Logic gates
Figure 2.3 is a schematic diagram of a transistor NOT gate. This circuit is powered by a 5 V supply. The input signal might come from a pushbutton 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 about 1,000 ohms. R2 might have a value of 5,000 ohms. R2 limits the current flowing from the collector to the emitter when the transistor is switched on:
Figure 2.3: Transistor NOT gate
The input terminal accepts voltage inputs over the range 0 to 5 V, but since we are interested in digital circuit operation, we are only interested in signals that are either near 0 V (low) or near 5 V (high). We will assume that all voltage levels between the low and high states are transient during near-instantaneous transitions between the low and high states.
A typical NPN transistor...
