# How is Alternating Current (AC) used in Bipolar Junction Transistor (BJT) without ruining its operation?

In a BJT (Bipolar Junction Transistor), say NPN in common base configuration, we forward bias the Base-Emitter Junction and reverse bias the Base-Collector Junction when using a constant DC supply.

Intuitively, when I replace DC with AC, the polarity changes after every half cycle. And this means, the Base-Emitter Junction is now reverse biased and Base-Collector is forward biased. This leads the BJT to be in Reverse-Active Mode.

But, we also know, the manufacturing of transistor is made uniquely, Emitter with medium area and highly doped and Collector being of largest in area and averagely doped. And changing the direction of current should might obstruct its function.

So, is there some transistors that can go in reverse-active mode and some breakdown, or it is a general property? And if later is the answer, how does it work?

• Please check out Q point. For a video lecture series I'd highly recommend " Razavi lectures "on YouTube. He is an award winning professor, Imho the best I've found on YouTube. Commented Jan 9, 2021 at 14:27
• Would Electrical Engineering be a better home for this question? Commented Jan 9, 2021 at 15:21

The resistors $$R_1$$, $$R_2$$ and $$R_\mathrm{E}$$ define the DC operating point and the base of the BJT Q will be at a certain potential $$V_\mathrm{B}$$ (I shall use the convention, common in electronics, that upper case letters denote bias quantities, whereas lower case letters denote variable quantities).
Suppose that the input voltage $$v_\mathrm{i}$$ is initially zero. The capacitor $$C_1$$ will then charge at the value $$V_\mathrm{B}$$. If we assume that $$C_1$$ is sufficiently large so that its voltage doesn't change when $$v_\mathrm{i}\neq 0$$, we then have, by Kirchhoff's voltage law applied at the input mesh, $$v_\mathrm{B} = V_\mathrm{B}+v_\mathrm{i}$$. That is, when there is a signal, the instantaneous base voltage $$v_\mathrm{B}$$ is the sum of the DC bias and the AC signal.