1. MALVINO
Electronic
PRINCIPLES
SIXTH EDITION

2. Power Amplifiers
Chapter 11

3. 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 C means that current flows for less than 180o of the ac cycle.
Class D amplifiers switch on and off and spend no time in the linear mode.

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

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

6. dc ac
Capacitive
Direct ac ac
Tuned
transformer
Transformer

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

8. 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 higher slope.

9. 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)

10. 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

11. 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.

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

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

14. Class B 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.
The transformers can be eliminated.

15. A
Class C amplifier
Amax 2p LC 1
fr =
+VCC C L f fr RL RB
vin

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

17. 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

18. 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

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

20. 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

21. 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