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Digital Gates and Logic

We have seen (sections 8.3.1 and 9.4.1) that 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 V_CC (or V_DD).

Let's now interpret the BJT switch in digital terms (recall Figure 96). Suppose V_BB is H (say 5 V). Then the transistor is turned on and will operate at the point Q_sat, so that V_CE is L (i.e. 0.2 V). Now if V_BB is L (say 0 V), the transistor is off, operates at Q_cutoff, so V_CE is H (i.e. 5 V). If we regard V_BB as the input and V_CE as the output then the circuit is behaving like a digital inverter.   This is summarised in the table.

INPUT	OUTPUT
L	H
H	L

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.

INPUT	OUTPUT
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 10.3.

   

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