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Some models base the collector current correction factor on the collector–base voltage V CB (as described in base-width modulation) instead of the collector–emitter voltage V CE. [3] Using V CB may be more physically plausible, in agreement with the physical origin of the effect, which is a widening of the collector–base depletion layer ...
Another useful characteristic is the common-base current gain, α F. The common-base current gain is approximately the gain of current from emitter to collector in the forward-active region. This ratio usually has a value close to unity; between 0.980 and 0.998. It is less than unity due to recombination of charge carriers as they cross the ...
Nonetheless, the voltage gain is appreciable even for small loads: according to the table, with R S = r E the gain is A v = g m R L / 2. For larger source impedances, the gain is determined by the resistor ratio R L / R S, and not by the transistor properties, which can be an advantage where insensitivity to temperature or transistor variations ...
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.
Generalised h-parameter model of an NPN BJT. The term "x" in the model represents a different BJT lead depending on the topology used. For common-emitter mode the various symbols take on the specific values as: x = 'e' because it is a common-emitter topology; Terminal 1 = Base; Terminal 2 = Collector; Terminal 3 = Emitter; i in = Base current (i b)
In other words, the circuit has current gain (which depends largely on the h FE of the transistor) instead of voltage gain. A small change to the input current results in much larger change in the output current supplied to the output load. One aspect of buffer action is transformation of impedances.
For transistors, the current-gain–bandwidth product is known as the f T or transition frequency. [4] [5] It is calculated from the low-frequency (a few kilohertz) current gain under specified test conditions, and the cutoff frequency at which the current gain drops by 3 decibels (70% amplitude); the product of these two values can be thought of as the frequency at which the current gain ...
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.