This is a demonstration of an NPN transistor. The emitter is at ground, and the base and collector voltages can be adjusted using the sliders on the right. Hover the mouse over the transistor to see labels for the three terminals. The base-emitter junction acts as a diode. A small current flows into the base once it exceeds 0.6 V. 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. Raising or lowering the collector voltage will have no effect as long as it is higher than the base voltage. This is the forward active mode. The transistor is often considered to be in saturation when the collector is below the base. But it still acts as if in the forward active mode as long as the voltage difference, V bc, does not exceed a diode drop (0.6 V). If the base is at 0.7 V and the collector drops to 0.14 V or below, the base current will increase and the collector current will drop, so that it is no longer 100 times the base current. This is saturation, when the transistor acts as a low-resistance switch with a small voltage drop from collector to emitter.
This page is a utility for simulating npn-транзистор (биполярный) online with specified initial values.
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