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.

Part I Power Transistor – BJT & MOSFET POWER AMPLIFIERS

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.

– 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) Power BJT (2N6078) V CE (max) (V) I C (max) (A) P D (max) (W)  35 – 1005 – 2012 – 70 f T (MHz) POWER TRANSISTOR

Current gain depends on I C and is smaller in power BJT. The maximum rated collector current, I C(rated) may be related to the following: 1.maximum current that the wires connecting the semiconductor to the external terminals can handle 2.The collector current at which the gain falls below a minimum specified value 3.current which leads to maximum power dissipation when the transistor is in saturation. – BJT POWER TRANSISTOR

Typical dc beta characteristics ( h FE versus I C ) for 2N3055 – BJT POWER TRANSISTOR

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. – BJT POWER TRANSISTOR

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

– BJT POWER TRANSISTOR I C – V CE characteristics showing breakdown effect Figure 1

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

– BJT POWER TRANSISTOR 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, P T The power handling ability of a BJT is limited by two factors, i.e. junction temperature, T J and second breakdown. Safe Operating Area (SOA) must be observed, i.e. do not exceed BJT power dissipation.

– BJT POWER TRANSISTOR The safe operating area (SOA) is bounded by I C(max) ; V CE(sus) and maximum rated power curve, P T and the transistor’s second breakdown characteristics curve (Figure 2) SOA of a BJT (linear scale) Figure 2

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

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

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

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

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

– BJT POWER TRANSISTOR 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.

– BJT POWER TRANSISTOR 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