The base current I B, which is limited by the base resistor R B, determines the collector current: I C = βI B. When V IN is approximately 0.6 V, the BE junction begins to conduct. Thus, the transistor is in the nonfunctional mode called cutoff, and it will remain in this mode until the base voltage is high enough to forward-bias the BE junction. No significant amount of current flows into the base, and consequently no significant amount of current flows from collector to emitter.

If V IN is connected to ground, the base-to-emitter (BE) junction is not forward-biased. Let’s imagine that we’re working with the simple BJT circuit shown below. We’ll start with the three most common operational modes, and then we’ll look at some plots that will help us to understand the relationships between BJT currents and voltages. This is a good place to start, but you’ll need to know quite a bit more if your goal is to analyze or design BJT-based circuits. The previous tutorial provided a low-level explanation of how a BJT functions when it is in the forward active mode.

Bjt clipping detector