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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.
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 ...
is forward common-emitter current gain at zero bias. 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]
The common collector amplifier's low output impedance allows a source with a large output impedance to drive a small load impedance without changing its voltage. Thus this circuit finds applications as a voltage buffer. In other words, the circuit has current gain (which depends largely on the h FE of the transistor) instead of voltage gain. A ...
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-emitter current gain, , and the common-base current gain, , Base and collector ideality factors, , Series resistances and leakage currents. Sometimes the DC current gain, , is plotted on the same figure as well.
The transition frequency of a bipolar transistor, , is the frequency at which the short-circuit common-emitter current gain falls to unity. [7] It is effectively the highest frequency for which a transistor may supply useful gain in an amplifier.
The current-follower stage presents a load to the common-source stage that is very small, namely the input resistance of the current follower (R L ≈ 1 / g m ≈ V ov / (2I D) ; see common gate). Small R L reduces C M. [2] The article on the common-emitter amplifier discusses other solutions to this problem.