Difference between class A, B and AB (practical uses)

Discussion in 'General Electronics Chat' started by u-will-neva-no, Apr 20, 2012.

  1. u-will-neva-no

    Thread Starter Member

    Mar 22, 2011
    230
    2
    Hey everyone, from what I have read, Class A, B and AB are all used in audio power amplifiers and output stages for operational amplifiers. I have also come across the idea of the conduction angle, (class A conducts all the time, class B conducts for half of the input signal etc) but other than efficiency, what is the difference in uses for these three classes?

    I ask because a question in a text book implies that there are different applications for A, B and AB but only see the generic idea of these three classes being used in the above situations.

    Thanks for reading!
     
  2. panic mode

    Senior Member

    Oct 10, 2011
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    class A conducts all the time (because it is biased so). for example your "idle" current is 0.5mA and peak is 0.35mA from idle. then positive peak is 0.5+0.35=0.85mA, negative peak is 0.5-0.35=0.15mA but in all of those cases current is greater than zero. so far we only mentioned biasing current but keep in mind that this translates to some serious collector current. if the hfe = 100 then 0.5mA input will mean 50mA idle current at the output. and actually the larger the amplifier, the bigger the idle current (it could be amps). this is terrible for battery powered device because even if device is doing nothing, significant current flows and drains the battery. the nice thing about class A is that there is no switchover, you get smooth response. one huge downside of class A is power consumption.

    class B uses bias that keeps device (transistor) at the edge of conduction. this means that if the AC signal comes in (think sinusoid), transistor will react only to positive part of the signal. negative would push transistor deeper in cutoff mode and no current would flow through it. so to get entire signal amplified, we need two separate stages, one for positive signals, one for negative art of the signal. then they are combined into one. in this case idle current can be zero (or very close). the problem is that there is quite noticeable step at the point where positive and negative portions of output are supposedly stitched together. so class B saves power, but does not not so well in maintaining signal quality.

    class AB is similar with B but here there is some (fairly small) bias all the time. overall this looks a lot like class B but the bias helps dramatically with the crossover distortion. It does not quite reach the perfection class A can do but - it gets close while saving power.

    then there is class C. in this class circuit is biased below the edge of conduction. signal coming in has to overcome certain threshold before transistor even starts to conduct. supposedly BJT 'edge' is at 0.6V and we set the bias at 0.2V. then we bring some signal in (think sinusoid). this input signal must be at least 0.4V just to get transistor to turn on. if the input signal is higher, transostor will be on longer because bigger portion of the sinusoid is above the 0.4V threshold.

    obviously this causes transistor to conduct only in short pulses instead of all the time (as the class A does). this means that distortion must be terrible. actually this is used quite often and - only on one side of the signal (say positive half period). there is silence during negative half period (and portion of positive one). so what good is this for? it is good for RF amplifiers. they have load that is usually tuned LC circuit. such circuits oscillate. imagine kids playground and swing. if you are parent or bigger sibling, you don't need to maintain contact with the swing all the time, it is enough to make brief push (pulse). even if you stop helping, swing will still continue to move for quite some time. in fact you can push only every second or every third time and swing will operate practically indefinitely. this is awesome news for class C amplifier designers. because transistor only conducts fraction of time, power is not wasted like in class A. more over you can push this transistor harder because of low duty cycle. this is one (of many) things that helped reduce size of RF circuits and improve energy efficiency (mobile phones for example).

    then there is even class D (digital). in this class transistors are either OFF or ON (just like the states in digital circuits). this has advantage that power dissipated by transistor is really small. when transistor is on (saturated) voltage drop is small. Since power is P=V*I, even if current I is considerable, power is low because V is low. Then when transistor is OFF, the voltage across transistor may be considerable but the current is zero and power is zero.
     
  3. u-will-neva-no

    Thread Starter Member

    Mar 22, 2011
    230
    2
    Fantastic post! Thanks!
     
  4. panic mode

    Senior Member

    Oct 10, 2011
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    you are welcome,

    as it may be hinted, different classes have different application.
    class A is used where even smallest crossover distortion is not acceptable. fortunately this restriction is usually not much of a problem because this class circuits are generally low power (opamp circuits, transducers, etc.) or maybe medium (ultimate quality audio amps).

    class B can be used where crossover distortion is not much of a issue but ultimate goal is efficiency. not much of a use in audio amps due high distortion.

    class C is normally only used in RF. ability to excite LC circuit with lower frequency is used to make frequency multipliers (2x, 3x sometimes up to 5x).

    class D is more recent, well not really but it is the more recent advancements in semiconductors (switching and processing) that make it usable.
     
    u-will-neva-no likes this.
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    A couple of further points.

    Class A amps are inherently short circuit proof.
    Some can be damaged or destroyed by open circuit at the load terminals however.

    Class B amps are subject to damage or destruction by short circuits.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Not sure where you got the idea that they are short circuit proof.:confused:

     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    I have to agree with that. It may be true to say that some class A amplifiers may have their possible range of output currents internally defined, such that no output short-circuit will provoke a damaging current increase. For instance, this may be the case for some valve (vacuum tube) type amplifiers.

    Other types of class A amplifier are made which are not inherently safe in this way. For instance, audio amplifiers may be made with designs very similar to those used for Class AB, but with the bias current raised until Class A operation is obtained over the normal range of power levels into the intended load impedance. Unless specific steps are taken to limit current into a short-circuit,such an amplifier is likely to go into a highly distorted class AB mode, with excessive peak currents. This may be very severe if heavy negative feedback is used, as the short-circuit reduces the feedback level so that the amplifier is driven much harder.
     
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