What is BJT Beta Rule?
Like transfer of primary and secondary winding impedances of a transformer, resistance from one part of a transistor circuit can be referred to another parts. For instance, resistance of collector circuit, RL can be referred to the base circuit and vice versa. Similarly resistance of emitter circuit RE may be referred to the base circuit and, reciprocally, RB can be referred to the emitter circuit. This rule makes the calculation of base current quite simple. The BJT Beta Rule may be used for transfer of resistance from one part of a transistor circuit to another part as studied below :
1. While referring to collector circuit resistance RL to base circuit, it is to be multiplied by β, the ratio of collector current IC and base current IB (Collector current, IC = βIB). While referring base circuit resistance RB to collector circuit, it is to be divided by β.
2. While referring to emitter circuit resistance RE to base circuit, it is to be multiplied by (β + 1), the ratio of emitter current IE and base current IB [Emitter current, IE = (β + 1)IB]. β may also be used in place of (β + 1) because (β + 1) ≈ β.
Similarly, while referring base circuit resistance, RB to emitter circuit, it is to divided by (β + 1) or β.
The precautionary point in application of BJT Beta Rule is that only those resistances can be transferred which lie in the path of the current under computation. Not otherwise.