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NOT gate (inverter) from an NPN transistor

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Open this experiment in the circuit simulator

Objective

Build a resistor-transistor inverter and verify that it inverts and restores the logic level.

Procedure

Use the Controls tab (or the batteries' Properties) to set each input to 0 V or 5 V and press Start Simulation; turn Live on and drag an input slider to watch the output.
Slide the input slowly from 0 to 1.2 V and watch the output fall through the active region.

Theory

With 0 V in the transistor is cut off and the 1 kΩ pull-up gives 5 V; with 5 V in the base current (≈ 0.4 mA) saturates the transistor and Vce(sat) ≈ 0.1–0.2 V. Between the two it is an amplifier: the switching threshold is near 0.7 V.

Questions to answer

Why is a base resistor needed?

Components used

2 × Battery / DC source, Panel voltmeter, 2 × Resistor 220 ohm 1/4W, Transistor BC547 (NPN)

About the model

Ideal wires; component models as listed in the catalogue (no parasitics, no temperature drift). Built without a breadboard so the schematic stays readable.