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Some hobbyists who prefer class-A amplifiers also prefer the use of thermionic valve (tube) designs instead of transistors, for several reasons: Single-ended and triode class-A amplifiers Class A power amplifiers are also used in some 'boutique' guitar amplifiers due to their unique tonal quality and for reproducing vintage tones. 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.

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Because the output devices are in full operation at all times (unlike a class A/B amplifier), they will not have as long a life unless the amplifier is specifically over-designed to take this into account, adding to the cost of maintaining or designing the amplifier.Ĭlass-A power amplifier designs have largely been superseded by more efficient designs, though their simplicity makes them popular with some hobbyists.

For high-power amplifiers this means very large and expensive power supplies and heat sinks.

For every watt delivered to the load, the amplifier itself, at best, uses an extra watt. If high output power is needed from a class-A circuit, the power supply and accompanying heat becomes significant. Inefficiency comes from the standing current, which 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. In a power amplifier, this not only wastes power and limits operation with batteries, but increases operating costs and requires higher-rated output devices. A maximum theoretical efficiency of 25% is obtainable using usual configurations, but 50% is the maximum for a transformer or inductively coupled configuration. Best for low signal levels of radio receivers due to low distortion.ĭisadvantage of class-A amplifiers.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.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 amplifying element is biased so the device is always conducting, the quiescent (small-signal) collector current (for transistors drain current for FETs or anode/plate current for vacuum tubes) is close to the most linear portion of its transconductance curve.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 output), each to handle one half of the waveform whereas class A can use a single device ( single-ended).This, however, incurs higher signal distortion. 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 ). A class-A amplifier is distinguished by the output stage devices being biased for class A operation.

The active element remains conducting all of the time.Īmplifying devices operating in class A conduct over the entire range of the input cycle. In a class-A amplifier, 100% of the input signal is used (conduction angle Θ = 360°). However, the same attributes are found with MOSFETs or vacuum tubes. In the illustrations below, a bipolar junction transistor is shown as the amplifying device. The angle of flow is closely related to the amplifier power efficiency. If it is on for only half of each cycle, the angle is 180°. If the device is always on, the conducting angle is 360°. The image of the conduction angle derives from amplifying a sinusoidal signal. The classes are based on the proportion of each input cycle (conduction angle) during which an amplifying device passes current. Power amplifier circuits (output stages) are classified as A, B, AB and C for linear designs-and class D and E for switching designs.

1.4 Suffix numbers for vacuum tube amplifiers.1.1.3 Single-ended and triode class-A amplifiers.1.1.2 Disadvantage of class-A amplifiers.

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