EKT 204 ANALOGUE ELECTRONICS CIRCUITS 1

1. EKT 204 ANALOGUE ELECTRONICS CIRCUITS 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 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:

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. 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 byIC(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