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A gain greater than one (greater than zero dB), that is, amplification, is the defining property of an active device or circuit, while a passive circuit will have a gain of less than one. [4] The term gain alone is ambiguous, and can refer to the ratio of output to input voltage (voltage gain), current (current gain) or electric power (power ...
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 ...
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.
A Darlington pair behaves like a single transistor, meaning it has one base, collector, and emitter. It typically creates a high current gain (approximately the product of the gains of the two transistors, because their β values multiply together). A general relation between the compound current gain and the individual gains is given by:
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.
Collector current is approximately β (common-emitter current gain) times the base current. It is typically greater than 100 for small-signal transistors but can be smaller in transistors designed for high-power applications. Unlike the field-effect transistor (see below), the BJT is a low-input-impedance device.
For a typical current of 10 mA, g m ≈ 385 mS. The input impedance is the current gain (β) divided by the transconductance. The output (collector) conductance is determined by the Early voltage and is proportional to the collector current. For most transistors in linear operation it is well below 100 μS.
In electronics the amplification factor, or gain, is the ratio of the output to the input of an amplifier, sometimes represented by the symbol A F. In numerical analysis the amplification factor is a number derived using Von Neumann stability analysis to determine stability of a numerical scheme for a partial differential equation.