BJT operation regions

When we talk about Bipolar Junction Transistor BJT operation regions, we mean ways in which Bipolar Junction Transistor is use.

Bipolar Junction Transistor (BJT) has basically three (3) operation regions. These are;

  1. Cut off region (FULLY OFF)
  2. Saturation region (FULLY ON)
  3. Active region.

In cut off or fully off region, the transistor is bias in such a way that both junctions are reverse so that no current will pass from the emitter (E) to the collector (C). In this mode, the transistor is in OFF mode.

When we study the below circuits, two PN-junction diodes are configure to form three layers for NPN and PNP BJT. In NPN layers; P-dope is the common junction while in PNP layers; N-dope is the common junction. Both the configurations are in reverse modes. By connecting the base (B) to the emitter (E) that is to say; the base (B) is lower than the collector (C); current will not flow from the emitter (E) to the collector (C). This is because the PN-Junction, from the base (B) to the collector is in reverse direction, at this state the transistor is “fully OFF.” Note that PNP emitter terminal is always connected to the positive potential difference while its collector READ THIS


A transistor is said to be saturated (Fully ON) when it is biased in such a way that current passes from the Emitter (E)  to the Collector (C). In NPN and PNP bipolar junction transistors (BJTs), connecting the base (B) to the collector (C) makes the PN-junction from the base (B) to the emitter (E) to be forward bias. As a result of this, the base (B) to the collector (C) of the transistor shorts thereby causing high current to flow from the emitter (E) to the collector (C). In this state the READ THIS


In active region the transistor is bias so that the base (B) measures a higher potential difference (Pd) to the emitter (E) but lower than the collector (C) to the emitter (E). In this region the transistor is always active. This is the biasing mode or region which transistor amplifiers use for their operations.


The below circuits explain and illustrate the operations of READ THIS



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