What is Class B Amplifier : Working & Its Applications

In electronics, an amplifier circuit is most commonly used in audio devices to strengthen the input signal. There are different types of amplifiers available which are classified based on different considerations like a number of stages, input signals, output, biasing conditions, frequency range, transistor configuration, and coupling methods. Amplifiers are classified based on operation modes like class A amplifier, class B amplifier, class C amplifier, class D, and class AB. Here, classes are letter symbols that are applied to different power amplifiers to indicate the performance and characteristics. This article discusses an overview of a class b amplifier and its working.

What is Class B Amplifier?

Class B amplifier can be defined as, whenever the collector current (Ic) supplies simply throughout the positive half cycle of the input signal, then this is called class B amplifier. This is is one kind of power amplifier that uses two complementary types of transistors (PNP &NPN) to conduct the half cycles of the i/p signal. So the conduction of this amplifier is 180 degrees. When one of the complementary transistors is positively biased, another transistor will be turned off, so it will conduct the positive signal.

Class B Amplifier Circuit
Class B Amplifier Circuit

Similarly, when the transistor is negative biased then it will conduct a negative signal so the positive transistor will be turned off and the negative transistor will be turned ON. These alternate two complementary transistors cause the crossover distortion in the output signal. But, this alternate switching generates low heat & increases efficiency to 78% so that it can be used in battery-operated devices.

Class B Amplifier Efficiency

The class B amplifier efficiency can be measured using the following equation:

% Efficiency = Po(ac)/Pi(dc) x 100 %

= [pi/4] x [VL(p)/VCC] x 100 %

In this amplifier, the peak voltage is larger, the circuit efficiency is higher up to a maximum value once VL(p)=VCC, so this maximum efficiency is expressed as;

Maximum efficiency = [pi/4] x 100% = 78.5%

Transformer Coupled Class B Amplifier Circuit

The class B power amplifier’s efficiency is higher as compared to class A because, in class B, there is no DC base bias current because its quiescent current (IQ) is zero so that the DC power is very small. In a Class-B amplifier, the transistors are biased to cutoff, so that there is no power dissipation of transistor when there is no i/p signal. So this gives great efficiency to Transformer Coupled Class B Amplifier than the Class-A circuit. The circuit diagram of the transformer-coupled class b amplifier is shown below.

Transformer Coupled Class B Push-Pull Amplifier
Transformer Coupled Class B Push-Pull Amplifier

This circuit can be configured by using two identical transistors like T1 & T2. The base terminals of these two transistors are simply connected to the center-tapped i/p transformer (Tr1) whereas the emitter terminals are shorted and the collector terminals are connected to VCC using a Tr2 transformer. The arrangement of this class-b push-pull amplifier circuit can be done similarly to the class A push-pull amplifier apart from the biasing resistors in class-A, biased transistors are used in class-B at the cut-off region.

Class B Amplifier Working

From the above circuit diagram, we can conclude that both the transformers like TR1 & TR2 are center-tapped. At the input, no signal is applied then both the transistors will be in the cut-off region, and thus no flow of current throughout the collector terminal. When no current is used from VCC, then there is no waste of power.

Once the input signal is applied to the circuit, then it is supplied to the Tr1 transformer then it divides the signal into two separate signals. These two signals are 180 degrees out of phase through each other.
These signals are directly given to the two T1 & T2 transistors. For the upper half cycle, the T1 transistor’s base terminal will become positive & current flows throughout the collector.

Simultaneously, the T2 transistor has a lower (-Ve) half cycle, which moves the T2 transistor into the cutoff region & thus there is no flow of current through the collector terminal. The output waveform is generated like the following. So for both the upper (+ve) and lower (-ve) cycles, every transistor will conduct alternately. The Tr3 output transformer serves to connect the two currents generating an almost exact o/p waveform.

Class B Amplifier Waveform
Class B Amplifier Waveform

Cross Over Distortion

Whenever the transistors start conduction just after the amplitude of the input signal has increased above 0.7V, then the input signal regions where the amplitude is below 0.7V will be not there with the output signal, known as cross-over distortion. The cross-over distortion waveform is shown below. So in this diagram, you can notice that the input waveform regions which are below 0.7V are not there within the o/p waveform.

Crossover Distortion
Crossover Distortion

Class B Power Amplifier Solved Problems

Example1: Calculate the input power, o/p power & the efficiency for the class B amplifier which provides a peak signal of 20 V to a 16ohms speaker & 30V power supply.

The i/p power is provided by

Pi (dc) = (2/π) Vcc Ip

The load current of peak collector can be found from

IL(p) = VL(p)/RL = 20V/16 Ohms = 1.25A

The i/p power is Pi (dc) = 2/π 30(1.25) 23.9 W

The o/p power is provided by

Po(ac) = V2L(p)/2RL = (20V)2/2 (16) =400/32 = 12.5w

The efficiency is %η = Po(ac)/Pin(ac) X 100 = (12.5W/23.9W)X100% = 52.3%

Example2: For class B amplifier, voltage supply VCC is 30V using a supply & driving a load is 16Ω, calculate the i/p power, o/p power & the efficiency.

The maximum o/p power can be provided by

Max Po(ac) = V^2CC/2RL = (30V)^2/2(16 Ohms) = 28.12W

The max i/p power drawn by using the supply is

Max Pi(ac) = 2V^2CC/ π RL = (30V)^2/ π 2(16 Ohms) = 35.81W

The efficiency can be calculated as

The max efficiency %η = Po(ac)/Pin(ac) X 100 = (28.125W/35.81W)X100% = 78.54%

The max dissipated power by every transistor is

Max PQ = Max P2Q/2 = 0.5 (2V^2CC/π^2 RL) = 0.5 (2(30V)^2/ π^216 ohm) = 5.7W

Advantages

The advantages of a class B amplifier include the following.

  • As compared to class A amplifier, this is more efficient
  • The cost and weight can be reduced simply when there is no requirement of center-tapped transformers.
  • Equivalent & opposite input signal voltages are not necessary.
  • Less standing bias current
  • Negligible power utilization without signal
  • Used for very powerful outputs.
  • High efficiency as compared to class A
  • The push-pull system within this amplifier removes even order harmonics within AC o/p signal.
  • There are no DC components within the output

Disadvantages

The disadvantages of class B amplifiers include the following.

  • Crossover distortion can be created in these types of amplifiers
  • Distortion is more as compared to class A
  • The current supply will be changed through signal so stabilized supply is necessary.
  • It is very difficult to obtain two complementary transistors with the same characteristics.
  • We need both positive & negative voltage supplies.
  • No need for self-bias.
  • When transformers are connected to this amplifier circuit then size & cost can be increased.
  • Transformer coupling can cause vibrations within the output, so it will affect the frequency response.

Applications

The applications of class B amplifiers include the following.

  • These are mainly used in the low-cost design.
  • These amplifiers are very significant as compared to the class-A amplifiers.
  • This type of amplifier suffers mainly from distortion if the level of signal is low.
  • These are mainly used in two complementary transistors like bipolar and FET.

Thus, this is all about an overview of the class b power amplifier and its working. These are preferred highly over Class A amplifier designs in high-power applications like PA systems & audio power amplifiers. Similar to a class-A amplifier, one method is used to boost the current gain of the Class B push-pull amplifier by using Darlington transistors pairs in place of single transistors within its output circuitry. Here is a question for you, what are Class b power amplifier types?

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