Voltage, current, and charge control
The collector emitter current can be viewed as being controlled by base-emitter current or by the base emitter voltage. These views are related by current-voltage relation of base-emitter junction, which is the usual exponential current- voltage curve of a PN junction (diode).
The physical explanation for the collector current is amount of the minority-carrier charge in the base region. Detailed models of the transistor action, like the Gummel-Poon model, account for distribution of this charge explicitly to explain the transistor behavior exactly. The charge-control view easily handles the phototransistors, where minority carriers in base region are created by absorption of the photons, and handles the dynamics of turn-off, or recovery time, which depends on the charge in base region recombining. However, as base charge is not a signal which is visible at the terminals, the current- and voltage-control views are used in circuit design and analysis.
In the analog circuit design, current-control view is sometimes used as it is approximately linear. That is, the collector current is nearly βF times the base current. Some basic circuits can be designed by supposing that the emitter-base voltage is approximately constant, and collector current is beta times the base current. However, to accurately and reliably design the production BJT circuits, the voltage-control (for instance, Ebers-Moll) model is needed. The voltage-control model requires an exponential function to be taken into explanation, but when it is linearized such that the transistor can be modeled as a transconductance, as in Ebers-Moll model, design for circuits like differential amplifiers again becomes a linear problem, so the voltage-control view is preferred. For the translinear circuits, in which exponential I-V curve is key to the operation, the transistors are typically modelled as voltage controlled with the transconductance proportional to collector current. Generally transistor level circuit design is performed using SPICE or the comparable analogue circuit simulator, so model complexity is not of much concern to designer.
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