1. 5. Design of Power Amplifiers
5.1 Classification of power amplifiers
5.2 Class A Amplifier design
5.3 Class B Amplifier design (OTL)
5.4 Class B Amplifier design (OCL)
5.5 Class B Amplifier Maximum Dissipation

2. 5.1 Classification of power amplifiers
Amplifier classes produce different power efficiency
Power efficiency is very important for high power circuits. Greater efficiency reduces power dissipation, and produce higher output power.
In electronics power amplifiers class “C” operation (used in radio frequency transmitters) is the most power efficient and class “A” operation (used in preamplifier of various applications) is the least power efficient
Class A
class “A” operation has a Q point collector current at the center between cutoff current and saturation current. Thus when the input vary the Base current, Collector current varies continuously between cutoff current and saturation for the whole cycle (360 degree) of the output signal.
R1 and R2 create collector current at the center between cutoff and saturation

3. Class B
class “B” operation has a Q point collector current at the cutoff current. Thus when the input vary the Base current, Collector current varies continuously between cutoff current and saturation for the half cycle (180 degree) of the output signal.
R1 to ground create collector current at the cutoff
Class AB
class “AB” operation has a Q point collector current above the cutoff current. Thus when the input vary the Base current, Collector current varies continuously between the above cutoff current and saturation for more than a half cycle (more than 180 degree) of the output signal. Class “AB” eliminates the crossover distortion usually occur in class”B” operations.
D1 creates collector current above but near the cutoff
Note: Both class “B” operation and class “AB” operation requires two BJTs or two FETs to produce one full cycle of the output waveform

4. Class C
class “C” operation has a Q point collector current at below cutoff current. Thus when the input vary the Base current, Collector current varies only for a small part above cutoff current for less than half cycle (<180 degree) of the output signal. Later this output can be converted into full sine wave by using LC tuned (resonant) circuit.
Negative VBB creates collector current away and below the cutoff

5. 5.2 Class A Power Amplifier Design
Inductively coupled class “A” power amplifier analysis
Radio Frequency Choke (RFC)
Should specify RB and RE

6. Transformer coupled class “A” power amplifier analysis
Should specify RB or Rin

7. Summary of Design Equations
Class A Power Amplifier
Inductively coupled Class A
Transformer coupled Class A

8. Example
(a) Draw an Inductively coupled power amplifier and Design the VCC and BJT maximum dissipation rating , R1, R2 to deliver 1W to a 16W load using RE = 2W., RB = 1kW . What is the Vo(pp) and what is the required Vin (pp)?
(b) Find efficiency and dissipation of the amplifier at an output power of 0.25W.
(b)

9. Example
(a) Draw Transformer coupled power amplifier and design the VCC and BJT maximum dissipation rating , R1, R2 to deliver 1W to a 4W load using n=2, b=50, RB = 300W . What is the Vo(pp) and what is the required Vin (pp)?
(b) Find efficiency and dissipation of the amplifier at an output power of 0.25W.
(b)

10. 5.3 Class B Amplifier design (OTL)
Output Transformerless (OTL) class “B” power amplifier
In class B, ICQ=0 for Q1 and Q2
In class B amplifier, R1 is negligible and VBE for Q1 and Q2 is created by input signal voltage
To design R1 in OTL class B, assume VBE=0.5V=IR1xR1

11. Output Transformer Less (OTL) class “AB” power amplifier
In class AB, small ICQ flows due to VBE from D1 and D2
In class AB amplifier, D1 and D2 create VBE for Q1 and Q2
To design ICQ in OTL class AB, assume ICQ=ID1=ID2=(VCC-1.4)/2R2

12. Analysis of OTL power amplifier

13. OTL Power and efficiency and dissipation at full output power

14. 5.4 Class B Amplifier design (OCL)
Output Capacitor Less (OCL) class “B” power amplifier
In OCL amplifier note that each BJT has VCE=VCC
In class B, ICQ=0 for Q1 and Q2
0V dc
In class B amplifier, R1 is negligible and VBE for Q1 and Q2 is created by input signal voltage
To design R1 in OCL class B, assume VBE=0.5V=IR1xR1

15. Output Capacitor Less (OCL) class “AB” power amplifier
In class AB amplifier, D1 and D2 create VBE for Q1 and Q2
In class AB, small ICQ flows due to VBE from D1 and D2
In OCL amplifier note that each BJT has VCE=VCC
To design ICQ in OCL class AB, assume ICQ=ID1=ID2=(2VCC-1.4)/2R2

16. Analysis of OCL power amplifier

17. OCL Power and efficiency and dissipation at full output power

18. 5.5 Class B Maximum dissipation of BJT
OTL power amplifier
For two BJT

19. OCL power amplifier
OCL power amplifier
For two BJT

20. For OCL use 2VCC in place of VCC and they will be cancelled later
Efficiency at the time of maximum dissipation (OTL and OCL)
Maximum dissipation to maximum output power ratio of the amplifier (OTL-OCL)

21. Summary of Design Equations
OTL- Class B and AB Power Amplifier
To design R1 in OTL class B, assume VBE=0.5V=IR1xR1
To design ICQ in OTL class AB, assume ICQ=ID1=ID2=(VCC-1.4)/2R2

22. OTL-Equations at Full Output power
OTL-Equations at Maximum power dissipation

23. Summary of Design Equations
OCL- Class B and AB Power Amplifier
To design R1 in OCL class B, assume VBE=0.5V=IR1xR1
To design ICQ in OCL class AB, assume ICQ=ID1=ID2=(2VCC-1.4)/2R2

24. OCL-Equations at Full Output power
OCL-Equations at Maximum power dissipation

25. Example
Draw OTL class AB power amplifier and design the required VCC , BJT maximum dissipation rating to deliver 100W to a 4W load. What is the BJT dissipation at the full output power?

26. Example
Draw OCL class AB power amplifier and design the required VCC , BJT maximum dissipation rating of each BJT to deliver 100W to a 4W load. What is the BJT dissipation at the full output power?
Note that if output power is the same, BJT ratings are the same in both OTL and OCL
The only difference between OTL and OCL is that in OCL, VCC is double and ICp is half

27. Example
Draw OCL class B power amplifier and design biasing resistances R1 and R2 and the required VCC , to deliver 100W to a 4W load. Assume b = 200. What is the maximum dissipation rating of each BJT and BJT dissipation at the full (maximum) output power? What is the input impedance?
IR1= (2VCC-1)/2R2 = 59/57.7 = 1.02A
R1 = 0.5/1.02 = 0.49W