Anyone who works with electronic systems or components will frequently encounter the words “analog” and “digital.” What exactly do these words mean? And how does the difference between analog and digital translate into the realm of physical circuits?
The physical environment in which we live is characterized by analog quantities, that is, quantities that change in a continuous fashion and are not restricted to a small number of discrete values. Temperature, position, light intensity, sound waves, colors, textures—our world is filled with gradations and displacements and variations that do not fit into restricted measurement systems such as “on vs. off,” “small vs. big,” “black vs. white,” or “soft vs. hard.” When we use a plot to visually represent the values of these analog quantities, the curve will be smooth. The most emblematic of these smoothly varying analog curves is the sinusoid:
If the world is an analog place, why do we hear so much about digital technology these days? How can we speak of a “digital revolution” if the human experience is still fundamentally analog? It turns out that engineered systems can provide vastly superior performance and functionality when electrical portions of these systems store, transmit, and process information using signals that are restricted to two values: on and off, otherwise known as one and zero.
Though the word “digital” refers in a general way to systems involving a limited set of discrete values, in the context of modern electronics, “digital” implies binary. In binary calculations, the only available digits are one and zero, and this mathematical construct is translated into the electronic domain through the use of digital circuitry in which voltages are always “high” or “low.”
In typical single-ended digital circuits, a logic-high signal has a voltage that is close to (ideally, equal to) the circuit’s supply voltage, and a logic-low signal has a voltage that is close to (ideally, equal to) the circuit’s ground voltage. Since the ground node is the reference for all voltages in the system, we say that logic low is 0 V. Thus, if the supply voltage for a digital circuit is 3.3 V, electrical signals present in this circuit would resemble rectangular waveforms that transition between 0 V and 3.3 V:
In many applications, digital storage, transmission, and processing are so advantageous that electrical engineers employ digital techniques even when this creates a need for additional circuitry that converts analog quantities to digital quantities and then digital quantities back to analog quantities. We’ll learn more about analog-to-digital converters and digital-to-analog converters in a later chapter.
Nowadays, a large proportion of the activity performed by an electronic device occurs inside integrated circuits. Consequently, the difference between analog and digital circuits is rooted in the difference between analog and digital integrated circuits.
Analog and digital ICs contain the same basic components: primarily transistors, but also diodes and passive elements. However, in analog ICs, transistors are intended to amplify or produce continuously varying signals. When we bias a transistor, we create circuit conditions that allow it to properly respond to small changes in voltage. For example, an input stage of an amplifier IC might employ the MOSFET differential-pair configuration shown below; note that the current source (IBIAS) is biasing the Q1 and Q2 transistors.
The next circuit, called a Colpitts oscillator, uses a biased bipolar junction transistor to generate a sinusoidal signal.
Digital ICs, in contrast, are designed in a way that allows input signals to turn transistors fully on or fully off. Whereas both MOSFETs and BJTs are found in analog ICs, the vast majority of transistors in digital ICs are MOSFETs. Designers interconnect MOSFETs in order to form relatively simple circuits that implement basic Boolean logic functions, and these logic gates can then serve as the building blocks for higher-level digital circuits such as flip-flops and even for exceedingly complex circuits such as microprocessors.
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