 | This article needs additional citations for verification .(August 2012) |
In signal processing, when describing a periodic function in the time domain, the DC bias, DC component, DC offset, or DC coefficient is the mean value of the waveform. A waveform with zero mean or no DC bias is known as a DC balanced or DC free waveform. [1]
The term originates in electronics, where DC refers to a direct current voltage. In contrast, various other non-DC frequencies are analogous to superimposed alternating current (AC) voltages or currents, hence called AC components or AC coefficients.
In the design of electronic amplifier circuits, every active device has biasing to set its operating point , the steady state current and voltage on the device when no signal is applied. In bipolar transistor biasing, for example, a network of resistors is used to apply a small amount of DC to the base terminal of the transistor. The AC signal is applied at the same terminal and is amplified. The bias network is designed to preserve the applied AC signal. Similarly, amplifiers using field-effect transistors or vacuum tubes also have bias circuits. The operating point of an amplifier greatly affects its characteristics of distortion and efficiency; power amplifier classes are distinguished by the operating point set by the DC bias.
DC offset is usually undesirable when it causes clipping or other undesirable change in the operating point of an amplifier. An electrical DC bias will not pass through a transformer or capacitor; thus a simple isolation transformer or series-wired capacitor can be used to block or remove it, leaving only the AC component on the other side. In signal processing terms, DC offset can be reduced in real-time by a high-pass filter. For stored digital signals, subtracting the mean amplitude from each sample will remove the offset. Very low frequencies can look like DC bias but are called "slowly changing DC" or "baseline wander".
DC-balanced signals are used in communications systems to prevent bit errors when passing through circuits with capacitive coupling or transformers. Bit errors can occur when a series of 1's create a DC level that charges the coupling capacitor, bringing the signal input down incorrectly to a 0-level. In order to avoid these kinds of bit errors, most line codes are designed to produce DC-balanced signals. The most common classes of DC balanced line codes are constant-weight codes and paired-disparity codes.
In audio recording, a DC offset is an undesirable characteristic. It occurs in the capturing of sound, before it reaches the recorder, and is normally caused by defective or low-quality equipment. It results in an offset of the center of the recording waveform that can cause two main problems. Either the loudest parts of the signal will be clipped prematurely since the base of the waveform has been moved up, or inaudible low-frequency distortion will occur. Low-frequency distortion may not be audible in the initial recording, but if the waveform is resampled to a compressed or lossy digital format, such as an MP3, those corruptions may become audible. [2]
A DC tape bias was used in early tape recorders to reduce distortion.
A DC bias is applied to the control grid of vacuum tubes in power amplifiers in order to regulate power. [3]
On a voltage-controlled oscillator (VCO), such as in a radio transmitter, selection of the center frequency of the carrier wave is done with a DC bias. For frequency modulation (FM), the AC component is the baseband audio signal plus any subcarriers. Frequency-shift keying can be done solely by changing the DC bias.
The concept has been extended to any representation of a waveform and to two-dimensional transformations like the discrete cosine transform used in JPEG.
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal. It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. An amplifier is defined as a circuit that has a power gain greater than one.
An operational amplifier is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op amp produces an output potential that is typically 100,000 times larger than the potential difference between its input terminals. The operational amplifier traces its origin and name to analog computers, where they were used to perform mathematical operations in linear, non-linear, and frequency-dependent circuits.
Capacitive coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) nodes, induced by the electric field. This coupling can have an intentional or accidental effect.
In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode designed to exploit the voltage-dependent capacitance of a reverse-biased p–n junction.
A valve amplifier or tube amplifier is a type of electronic amplifier that uses vacuum tubes to increase the amplitude or power of a signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers in the 1960s and 1970s. Valve amplifiers can be used for applications such as guitar amplifiers, satellite transponders such as DirecTV and GPS, high quality stereo amplifiers, military applications and very high power radio and UHF television transmitters.
A voltage regulator is a system designed to automatically maintain a constant voltage. It may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.
A push–pull amplifier is a type of electronic circuit that uses a pair of active devices that alternately supply current to, or absorb current from, a connected load. This kind of amplifier can enhance both the load capacity and switching speed.
Power electronics is the application of electronics to the control and conversion of electric power.
Clipping is a form of waveform distortion that occurs when an amplifier is overdriven and attempts to deliver an output voltage or current beyond its maximum capability. Driving an amplifier into clipping may cause it to output power in excess of its power rating.
A radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. Radio waves are electromagnetic waves with frequencies between about 30 Hz and 300 GHz. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves. Transmitters are necessary parts of all systems that use radio: radio and television broadcasting, cell phones, wireless networks, radar, two way radios like walkie talkies, radio navigation systems like GPS, remote entry systems, among numerous other uses.
In radio, a detector is a device or circuit that extracts information from a modulated radio frequency current or voltage. The term dates from the first three decades of radio (1888-1918). Unlike modern radio stations which transmit sound on an uninterrupted carrier wave, early radio stations transmitted information by radiotelegraphy. The transmitter was switched on and off to produce long or short periods of radio waves, spelling out text messages in Morse code. Therefore, early radio receivers did not have to demodulate the radio signal, but just distinguish between the presence or absence of a radio signal, to reproduce the Morse code "dots" and "dashes". The device that performed this function in the receiver circuit was called a detector. A variety of different detector devices, such as the coherer, electrolytic detector, magnetic detector and the crystal detector, were used during the wireless telegraphy era until superseded by vacuum tube technology.
A test probe is a physical device used to connect electronic test equipment to a device under test (DUT). Test probes range from very simple, robust devices to complex probes that are sophisticated, expensive, and fragile. Specific types include test prods, oscilloscope probes and current probes. A test probe is often supplied as a test lead, which includes the probe, cable and terminating connector.
A clamper is an electronic circuit that fixes either the positive or the negative peak excursions of a signal to a defined voltage by adding a variable positive or negative DC voltage to it. The clamper does not restrict the peak-to-peak excursion of the signal (clipping); it moves the whole signal up or down so as to place its peaks at the reference level.
A multistage amplifier is an electronic amplifier consisting of two or more single-stage amplifiers connected together. In this context, a single stage is an amplifier containing only a single transistor or other active device. The most common reason for using multiple stages is to increase the gain of the amplifier in applications where the input signal is very small, for instance in radio receivers. In these applications a single stage has insufficient gain by itself. In some designs it is possible to obtain more desirable values of other parameters such as input resistance and output resistance.
In electronics, biasing is the setting of DC operating conditions of an electronic component that processes time-varying signals. Many electronic devices, such as diodes, transistors and vacuum tubes, whose function is processing time-varying (AC) signals, also require a steady (DC) current or voltage at their terminals to operate correctly. This current or voltage is called bias. The AC signal applied to them is superposed on this DC bias current or voltage.
In electronics, a plate detector is a vacuum tube circuit in which an amplifying tube having a control grid is operated in a non-linear region of its grid voltage versus plate current transfer characteristic, usually near plate current cutoff, to demodulate amplitude modulated carrier signal. This differs from the grid leak detector, which utilizes the non-linearity of the grid voltage versus grid current characteristic for demodulation. It also differs from the diode detector, which is a two-terminal device.
Tube sound is the characteristic sound associated with a vacuum tube amplifier, a vacuum tube-based audio amplifier. At first, the concept of tube sound did not exist, because practically all electronic amplification of audio signals was done with vacuum tubes and other comparable methods were not known or used. After introduction of solid state amplifiers, tube sound appeared as the logical complement of transistor sound, which had some negative connotations due to crossover distortion in early transistor amplifiers. However, solid state amplifiers have been developed to be flawless and the sound is later regarded neutral compared to tube amplifiers. Thus the tube sound now means 'euphonic distortion.' The audible significance of tube amplification on audio signals is a subject of continuing debate among audio enthusiasts.
This glossary of electrical and electronics engineering is a list of definitions of terms and concepts related specifically to electrical engineering and electronics engineering. For terms related to engineering in general, see Glossary of engineering.
In electronics, power amplifier classes are letter symbols applied to different power amplifier types. The class gives a broad indication of an amplifier's characteristics and performance. The classes are related to the time period that the active amplifier device is passing current, expressed as a fraction of the period of a signal waveform applied to the input. A class A amplifier is conducting through all the period of the signal; Class B only for one-half the input period, class C for much less than half the input period. A Class D amplifier operates its output device in a switching manner; the fraction of the time that the device is conducting is adjusted so a pulse-width modulation output is obtained from the stage.
The Blackmer gain cell is an audio frequency voltage-controlled amplifier (VCA) circuit with an exponential control law. It was invented and patented by David E. Blackmer between 1970 and 1973. The four-transistor core of the original Blackmer cell contains two complementary bipolar current mirrors that perform log-antilog operations on input voltages in a push-pull, alternating fashion. Earlier log-antilog modulators using the fundamental exponential characteristic of a p–n junction were unipolar; Blackmer's application of push-pull signal processing allowed modulation of bipolar voltages and bidirectional currents.
The dc-balanced or dc-free codes, as they are often called, have a long history and their application is certainly not confined to recording practice.
