A device that supplies electricity to an electric circuit is called a power supply. Portable electronic devices are typically powered by batteries, which provide direct current. Household current is alternating current, and the power supply of electronic equipment designed to operate on household current must convert it into direct current. In addition, the power supply provides the current at one or more specific voltages. A power supply performs these functions with a rectifier, filter, and voltage regulator. Most power supplies also contain a transformer to increase or decrease the voltage of the current.

A typical diode can rectify an alternating current—that is, it is able to block part of the current so that it will pass through the diode in only one direction. However, in blocking part of the current, the diode reduces the amount of electric power the current can provide.

A full-wave rectifier is able to rectify an alternating current without blocking any part of it. The voltage between two points in an AC circuit regularly changes from positive to negative and back again. In the full-wave rectifier shown in the illustration on this page at lower left, the positive and negative halves of the current are handled by different pairs of diodes. When the input signal to the rectifier is positive, point x is at a higher voltage than point y. This situation creates a reverse bias in diodes 2 and 3 and a forward bias in diodes 1 and 4. Therefore, current flows only through diodes 1 and 4. When the input to the rectifier is reversed, the input signal becomes negative (drops below the horizontal line) and the voltages at x and y are reversed. This situation creates a reverse bias in diodes 1 and 4 and a forward bias in diodes 2 and 3, so only diodes 2 and 3 conduct.

The output signal produced by the full-wave rectifier is a DC voltage, but it pulsates. To be useful, this signal must be smoothed out to produce a constant voltage at the output. A simple circuit for filtering the signal is one in which a capacitor is in parallel with the output. With this arrangement, the capacitor becomes charged as the voltage of the signal produced by the rectifier increases. As soon as the voltage begins to drop, the capacitor begins to discharge, maintaining the current in the output. This discharge continues until the increasing voltage of the next pulse again equals the voltage across the capacitor.

A voltage regulator is used to help keep the output voltage at a desired level. A simple type of voltage regulator consists of a resistor in series with and a zener diode in parallel with the output of the power supply. The diode is used with a reverse bias above its breakdown voltage so that it will conduct a reverse current. In this mode of operation, a very small change in voltage across the diode causes a very large change in the current through the diode. Should the voltage at the input to the regulator increase, the diode conducts more current and the current through the resistor increases. This action increases the voltage drop across the resistor in such a way that the voltage at the output remains the same.

An amplifier is an electronic device that magnifies an electrical signal by increasing its voltage or current. Amplifiers are essential components of most electronic devices. For example, a radio receiver contains one or more amplifiers to boost the strength of the feeble signal generated by radio waves in an antenna and to produce a signal that can power an earphone or loudspeaker.

In virtually all amplifiers in use today, the signal to be amplified is fed to a transistor. In the transistor, the signal imposes its waveform or some other characteristic onto a stronger electric current, which is produced by the amplifier's power supply.

Amplifiers are grouped into several classes based on the portion of the input signal they reproduce. A class A amplifier conducts for the entire signal; a class B amplifier, for only the positive half of the signal; and a class C amplifier, for only the peaks of the signal. Class A amplifiers are used when a signal needs to be reproduced as accurately as possible, such as in audio equipment. Class C amplifiers are the most efficient of the three; they can produce a large amount of amplification but with distortion. Class B amplifiers are more efficient than Class A amplifiers, but produce less distortion than Class C types.

In many amplifiers part of the output signal is redirected to the input as, feedback. Feedback is used in such a way that it helps in adjusting the input signal. Negative feedback is out of phase with the input and therefore decreases the output signal. It is used to help maintain the output of the amplifier within desired levels. Positive feedback is in phase with the input and therefore increases the output signal. It is used to maximize the output of the amplifier.

or Op Amps, are amplifiers that provide a high level of amplification. Operational amplifiers are so named because they can be used to perform mathematical operations using analog signals. Operational amplifiers are linear devices—that is, their output varies directly with their input. A common type, the differential operational amplifier, will produce a signal whose strength is proportional to the difference in the strength of two input signals. Such amplifiers are used in controlling the operation of automation equipment.

An oscillator is an electronic circuit that produces a regularly varying electrical signal from a direct current. Oscillators are at the heart of all radio, television, and radar transmitters. They also play a vital role in the operation of most radio and all television receivers. In a computer, an oscillator circuit called a clock produces regularly spaced pulses that coordinate the operations performed by logic circuits. Such pulses are also used for keeping time in electronic watches.

One common type of oscillator contains a tank circuit, a circuit formed by an inductor and capacitor. Such a circuit produces an oscillating signal whose frequency depends on the capacitance and inductance of the circuit. Within the circuit a DC power supply and a transistor are used to maintain the strength of the oscillating current by means of positive feedback.

Some oscillators contain variable capacitors, which permit the frequency of the oscillations to be modified. A quartz-controlled oscillator contains a quartz crystal to help eliminate any variations in the frequency of the signal.

Logic circuits are essentially arrays of switches called logic gates. In general, a switch is a device whose inputs and outputs have a limited number of distinct states, usually “on” or “off.” In many logic circuits, these states are referred to as high or low, because the state corresponds to either a high or a low voltage level. Logic circuits are essential for the operation of computers and most other digital equipment.

A major advantage of electronic switches is that their “on” and “off” states can be changed very quickly, in as little as one billionth of a second. Computations in digital electronic computers and other digital devices may involve complex arrays of logic gates and an enormous number of steps. The time required to perform such switching operations must be small or the time required for a calculation would become prohibitively long.

Logic circuits can be built from a variety of electronic components. Circuits built from compatible types of components are referred to as logic families. Two of the most important such families are TTL (transistor-transistor logic), which uses combinations of bipolar transistors to build logic gates; and CMOS (complementary metal-oxide silicon), which uses combinations of metal-oxide field-effect transistors. MOSFET's are particularly suited for use in integrated circuits because they consume little power.

Most logic gates are switched on or off depending on the value of two inputs. For example, an AND gate will produce an output signal only if it receives a signal from all of its inputs. An OR gate will produce an output signal if it receives an input signal from any of its inputs.

For many applications logic circuits are needed that will retain an “on” or “off” state. The simplest such circuit is a two-transistor circuit called a flip-flop. Large arrays of flip-flops and other types of logic circuits are used to form memory circuits— circuits that hold digital information. These arrays are built as integrated circuits. There are two basic kinds: RAM (random-access memory) and ROM (read-only memory).

A microprocessor is a highly complex integrated circuit that contains a large number of logic gates. The gates are organized into various specialized subcircuits for receiving, processing, storing, and relaying data (information) in the form of digital signals. Such logic circuits can be programmed to work with the data in predetermined ways, serving as a computer's central processing unit.

All but the simplest electronic products are made up of a combination of circuits, each designed to perform a specific function. The same basic circuit can be used in a wide variety of products; for example, the power-supply circuit of an audio amplifier is essentially the same as that of a personal computer. Some circuits are designed as self-contained devices, forming units that can be easily interconnected in assembling an electronic product and easily replaced if they become defective.

In conventional circuits, the active and passive components are individual parts that are connected by wires. The connections are greatly simplified in a printed circuit, a circuit in which the components are mounted on a stiff board and connected by strips of metal deposited on the board by a printing process. In integrated circuits, all the various components of the circuit are formed on a single chip of silicon or other semiconductor. Most complex electronic circuits today are built as integrated circuits.

Integrated circuits are very small—their individual components are typically microscopic in size. The entire circuit is generally no more than 1/16 of an inch (1.6 mm) square. Integrated circuits are highly reliable and, compared with the components they replace, inexpensive. Their use reduces the overall size of electronic equipment and helps in designing electronic products as systems made up of separate units, each performing a distinct operation. This construction simplifies manufacturing and maintenance procedures.