1. LARGE SIGNAL AMPLIFIERS CLASS A , B AND C POWER AMPLIFIERS
S.PREMA AP/ECE

2. Large signal amplifier or Power Amplifier
The stage which develops and feeds sufficient power to the load like loudspeaker, servomotor, handling the large signals is called Large signal Amplifier or Power amplifier.
Applications: Radio receivers, Industrial control systems, tape players, TV receivers etc..

3. 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

4. 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

5. Class A Ic Vce B Q A 10mA 6mA Ib = 60μA 10mA 4mA Ib = 50μA 8mA

6. 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

7. Class B Ic Vce Q 10mA Ib = 60μA 10mA Ib = 50μA 8mA Ib = 40μA 6mA

8. 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

9. Class C Ic Vce Q` 10mA Ib = 60μA 10mA Ib = 50μA 8mA Ib = 40μA 6mA

10. Class A
Class B
Class C

11. 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

12. 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

13. 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

14. Class A Audio Amplifier
Vcc
Rb1 Rc
Rb1
270 K
5.6 K
270 K
Rb2 Re Ce
Rb2 Re Ce

15. 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

16. Class A Ic Vce B Q A Ib = 60μA 10mA Ib = 50μA 8mA Ib = 40μA 6mA

17. To find base current and collector current
Apply KVL
IBQ = VCC-07 / RB
ICQ = ßibq
VCEQ = VCC-ICQRL
DC POWER OUTPUT : Pdc = VCC.ICQ
AC POWER OUTPUT : Vm = Vpp / 2 = (Vmax –Vmin )/2
Im = Ipp/2 = (Imax-Imin)/2
RL = Vm/Im
Pac = Vrms Irms.
Efficiency = (Vmax –Vmin)(Imax –Imin) / 8Vcc IcQ X 100

18. Power Dissipation
The Difference between the dc power input Pdc and the ac power delivered to the load Pac
Pd = Pdc –Pac
Dc power input without ac input signal is the maximum power dissipation
Pd max = VCC ICQ

19. Vcc T1
TR1
TR2 T3 T2
Class B Push-Pull

20. 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

21. 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

22. 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

23. 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

24. Working of Push-Pull Circuit
Vcc T1 T2 T3
TR2
TR1
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

25. Working of Push-Pull Circuit
Vcc T1 T2 T3
TR2
TR1
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

26. 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

27. Complementary Symmetry Amplifier
Vcc T1 T2

28. 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

29. 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

30. 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

31. Cross – over distortion
Vbe
-Vbe

32. 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

33. Class AB amplifier
Vcc R1 T1 D1 D2 T2 R2

34. 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

35. 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

36. 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

37. 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

38. 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