COMMUNICATION SYSTEMS IT DESCRIBES ABOUT COMPONENTS
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Power amplifiers are classified based on their operating point as Class A, B, or C. Class A amplifiers have the operating point at the midpoint of the load line, resulting in distortionless output but lower efficiency. Class B amplifiers have the operating point at cutoff, resulting in higher efficiency but distorted output. Class C amplifiers operate beyond cutoff, with severe distortion but ability to handle large signals. Push-pull and complementary symmetry amplifier configurations are used to reduce distortion in Class B amplifiers by using two transistors that conduct alternately. Class AB amplifiers add biasing to reduce crossover distortion during signal transitions.
COMMUNICATION SYSTEMS IT DESCRIBES ABOUT COMPONENTS
- 1. Power Amplifiers Unit – 4.1
- 2. Classification of Power Amplifiers Power amplifiers are classified based on the Q point If the operating point is chosen at the middle of the load line, it is called Class A amplifier If the operating point is chosen at the cut-off point it is called Class B amplifier If the operating point is chosen beyond the cut-off point it is called Class C amplifier It conducts for 3600
- 3. Class A amplifier The Q point is chosen at the middle of load line This will give equal swing on either direction Both halves of the input comes at the output Hence Class A will give (amplitude) distortionless output It can handle only small signals Its efficiency is less
- 4. Ic Vce Ib = 60μA Ib = 50μA Ib = 20μA Ib = 30μA Ib = 40μA 10mA 2mA 4mA 6mA 8mA 24 V 0 V Q A B Class A
- 5. Class B amplifier The Q point is chosen at the cut-off point This will give swing only on one direction Only one half of the input comes at the output Hence Class B will give (amplitude) distorted output It can handle large signals Its efficiency is high It conducts for 1800
- 6. Ic Vce Ib = 60μA Ib = 50μA Ib = 20μA Ib = 30μA Ib = 40μA 10mA 2mA 4mA 6mA 8mA 24 V 0 V Q Class B
- 7. Class C amplifier The Q point is chosen at the beyond the cut-off point This will give only a partial swing in one direction Only a portion of the input comes at the output Hence Class C will give (amplitude) severely distorted output It can handle large signals It conducts for less than 1800
- 8. Ic Vce Ib = 60μA Ib = 50μA Ib = 20μA Ib = 30μA Ib = 40μA 10mA 2mA 4mA 6mA 8mA 24 V 0 V Q` Class C
- 10. Distortionless amplifier Out of the 3 amplifiers, Class C is unsuitable as the distortion is very heavy Class A is the best, as it gives distortionless output But Class A cannot handle large signals as required by the Power Amplifier Though Class B gives heavy distortion, it gives out one half of the signal perfectly And Class B can handle large signals
- 11. Class A Audio Amplifier As we have seen out of the 3 classifications, Class A is the best, as it does not give any distortion Among the configurations, we know that CE is the best as it gives maximum power gain A CE amplifier will have high output impedance Unfortunately for an audio amplifier, the output device is the speaker which has a low impedance
- 12. Impedance Matching The speaker impedance is typically about 4 Ω Hence there is a mismatch between the high Zo of the amplifier and the low impedance of the speaker This will result in loss of gain This can be avoided by connecting a transformer at the output stage The primary winding will match the high Zo of the amplifier while the secondary will match the low impedance of the speaker
- 13. Class A Audio Amplifier 270 K 5.6 K Re Rb1 Rb2 Ce Rc Vcc 270 K Re Rb1 Rb2 Ce
- 14. Drawback The drawback of this circuit is that it cannot handle large signals In a Class A amplifier, the operating point is chosen around the middle of the load line If the signal exceeds the cut-off point, the output current stops and any signal with a lower amplitude will not come at the output Similarly, if the signal exceeds the saturation point, the output current cannot increase any further, even if the input signal increases
- 15. Ic Vce Ib = 60μA Ib = 50μA Ib = 20μA Ib = 30μA Ib = 40μA 10mA 2mA 4mA 6mA 8mA 24 V 0 V Q A B Class A
- 16. Class B Push-Pull Amplifier To avoid this we can use Class B which has a greater signal handling capacity But Class B will give only one half of the signal Hence we can use 2 Class B amplifiers One for one half and one for the other half This type of amplifier is called Push-Pull Amplifier
- 18. Push-Pull Circuit TR1 and TR2 are output transistors connected back to back, with their emitters grounded The output transformer TR1 couples the push-pull output to the speaker In the Push-Pull arrangement T1 conducts for one half of the signal & T2 conducts for the other half Both are biased in Class B and each gives one half of the signal & the combined output is coupled to the speaker
- 19. Push-Pull Circuit The Driver Transformer TR2 gives 2 out of phase signals During one half, the +ve half forward biases T1 while the –ve half reverse biases T2 Thus when T1 conducts, T2 is cut-off & vice- versa This way both the transistors conduct alternately to give the full signal output
- 20. Class D Amplifier During the +ve half cycle Q1 gets Forward Bias and it conducts During the -ve half cycle Q2 gets Forward Bias and it conducts Thus both the transistors conduct alternately The amplifier works for 3600 No distortion 100% efficiency
- 21. During the first half T1 conducts Ic flows from the centre-tapping through T1 to ground This half is coupled to the speaker through TR1 Working of Push-Pull Circuit Vcc T1 T2 T 3 TR2 TR1
- 22. During the second half T2 conducts Ic flows from the centre-tapping through T2 to ground This half is coupled to the speaker through TR1 Working of Push-Pull Circuit Vcc T1 T2 T 3 TR2 TR1
- 23. Drawbacks Though this circuit functions well it has a few drawbacks Transformer coupling affects the quality of output Phase shifting circuit is a must Both these drawbacks can be avoided if we use one pair of PNP and NPN transistors at the output
- 24. T1 T2 Vcc Complementary Symmetry Amplifier
- 25. Complementary Symmetry Amplifier This circuit uses one NPN transistor & one PNP transistor at the output stage During the +ve half, T1(NPN) base gets forward bias & it conducts while T2 (PNP) gets reverse biased and does not conduct This gives one half of the signal at the speaker coupled to the emitter
- 26. Complementary Symmetry Amplifier During the other half, T2 gets forward bias and conducts while T1 gets reverse biased and does not conduct Thus T1 & T2 conduct alternately giving a distortionless output This circuit does not require a phase shifter
- 27. Cross – over distortion Class B Push-Pull amplifier has one limitation As the phase of the signal changes from +ve to –ve (or vice-versa) one transistor stops conducting while the other begins conducting But the transistor cannot conduct instantaneously as it requires a minimum Vbe before it starts conducting Thus as the signal crosses over zero, a distortion occurs This is called Cross over distortion
- 28. Cross – over distortion Vbe -Vbe
- 29. Class AB amplifier This circuit overcomes cross-over distortion Biasing is done such that even if there is no input signal, a small current keeps the output transistor conducting This circuit uses 2 diodes whose characteristics matches with that of the BE junction of the output transistors Biasing resistors R1 & R2 are also identical values
- 30. T1 T2 Vcc Class AB amplifier R1 R2 D1 D2
- 31. Symmetrical components Since R1 & D1 are identical to R2 & D2, the diode junction as well as the output point will be at half the supply voltage Because of symmetry both T1 & T2 will conduct equally Even when there is no input signal, there will be a current Icq = (I/2 Vcc – 0.6) / R1 This will keep the output transistors conducting
- 32. Elimination of cross-over distortion Normally, during cross-over there will not be any output till the non-conducting transistor gets the minimum Vbe This causes distortion This has been eliminated here, since the 0.6 V across the diodes keep the transistors on and gives a continuous output signal without producing cross-over distortion
- 33. Thermal stability In addition, the two diodes also provide thermal stability They prevent the output transistors going to Thermal Run Away When the output current is high, heat dissipation is more The increase in temperature produces more charge carrier in the BE junction of T1 & T2
- 34. This increases Ib & hence Ic This in turn increases the power dissipation & hence the heat This chain goes on till too much current flows and destroys the transistors This is called Thermal Run Away This is arrested by the diodes in the output circuit
- 35. When the charge carriers increase in the B-E junction of T1 & T2, a similar increase takes place in D1 & D2, due to matching characteristics This increase in the diode current, produces more drop across R1 & R2 and brings down the forward bias at the base of T1 & T2 Thus the 2 diodes prevent cross-over distortion as well as provide thermal stability
- 36. End of Unit – 4.1