1. Electronic PRINCIPLES
MALVINO & BATES
Electronic
PRINCIPLES
SEVENTH EDITION

2. Chapter 12
Power Amplifiers

3. Topics Covered in Chapter 12
Amplifier terms
Two load lines
Class A operation
Class B operation
Class B push-pull emitter follower

4. Topics Covered in Chapter 12 (Continued)
Biasing Class B/AB amplifiers
Class B/AB driver
Class C operation
Class C formulas
Transistor power rating

5. Classes of operation
Class A means that the transistor collector current flows for 360o of the ac cycle.
Class B amplifiers have collector current for 180o of the ac cycle.
Class AB amplifiers have collector current between 180° and 360°
Class C means that current flows for less than 180o of the ac cycle.

6. IC IC A B t t IC IC
ISAT C D t t

7. AB
Class AB is biased near but not at cutoff

8. Types of coupling Direct (dc amplifier) Capacitive (ac amplifier)
Untuned transformer (wideband ac amplifier)
Tuned transformer (narrowband ac amplifier)

9. dc ac
Capacitive
Direct ac ac
Tuned
transformer
Transformer

10. Load lines
Every amplifier has a dc and an ac equivalent and therefore has two load lines
The Q point location for a small signal amplifier is not critical
The Q point for a large signal amplifier must be at the center of the ac load line

11. This amplifier has two load lines.
+VCC RE R2 RC R1 RL
vin
vout

12. VCC
IC(sat) =
The dc load line
VCE(cutoff) = VCC
RC + RE
100 mA 14 12
80 mA 10
60 mA
IC in mA 8
40 mA 6 Q 4
20 mA 2
0 mA 2 4 6 8 10 12 14 16 18
VCE in Volts
The ac load line has a steeper slope.

13. AC load line

14. Large Signals
When the Q point is at the center of the dc load line, the signal cannot use all of the ac load line without clipping.
MPP < VCC (max. peak-to-peak output)
MPP = 2MP
MP = ICQrc or VCEQ (whichever is smaller)

15. Class A operation
Power gain equals ac output power divided by ac input power
The power rating of a transistor must be greater than the quiescent power dissipation
Efficiency of the amplifier equals ac output power divided by dc input power times 100 percent

16. 100 mA 14 12
80 mA 10
60 mA
IC in mA 8
40 mA Q 6 4
20 mA 2
0 mA 2 4 6 8 10 12 14 16 18
VCE in Volts
pout =
vpp2
8RL
pout(max) =
MPP2
8RL

17. Efficiency The dc power supplied to an amplifier is Pdc = VCCIdc
Efficiency = h = pout/Pdc x 100%
The maximum efficiency for Class A amplifiers with a dc collector resistance and a separate load resistance is 25%.
Class A is usually not acceptable when watts of power are required.

18. Class B Collector current flows for 180° of the ac cycle
Q point is located at cutoff on both the dc and ac load lines
An advantage is lower current drain and higher stage efficiency

19. Push-pull Class B amplifierQ1 T2 T1
+VCC
vin Q2
On the positive half-cycle of the input, Q1 is on.

20. Q1 T2 T1
+VCC
vin Q2
On the negative half-cycle of the input, Q2 is on.

21. Class B push-pull operation
Each transistor conducts for half a cycle.
There is no bias so each transistor is at cutoff with no input signal.
The maximum efficiency is 78.5 percent.
Transformers are bulky and expensive and can be eliminated by using a push-pull emitter follower.

22. Class B push-pull emitter follower
Complementary npn and pnp transistors are used

23. Class AB The transistor is biased slightly above cutoff
The conduction angle is slightly greater than 180°
Crossover distortion is reduced

24. Biasing Class B/AB amplifiers
Crossover distortion is reduced by setting the Q point slightly above cutoff in Class AB
With voltage-divider bias (VDB), the Q point is unstable and may result in thermal runaway
Diode bias is preferred

25. Class B/AB driver Direct rather than capacitive coupling can be used
The driver’s collector current establishes the quiescent current through complementary diodes

26. Class AB Q point

27. Class C operation Most Class C amplifiers are tuned RF amplifiers
The input signal is negatively clamped which produces narrow collector current pulses
The tank circuit is tuned to the fundamental frequency

28. AV
Class C amplifier
AVmax 2p LC 1
fr =
+VCC C L f fr RL RB
vin

29. Class C amplifier dc and ac load linesIC
dc load line
VCC rc Q
VCE
VCC

30. Negative clamping at the base causes the
collector current to flow in brief pulses. IC
+VCC C L t
-2 VP
+ VP
- VP RL RB

31. Class C formulas BW = f2 - f1 = fr/Q QL = XL/RS (inductor only)
RP = QLXL (equiv. parallel resistance)
rc = RP RL (ac resistance)
Q = rc/XL (overall circuit)
PD = MPP2/40rc

32. Duty cycle and conduction anglef
360o W T
D = =
f = 180o
f = 90o

33. The effects of conduction angle in a Class C amplifierPD f
The effects of conduction
angle in a Class C amplifier
MPP2
40rc
180o
Idc f h
180o
100% f
180o

34. Transistor power rating
The power rating of a transistor decreases as temperature increases
Data sheets list derating factors or show graphs
Heat sinks can remove heat more rapidly and produce a higher power rating

35. Derating factor Power devices generate heat
PD(max) rating good up to 25 oC
Decrease in rating = DP
Derating factor = D (W/oC)
TA = ambient temperature (oC)
DP = D(TA - 25 oC)
PD(safe) = PD(max) - DP