Analog and Digital Electronics

Generally, every electronic circuit is either an analog circuit or a digital circuit. Analog circuits are often referred to as “Linear” circuits, we shall get to know why as we proceed.

In this tutorial, you will learn:

  • What an analog electronic circuit is
  • What a digital electronic circuit is
  • The meaning of linearity in analog electronics

The components you need for this tutorial include:

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You may have seen the speedometer on a car dash board or the speed level graphics on your car race computer game. All these are analog systems obtained from analog electronic circuits. In the car, as you apply pressure on the throttle or accelerator, the speed of the car increases with time and you can view the rate of this increase on your speedometer. Also, in your computer game, as you press the throttle button more, the speed level increases as well. In the two systems, if you reduce the pressure on the throttles, the speed levels go down. Such a system that varies with time is called an analog system.

Analog vs digital Electronic Circuit

In analog electronic systems, voltage or current is used to represent various physical measurements; like the speed of the car, temperature, humidity, pressure, amount of light, etc.

When we want to electronically measure all these physical quantities that vary with time, we do so using an electronic system. An electronic system will convert these varying physical quantities, to varying voltages or currents that are proportional to the physical quantities being measured. This can be achieved by using analog electronic circuits that contain transducers. Transducers can be sensors, devices or systems that can convert one form of energy to another or one form of system to another.

block diagram of anolog electronic circuit acting on a varying physical measurement to produce varying voltage or current
Figure1: Block diagram of analog electronic circuit acting on a varying physical measurement to produce varying voltage or current
A simple light intensity tracking circuit
Figure 2: A simple light intensity tracking circuit.

Figure 1 above shows a block diagram of a physical change measurement being converted to voltage or current using analog electronic circuit. Figure 2 shows an electronic circuit that can convert a varying physical measurement to varying current.

The circuit contains a light dependent resistor (LDR). A light dependent resistor is a resistor whose resistance varies depending on the amount of light shining on it. As the light intensity on the LDR increases, its resistance decreases, and as the light intensity decreases its resistance increases. This constitutes a varying physical system.

With the help of an analog electronic circuit, we can measure this varying light intensity in the form of current that flows through a light emitting diode (LED). When you turn on the switch S1, and gradually cover the LDR (R1),  the light intensity on the LDR decreases, the resistance of the LDR increases up to 10,000,000Ω = 10M, when this happens, current will not follow the LED path because it sees a very high resistance on that path. Rather current will follow the least resistive path, which is the path of the 10000Ω = 10K. When this happens, the LED will gradually go OFF. But, when you gradually remove your hand from the LDR and have the LDR exposed in such a way that reasonable amount of light shines on it, the resistance of the LDR will decrease tending to 0Ω. When this happens the current following in the circuit will see only the 220Ω (R3) resistance on the path the LED is connected, while it will also see 10k (R2) on the other path, in this situation, most of the current will definitely follow the LED path, which is the least resistive path, thereby gradually lighting up the LED as you gradually remove your hand. If this action of covering and uncovering of the LDR is gradually repeated, the LED will be gradually going ON and OFF.

With this analog electronic circuit, we are actually monitoring and tracking light intensity, if we connect an ammeter in series with the LDR, we can measure the intensity of light on the LDR by multiplying the measured current by a factor. If we connect a voltmeter across the LDR, we can measure the voltage drop on the LDR. By  multiplying this voltage value by a factor, we can measure the varying intensity of light on the LDR. The bottom line is this: we are tracking varying physical system (change in light intensity) with varying voltage and current.

This electronic circuit that takes in a varying input and sequentially produces a varying output is called an analog electronic circuit. But if the system takes in a varying input and produces a steady output like ON or OFF without a lower value ON (like a LED dimming as in the case above), such electronic circuit is called a digital electronic circuit. See the figure below:

Circuit diagram explaining digital electronics
Figure 3: Circuit diagram explaining digital electronics

Figure 3 above is similar to figure 2; the only difference being the addition of a resistor and a relay. Also, the position of the LED has been changed. Here is what happens in the circuit, when the LDR is gradually covered, resistance will increase on the path of the relay and it will not be energized; as it is not energized the LED connected to its normally open lead will not come ON, because no current flows to it from the battery, it keeps staying OFF even as light intensity on the LDR varies as you gradually cover it. But as you gradually uncover the LDR, reasonable current starts to flow to the relay, after some time, the relay will be energized and the LED will come ON and keep staying ON, even as the current interring the relay keeps increasing while you remove your hand. From this we will observe that; though the intensity of light varies as we gradually cover and uncover the LDR, the LED only comes ON once, and stays ON, or  goes OFF once and stays OFF without dimming or exhibiting varying brightness.

The LED having only an OFF or ON state is a typical digital electronic circuit implementation. In digital electronics, no matter the variation of the measured system, the output or result is either 0 or 1, HIGH or LOW, ON or OFF, 5 volts or 0 volts, 3.3 volts or 0 volts.

Components Used For Analog Circuit Design

The design of analog electronic circuit requires bunch of discrete components and some integrated circuits (IC) listed below:

  • Resistors
  • Diodes
  • Capacitors
  • Inductors
  • Transistors
  • Operational amplifiers (Op Amp)

We shall discuss all these components and many more in the future. Please don’t forget to buy electronic components from us.

Diagram showing various analog electronic circuit design components
Figure 4: Diagram showing various analog electronic circuit design components

On the other hand, digital electronic circuits are built from logic gate integrated circuits. It’s quite disappointing that some people do not know that the various logic gates taught in classrooms are integrated circuits (IC). The AND gate has an integrated circuit that carries out the function it is known to perform.

AND Gate Integrated circuit, Symbol and Truth Table
Figure 5: AND Gate Integrated circuit, Symbol and Truth Table
Various logic gates and their truth tables
Figure 6: Various logic gates and their truth tables

With the logic gates we obtain combinational logic gates, and with the combinational logic gates we obtain the sequential logic gates. All of these we shall cover in the digital electronics tutorials in future.

Linearity in Analog Electronics

Most people refer to analog electronic circuits as “Linear” electronic circuits. Now, here is the reason, if you look back to the analog electronic circuit we have in figure 2, you will observe that as the intensity of light on the LDR increases, the current flowing into the LED increases. This is a proportionate relationship, whereby increase in input brings about increase in output. Such a relationship is called a “Linear Relationship.” Analog systems are used to depict linear systems. Hence, we can call analog systems linear systems, and analog electronic circuits can as well be termed linear electronic circuits.

Block diagram of a system depicting linearity
Figure 7: Block diagram of a system depicting linearity

The figure above shows a linear relationship between the input and output.  When an analog system acts on the input, the output or result obtained is linearly proportionate with the input.

Summary:

  • Analog electronic circuit is the same as linear electronic circuit
  • Analog electronic circuit is an electronic circuit that handle varying signals (physical measurements) as input, and bring out proportionate varying signals (voltage and current) as output.
  • Analog electronic circuits are designed with discrete electronic components and operational amplifiers.
  • Digital electronic circuits are circuits that operate on the principles of binary digits (0 or 1).
  • Logic gates are the building blocks of digital electronic circuits, followed by combinational logic gates and sequential logic gates.
  • Logic gates can be implemented with their corresponding logic gate integrated circuits.

Assignment:

How can you connect the circuit in Figure 1 to do the opposite of what it does? I.e. how can you design the circuit to make the LED gradually come ON when you gradually cover the LDR?

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