[ENGN3213 Home]

##

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 *V*_{CC}
(or *V*_{DD}).

Let's now interpret the BJT switch in digital terms.
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.

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.

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.

[ENGN3213 Home]

*ANU Engineering - ENGN3213*