forwardactive

Forward-active is one of the primary operating regions of a bipolar junction transistor (BJT). In this region, the base-emitter junction is forward biased while the base-collector junction is reverse biased. For an NPN transistor this means the base is roughly 0.6–0.7 V above the emitter and the collector is at a higher potential than the base; for a PNP device the polarities are reversed. In forward-active operation the transistor acts as an amplifier: small changes in base current produce larger changes in collector current. The collector current is approximately IC ≈ β_F · IB, where β_F is the forward current gain. In addition, small-signal parameters such as gm ≈ IC / V_T and r_pi ≈ β_F · r_e describe its response to AC signals.

A key requirement is that the collector-base junction remains reverse biased, which needs a sufficiently high V_CE. If V_CE becomes too low, the device enters saturation, where both junctions are forward biased and gain falls. If the base-emitter junction is not forward biased, the device is in cutoff.

The forward-active region is central to most analog BJT amplifiers, particularly in common-emitter configurations, where it provides high gain with relatively predictable behavior over a range of collector voltages. β_F and V_BE vary with manufacturing, temperature, and operating point, so biasing networks are used to stabilize operation within this region. The term is contrasted with saturation and cutoff, the other primary BJT operating regions.