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Explore the people, places, events, achievements, struggles and stories that shaped our journey.
Movements, leaders, victories and the continuing fight for equality.
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Meet the people whose lives, choices and achievements shaped the journey.
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Moments that changed communities, movements, institutions and the nation.
Wilmington, North Carolina once had a thriving Black middle class and an elected interracial government. In 1898 white supremacists used violence to overthrow that government, kill Black residents and drive many others from the city.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
In electronics, power amplifier classes are letter symbols applied to different power amplifier types. The class gives a broad indication of an amplifier's efficiency, linearity and other characteristics. Broadly, as you go through the alphabet, the amplifiers become more efficient but less linear, and the reduced linearity is dealt with through other means. The first classes, A, AB, B, and C, 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. This metric is known as conduction angle ( θ {\displaystyle \theta } ). A class-A amplifier is conducting through the entire period of the signal ( θ = 360 ∘ {\displaystyle \theta =360^{\circ }} ), class-B only for one-half the input period ( θ = 180 ∘ {\displaystyle \theta =180^{\circ }} ), and class-C for much less than half the input period ( θ ≪ 180 ∘ {\displaystyle \theta \ll 180^{\circ }} ). Class AB is between class A and class B ( 360 ∘ > θ > 180 ∘ {\displaystyle 360^{\circ }>\theta >180^{\circ }} ); Class-D and E amplifiers operate their output device in a switching manner; the fraction of the time that the device is conducting may be adjusted so a pulse-width modulation output (or other frequency based modulation) can be obtained from the stage. Additional letter classes are defined for special-purpose amplifiers, with additional active elements, power supply improvements, or output tuning; sometimes a new letter symbol is also used by a manufacturer to promote its proprietary design. By December 2010, classes AB and D dominated nearly all of the audio amplifier market with the former being favored in portable music players, home audio and cell phone owing to lower cost of class-AB chips. In the illustrations below, a bipolar junction transistor is shown as the amplifying device. However, the same attributes are found with MOSFETs or vacuum tubes.
A class-D amplifier, or switching amplifier, is an electronic amplifier in which the amplifying devices (transistors, usually MOSFETs) operate as electronic switches, and not as linear gain devices as in other amplifier classes. Class-D amplifiers operate by rapidly switching back and forth between the power supply rails, using pulse-width modulation, pulse-density modulation, or related techniques to produce a pulse train output. Little energy is dissipated in the amplifying transistors because they are always either fully on or fully off, so efficiency can exceed 90%. A low-pass filter is typically used to attenuate the pulse train output high-frequency content to reduce quiescent current and electromagnetic interference, resulting in the final analog output current and voltage to be sent to the speaker.
A linear amplifier is an electronic circuit whose output is proportional to its input, but capable of delivering more power into a load. The term usually refers to a type of radio-frequency (RF) power amplifier, some of which have output power measured in kilowatts, and are used in amateur radio. Other types of linear amplifier are used in audio and laboratory equipment. Linearity refers to the ability of the amplifier to produce signals that are accurate copies of the input. A linear amplifier responds to different frequency components independently, and tends not to generate harmonic distortion or intermodulation distortion. No amplifier can provide perfect linearity however, because the amplifying devices—transistors or vacuum tubes—follow nonlinear transfer function and rely on circuitry techniques to reduce those effects. There are a number of amplifier classes providing various trade-offs between implementation cost, efficiency, and signal accuracy.
An OCL amplifier (output capacitor-less amplifier) is any audio amplifier with direct-coupled capacitorless output. Typically, OCL amplifiers can be any of several amplifier classes, and typically have a push–pull output stage. Advantages of OCL amplifiers over capacitor-coupled amplifiers include Avoiding the cost and bulk of an output capacitor Better immunity to motorboat oscillation Greater output power at very low frequencies and DC Disadvantages of OCL amplifiers include Larger power dissipation and passing DC through the load, in the minority of designs with poorly controlled DC bias point Increased sensitivity of the output DC bias point to process variations, although the last disadvantage is less important for older bipolar processes.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Joe Louis.
