Class A Amplifiers, Physics tutorial

Introduction:

Class A amplifiers amplify over entire input cycle such that output signal is the exact magnified copy of input. Though class A amplifiers have the conduction angle of 360o, they are not proficient being no more than 50% efficiency at most. This is due to device is always conducting whether or not input signal is applied.

Class A amplifiers find application where efficiency is not primary design criteria, but linearity. Many small signal linear amplifiers are developed as class A amplifiers. Class A amplifiers are usually more linear and less complex than other kinds, but are very incompetent. There exists the subclass designated A2 and that refers to vacuum tube class A stages where grid is permitted to be driven slightly positive on signal peaks, resulting in slightly more power than in normal class A.

Class A amplifier:

Class A amplifiers give lowest distortion but are most expensive, and are least practical to apply for high power applications. Class A amplifiers waste power but generate very clean signal output. Class AB amplifiers dominate market and rival best Class A amplifiers in sound quality. They employ less power than Class A, and can be cheaper, smaller, cooler, and lighter. Class D amplifiers are even smaller than Class AB amplifiers and more competent, as they use high-speed switching rather than linear control.

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In some amplifiers, output devices are tubes. Many amplifiers use more than one transistor or tube per function in output stage to increase power. Class A refers to the output stage with bias current greater than maximum output current, so that every output transistors are always conducting current. Biggest benefit of Class A is that it is most linear, it has lowest distortion. Biggest drawback of Class A is that it is inept, it takes a very large Class A amplifier to deliver 50 watts, and that amplifier utilizes lots of electricity and gets very hot. Few high-end amplifiers are Class A, but true Class A only accounts for maybe 10% of small high-end market and none of the middle or lower-end market. Class B amplifiers have output stages that have zero idle bias current. Naturally, a Class B audio amplifier has zero bias current in the very small part of power cycle, to avoid nonlinearities. Class B amplifiers have important benefit over Class A in efficiency as they use almost no electricity with small signals.

Advantages and drawbacks of class A amplifiers:

The merits of the Class A amplifier are as follows:

  • Class A designs are simpler to design and analyze than all other classes of amplifiers; mainly as it uses the single device to control 360o conduction cycle of waveform.
  • Class A amplifiers are single ended amplifiers by using single device.
  • Amplifying element in the class A amplifier is biased so device is always conducting to some extent to make sure its operation is close to most linear portion of transconductance curve.
  • There is no warm up or turn on time as amplifying device is always on in class A amplifier. This signifies no charge storage and better frequency performance.
  • Class A amplifiers are immune to problem of crossover distortion related with class AB and B amplifiers.
  • Class A amplifiers show total absence of crossover distortion, offer decreased odd-harmonic and high-order harmonic distortion and is preferably suited for high quality audio applications.
  • Bias current for each of active elements flow in opposite directions in primary of output transformer such that they efficiently cancel each other out. This lack of static, offset direct current in output transformer signifies that core can be made smaller as it needs no air gap to prevent core saturation from static offset current.
  • Also, in single-ended transformer coupled class A amplifier output, output transformer is huge compared to the push-pull class A amplifier of same power level. Air gap needed to prevent core saturation drastically decreases primary inductance, so transformer should have larger core and more windings to get same primary inductance.

Drawbacks when the class A amplifier is transformer coupled are:

  • Need for the phase splitter stage to produce oppositely phased drive signals.
  • Even-order harmonics produced in output stage are cancelled out in push-pull output stage. This doesn't mean that push-pull amplifier produces no even order harmonics, but as even-order harmonics produced in preamplifier stages are amplified by output stage and will pass right through to output. Only those even-order harmonics produced in output stage itself are cancelled out.
  • Class A amplifiers are the most ineffective class of amplifiers boasting the theoretical maximum efficiency of 50% with inductive output coupling and only 25% with capacitive coupling.
  • In the power amplifier this inefficiency not only wastes power but also restricts battery operation in battery operated appliances. It place limitations on output devices which can be utilized and increases costs.
  • Inefficiency is not just the consequence of device always conducting current to some extent as class A amplifiers are not exclusive in that respect, but that standing current is at least half of maximum output current if distortion through clipping is to be avoided on one hand, and also that large part of power supply voltage is developed across output device at low signal levels.
  • The main disadvantage with Class A amplifiers is that for every watt delivered to load, amplifier itself will, at best, dissipate another watt and where large power output delivery is to be attained; very large and expensive power supplies and heat sinking should also be used.

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