1. BY-Udale Rupesh -130370109112 Zala Kishan -130370109125 Vivek Vaghasiya-130370109115 GUIDE :- ANKIT BRAHMBHATT DIVYESH MANGROLIYA

2. POWER AMPLIFIERS usually used small scale amplifiers, which are also known as “Voltage amplifier” are not so capable of supplying a large power to load. This is because these amplifiers are used primarily for voltage amplification. Whenever the load demands a large power, we have to use specially designed amplifiers called “Power amplifier”. These amplifier converts low power signal at their input to high power signals.

3. Continue… The input signal to the large signal amplifier is a high voltage low current signal obtained from the chain of voltage amplifiers. Power is equal to the product of voltage and current. Hence high voltage, low current signals at the input corresponds to a low power signal. The power amplifier will increase the current sourcing and sinking capability. So, at it’s output we get a high voltage, high current signal that means a high power signal. Thus the power amplifier is basically a current amplifier.

4. Application of power amplifier 1. Radio receivers 2. Public address (P.A) system 3. CD/cassette players 4. TV receivers.

5. Classification of power amplifiers Depending on the position of the Q-point or operating point on the load line, the power amplifiers are classified into four categories: 1. Class A amplifier 2. Class-B amplifier 3. Class-C amplifier 4. Class-AB amplifier This classification has been done on the basis of position of Q-point on the load line.

6. Sr no.Type of power amplifierPosition of Q-point 1.Class-AAt the center of load line 2.Class-BIn the cut off region 3.Class-ABJust above of cut off 4.Class-CBelow the cut off

7. Ref:080327HKNEE3110 Power Amplifier (Class A)7 Efficiency Ratings The maximum theoretical efficiency ratings of class-A, B, and C amplifiers are: AmplifierMaximum Theoretical Efficiency,  max Class A25% Class B78.5% Class C99%

8. Class-A power amplifier A power amplifier is referred to as a class A power amplifier if the transistor used for amplification conducts for the full cycle duration of the input ac signal. The Q-point is adjusted exactly at the center of the load line. Due to this output signal is obtained for the full cycle of the ac input i.e. for 360 ˚. As we apply the ac signal to the base of the transistor, the base current changes sinusoidal above and below the quiescent base current I BQ as shown in the figure.

9. Circuit For Class-A

10. Transformer Coupled Class-A amplifier

11. Advantages of class-A amplifier 1. Simple construction 2. Distortion less output voltage DISADVANTAGES of class-A amplifier 1. Very low efficiency (25% or 50%) 2. Large power dissipation in the power transistor.

12. Class-B power amplifier A power amplifier is referred to as a class B amplifier if the output signal is obtained only for one half cycle period of the input signal. The transistor consists only in one half cycle of the input and the collector current therefore flows for only 180 ˚. For this the Q-point is adjusted to be at cutoff. i.e. on the x axis as in the figure. Thus in the absence of ac input signal the transistor remains in the off state.

13. Observation for class-B operation 1. As we apply a sinusoidal input signal to the base of the transistor, the B-E junction of the transistor is forward biased only during the positive half cycle of the input and base current starts flowing. 2. The transistors remain in the active region, only for the positive half cycle, the transistor remains in the cut off state. Thus collector current flows for only 180 ˚ of the input signal.

14. Continue… 1. As seen in the figure only half cycle is obtained at the output. Therefore the output voltage waveform is distorted, because it is no more a sine wave. 2. The distortion can be eliminated by using two transistor in alternate half cycle of input signal to produce the complete cycle of signal at the output. Each transistor conducts only 180 ˚.

15. As the transistor remains off for the complete negative half cycle, the power dissipation in the transistor reduced as compare to that in the class A amplifier. Therefore the efficiency of class B amplifier is higher than that of the class A power amplifier. The maximum efficiency of the class-B configuration can be 78.5% which is much higher than of class A amplifier.

16. Types of class-B power amplifier The class-B amplifier using two transistors can be of two types: 1. Class-B push pull amplifier 2. Class-B complimentary symmetry amplifier

17. Crossover distortion The waveform distortion near the zero crossing point is observed in class B power amplifiers. This happens because of transistor are biased at cut-off. The crossover distortion can be eliminated by using the class-AB type power amplifier.

18. Class-B Push Pull Amplifier The push pull class-B amplifier uses two center tapped transformers. The transformer on the input side is known as the driver transformer and the other connected on the load is called as the output transformer. Both the transistor are of identical characteristics. The input signal Vs is applied to the primary winding of the input transformer. Both the transistor can be replaced by identical p-n-p transistors, only thing is that then we must use a negative supply voltage for them.

19. Complimentary Symmetry class-B Amplifier One p-n-p and the other n-p-n transistor are being used. The circuit does not use the driver and output transformers. Both the transistor operate in the emitter follower configuration for the purpose of impedance matching. The low load can be matched with the low output resistance of emitter follower configuration.

20. The two transistor should have matched characteristics. This circuit requires a dual polarity supply(+- Vcc). Both the transistor are biased at cutoff. Cc are the two identical coupling capacitors which couple the ac input signal Vs to the base of both the transistors.

21. Input and output voltage waveform for complimentary symmetry class- B amplifier. Complentary symmetry class B amplifier.

22. Advantages of Complimentary Symmetry Amplifier Impedance matching with the load is possible as both the transistor operate in the emitter follower configuration. Transformer are not being used. This makes the circuit less expensive and less bulky. The practical value of %n max is higher than that of class B push pull amplifier as the transformer are not used. Even harmonics are automatically balanced out. Hence only odd harmonics are present. Disadvantages of Complimentry symmetry amplifier:-

23. Disadvantages of Complimentary symmetry amplifier A dual polarity supply (+- Vcc ) is essential It is necessary to have transistor Q1 and Q2 with matched characteristics, otherwise output waveform will be distorted. Crossover distortion will be present in output in the present in the output.

24. Advantages of class-B amplifier: 1. Higher efficiency(78.5%) 2. Zero power dissipation under quiescent condition 3. Impedance matching with load is possible. 4. Second harmonic component gets automatically cancelled. Disadvantages of class-B amplifier: 1. Cross over distortion is present in the output waveform. 2. Efficiency is not so high.

25. Class-AB amplifier A power amplifier is referred to as a class AB amplifier if the output signal is obtained for more than 180 degree but less than degree of the input signal. That means the power transistor connected in class AB power amplifier will conduct for more than 180 and less than 360 degree of the ac input. For this the Q-point is positioned slightely above the cutoff region.

26. Observation from class-AB amplifier In order to obtain the signal for more than 180 and less than 360 degree of the ac input signal, the Q- point is positioned slightly above the X-axis but below the mid-point of the load line. The output signal is distorted. However this distortion can be eliminated by using two transistors which can conduct in the alternate half cycle of the input. The class AB operation is helpful in eliminating the crossover distortion.

27. Continue… The Q point is neither in middle of the load line like class A operation nor on the X-axis like classB. It is in between the two. Therefore the name class AB operation. The transistor conducts for more than 180(Class-B) but less than 360 (class-A), therefore the power dissipation in the transistor is less than that in class A operation and more than that in the class B operation. Hence efficiency of class AB is higher than class A but less than class B amplifier. The transistor conducts for the complete positive half cycle of the input signal.

28. Advantages of class-AB amplifier 1. The biggest advantage of class AB amplifier is eliminating of the crossover distortion. 2. Hence this configuration is preferred in all the audio systems, radio, TV receivers etc.

29. Class-AB Push Pull Amplifier The class B push -pull amplifier by connecting a resistor R and diode D in the circuit. The DC voltage developed across the diode Disconnected to the bases of both the transistors through the secondary winding of the input transformer. This voltage acts as a dc bias for the transistors because it is equal to the cut-in voltage and they will conduct for complete half cycle period of the input to eliminate the cross-over distortion. Due to this type of biasing, the Q point is shifted slightly above the X-axis. However for all the practical purpose, class AB amplifier is referred to as class B simplifier only.

30. Shifting of Q-point Class-AB push-pull amplifier A voltage of 1.4V between the base will forward bias both transistor Complimentary class-AB amplifier

31. Complimentary Symmetry Class-AB Amplifier In the complimentary circuit the voltage drop across an diode like the push-pull circuit is not sufficient to forward bias the B-E junctions of the transistors. In fact a voltage of 1.4V must be appear between their bases to ensure that both the BE junctions are forward biased. The circuit arrangements of complimentary symmetry class AB circuit as in the figure. The values of resistor R1,R2,R3 are adjusted in such a way that a constant voltage of 1.4V appears across resistor R2. The operation of complimentary symmetry class AB is identical to the complimentary symmetry class B amplifier.

32. Thank you