Difference between Analog and Digital Electronics

Many of you have doubt, what’s the difference between analog and digital electronics? In this article, you will find an overview of analog and digital electronics.

Analog Electronics

Analog circuits are conventionally where most beginners to electronics start because we can identify much more easily with analog concepts in electronics. Take, for example, the simplest and best analog scenario— audio amplifiers. Everyone is familiar with audio amplifiers in one form or another, from TV, stereo sound systems, cassette recorders, Walkmans, CD players, and so on.

Audio is a concept we can readily identify with. You can be guaranteed that almost all people, at some point in their lives, have encountered audio. If our ears pick up the sound, we know it’s there (so to speak).

We can distinguish easily between clear, clean sounds and distorted sounds, low sound levels, and high sound levels. Turn up the bass on your stereo system and everyone knows (you don’t have to be electronics knowledgeable) what that sounds like. Jimi Hendrix’s screaming guitar work (perhaps not to everyone’s taste) is clearly different from a classical Mozart rendition. Once more, no electronics appreciation is needed to perceive the difference.

That’s why analog electronics is so satisfying to work with—we can see (well, actually hear) the results and appreciate the differences instantly. Video circuits, although still analog, are much more difficult to work with. There are no equivalent easy-to-use circuits as we have in the audio case, and even if there were, they would still not be the same clarity in being able to distinguish between different video characteristics. Video gets a little far removed from the user being able to distinguish differences. Why do I mention video here?

Well, digital is still one more step removed that video and is, therefore (in my opinion), an even more faceless entity—and therefore a more difficult concept for the beginner to appreciate.

Temperature is another entity that is an analog parameter. We all are familiar with reading temperatures, from the thermometer on your wall used to monitor room temperature changes, to taking your temperature with a medical thermometer. The temperature reading changes smoothly; there is no jump from one level to another. Depending on the type of thermometer deployed, the incremental changes in temperature will be different, but it is always continuous. That’s one of the key properties of an analog signal—it’s continuously varying.

We (i.e., humans) are familiar with entities that change in this manner. Analog electronics is concerned with the processing of analog signals, which is usually amplification, filtering, or signal generation. And once more, the electronic amplifier is the prime example of analog electronics. Most beginners’ introduction to electronics (and it is usually analog electronics) is the encounter of the basic amplifier block.

Amplifiers, by implication of the title, change a small signal into a larger signal. Let’s say we monitor (by one means or another) the starting signal, pass it through an amplifier, and then look at the resultant signal; if it has increased in magnitude, then we know it’s been amplified. Anything from preamplifier circuits, which generally increase the voltage level (but not the current level) of the starting signal, to power amplifiers, which increase the drive (i.e., current) capacity of the signal, is included in this category.

Preamplifiers, although increasing the signal amplitude, don’t have any capability to drive a low impedance load, such as a speaker, and many applications (e.g., audio systems) have a final requirement of needing to do that. Where we’re going to ultimately drive a speaker (this is a low-impedance load, meaning that the resistance of the load is low, typically less than 10ohms), we’re going to consume significant current. From Ohm’s law, we know that when a voltage is applied to a resistance, the resultant current flow increases in inverse proportion to the load resistance.

A low load resistance draws more current, which in turn means that the power amplifier driving this load must be capable of supplying the load current. That’s the principal difference between a preamplifier (which is essentially a voltage amplifier) and a power amplifier (although it has some voltage gain, too), which is essentially a current-supplying (or power) amplifier.

Analog amplifier circuits are always characterized by having the input signal and output signal capacitively coupled. The presence of the capacitors removes the dc component of the signal and passes only the ac component for processing (i.e., amplification in this case) since the information we’re interested in is contained within the ac signal. Analog signals are typically sinusoidal in form, which means that apart from the dimension of amplitude, they are also characterized by a dimension of frequency.

Frequency is the measure of the number of complete cycles of the sine wave that occurs in one second, where the second is defined as the period of reference. A low-frequency audio signal, for example, could be 100Hz, which means that over a 1-second duration there are 100 complete cycles. For a sine wave, a cycle is defined as the difference between a reference point on the waveform to the same reference point on the next cycle. A sine wave basically makes an excursion from zero volts, up to a maximum, back to zero volts, down to a minimum, and back to zero again; this excursion is defined as one cycle.

The waveform just repeats in time and is defined as a periodic waveform. This waveform can be easily seen by coupling it into an oscilloscope, which is basically an instrument for monitoring periodic waveforms. A high-frequency signal (e.g., 10kHz) would have more cycles contained in a 1second reference measure—10,000 cycles to be exact. The reference measure (whatever it is) of course needs to be defined in order for a measure of the signal of interest to be made. The most popular and commonly used analog circuit today is undoubtedly the operational amplifier, which is most generally encountered in the inverting amplifying mode.

Amplifier circuits are one of the most common circuits seen in all of the electronics because all electronic signals of interest, such as microphone signals or radio signals, are very weak (i.e., small in amplitude), and hence need to be amplified to a useful level. Using the operational amplifier as the basis of construction, it is extremely easy to design and build a stable circuit. That’s the beauty of operational amplifier circuits—the circuit performance is predictable and unaffected, in the main by the operational amplifier itself. The operational amplifier gain is just determined by the ratio of two resistors.

Contrast this with building up an amplifier from discrete circuits (transistors, resistors, and capacitors), where the circuit performance is going to be affected by the transistor characteristics and a mix of the circuit components used. As your electronics expertise and enthusiasm grow through the practical learning process of learning by doing, you can move on to other circuit projects at a more intermediate, but still understandable, level.

Digital Electronics

Digital electronics, and especially digital signals, on the other hand, are quite different (from analog signals). The amplitude excursion (for digital signals) is no longer continuous, but discrete; that is, moving between two clearly defined levels, which are generally defined as the starting zero voltage point and a positive maximum. The digital signal is also periodic (like the analog sine wave), but instead of being an essentially smoothly moving continuous signal, it just goes from zero level to a positive maximum level and back to zero again.

This is like a series of sharp quantum changes in signal amplitude. This sharp change can also be discerned in an audio sense. If you were to listen to a digital square wave (which is what it’s commonly known as), then the sound would be rough and harsh.

The harshness can be thought of as being aligned with the quantum nature of the digital signal. On the other hand, the analog counterpart sounds smooth—an artifact that can be associated with the inherent continuum of the analog waveform’s property. The same time reference measure (one second) is used in order to characterize the digital signal of interest.

Digital signals are defined by repetition rate rather than by frequency, but it is essentially the same thing. A 1-kHz repetition rate digital signal has 1,000 cycles (defined in exactly the same way as the analog signal) per second, whereas a 10-kHz digital signal has 10,000 cycles contained in a 1-second measure. The oscilloscope can also provide a visual display of a digital signal because it is also a periodic signal (regardless of waveform shape).

Digital electronics are not involved with amplification or filtering of signals (as in the analog case), but rather concern issues such as counting pulses and triggering subsequent circuits when a number of predetermined pulses have been generated. A pulse is just a single digital signal going from a zero value, to a maximum, and returning to a zero value, as opposed to the alternative, which is a train of continuous pulses.

The occurrence of a pulse event is based on detecting either the positive-going pulse edge (where the pulse goes from a zero voltage level to a positive maximum voltage level), which is the more usual case, or alternatively, the negative-going edge (where the pulse goes from a maximum voltage to a zero voltage level) of the input digital signal. Digital circuits are also known as logic circuits because they essentially traverse two logic states, defined as either a logic low state (a descriptor of when the digital signal sits at the zero volts level) or a logic high state (a descriptor of when the signal sits at the positive maximum volts level).

In a typical counting application, we would see a digital circuit performing a sequential count on the number of pulses being generated. When the count total reaches a predetermined value, a single resultant pulse is generated, and this pulse can then act as a trigger for a further series of logic events to take place.

Digital circuits are typically a sequence of logic events taking place. Because a digital signal has a clean edge (these are the rising and falling edges), it is easy for logic circuits to respond to the edge change and, consequently, to produce a resultant pulse. Analog signals don’t have this clean characteristic edge.

There are two main types of digital circuits popularly in use: TTL digital and CMOS digital circuits. The first of these circuits, TTL technology, is characterized by having a power supply voltage that operates off 5 volts. This technology is the most commonly used digital type and can be found in most logic designs. Although the digital schematic is essentially transparent to the technology type used, the parts list will define the technology type.

The second digital technology type in use is CMOS (complementary metal oxide semiconductor) technology, which runs off a supply voltage range from 3 volts to 15 volts. But the main distinction between CMOS and TTL is the considerably lower power supply current requirements of CMOS. The consequent lower power consumption (for CMOS) translates to a longer, more desirable operating life (from battery-powered circuits). A continuous drive toward a lower supply voltage means a more compact battery requirement.

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