This is an inverter implemented using resistor-transistor logic, the earliest form of logic implemented with transistors. Click on the input on the left to toggle its state. When the input is high, the output is low, and vice versa.
When the input is high (3.6 V), a current flows from the base to the emitter. The transistor wants the collector-emitter current to be 100 times the base current, but it can't, because the collector is connected to the same voltage through a larger resistor. So, the transistor is in saturation mode; it gets the collector voltage down to the saturation voltage of 9.7 mV.
When the input is low (at ground), no current flows through the base, so the transistor is off, and the collector stays at 3.6 V.
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