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Digital Gates and Logic


Transistors can be made to switch between the extreme operating points, essentially from ON to OFF and vice versa. This forms the basis for the design of digital circuits, which only use two voltage states, L and H. Here, L stands for a range of voltages close to zero, while H stands for voltages near VCC (or VDD).

Let's now interpret the BJT switch in digital terms. Suppose VBB is H (say 5 V). Then the transistor is turned on and will operate at the point Qsat, so that VCE is L (i.e. 0.2 V). Now if VBB is L (say 0 V), the transistor is off, operates at Qcutoff, so VCE is H (i.e. 5 V). If we regard VBB as the input and VCE as the output then the circuit is behaving like a digital inverter.   This is summarised in the table.


The voltage levels L and H can be interpreted in terms of logic levels 0 and 1, or TRUE and FALSE.

    In positive logic, we have the correspondence

Physical (voltage) level logic level (0 or 1)
L 0
H 1
whereas in negative logic, we have
Physical (voltage) level logic level (0 or 1)
L 1
H 0
In mixed logic, parts of the circuit use positive logic, while other parts use negative logic.

Therefore the transistor switch is an implementation of a logical inverter, as shown in the table in positive logic.

0 1
1 0
The FET switch has similar inverter function.

The inverter is an example of a logic gate.   A logic gate is a circuit which has one or more inputs and an output and performs logical operations (assuming we interpret the voltages as logic levels).

There are a number of different digital integrated circuit technologies, and two of the most common are transistor-transistor logic (TTL) and complementary metal-oxide semiconductor (CMOS). Examples are discussed in Section 2.4.


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