1. EMT 112/4 ANALOGUE ELECTRONICS 1 Power Amplifiers Syllabus
Power amplifier classification; class A, class B, class AB and class C, amplifier distortion, transistor power dissipation, thermal management.

2. Power Transistor – BJT & MOSFET
POWER AMPLIFIERS
Part I
Power Transistor – BJT & MOSFET

3. POWER TRANSISTOR Transistor limitations Maximum rated current,
Maximum rated voltage,
Maximum rated power.
The maximum rated power is related to the maximum allowable temperature of the transistor.

4. POWER TRANSISTOR
– BJT
Large-area devices – the geometry and doping concentration are different from those of small-signal transistors
Examples of BJT rating:
Parameter
Small-signal BJT
(2N2222A)
Power BJT
(2N3055)
(2N6078)
VCE (max) (V) 40 60
250
IC (max) (A)
0.8 15 7
PD (max) (W)
1.2
115 45 
35 – 100
5 – 20
12 – 70
fT (MHz)
300 1

5. POWER TRANSISTOR
– BJT
Current gain depends on IC and is smaller in power BJT.
The maximum rated collector current, IC(rated) may be related to the following:
maximum current that the wires connecting the semiconductor to the external terminals can handle
The collector current at which the gain falls below a minimum specified value
current which leads to maximum power dissipation when the transistor is in saturation.

6. POWER TRANSISTOR
– BJT
Typical dc beta characteristics ( hFE versus IC) for 2N3055

7. POWER TRANSISTOR – BJT The maximum voltage limitation:
Avalanche breakdown in the reverse-biased base-collector junction (involves gain and breakdown at the p-n junction)
Second breakdown – nonuniformities in current density which inreases temperature in local regions in semiconductor.

8. POWER TRANSISTOR – BJT Avalanche Breakdown (Figure 1)
In Figure 1, the breakdown voltage when the base terminal is open-circuited (IB=0) is VCEO, approx. 130V (Figure 1).
All the curves tend to merge to the same collector-emitter voltage, denoted as VCE(sus) once breakdown has occurred.
VCE(sus) is the voltage necessary to sustain the transistor in breakdown.
In Figure 1, VCE(sus) is approx. 115V

9. IC–VCE characteristics showing breakdown effect
POWER TRANSISTOR
– BJT
IC–VCE characteristics showing breakdown effect
Figure 1

10. POWER TRANSISTOR
– BJT
The total instantaneous power dissipation in transistor
The second term is usually small, hence;
The average power over ONE CYCLE of the signal:

11. POWER TRANSISTOR
– BJT
The average power dissipated in a BJT must be kept below a specified maximum value to ensure that the temperature of the device does not exceed the maximum allowable value.
If collector current and collector-emitter voltage are dc quantities, the maximum rated power, PT
The power handling ability of a BJT is limited by two factors, i.e. junction temperature, TJ and second breakdown. Safe Operating Area (SOA) must be observed, i.e. do not exceed BJT power dissipation.

12. POWER TRANSISTOR
– BJT
The safe operating area (SOA) is bounded by IC(max); VCE(sus) and maximum rated power curve, PT and the transistor’s second breakdown characteristics curve (Figure 2)
SOA of a BJT (linear scale)
Figure 2

13. POWER TRANSISTOR
– BJT
SOA of a BJT (log scale)
Figure 3

14. POWER TRANSISTOR – BJT EXAMPLE 8.1
Determine the required ratings (current, voltage and power) of the BJT.

15. POWER TRANSISTOR – BJT EXAMPLE 8.1 – Solution
For the maximum collector current;
For the maximum collector-emitter voltage;

16. POWER TRANSISTOR – BJT EXAMPLE 8.1 – Solution
The load line equation is;
The load line must lie within the SOA
The transistor power dissipation;

17. POWER TRANSISTOR – BJT EXAMPLE 8.1 – Solution
The maximum power occurs when
i.e. when
Differentiating
or when
At this point;
and;

18. POWER TRANSISTOR – BJT EXAMPLE 8.1 – Solution
Thus the transistor ratings are;
In practice, to find a suitable transistor for a given application, safety factors are normally used. The transistor with will be required.

19. POWER TRANSISTOR – BJT Physical structure;
Large emitter area to handle large current densities
Narrow emitter width to minimize parasitic base resistance
May include small resistors (ballast resistor) in emitter leg to help maintain equal currents in each B–E junction.
Top view
Cross-sectional view