This is an NAND gate implemented using diode-transistor logic. 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), the only path to ground through the 4.7k resistor is through the base of the transistor. So a base current flows. The transistor wants its collector-emitter current to be 100 times the base current. By attempting to bring the current up to this level, it brings the collector voltage down near ground.
When any of the inputs are low (at ground), the easiest path to ground through the 4.7k resistor is through the low input(s). This brings the bottom of the 4.7k resistor within one diode drop of ground. At this voltage, very little base current can flow to the right through both the diode and the base of the transistor. With no base current, the transistor is off, keeping the output high.
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