Class B power amplifier

Class B power amplifier.

Class B amplifier is a type of power amplifier where the active device (transistor) conducts only for one half cycle of the input signal. That means the conduction angle is 180° for a Class B amplifier. Since the active device is switched off for half the input cycle, the active device dissipates less power and hence the efficiency is improved. Theoretical maximum efficiency of Class B power amplifier is 78.5%. The schematic of a single ended Class B amplifier and input , output waveforms are shown in the figure below.

class b amplifier output waveformFrom the above circuit it is clear that the base of the transistor Q1 is not biased and the negative half cycle of the input waveform is missing in the output. Even though it improves the power efficiency, it creates a lot of distortion. Only half the information present in the input will be available in the output and that is a bad thing.Single ended Class B amplifiers are not used in present day practical audio amplifier application and they can be found only in some earlier gadgets. Another place where you can find them is the RF power amplifiers where the distortion is not a matter of major concern. Anyway, Class C amplifiers are more often used in RF power amplifier applications. Output characteristics of a  single ended Class B power amplifier is shown in the figure below.

class b power amplifier output characteristics

One way to realize a practical Class B amplifier is to use a pair of active devices (transistors)  arranged in push-pull mode where one transistor conducts one half cycle and the other transistor conducts the other half cycle. The output from both transistors are then combined together to get a scaled replica of the input. But there is a snag – there must be some way to split the input wave form to feed the individual transistors and there must be some way to put together the output of the individual transistors. Transformer coupling is solution for this problem and such amplifiers are called transformer coupled Class B amplifiers.

Transformer coupled Class B amplifier.

class b power amplifier

The circuit diagram of a simple transformer coupled class B power amplifier is shown in the figure above. Transistor Q1 and Q2 are the active elements. The transformer T1 reproduces the input signal into two copies which are 180° out of phase. From the above figure you can see that the transistor Q1 amplifies the positive half of the input signal and transistor Q2 amplifies the negative half of the input signal. Current flow path of the two transistors are also depicted in the above figure. The amplified two halves are joined together by the transformer T2. If an ideal transformer is used the DC components of the collector current of each transistors will flow in opposite directions through the transformer primary and they will cancel each other. That means there is no core saturation and there will be no DC components in the output.

Since the transistors are not biased they remains OFF when there is no input signal and no current flows through the load. Each transistor starts conduction only when the amplitude of the input signal goes above the base-emitter voltage (Vbe) of the transistor which is about 0.7 V. This improves the efficiency but creates a problem called cross-over distortion.

Cross over distortion.

Since the active elements start conduction only after the input signal amplitude has risen above 0.7V, the regions of the input signal where the amplitude is less than 0.7V will be missing in the output signal and it is called cross over distortion. The schematic representation of cross-over distortion is shown in the figure below. In the figure, you can see that the regions of the input waveform which are under 0.7V are missing in the output waveform.

crossover distortion

Advantages of Class B amplifier.
  • High efficiency when compared to the Class A configurations.
  • Push-pull mechanism avoids even harmonics.
  • No DC components in the output (in ideal case).
Disadvantages of Class B amplifier.
  • The major disadvantage is the cross-over distortion.
  • Coupling transformers increases the cost and size.
  • It is difficult to find ideal transformers.
  • Transformer coupling causes hum in the output and also affects the low frequency response.
  • Transformer coupling is not practical in case of huge loads.

Class AB power amplifier.

In Class AB configuration, the active elements (transistors)  are slightly biased so that the conduction angle is slightly more than 180° but much less than 360°. The transistors conduct for more than a half cycle but much less than the full cycle. That means there will be no instant where both transistors are OFF simultaneously and thus cross-over distortion is eliminated. Class AB configuration is actually a trade-off between Class A and Class B configurations where efficiency is slightly compromised for fidelity. Class AB power amplifiers are slightly inefficient than the Class B configurations but far better in terms of distortion when compared to Class A configurations. Since the active devices are slightly pre-biased there will be a small amount of collector current flowing and this is the reason behind the slightly reduced efficiency. Typical efficiency of a well designed class AB power amplifier is around 70%. The output characteristics of a single ended Class AB power amplifier is shown in the figure below.
class ab amplifier output characteristicsFrom the above figure it is clear that the Q-point is not positioned at cut-off unlike the Class B characteristics and there will be a small amount of collector current flowing at zero input. As a result, some part of the negative going half cycle will be also reproduced at the output. The amount of negative going half cycle reproduced at the output depends on the amount of pre-bias given to the transistor.
Practical Class AB power amplifier.
Single ended Class AB configurations are not practical just because a major portion of one half cycle will be missing at the output. Just like the Class B configuration, push-pull mechanism is essential for realizing practical Class AB power amplifiers. Circuit diagram of a typical Class AB push-pull amplifier is shown in the figure below.
transformer less class ab push pull amplifier
The exact technical designation of the above circuit is ” Complementary- symmetry Class AB power amplifier”. The active elements used in this circuit (transistor Q1 and Q2) are complementary symmetric and it means the the transistor are similar in all aspects except one is NPN and the other is PNP. The use of this complementary pair eliminates the bulky transformer for phase splitting the input signal for driving the individual transistors. The NPN transistor alone will conduct the positive half cycle and PNP transistor alone will conduct the negative half cycle.
Slight pre-biasing is given to the transistors using the network comprising of resistors R1, R2 and biasing diodes D1 and D2. As you know, an NPN transistor will start conducting when its base voltage is above the base emitter voltage (Vbe~0.7V)  and a PNP transistor will start conducting when its base  voltage is below the base emitter voltage (Vbe~ -0.7V).  A forward biased diode will drop approximately 0.7V across it and the biasing diodes used here will keep the transistor slightly forward biased even if there is no input signal.
One important thing while choosing the biasing diodes (also called compensating diodes) is that their characteristics must match as close as possible to the transistors. Resistors R1 and R2 are actually used for forward biasing the diodes so that they drop 0.7V across it for biasing the individual transistors. C1 and C2 are input DC decoupling capacitors. Input and output waveforms of a typical class AB push pull amplifier is shown in the figure below.
class ab push pull amplifier input and output waveforms
Since both the transistors are slightly conducting at zero input, no information in the input signal  is lost at the output during the zero-crossing of the input signal and thus cross-over distortion is completely eliminated at a cost of slightly reduced efficiency.
Advantages of Class AB power amplifier.
  • No cross over distortion.
  • No need for the bulky coupling transformers.
  • No hum in the output.
Disadvantages of Class AB power amplifier.
  • Efficiency is slightly less when compared to Class B configuration.
  • There will be some DC components in the output as the load is directly coupled.
  • Capacitive coupling can eliminate DC components but it is not practical in case of heavy loads.

 

 

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