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Modern Computer Architecture and Organization - Third Edition
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Figure 2.3 is a schematic diagram of a transistor NOT gate. A 5 VDC supply powers this circuit. The input signal might come from a push-button circuit that produces 0 V when the button is not pressed and 5 V when it is pressed. R1 limits the current flowing from the input terminal to the transistor base terminal when the input is high (near 5 V). In a typical circuit, R1 has a value of 1,000 ohms, and R2 has a value of 5,000 ohms. R2 limits the current flowing from the collector to the emitter when the transistor is switched on, with very little resistance between the C and E terminals.

Figure 2.3: Transistor NOT gate
The input terminal accepts voltages between 0 and 5 V. Since we are interested in digital circuit operation, we are only concerned with signals near 0 V (low) or near 5 V (high). We assume that all voltage levels between the low and high states are transient during rapid transitions between these states.
A typical NPN transistor has a switching voltage of 0...