History[ edit ] Before the commercial introduction of transistors in the s, electronic amplifiers used vacuum tubes known in Great Britain as "valves". By the s, solid state transistorized amplification had become more common because of its smaller size, lighter weight, lower heat production, and improved reliability. Tube amplifiers have retained a loyal following amongst some audiophiles and musicians.
The classes are based on the proportion of each input cycle conduction angle during which an amplifying device passes current. The angle of flow is closely related to the amplifier power efficiency.
In the illustrations below, a bipolar junction transistor is shown as the amplifying device. The active element remains conducting  all of the time. Amplifying devices operating in class A conduct over the entire range of the input cycle.
A class-A amplifier is distinguished by the output stage devices being biased for class A operation. Subclass A2 is sometimes used to refer to vacuum-tube class-A stages that drive the grid slightly positive on signal peaks for slightly more power than normal class A A1; where the grid is always negative  .
This, however, incurs higher signal distortion[ citation needed ]. Advantages of class-A amplifiers[ edit ] Class-A designs can be simpler than other classes insofar as class -AB and -B designs require two connected devices in the circuit push—pull outputeach to handle one half of the waveform whereas class A can use a single device single-ended.
Because the device is never 'off' there is no "turn on" time, no problems with charge storage, and generally better high frequency performance and feedback loop stability and usually fewer high-order harmonics. The point where the device comes closest to being 'off' is not at 'zero signal', so the problems of crossover distortion associated with class-AB and -B designs is avoided.
Best for low signal levels of radio receivers due to low distortion.
Disadvantage of class-A amplifiers[ edit ] Class-A amplifiers are inefficient. Inefficiency comes from the standing current that must be roughly half the maximum output current, and a large part of the power supply voltage is present across the output device at low signal levels.
If high output power is needed from a class-A circuit, the power supply and accompanying heat becomes significant. For every watt delivered to the loadthe amplifier itself, at best, uses an extra watt.
For high power amplifiers this means very large and expensive power supplies and heat sinks. Class-A power amplifier designs have largely been superseded by more efficient designs, though their simplicity makes them popular with some hobbyists.
There is a market for expensive high fidelity class-A amps considered a "cult item" among audiophiles  mainly for their absence of crossover distortion and reduced odd-harmonic and high-order harmonic distortion.
Class A power amps are also used in some "boutique" guitar amplifiers due to their unique tonal quality and for reproducing vintage tones. Single-ended and triode class-A amplifiers[ edit ] Some hobbyists who prefer class-A amplifiers also prefer the use of thermionic valve tube designs instead of transistors, for several reasons: Single-ended output stages have an asymmetrical transfer functionmeaning that even-order harmonics in the created distortion tend to not cancel out as they do in push—pull output stages.
For tubes, or FETsmost distortion is second-order harmonics, from the square law transfer characteristicwhich to some produces a "warmer" and more pleasant sound. Historically, valve amplifiers were often used as a class-A power amplifier simply because valves are large and expensive; many class-A designs use only a single device.
Transistors are much less expensive than tubes so more elaborate designs that use more parts are still less expensive to manufacture than tube designs. A classic application for a pair of class-A devices is the long-tailed pairwhich is exceptionally linear, and forms the basis of many more complex circuits, including many audio amplifiers and almost all op-amps.
Class-A amplifiers may be used in output stages of op-amps  although the accuracy of the bias in low cost op-amps such as the may result in class A or class AB or class B performance, varying from device to device or with temperature. They are sometimes used as medium-power, low-efficiency, and high-cost audio power amplifiers.
The power consumption is unrelated to the output power. At idle no inputthe power consumption is essentially the same as at high output volume. The result is low efficiency and high heat dissipation. Class B[ edit ] Ideal class-B push—pull amplifier.
In practice, distortion occurs near the crossover point. In a class-B amplifier, the active device conducts for degrees of the cycle. These are used in linear amplifiers, so called because the radio frequency output power is proportional to the square of the input excitation voltage.
This characteristic prevents distortion of amplitude-modulated or frequency-modulated signals passing through the amplifier. Efficiency is much improved over class-A amplifiers.
Complementary devices are each used for amplifying the opposite halves of the input signal, which is then recombined at the output. This is called crossover distortion. An improvement is to bias the devices so they are not completely off when they are not in use.
This approach is called class AB operation. In a class-AB amplifier, the conduction angle is intermediate between class A and B; the two active elements conduct more than half of the time.srmvision.com Contents • Class D Amplifier Introduction • The latest Digital Audio MOSFET, DirectFET® MOSFET • Designing Dead-time and Overload Protection with Digital Audio Gate Driver IC • Design Example Theory of class D operation, Points of design Designing with built-in dead-time generation.
Type or paste a DOI name into the text box. Click Go. Your browser will take you to a Web page (URL) associated with that DOI name. Send questions or comments to doi. C, class D and class AB. This thesis mainly deals with class B power amplifiers.
In Class B amplifier, the positive and negative halves of the signal are dealt with by different. HARMONIC DISTORTION TEST DEVELOPMENT FOR CLASS-D AMPLIFIER by KIRT T.
WILLIAMS, B.S. A THESIS IN ELECTRICAL ENGINEERING Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfilhnent of the Requirements for the Degree of.
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