pcb-guide

Basics of Electronics:

What is Electric Current?

An electric current is a flow of electric charge in a circuit. More specifically, the electric current is the rate of charge flow past a given point in an electric circuit. The charge can be negatively charged electrons or positive charge carriers including protons, positive ions or holes.

What is Conventional current flow?

  The conventional current flow is from positive to the negative terminal and indicates the direction that positive charges would flow.

What is Electron flow?

The electron flow is from negative to positive terminal. Electrons are negatively charged and are therefore attracted to the positive terminal as unlike charges attract.

Effects of current

When an electric current flows through a conductor there are a number of signs which tell that a current is flowing.

• Heat is dissipated: 

If the current is small then the amount of heat generated is likely to be very small and may not be noticed. However if the current is larger then it is possible that a noticeable amount of heat is generated. An electric fire is a prime example showing how a current causes heat to be generated. The actual amount of heat is governed not only be the current, but also be the voltage and the resistance of the conductor.

• Magnetic effect: 

Another effect which can be noticed is that a magnetic field is built up around the conductor. If a current is flowing in conductor then it is possible to detect this. By placing a compass close to a wire carrying a reasonably large direct current, the compass needle can be seen to be deflect. Note this will not work with mains because the field is alternating too fast for the needle to respond and the two wires (live and neutral) close together in the same cable will cancel out the field. The magnetic field generated by a current is put to good use in a number of areas. By winding a wire into a coil, the effect can be increased, and an electro-magnet can be made. Relays and a host of other items use the effect. Loudspeakers also use a varying current in a coil to cause vibrations to occur in a diaphragm which enable the electronic currents to be converted into sounds.

Unit of Electric Current: ampere, amp

What is resistance?

Resistance is the hindrance to the flow of electrons in material. While a potential difference across the conductor encourages the flow of electrons, resistance discourages it. The rate at which charge flows between two terminals is a combination of these two factors.

Units: ohms

Ohm's Law:

Ohm's law states that the current flowing in a circuit is directly proportional to the applied potential difference and inversely proportional to the resistance in the circuit.

Ohm's Law formula

                                       V=IR

Where:

    V = voltage expressed in Volts     I = current expressed in Amps     R = resistance expressed in Ohms

Resistivity definition:

The resistivity of a substance is the resistance of a cube of that substance having edges of unit length, with the understanding that the current flows normal to opposite faces and is distributed uniformly over them. The electrical resistivity is the electrical resistance per unit length and per unit of cross-sectional area at a specified temperature.

Resistivity formula / equation:

The resistivity of a material is defined in terms of the magnitude of the electric field across it that gives a certain current density. It is possible to devise an electrical resistivity formula. ρ=EJ

Where:

    ρ is the resistivity of the material in ohm metres, Ω⋅m     E is the magnitude of the electric field in volts per metre, V⋅m^-1     J is the magnitude of the current density in amperes per square metre, A⋅m^-2

Capacitors

Capacitors are simple passive device that can store an electrical charge on their plates when connected to a voltage source.

Standard Units of Capacitance

• Microfarad  (μF)   1μF = 1/1,000,000 = 0.000001 = 10-6 F • Nanofarad  (nF)   1nF = 1/1,000,000,000 = 0.000000001 = 10-9 F • Picofarad  (pF)   1pF = 1/1,000,000,000,000 = 0.000000000001 = 10-12 F

SI unit: farad

Other units: μF, nF, pF

Inductance:

Inductance is used in many areas of electrical and electronic systems and circuits. Components can be in a variety of forms and may be called by a variety of names: coils, inductors, chokes, transformers, . . . Each of these may also have a variety of different variants: with and without cores and the core materials may be of different types.

There are two ways in which inductance is used:

Self-inductance:

Self-inductance is the property of a circuit, often a coil, whereby a change in current causes a change in voltage in that circuit due to the magnetic effect of caused by the current flow. It can be seen that self-inductance applies to a single circuit - in other words it is an inductance, typically within a single coil. This effect is used in single coils or chokes.

Mutual-inductance: 

Mutual inductance is an inductive effect where a change in current in one circuit causes a change in voltage across a second circuit as a result of a magnetic field that links both circuits. This effect is used in transformers.

SI unit : henry, H

Voltage:

he standard unit of potential difference and electromotive force in the International System of Units(SI), formally defined to be the difference of electric potential between two points of a conductor carrying a constant current of one ampere, when the power dissipated between these points is equal to one watt.

Units: Volt

Power:

Electric power is the rate, per unit time, at which electrical energy is transferred by an electric circuit. It is the rate of doing work.

Formula:

W=V I I = Q/t

Where:

    W = power in watts     V = potential in volts     I = current in amps     Q = charge in coulombs     t = time in seconds

Unit: watt

watt:

The watt is the SI unit of power defining the rate of energy conversion and it is equivalent to one joule per second. The watt can be defined according to the application:

• Electrical definition of the watt:

One watt is the rate at which work is done when a current of one ampere, I of current flows through a network which has an electrical potential difference of one volt, V. W = V I

• Mechanical definition of the watt:  

One watt is the rate at which work is done when the velocity of an object is held constant at one metre per second against constant opposing force of one newton.

Semiconductor:

List of common semiconductor terms

Charge carrier

Charge carrier is a free a free (mobile, unbound) particle carrying an electric charge, e.g. an electron or a hole.

Conductor

A material in which electrons can move freely and electricity can flow.

Electron

A sub-atomic particle carrying a negative charge.

Hole

The absence of a valence electron in a semiconductor crystal. The motion of a hole is equivalent to motion of a positive charge, i.e. opposite to the motion of an electron.

Insulator

A material in which there are no free electrons available to carry electricity.

Majority carrier

Current carriers, either free electrons or holes that are in excess i.e. in the majority in a specific area of a semiconductor material. Electrons are the majority carriers in N-type semiconductor, and holes in a P-type area.

Minority carrier

Current carriers, either free electrons or holes that are in the minority in a specific area of a semiconductor material

N-type

An area of a semiconductor in which there is an excess of electrons.

P-type

An area of a semiconductor in which there is an excess of holes.

Semiconductor

A material, that is neither an insulator nor a full conductor that has an intermediate level of electrical conductivity and in which conduction takes place by means of holes and electrons.

Semiconductors types / classifications

There are two basic groups or classifications that can be used to define the different semiconductor types:

• Intrinsic material:

An intrinsic type of semiconductor material made to be very pure chemically. As a result it possesses a very low conductivity level having very few number of charge carriers, namely holes and electrons, which it possesses in equal quantities.

• Extrinsic material:

Extrinisic types of semiconductor are those where a small amount of impurity has been added to the basic intrinsic material. This ‘doping’ uses an element from a different periodic table group and in this way it will either have more or less electrons in the valence band than the semiconductor itself. This creates either an excess or shortage of electrons. In this way two types of semiconductor are available: Electrons are negatively charged carriers.

• N-type:

•   An N-type semiconductor material has an excess of electrons. In this way, free electrons are available within the lattices and their overall movement in one direction under the influence of a potential difference results in an electric current flow. This in an N-type semiconductor, the charge carriers are electrons.

• P-type:

In a P-type semiconductor material there is a shortage of electrons, i.e. there are ‘holes’ in the crystal lattice. Electrons may move from one empty position to another and in this case it can be considered that the holes are moving. This can happen under the influence of a potential difference and the holes can be seen to flow in one direction resulting in an electric current flow. It is actually harder for holes to move than for free electrons to move and therefore the mobility of holes is less than that of free electrons. Holes are positively charged carriers. • semiconductor materials are crystalline inorganic solids. These materials are often classified according to their position or group within the periodic table. These groups are determined by the electrons in the outer orbit the particular elements. • While most semiconductor materials used are inorganic, a growing number of organic materials are also being investigated and used. Semiconductor materials list

Quality factor definition

• For electronic circuits, Q is defined as the ratio of the energy stored in the resonator to the energy supplied by a to it, per cycle, to keep signal amplitude constant, at a frequency where the stored energy is constant with time.

• It can also be defined for an inductor as the ratio of its inductive reactance to its resistance at a particular frequency, and it is a measure of its efficiency.

Effects of Q factor

When dealing with RF tuned circuits, there are many reasons why Q factor is important. Usually a high level of Q is beneficial, but in some applications a defined level of Q may be what is required. Some of the considerations associated with Q in RF tuned circuits are summarised below:

• Bandwidth:

With increasing Q factor or quality factor, so the bandwidth of the tuned circuit filter is reduced. As losses decrease so the tuned circuit becomes sharper as energy is stored better in the circuit.

It can be seen that as the Q increases, so the 3 dB bandwidth decreases and the overall response of the tuned circuit increases. In many instances a high Q factor is needed to ensure that the required degree of selectivity is achieved.

• Wide bandwidth:

In many RF applications there is a requirement for wide bandwidth operation. Some forms of modulation require a wide bandwidth, and other applications require fixed filters to provide wide band coverage. While high rejection of unwanted signals may be required, there is a competing requirement for wide bandwidths. Accordingly in many applications the level of Q required needs to be determined to provide the overall performance that is needed meeting requirements for wide bandwidth and adequate rejection of unwanted signals.

• Oscillator phase noise:

Any oscillator generates what is known as phase noise. This comprises random shifts in the phase of the signal. This manifests itself as noise that spreads out from the main carrier. As might be expected, this noise is not wanted and therefore needs to be minimised. The oscillator design can be tailored to reduce this in a number of ways, the chief one being by increasing the Q, quality factor of the oscillator tuned circuit.

• General spurious signals:

Tuned circuits and filters are often used to remove spurious signals. The sharper the filter and the higher the level of Q, the better the circuit will be able to remove the spurious signals.

• Ringing:

As the Q of a resonant circuit increases so the losses decrease. This means that any oscillation set up within the circuit will take longer to die away. In other words the circuit will tend to “ring” more. This is actually ideal for use within an oscillator circuit because it is easier to set up and maintain an oscillation as less energy is lost in the tuned circuit.

formula

From the definition of quality factor given above, the Q factor can be mathematically expressed in the Q factor formula below:

                                        Q=Estored /ELost per cycle

When looking at the bandwidth of an RF resonant circuit this translates to the Q factor formula:

                                            Q=F0/F3dB

Inductor Q factor basics

When using an inductor in a circuit where the Q or quality factor is important its resistance becomes an important factor. Any resistance will reduce the overall inductor Q factor. An inductor can be considered in terms of its equivalent circuit. This can be simply expressed as a perfect inductor with a series resistor. Where:     L is a perfect inductor     R is the resistance of the inductor

The resistance within an inductor is caused by a number of effects:

• Standard DC resistance:

The most obvious constituent of the resistance in an inductor results from the standard DC resistance. This is always be present (except in superconductors which are not normally encountered). This is one of the major components of resistance in any coil or inductor and one that can sometimes be reduced. Thicker wires, and sometimes silver or silver plated wires may be used to reduce this and improve the overall inductor Q factor.

• Skin effect:

The skin effect affects the inductor Q because it has the effect of raising the resistance. The skin effect results from the tendency of an alternating current flow through the outer areas of a conductor rather than through the middle. This has the effect of reducing the cross sectional area of the conductor through which the current can flow, thereby effectively increasing its effective resistance. It is found that the skin effect becomes more pronounced as the frequency increases.

Core losses:  

Many inductors have ferrite or other forms of core. These cores introduce losses as a result of various factors, each of which affects the inductor Q factor:

• Hysteresis losses:

Magnetic hysteresis is another effect that causes losses and can reduce inductor Q factor values. The hysteresis of any magnetic material use as a core needs to be overcome with every cycle of the alternating current and hence the magnetic field. This expends energy and again manifests itself as another element of resistance. As ferrite materials are known for hysteresis losses,, the effect on the inductor quality factor can be minimised by the careful choice of ferrite or other core material, and also ensuring that the magnetic field induced is within the limits of the core material specified.

• Eddy currents:

It is a commonly known fact that eddy currents can flow in the core of an inductor. These are currents that are induced within the core of the inductor. The eddy currents dissipate energy and mean that there are losses within the inductor which can be seen as an additional level of resistance that will reduce the inductor Q factor. Radiated energy:   When an alternating current passes through an inductor, some of the energy will be radiated. Although this may be small, it still adds to the losses of the coil and in exactly the same way as occurs in an antenna this is represented by a radiation resistance. Accordingly this is a component of the inductor resistance and will reduce the inductor Q factor.

Inductor Q factor formulas

In order to calculate the Q, quality factor for an inductor, the formula or equation below can be used:

                                    Q=XL/ R

As the resistance is equal to 2 π f L, this can be substituted in the formula to give:

                                  Q=2πfL/R

Where,

inductive reactance, X varies according to the frequency. This means that the inductor Q factor will also change with frequency.

Tuned Circuit Filter Quality Factor

Q factor and LCR tuned circuits

One of the key features of an LC tuned circuit is that at resonance the inductive and capacitive reactances become equal. However dependent upon the type of tuned circuit, the effect is slightly different. There are two basic types of tuned circuit:

• Parallel tuned circuit:

At resonance the impedance of a parallel tuned circuit peaks, decreasing either side of resonance. Below resonance the inductive reactance dominates and above resonance it becomes capacitive. As a result of its action any alternating or RF signal voltage placed across the circuit will peak at resonance.

• Series tuned circuit:

The series tuned circuit is very much the inverse of the parallel tuned circuit in that rather than showing a peak in impedance at resonance there is a minimum.

LC Q factor equations

When determining the Q of an LC tuned circuit it is necessary to determine whether the circuit is series or parallel tuned. The LC Q factor for a series tuned circuit is:

                                    Q=1/R √L/C

The LC Q factor for a parallel tuned circuit is:

                                        Q=R/√C/L

Where series or parallel tuned the resistance has a marked affect on the filter Q factor.