Electronic Circuits

FIELDS OF STUDY

Computer Engineering

ABSTRACT

Electronic circuits actively manipulate electric currents. For many years these circuits have been part of computer systems and important home appliances such as televisions. In information processing, the most important innovation has been the integrated circuit, which is power efficient, small, and powerfully capable. Integrated circuits form the basic operational units of countless everyday gadgets.

PRINCIPAL TERMS

• BCD-to-seven-segment decoder/driver: a logic gate that converts a four-bit binary-coded decimal (BCD) input to decimal numerals that can be output to a seven-segment digital display.
• counter: a digital sequential logic gate that records how many times a certain event occurs in a given interval of time.
• inverter: a logic gate whose output is the inverse of the input; also called a NOT gate.
• negative-AND (NAND) gate: a logic gate that produces a false output only when both inputs are true
• programmable oscillator: an electronic device that fluctuates between two states that allows user modifications to determine mode of operation.
• retriggerable single shot: a monostable multivibrator (MMV) electronic circuit that outputs a single pulse when triggered but can identify a new trigger during an output pulse, thus restarting its pulse time and extending its output.
ELECTRIC VERSUS ELECTRONIC CIRCUITS

Electrical circuits have developed over the years since the discovery of the Leyden jar in 1745. An electrical circuit is simply a path through which electric current can travel. Its primary components include resistors, inductors, and capacitors. The resistor controls the amount of current that flows through the circuit. It is so called because it provides electrical resistance. The inductor and the capacitor both store energy. Inductors store energy in a magnetic field, while capacitors store it in the electric field. The Leyden jar was the first capacitor, designed to store static electricity.

Electronic circuits are a type of electrical circuit. However, they are distinct from basic electrical circuits in one important respect. Electrical circuits passively conduct electric current, while electronic circuits actively manipulate it. In addition to the passive components of an electrical circuit, electronic circuits also contain active components such as transistors and diodes. A transistor is a semiconductor that works like a switch. It can amplify an electronic signal or switch it on or off. A diode is a conductor with very low electrical resistance in one direction and very high resistance in the other. It is used to direct the flow of current.

INTEGRATED CIRCUITS

The most important advance in electronic circuits was the development of the integrated circuit (IC) in the mid-twentieth century. An IC is simply a semiconductor chip containing multiple electronic circuits. Its development was enabled by the invention of the transistor in 1947. Previously, electric current was switched or amplified through a vacuum tube. Vacuum tubes are much slower, bulkier, and less efficient than transistors. The first digital computer, ENIAC (Electronic Numerical Integrator and Computer), contained about eighteen thousand vacuum tubes and weighed more than thirty tons. Once the transistor replaced the vacuum tube, electronic circuits could be made much smaller. Electronic circuits are designed to use a series of logic gates to send a charge through the circuit in a particular manner. These logic gates control the charge output and thus the output of the circuits. In this example, the circuit is designed to add two binary numbers together using a series of AND, OR, and NOT commands to determine the route of the charge and the resulting output from each circuit component.

By 1958, several scientists had already proposed ideas for constructing an IC. That year, Jack Kilby, a scientist at Texas Instruments, was the first to put the idea into practice. He designed a chip constructed entirely from a single block of semiconductor material. Because there were no individual components, the circuits did not have to be large enough to assemble manually. The number of circuits in the chip was limited only by the number of transistors that could fit in it. Early ICs contained only a few transistors each. By the twenty-first century, the maximum possible number of transistors per IC was in the billions.

LOGIC GATES

Active manipulation of electric current is accomplished through logic gates. A logic gate is an electronic circuit that implements a Boolean function. Broadly speaking, a Boolean function is a function that produces one of two potential outputs—either 0 or 1—based on a given rule. Because transistors work as switches, which can take one of two values (e.g., “on” or “off”), they are ideal for implementing logic gates. Most logic gates take in two inputs and produce a single output.

There are seven basic types of logic gates: AND, OR, NOT, XOR, NAND, NOR, and XNOR. These logic gates only accept two input values, 0 and 1, which represent “false” and “true” respectively. They are distinguished from one another based on what output is produced by each combination of inputs:

AND gate: output is only true (1) if both inputs are true; otherwise, output is false (0).

OR gate: output is false (0) only if both outputs are false; otherwise, output is true (1).

NOT gate: output is true (1) if input is false (0), and vice versa. A NOT gate is also called an inverter, because it takes in only one input and outputs the inverse.

exclusive-OR (XOR) gate: output is true (1) if the inputs are different, that is, if only one input is true; if both inputs are the same, output is false (0).

negative-AND (NAND) gate: output is false (0) if all inputs are true (1); otherwise output is true. A NAND gate is essentially an AND gate followed by an inverter

NOR gate: output is true (1) only if both inputs are false (0); otherwise, output is false. A NOR gate is an OR gate followed by an inverter.

exclusive-NOR (XNOR) gate: output is true (1) if both inputs are the same and false (0) if they are different. An XNOR gate is a XOR gate followed by an inverter.

Electronic circuits transmit binary data in the form of electric pulses, where, for example, 0 and 1 are represented by pulses of different voltages. These seven gates can be combined in different ways to complete more complex operations. For example, a BCD-to-seven-segment decoder/driver is a logic gate that converts binary data from a counter to a decimal number display. The “seven segment” refers to the number display system common in digital clocks and other devices, where each numeral is represented by up to seven short parallel or perpendicular line segments. Another complex circuit is a retriggerable single shot. This is a type of time-delay relay circuit that can generate an output pulse of a predetermined length and then extend the output indefinitely if the input is repeated. The purpose of this is to change the timing of another circuit, such as a programmable oscillator.

LIFE WITHOUT INTEGRATED CIRCUITS

Whether in home appliances, computer systems, or mobile devices, electronic circuits make modern life possible. Without advanced electronic circuits such as ICs, personal computers and small, portable electronic devices could not exist. Despite improvements over the years, ICs have maintained their silicon-based design. Scientists predict that the only thing to replace ICs would be a new kind of biologically based circuit technology.

SAMPLE PROBLEM

Determine the output of a NAND logic gate for all possible combinations of two input values (0 and 1).

The combination of an AND gate and a NOT gate forms a NAND logic gate. The output of each input combination is the inverse of the AND gate output. The NAND gate accepts four possible combinations of two inputs and produces outputs as shown: —Melvin O

Frenzel, Louis E., Jr. Electronics Explained: The New Systems Approach to Learning Electronics. Burlington: Elsevier, 2010. Print.

Harris, David Money, and Sarah L. Harris. Digital Design and Computer Architecture. 2nd ed. Waltham: Morgan, 2013. Print.

“The History of the Integrated Circuit.” Nobelprize.org. Nobel Media, 2014. Web. 31 Mar. 2016.

Kosky, Philip, et al. Exploring Engineering: An Introduction to Engineering and Design. 4th ed. Waltham: Academic, 2016. Print.

Tooley, Mike. Electronic Circuits: Fundamentals and Applications. 4th ed. New York: Routledge, 2015. Print.

Wilson, Peter. The Circuit Designer's Companion. 3rd ed. Waltham: Newnes, 2012. Print.