Class B and AB Amplifiers, Physics tutorial

Introduction:

Class B amplifier is one whose operating point is at the extreme end of characteristic, so that either quiescent current or quiescent voltage is almost zero and if the sinusoidal input voltage is utilized, amplification of the Class B amplifier occurs only for 50% of cycle. This signifies that amplifying device is switched off half of time.

Up to 78.5% efficiency can be achieved by the Class B amplifier with main disadvantage that class B amplifier shows higher distortion than equivalent Class A amplifier.

Class AB amplifier is the amplifier which operates between two extremes defined for Class A and class B amplifiers signal that signifies that it conducts more than 50% but less than 100% of signal cycle.

Class B amplifier

Class B amplifiers only amplify half of input wave cycle, therefore creating the large amount of distortion, but their efficiency is greatly enhanced and is much better than class A. Class B has maximum theoretical efficiency of 78.5% (i.e., π/4). This is due to amplifying element is switched off altogether half of the time, and so can't dissipate power.

The single element class B amplifier is hardly ever found in practice but good example of it is its application as the loudspeaker driver in early IBM Personal Computers where it is utilized to generate beeps. Active element of Class B amplifiers is active for 50% of signal cycle within its linear range while it is more or less turned off for other half. In most class B, there are two output devices each of which conducts alternately in the push-pull arrangement for exactly 180° of input signal.

Class B amplifiers are subject to crossover distortion if transition from one active element to other is not perfect, as when two complementary transistors that may be a PNP and NPN pair in case of Bipolar junction Transistor application; are joined as two emitter followers with their base and emitter terminals in common, requiring base voltage to slew across region where both devices are turned off.

More frequently than not a class B stage is preceded by the class a amplifier. Now, class A amplifiers are not very power efficient that describes use of class A amplifier to process small signal while class B amplifier stage with higher power efficiency acts to boost processed signal.

Class B stage is thus to enhance full power efficiency of previous Class A amplifier by decreasing wasted power in form of heat and, it is possible to design power amplifier circuit with two transistors in its output stage producing what is commonly expressed as push-pull type amplifier configuration. Push-pull amplifiers utilize two complementary or matching transistors, one being NPN-type and other being a PNP-type with both power transistors receiving same input signal together which is equal in magnitude, but in opposite phase to each other.

This results in one transistor only amplifying one half or 180o of input waveform cycle while other transistor amplifies other half or remaining 180o of input waveform cycle with resulting two-halves being put back together again at output terminal. Then conduction angle for this kind of amplifier circuit is only 180o or 50% of input signal. This pushing and pulling effect of alternating half cycles by transistors provides this kind of circuit its name, but such kinds of audio amplifier circuit are more usually called as Class B Amplifier.

1090_Class B Push-Pull Transformer Amplifier.jpg

Circuit above shows a Class B Amplifier circuit which uses the balanced centre-tapped input transformer, that splits incoming waveform signal in two equal halves and that are 180o out of phase with each other. Another centre-tapped transformer on output is utilized to recombine two signals giving increased power to load. Transistors utilized for this kind of transformer push-pull amplifier circuit are both NPN transistors with their emitter terminals joined together. Here, load current is shared between two power transistor devices as it reduces in one device and increases in other throughout signal cycle decreasing output voltage and current to zero. Result is that both halves of output waveform now swing from zero to twice quiescent current thereby decreasing dissipation. This has effect of almost doubling efficiency of amplifier to around 70%.

Assuming that no input signal is present, and then each transistor carries normal quiescent collector current, value of that is determined by base bias that is at cut-off point. If transformer is correctly centre tapped, then two collector currents will flow in opposite directions (ideal condition) and there will be no magnetization of transformer core, therefore minimizing possibility of distortion. When the signal is present across secondary of driver transformer T1, transistor base inputs are in anti-phase to each other as shown, therefore if TR1 base goes positive driving transistor in heavy conduction, its collector current will increase but at same time base current of TR2 will go negative further in cut-off and collector current of this transistor decreases by the equal amount and vice versa. Therefore negative halves are amplified by one transistor and positive halves by other transistor giving push-pull effect. Unlike DC condition, such AC currents are additive resulting in two output half-cycles being joined to reform sine-wave in output transformers primary winding that then seems across load. Characteristic curve below displays input and output signal excursions.

1228_Class B output Characteristic Curve.jpg

Class B Amplifier operation has zero DC bias as transistors are biased at cut-off, thus each transistor only conducts when input signal is greater than base-emitter voltage. At zero input there is zero output and no power is being consumed. This then signifies that actual Q-point of the Class B amplifier is on Vce part of load line.

One of the main drawbacks of Class B amplifier circuit above is that it utilizes balanced centre-tapped transformers in its design, making it expensive to build. Though, there is another type of Class B amplifier known as Complementary-Symmetry Class B Amplifier which doesn't utilize transformers in its design thus; it is referred to as transformerless utilizing instead complementary or matching pairs of power transistors. As transformers are not required this makes amplifier circuit much smaller for same amount of output, also there are no stray magnetic fields or non-linear transformer distortion to effect quality of output signal.

The main drawback of class B type push-pull amplifiers is that they suffer from crossover distortion. Simple method to eliminate crossover distortion in Class B amplifier is to add two small voltage sources to circuit to bias both transistors at point slightly above their cut-off point, as it is not practical to add extra voltage sources to amplifier circuit PN-junctions are utilized to give additional bias in form of silicon diodes. Pre-biasing a class B amplifier in this way transforms it in a class AB amplifier.

Class AB amplifier

Class AB amplifiers are almost same as Class B amplifiers in that they have two driven transistors. Though, Class AB amplifiers vary from Class B amplifiers in that they have small idle current flowing from positive supply to negative supply even when there is no input signal. This idle current somewhat increases power consumption. This idle current also corrects approximately all the nonlinearity related with crossover distortion. These amplifiers are known as Class AB rather than Class A as with large signals, they behave like Class B amplifiers, but with small signals, they behave like Class A amplifiers.

The base-emitter voltage is needed to be greater than 0.7v for silicon bipolar transistor to begin conducting, so if you were to replace two voltage divider biasing resistors joined to base terminals of transistors with two silicon Diodes, biasing voltage applied to transistors would now be equivalent to forward voltage drop of diode. Such two diodes are usually known as Biasing Diodes or Compensating Diodes and are selected to match characteristics of matching transistors. Circuit exhibits diode biasing.

1708_Diode Biasing in Class AB Amplifier.jpg

 The Class AB Amplifier circuit is compromise between Class A and Class B configurations. This very small diode biasing voltage causes both transistors to somewhat conduct even when no input signal is present. The input signal waveform will cause transistors to operate usually in their active region thereby eliminating any crossover distortion present in pure Class B amplifier designs. The small collector current will flow when there is no input signal but it is much less than that for Class A amplifier configuration. This signifies that transistor will be ON for more than half a cycle of waveform but much less than full cycle giving conduction angle of between 180 to 360o or 50 to 100% of input signal depending on amount of extra biasing used. Amount of diode biasing voltage present at base terminal of transistor can be increased in multiples by adding extra diodes in series.

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