression of collect current in the second fly-back point. Through the analytical formulations, it is found that a smaller emitter ballast resistance can make the collector current achieve the second fly-back point before transistors are burned-out, and then returns to a stable situation. Such an emitter ballast resistance is beneficial to both of the output power and thermal stability at the same time. The experiment result also verifies our conclusions.