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The collector–emitter current can be viewed as being controlled by the base–emitter current (current control), or by the base–emitter voltage (voltage control). These views are related by the current–voltage relation of the base–emitter junction, which is the usual exponential current–voltage curve of a p–n junction (diode).
A load line diagram, illustrating an operating point in the transistor's active region.. Biasing is the setting of the DC operating point of an electronic component. For bipolar junction transistors (BJTs), the operating point is defined as the steady-state DC collector-emitter voltage and the collector current with no input signal applied.
Current gain in the common emitter circuit is obtained from the base and the collector circuit currents. Because a very small change in base current produces a large change in collector current, the current gain (β) is always greater than unity for the common-emitter circuit, a typical value is about 50.
Full hybrid-pi model. The full model introduces the virtual terminal, B′, so that the base spreading resistance, r bb, (the bulk resistance between the base contact and the active region of the base under the emitter) and r b′e (representing the base current required to make up for recombination of minority carriers in the base region) can be represented separately.
Negative feedback causes the opamp to output enough voltage on the base-emitter junction of the bipolar junction transistor (BJT) to ensure that all available input current is drawn through the collector of the BJT, so the output voltage is then referenced relative to the true ground of the transistor's base rather than the virtual ground.
The narrowing of the collector does not have a significant effect as the collector is much longer than the base. The emitter–base junction is unchanged because the emitter–base voltage is the same. Base-narrowing has two consequences that affect the current: There is a lesser chance for recombination within the "smaller" base region.
The reverse bias safe operating area (or RBSOA) is the SOA during the brief time before turning the device into the off state—during the short time when the base current bias is reversed. As long as the collector voltage and collector current stay within the RBSOA during the entire turnoff, the transistor will be undamaged.
Figure 1: Basic NPN common base circuit (neglecting biasing details). In electronics, a common-base (also known as grounded-base) amplifier is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a current buffer or voltage amplifier.