This is a demonstration of an NPN transistor. The emitter is at ground, and the base and collector voltages can be controlled using the sliders at right. Move the mouse over the transistor to see labels for the three terminals.
The base-emitter junction acts like a diode. Little current flows into the base unless it is above about 0.6V. Assuming the collector is at a higher voltage than the base, the collector-emitter current is 100 times the base current. So, this transistor has a beta (current gain) of 100. Moving the collector voltage higher or lower won't have any effect as long as it's higher than the base voltage. This is forward active mode.
A transistor is often considered to be in saturation mode when the collector is lower than the base. But it still acts like forward active mode unless the voltage difference, Vbc, is on the order of a diode drop (.6 V). If the base is at .7V and the collector is dropped to about .14V or lower, the base current will go up and the collector current will go down, so it will no longer be 100 times the base current. This is saturation, where the transistor acts like a low-resistance switch, with a small voltage drop from the collector to the emitter.
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