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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)
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 ...
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 ...
The current gain is very nearly unity as long as R S ≫ r E. An alternative analysis technique is based upon two-port networks . For example, in an application like this one where current is the output, an h-equivalent two-port is selected because it uses a current amplifier in the output port.
A small voltage change on the input terminal will be replicated at the output (depending slightly on the transistor's gain and the value of the load resistance; see gain formula below). This circuit is useful because it has a large input impedance,
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.
SiGe HBT Gummel plot. In electronics, the Gummel plot is the combined plot of the base and collector electric currents, and , of a bipolar transistor vs. the base–emitter voltage, , on a semi-logarithmic scale.
The current gain is unity, so the same current is delivered to the output load R L, producing by Ohm's law an output voltage v out = v Thév R L / R S, that is, the first form of the voltage gain above. In the second case R S << 1/g m and the Thévenin representation of the source is useful, producing the second form for the gain, typical of ...