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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.
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.
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 ...
A typical Darlington transistor has a current gain of 1000 or more, so that only a small base current is needed to make the pair switch on much higher switched currents. [2] Another advantage involves providing a very high input impedance for the circuit which also translates into an equal decrease in output impedance.
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.
The common base current gain (or α) of a point-contact transistor is usually around 2 to 3, [4] whereas α of bipolar junction transistor (BJT) cannot exceed 1. The common emitter current gain (or β) of a point-contact transistor does not usually exceed 1, [4] whereas β of a BJT is typically between 20 and 200. Negative differential ...