Direct Current (DC)

Direct Current, electrical current that flows in one direction only. In wikipedia it's defined as:

Direct Current (DC or "continuous current") is the constant flow of electric charge. This is typically in a conductor such as a wire, but can also be through semiconductors, insulators, or even through a vacuum as in electron or ion beams. In direct current, the electric charges flow in the same direction, distinguishing it from alternating current (AC). A term formerly used for direct current was Galvanic current.

The first commercial electric power transmission (developed by Thomas Edison in the late 19th century) used direct current. Because of the advantage of alternating current over direct current in transforming and transmission, electric power distribution today is nearly all alternating current. For applications requiring direct current, such as third rail power systems, alternating current is distributed to a substation, which utilizes a rectifier to convert the power to direct current.

Types of Direct Current

Within electrical engineering, the term DC is a synonym for "constant". For example, the voltage across a DC voltage source is constant as is the current through a DC current source. The DC solution of an electric circuit is the solution where all voltages and currents are constant. It can be shown that any voltage or current waveform can be decomposed into a sum of a DC component and a time-varying component. The DC component is defined to be the average value of the voltage or current over all time. The average value of the time-varying component is zero.

Although DC stands for "Direct Current", DC sometimes refers to "constant polarity." With this definition, DC voltages can vary in time, such as the raw output of a rectifier or the fluctuating voice signal on a telephone line.

Some forms of DC (such as that produced by a voltage regulator) have almost no variations in voltage, but may still have variations in output power and current.

Direct current installations usually have different types of sockets, switches, and fixtures, mostly due to the low voltages used, from those suitable for alternating current. It is usually important with a direct current appliance not to reverse polarity unless the device has a diode bridge to correct for this. (Most battery-powered devices do not.)

High voltage direct current is used for long-distance point-to-point power transmission and for submarine cables, with voltages from a few kilovolts to approximately one megavolt.

DC is commonly found in many low-voltage applications, especially where these are powered by batteries, which can produce only DC, or solar power systems, since solar cells can produce only DC. Most automotive applications use DC, although the alternator is an AC device which uses a rectifier to produce DC. Most electronic circuits require a DC power supply. Applications using fuel cells (mixing hydrogen and oxygen together with a catalyst to produce electricity and water as byproducts) also produce only DC.

Most telephones connect to a twisted pair of wires, and internally separate the AC component of the voltage between the two wires (the audio signal) from the DC component of the voltage between the two wires (used to power the phone).

Telephone exchange communication equipment, such as DSLAM, uses standard -48V DC power supply. The negative polarity is achieved by grounding the positive terminal of power supply system and the battery bank. This is done to prevent electrolysis depositions.

An electrified third rail can be used to power both underground (subway) and overground trains.


The Difference with AC (Alternate Current)

In 1887 direct current (DC) was king. At that time there were 121 Edison power stations scattered across the United States delivering DC electricity to its customers. But DC had a great limitation -- namely, that power plants could only send DC electricity about a mile before the electricity began to lose power. So when George Westinghouse introduced his system based on high-voltage alternating current (AC), which could carry electricity hundreds of miles with little loss of power, people naturally took notice. A "battle of the currents" ensued. In the end, Westinghouse's AC prevailed.

This special feature isn't about the two electrical systems and how they worked. Rather, it's a simple explanation that shows the difference between AC and DC.

To find out more about alternating and direct current, what exactly an electric current is, and two ways that the currents can be produced, check out the interaction by clicking on checkboxes below, and see the current flows on the right illustration...

 All Off
 Alternate Current (AC)
 Direct Current (DC)

Source: Wikipedia, Microsoft Encarta, and other electricity resource.

The Differential Amplifier

Differential amplifiers are everywhere: input stages of Op Amps; comparator inputs; some video amps; balanced line receivers for digital data transmission ; etc...

A differential amplifier is an amplifier that has two inputs, each of which is sensitive to the opposite polarity of the other, i.e., if the inverting input has a positive going signal, and the non-inverting input has the negative version, then there is an output equal their difference (multiplied by some gain, Gv). Conversely, if both inputs happen to be at the same value, then there is no output signal: they cancel one another, i.e., both signals (being the same polarity and amplitude) make no change is the shared emitter resistor's current, therefore, neither signal affects the other: there is "cancellation," otherwise known as Common Mode Rejection, CMR. Another way of saying the same thing is: if both inputs have the opposite polarity (or phase) signal, the shared emitter resistor draws current equal to the algebraic summation of both transistors.

The configuration of the transistors in a differential amplifier are a combination of common emitter and emitter follower.

• A signal into either input's base, causes an inverted signal at its collector, and simultaneously, a smaller, non-inverted output at the (shared) emitter resistor.
• Any signal at the emitter will appear at the collector as a non-inverted version of this signal--but amplified
• Therefore, any signal at one transistor's input is not only seen at its collector, but is also seen at the other transistor's collector, enabled by the action of the shared emitter resistor

This amplifier consists of two or three transistors (two in the simple version, three or more in the more precision version). These two input transistors are coupled to each other, via each emitter, and share the same emitter resistor . At this common connection each input transistor affects the output of itself, as well as, the other transistor's output.

As the two input transistors share the same emitter resistor and leaving the impression that a signal voltage was at the junction of the emitters and Re, when one transistor is increasing in current, e.g., positive alternation of a sine wave; the other transistor is decreasing in current, by an equal amount, for the negative alternation. Since the pair is sharing the one resistor, one can deduce that, ideally, there is always a constant current in that resistor. Ideally, it is desired that the emitters transfer all of their signal to the other transistor's emitter.

Gain Bandwidth Product

Using several lower gain stages in cascade is a strategy that also works. And, a very direct and effective solution is a common base configuration, in which the input signal drives the emitter, and the base is grounded, which has the effect of breaking the collector/base feedback path.

Frequency dependent feedback in the figure, the capacitor, Ce, across the emitter resistor, Re, causes the gain of this device to be greater at higher frequencies. As capacitive reactance, Xc, approaches the value of Re, a rapid increase in gain occurs. The effect, of course, is to reduce the negative feedback at higher frequencies. This is often done to compensate for the limited bandwidth of the transistor stage.

Common Base Stage

Because the base is "grounded", this configuration does not suffer from the Miller Effect, thus yielding the widest bandwidth of all configurations.

Note that the drive is to the Emitter, and there is no signal inversion.

The Miller Effect

In a gain stage (common emitter) there is a limit to the achievable bandwidth at some set gain: i.e., the higher the gain, the lower the bandwidth; conversely, the lower the gain, the wider the bandwidth. This is the now famous, Gain Bandwidth Product.

The dominant mechanism for this is found in the intrinsic feedback capacitance, C_cb, between the collector and the base. The effect -- as frequency increases -- is to increase feedback via C_cb's capacitive reactance, XC_cb, thus reducing the overall gain.

To compound this problem: XC_cb is dependent on the intrinsic capacitance, C_cb, multiplied by the gain, i.e., as the gain is reduced, the bandwidth is increased.

There are ways of reducing this effect, such as peaking coils in the collector (X_l cancels X_c); pre-emphasis of the signal's higher frequencies at the input; frequency selective feedback, etc...

Electrical Glossary

amplifier a system where an input signal is altered (usually in amplitude)

amplitude modulation (AM) a process of modulating the carrier wave in which the amplitude of the carrier wave is changed in sympathy with the modulating signal (information)

analogue a system in which changing values are represented by a continuously variable electrical signal

anti-intrusion system a system or technique used to monitor and prevent unlawful entry to premises by means of, for example, closed-circuit television, electric fencing, gate control, intercom system

astable a circuit which has no stable condition, and which changes at a rate determined by circuit values

audio relating to a system concerned with frequencies within the range of human hearing

base one terminal of a transistor through which 2% of the supply current will flow

binary logic an assembly of digital logic elements which operate with two distinct states

binary number a number system to the base 2

bandwidth the range of frequencies to which a system will respond in the required manner

bipolar-transistor a transistor in which current is carried through the semiconductor both by holes and electrons; it is a current operated device

bistable a circuit which can have two stable states, and which can remain in either state indefinitely

Boolean algebra a system of formal logic used for minimising complex digital systems

capacitor a component used in electronic circuits that has the ability to store electrical charge for a period of time

class A amplifier a system of which the output is identical to the input in terms of frequency and shape but not in amplitude

class B amplifier a system of which the output signal is altered in amplitude and shape representing half of the output signal

class C amplifier a system of the output signal is altered in amplitude and shape to represent less than half of the input signal

collector one terminal of a bipolar transistor through which 98% of the supply current will flow

conductor a material through which an electric current can flow relatively easily

construction encompasses the study of residential, commercial, industrial and recreational applications of Technology, including systems required to maintain and service them

conventional current electric current, regarded as flowing from positive to negative

central processing unit (CPU) the main number-processing and control section of a computer. In a microcomputer the CPU will be a microprocessor.

crystal usually refers to quartz crystal, used as a precision timing element in many circuits; may refer to the term piezoelectric

Darlington pair transistors used in cascade, giving high gain and high input impedance

decibel (dB) one-tenth of a bell; a measure of power gain, on a logarithmic scale (e.g. a power level equal to 10 times the common logarithm of the ratio of the output power to the input power). The decibel is a convenient unit for representing a very large range of power gain.

delta network a combination of three components connected to form a triangular configuaration like the Greek letter delta; also known as a mesh connection

demodulation the process ofrecovery of a modulating signal from a modulated carrier

design includes the study of a variety of basic design processes that are used in solving technological challenges

diac a bi-directional breakover diode; often used for triggering a triac

digital electronics the branch of electronics concerned with the processing of digital systems, usually in binary

diode a component, either semiconductor or thermionic, that permits current to flow through it in one direction only

electronflow regarded as the flow of electrons from negative to positive

emitter one terminal of a bipolar transistor through which 100% of the supply current will flow

energy focuses on domestic, industrial, generation and transmission services

farad unit of capacitance; a very large unit, the largest practical unit being the microfarad

fibre-optic a glass or plastic fibre used for the transmission of information through light

field-effect transistor a type of transistor characterised by a very high input resistance that is a voltage- operated device

flip-flop general term for a bistable, astable or monostable circuit

frequency the number of complete repetitions of a wave form in one second, expressed in Hertz

gain the factor by which the output of a system exceeds the input

gate a component in a digital logic circuit or one terminal of a field-effect transistor, or other semiconductor device

henry unit of inductance

integrated circuit an electronic system, or part of a system, produced on a silicon chip using microelectronic techniques

impedance the ratio of the voltage applied to a circuit to the current flowing in the circuit; similar to resistance, but applicable to alternating currents and voltages

insulator a material through which electric current will not easily flow

Karnaugh mapping a visual technique used in the planning of digital systems for the minimisation of logic circuits

liquid crystal display (LCD) a reflective display used in digital systems for the presentation of output; characterised by a very low power consumption

light-emitting diode (LED) an electronic component in which electric current is converted directly into visible or infrared light

logic the basic principles and applications of truth tables, interconnections of on/off circuit elements, and other factors involved in mathematical computation in a computer; also used as a general term for various types of gates, flip-flops, and other on/off circuits used to perform problem-solving functions in a digital computer

manufacturing and materials product design, process and production planning, raw materials, manufacturing processes and quality control

microcomputer a computer in which the central processing unit is a microprocessor

microprocessor a central processing unit constructed using large-scale integration in which all the CPU circuits are fitted into a single integrated circuit

modulation variation of the frequency, phase or magnitude of a high frequency waveform in accordance with a waveform of lower frequency

monostable a circuit with a single stable state

multimeter a general-purpose measuring instrument, usually able to measure resistance, current and voltage

negative feedback feedback applied to a system in such a way that it tends to reduce the output

NPN one of the two alternative types of bipolar transistors

ohm the unit of resistance ()

operational amplifier a highly stable, gain, DC amplifier, usually produced as a single integrated circuit

opto-electronics electronic systems or devices that involve the use of light

oscillator an electronic system that produces a regular periodic output

oscilloscope an instrument for displaying electrical waveforms on a cathode ray tube

photo-resistor also known as an LDR (light-dependent resistor); a resistor whose value depends upon the amount of light falling on it

piezoelectric effect the direct conversion of electrical to mechanical energy or vice versa in some crystalline materials

programmable logic controller (PLC) a control device, normally used in industrial control applications, that employs the hardware architecture of a computer and a programming language

PNP one of the two alternative types of bipolar transistor

power supply source of electrical energy

positive feedback feedback applied to a system in such a way that it tends to increase the output

protective device a particular type of equipment used in electric power systems to detect abnormal conditions and to initiate appropriate corrective action

rectification the process of changing an alternating current to a unidirectional current

relay an electromechanical device in which an electric current controls a switch

resistance the property of a material that resists the flow of electrical current

speaker an electromechanical device for converting electrical energy into sound (e.g. a loudspeaker)

star network a set of three or more branches with one terminal of each connected at a common node to give the form of a star; also known as a Y connection

technological process the identification, design, development and evaluation of processes and products related to Electrical Technology

thyristor a component similar to a semiconductor diode but having in addition a gate connection by which the component, normally non-conducting, can be triggered into conduction

tolerance generally the amount by which a specified component value can vary from the marked value

triac a semiconductor component similar to the thyristor but which will conduct in either direction

uni-junction transistor a semiconductor device used in some oscillators

voltage regulation a process to maintain the terminal voltage within required limits despite variations in input voltage or load

wavelength the physical distance between two similar and successive points on an alternating wave

zener diode a semiconductor diode, used for voltage regulation. When the zener diode is reverse-biased, it exhibits a sudden increase in conductivity at a certain specific voltage

Darlington Pair

Darlington Pair is transistors used in cascade, giving high gain and high input impedance.

The maximum input impedance one can expect from an emitter follower, is limited by the finite gains of individual transistors (~ 50 to ~ 350). However, there is a way to increase the effective gain or transistors by using two transistors. The total gain of this transistor pair is Gv1 x Gv2 = Gvtotal (Gv ~ 2k - 100k). This is achieved by arranging the transistors such that the emitter of one is driving the base of the next and connecting the collectors together. This is known as a Darlington pair, and can be used as any single transistor would be: common emitter, emitter follower, etc.

The down side of this arrangement, is reduced speed: because of the very high gain's effect on the collector to base capacitance, C_o ( Ctotal = Co x Hfe ).

Transistor

A Transistor can be thought of as a device that is active in only one direction: it can draw more or less current through its load resistor (sometimes referred to as a pull-up resistor).


It can either Source Current or it can Sink Current, it cannot do doth.

Since the Transistor is a Current device, any signal Voltage must first be converted to a Current.

Voltage to Current Convertor

First, you must convert the input voltage to a current by using a Voltage to Current Convertor -- a resistor.

Current to Voltage Convertor

Next, convert the output current into a voltage by using a Current to Voltage Convertor in the collector circuit -- also a resistor.



Note voltage to current convertor in the base circuit, a.k.a. current limiting resistor. This following animation is illustrating current limiting resistor...


The results when driving the transistor's base directly with no voltage to current convertor: