Digital electronics for bca, Lecture notes of Digital Electronics

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DEPARTMENT OF APPLIED CIENCES

LASER DIVISION

DIGITAL

ELECTRONICS

LECTURE NOTES

TH

YEAR

WALID K. HAMOUDEI

Summary of basic electronic parameters and components

Voltage: Voltage is the difference in charge between two points, measured in Volts. Current: Current is the flow of electrons through a conductor or semiconductor, measured in Amperes or Amps. Some materials conduct current better than others; these are known as conductors, semiconductors, and insulators. Current flow is from positive to negative. Power: Power determines how much work a circuit can do. It is measured in Watts (Watts = Volts * Amps).

Ground: Minimum voltage reference level. True ground connects to the earth but the circuits we work with may not actually be connected to the earth, especially if they are battery powered. Technically this is known as a floating ground.

Resistance Resistors are measured in Ohm and come between conductors, which conduct easily and insulators which don't conduct. The main function of resistors in a circuit is to control the flow of current and voltage drops to other components. For example; if too much current flows through an LED it is destroyed and will not light, so a resistor is used to limit the current but not so big as it will limit all the current. When a current flows through a resistor, energy is wasted and the resistor heats up. This will only be noticed if the resistor is working at its maximum power rating. The greater the current flowing through the resistor the hotter it gets. A battery or power supply has to do work to force electrons through the resistor and this work ends up as heat energy in the resistor. An important property to know about resistors is how much heat energy it can withstand before it's damaged or causes a fire. Resistors can dissipate different powers (Watts) depending on its power rating and the current passing through. It is difficult to make a resistor to an exact value, so resistances are given a tolerance. This is expressed as being plus or minus a percentage. A ±10% resistor with a stated value of 100 ohms could have a resistance anywhere between 90 ohms and 110 ohms. In circuit diagrams you will often see an 'R' instead of omega to represent ohms. The symbol and a few examples of this type are shown below:

Examples:

*Yellow, Purple, Red, Gold = 47 x 100 = 4 700 = 4.7 + 5% *Brown, Black, Yellow, Gold = 10 x 10 000 = 100 + 5% *Yellow, Purple, Black, Silver = 47 x 1 = 47 + 10% *Brown, Black, Red, Red = 10 x 100 = 1 000 = 1 + 1% *Brown, Black, Green, Gold = 10 x 100 000 = 1 000 = 1 M + 5%

Potentiometer: It is a variable resistor, a wiper moves between two leads and the resistance between wiper and lead determines resistance. Resistance between leads is maximum resistance of potentiometer. With linear pots, resistance varies directly with the rotation of the knob while with logarithmic pots; resistance varies exponentially with the rotation of the knob.

Ohms Law: Every circuit has Voltage, Current and Resistance. V=IR. Voltage = Current * Resistance. I=V/R. Current = Voltage/Resistance. R=V/I. Resistance = Voltage/Current.

Circuits: A working circuit must have a closed loop of current flow through a load. The sum of the current entering a junction equals the sum of the current exiting a junction. Kirchoff's current law is: I (^) in = I (^) out. A circuit with a break in it is called an open circuit. A circuit without enough resistance in its load is called a short circuit. Switching an LED is shown at the example schematic below.

Series Circuits: All components are connected end to end. Single path for electrons to flow - all components share the same current. Total resistance of circuit is equal to sum of individual resistances. Total voltage in the circuit is equal to the sum of individual voltage drops.

Parallel Circuits: All components are connected in parallel and share the same voltage. The total resistance of circuit is less than the value after adding individual resistances. Total current in circuit is equal to sum of individual branch currents.

Inductors: Their values are measured in Henry and are commonly used as AC filters. By coiling wire we can increase strength of magnetic field created by current. This is called an inductor. A large inductor functions as an electromagnet. Strength of the magnetic field depends on number of coil turns, coil size, coil spacing, winding arrangement, core material, and shape of inductor.

Transformers: Four terminal device which turns ac input voltage into a higher or lower output voltage. Transformers consist of two coils called primary and secondary sharing a common iron core. Ratio of turns between primary and secondary coil determines step up/step down value. Power (V*I) is the same in the primary and secondary coil. Stepping down the voltage increases current while Stepping up the voltage decreases the current.

Relays: It is switch operated by an electromagnet and controlled by electrically isolated signal from switched current. It is slow, noisy and can pass AC or DC current. It generates unfriendly voltage spike when magnetic field in coil collapses.

Inductive versus Resistive loads Inductive loads use magnetic fields as in motors, solenoids, and relays. If it moves, it's probably an inductive load. They can cause blowback voltage and circuits should be protected from this by diodes. Blowback is caused by a surge of voltage created by the collapsing magnetic field in an inductor. Resistive loads convert current into other forms of energy, such as heat.

Capacitors Capacitors are components that store an electrical charge. They can be charged up with energy from a battery, then return that energy back later. The capacitance of a capacitor is a measure of how much energy/charge it can store. In its simplest form a capacitor consists of two separated metal plates with air or another non-conductive material filling the gap, the bigger the plates the bigger the capacitance. To stop capacitors becoming impractically large they can be rolled up. Another way of increasing the capacitance is to put some non-conducting material between the plates. This is called a dielectric material. When a capacitor charges up, the protons and electrons in the dielectric separate out a little, this allows more charge to be stored on the plates than usual. Dielectrics are made of various materials Ceramic, paper, polyester, polystyrene, mica, etc. Capacitance is measured in Farads; one Farad is a very big unit and is usually found in the range of picot-to-micro farads. Capacitors come in two types, electrolytic and non-electrolytic. Electrolytic capacitors use special dielectrics sometimes a solid but the most common types are a liquid or paste which is formed into a very thin dielectric in the factory. Non-electrolytic capacitors have solid dielectrics. The symbol for electrolytic capacitors and a few examples of this type is shown below:

The symbol for non-electrolytic capacitors is shown below:

Capacitors hold charge when disconnected from power supply. Dielectric keeps charge from jumping from one plate to another. Lightening is a giant capacitive charge discharging. 1 Farad is equal to 1 amp of current at 1 volt for 1 second. Capacitors we work with are typically measured in Micro- Farads (μF) and Pico Farads (pF). Common uses of capacitors are camera flashes, lasers, decoupling noise, smoothing power supplies, timing etc.

Capacitors - RC Time: Capacitors take time to charge and discharge, according to the amount of current. The charge/discharge time of capacitors is controlled using resistors. Charge time (to 63.2% of supply voltage) and discharge time (to 36.8% of supply voltage) is nicely equal to R*C (in seconds). RC Time allows us to control the rate that things happen in circuits, which turns out to be very useful.

Capacitor Types: Three major types of capacitors are ceramic, electrolytic, and tantalum. Ceramic capacitors are small in size and value, ranging from a few Pico Farads to 1 μF. Not polarized, so either end can go to ground. Value is given by a code somewhat like that of resistors.

Electrolytic capacitors look like small cylinders and range in value from 1 μF to several Farads. Very inaccurate and change in value as the electrolytic ages. Polarized, cathode must go to ground. Cathode is marked with a minus sign on case. Value is usually written on case.

Tantalum capacitors are similar in size to ceramic but can hold more charge, up to several hundred μF. Accurate and stable, but relatively expensive. Usually polarized anode is marked with a plus sign.

Semiconductors: It is probably the most important discovery in electronics which happened last century. Without this discovery we wouldn't have televisions, computers, space rocket, CD players, etc. Unfortunately it's also one of the hardest areas to understand in electronics. The reason that makes metals such good conductors is that they have lots of electrons which are so loosely held that they're easily able to move when a voltage is applied. Insulators have fixed electrons and so are not able to conduct. Certain materials, called semiconductors, are insulators that have a few loose electrons. They are partly able to conduct a current. The free electrons in semiconductors leave behind a fixed positive charge when they move about (the protons in the atoms they come from). Charged atoms are called ions. The positive ions in semiconductors are able to capture electrons from nearby atoms. When an electron is captured another atom in the semiconductor becomes a positive ion. This behavior can be thought of as a 'hole' moving about the material, moving in just the same way that electrons move. So now there are two ways of conducting a current through a semiconductor, electrons moving in one direction and holes in the other. The holes don't really move of course. It is just fixed positive ions grabbing neighboring electrons, but it appears as if holes are moving.

Diode applications examples

1. Reverse polarity protection.

2. Reverse biased diode in parallel with an inductive load will snub the blowback current generated by the collapsing magnetic field.

3. Rectifier converts AC into DC.

A diode consists of a piece of n-type and a piece of p-type semiconductor joined together to form a junction. Electrons in the n-type half of the diode are repelled away from the junction by the negative ions in the p-type region, and holes in the p-type half are repelled by the positive ions in the n-type region. A space on either side of the junction is left without either kind of current carriers. This is known as the depletion layer because there are no current carriers in this layer, so current can flow. The depletion layer is, in effect, an insulator.

Consider what would happen if we connected a small voltage to the diode. Connected one way it would attract the current carriers away from the junction and make the depletion layer wider. Connected the other way it would repel the carriers and drive them towards the junction, so reducing the depletion layer. In neither case would any current flow because there would always be some of the depletion layer left.

Now consider increasing the voltage. In one direction there is still no current because the depletion layer is even wider (reverse biased), but in the other direction the layer disappears completely and current can flow (forward biased). Above a certain voltage the diode acts like a conductor. As electrons and holes meet each other at the junction they combine and disappear.

Thus a diode is a device which is an insulator in one direction and a conductor in the other. Diodes are extremely useful components. We can stop currents going where we don't want them to go. For example we can protect a circuit against the battery being connected backwards which might otherwise damage it.

Zener Diodes: Conducts in reverse-bias direction at a specific breakdown voltage. It is used to provide reference voltage.

and if the thin slice is p-type it is called an n-p-n transistor. The middle layer is always called the base, and the outer two layers are called the collector and the emitter. In an n-p-n transistor (more common), electrons are the main current carriers (because n- type material predominates). When no voltage is connected to the base then the transistor is equivalent to two diodes connected back to back. Recall that current can only flow one way through a diode. A pair of back-to-back diodes can't conduct at all. If a small voltage is applied to the base (enough to remove the depletion layer in the lower junction), current flows from emitter to base like a normal diode. Once current is flowing however it is able to sweep straight through the very thin base region and into the collector, only a small part of the current flows out of the base. The transistor is now conducting through both junctions. A few of the electrons are consumed by the holes in the p-type region of the base, but most of them go straight through.

Electrons enter the emitter from the battery and come out of the collector. To see how a transistor acts as a switch, a small voltage applied to the base will switch the transistor on, allowing a current to flow in the rest of the transistor. NPN and PNP Transistor components look identical to each other the only way to tell the difference is by the component number. The symbol and a few examples of this type are shown below:

Transistor Basics: Use three layers of silicon and can be used as a switch or an amplifier. Processor chips are lots and lots of transistors in one package. Transistors have three leads - the base, collector and emitter.

Bipolar versus Field Effect Transistors : There are two main families of transistors, Bipolar and FET. FETs are more popular, waste less power (therefore run cooler), and are cheaper than bipolar. FETs can be easily damaged by static electricity, so this explains why bipolar types are used for teaching and training students. The basic operation of bipolar and FETs are the same.

NPN versus PNP: In NPN, the base is at a higher voltage than the emitter, current flows from collector to emitter. A small amount of current also flows from base to emitter. NPN Voltage at base controls amount of current flow through transistor (collector to emitter).

In PNP, the base is at a lower voltage than the emitter, current flows from emitter to collector. A small amount of current also flows from emitter to base. PNP. Voltage at base controls amount of current flow through transistor (emitter to collector). The arrow represents the direction of current flow.

Transistor as switch: Most sensors, processors, microcontrollers can't source enough power to make things happen in the real world. Transistors allow a large amount of current to be controlled by a small change in voltage. Grounds between control circuit and transistor must be common.

Analog and Digital Signals

There are two different methods of sending an electronic signal from A to B. ANALOG signals are continuous, and can take any value. DIGITAL signals encode values into binary numbers. As a binary number is made up entirely from 0's and 1's, it may be transmitted in the form of electronic on/off pulses (on =1, off =0). When these pulses are received, they are processed. A digital signal is made up of discretely variable physical quantities.

Whilst these two types of signal both transmit information in electrical voltages, they each have their advantages and disadvantages. In recording audio signals, analog systems are useful, because they can give a faithful electronic representation of a complex waveform. However, because of the need for amplification of the electronic signal, 'noise' can be added along the signal path. This noise is due to unavoidable electron activity in the circuitry. Unfortunately, there is no easy way to get rid of noise from the original signal. Consequently, the noise (audible as a 'hiss') is added to the signal with each stage of transmission.

A digital equivalent to this system would sample the sound wave at selected intervals and transmit the values that correspond to the sound wave in binary code. The digital representation of the sound wave could then be moved around or processed within the system without picking up any additional noise. Although the electron (noise) activity is still taking place, whenever the digital signal is repeated, during each stage of the transmission, the noise can be omitted.

Analog Signal Digital Signal Accurate reproduction of signal needs extra work

Very immune from noise

Suffers from noise and distortion Output is accurate but can have errors from the sampling process Simple technique Complicated but can operate at long distance

Table 1 outlines the basic characteristics of 3 modulation (encoding transmission signal) schemes: Amplitude modulation (AM), frequency modulation (FM) (both analog schemes) and digital modulation.

Table 1 - Comparison of AM, FM, and Digital Encoding Techniques Parameter AM FM Digital Signal-to-Noise Ratio Low-to-Moderate^ Moderate-High^ High Performance vs. Attenuation Sensitive^ Tolerant^ Invariant Transmitter Cost Moderate-High Moderate High Receiver Cost Moderate Moderate-High High Receiver Gain Adjustment Often Required Not Required Not Required

Installation Adjustments Required

No Adjustments Required

No Adjustments Required Multi-channel Capabilities

Require High Linearity Optics Fewer Channels Good Performance Over Time Moderate Excellent Excellent

Environmental Factors

Moderate Excellent Excellent

One difference between analog and digital transmission involves the bandwidth, or transmission capacity required for both schemes. Analog signals require much less bandwidth, only about 4.5 MHz with a 143.2 Mb/s data rate for the average video signal. By comparison, some digital video transmission standards require as much as 74.25 MHz with a data rate of 1485 Mb/s.

Another difference between analog and digital transmission deals with the hardware’s ability to recover the transmitted signal. Analog modulation, which is continuously variable by nature, requires adjustment at the receiver end in order to reconstruct the transmitted signal. Digital transmission, however, because it uses only 1 ’s and 0’s to encode the signal, offers a simpler means of reconstructing the signal.