This is an NAND gate implemented using resistor-transistor logic, the earliest form of logic implemented with transistors. Click on the inputs on the left to toggle their state. When all of the inputs are high, the output is low; otherwise, the output is high.
When all the inputs are high (3.6 V), a current flows from the base to the emitter of all the transistors. Each transistor wants its 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 transistors are in saturation mode; they maximize the current to bring the output voltage down as low as possible.
When any of the inputs are low (at ground), no current flows through the base of the corresponding transistor, so it switches off. With no path to ground, the output stays at 3.6 V.
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