Physics · Ch 9 — Semiconductor Electronics
Analog and Digital Signals
Analog and Digital Signals
There are two fundamentally different kinds of electrical signal. An ANALOG signal is a continuously varying voltage or current with respect to time -- rectifier outputs (9.3.4) and transistor amplifier outputs (9.4.6) are both analog signals, since they can take literally any value within their range at any instant. A DIGITAL signal, by contrast, contains only DISCRETE values of voltage -- it needs only two states, conventionally called ON and OFF, equivalently HIGH and LOW voltage, or a CLOSED and OPEN switch. These two states are represented in Boolean algebra by the binary digits 1 and 0: state 1 stands for circuit-ON, high voltage, or a closed switch; state 0 stands for circuit-OFF, low voltage, or an open switch. In digital systems there are commonly two defined voltage levels, 5 V (high) and 0 V (low); a POSITIVE logic system assigns binary 1 to the higher voltage (5 V) and 0 to the lower voltage (0 V), while a NEGATIVE logic system reverses the assignment, with 1 …
What this figure shows. Two small pulse-train diagrams, each swinging between a marked V level and a marked 0 V level over three time intervals. Panel (a), labelled 'Positive logic', annotates the three intervals as the bit sequence 0, 0, 1 -- reading the LOW (0 V) segments as binary 0 and the HIGH ( V) segment as binary 1. Panel (b), labelled 'Negative logic', shows the identical physical waveform but annotates the SAME three intervals as the bit sequence 1, 1, 0 instead -- reading the LOW segments as binary 1 and the HIGH segment as binary 0 -- making concrete that positive and negative logic are just two differ …