Download Lecture 14: Power Amplifiers - BJT

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.
POWER AMPLIFIERS
Part I
Power Transistor –
BJT & MOSFET
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.
POWER TRANSISTOR – BJT
Large-area devices – the geometry and doping
concentration are different from those of small-signal
transistors
Examples of BJT rating:
Parameter
VCE (max) (V)
IC (max) (A)
PD (max) (W)

fT (MHz)
Small-signal
BJT
(2N2222A)
40
0.8
1.2
35 – 100
300
Power BJT Power BJT
(2N3055)
(2N6078)
60
250
15
115
7
45
5 – 20
0.8
12 – 70
1
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:
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.
POWER TRANSISTOR – BJT
Typical dc beta
characteristics
( hFE versus IC)
for 2N3055
POWER TRANSISTOR – BJT
The maximum voltage limitation:
• Avalanche breakdown in the reverse-biased basecollector junction (involves gain and breakdown at the
p-n junction)
• Second breakdown – nonuniformities in current
density which inreases temperature in local regions in
semiconductor.
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 collectoremitter 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
POWER TRANSISTOR – BJT
IC–VCE characteristics
showing breakdown
effect
Figure 1
POWER TRANSISTOR – BJT
The total instantaneous power dissipation in transistor
pQ  vCE iC  vBE iB
The second term is usually small, hence;
pQ  vCE iC
The average power over ONE CYCLE of the signal:
1
PQ 
T

T
0
vCE iC dt
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
PT  VCE I C
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.
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
POWER TRANSISTOR – BJT
SOA of a BJT
(log scale)
Figure 3
POWER TRANSISTOR – BJT
EXAMPLE 8.1
Determine the required ratings
(current, voltage and power) of
the BJT.
POWER TRANSISTOR – BJT
EXAMPLE 8.1 – Solution
For VCE  0 the maximum
collector current;
VCC 24
I C max  

3A
RL
8
For I C  0 the maximum collectoremitter voltage;
VCE max   VCC  24 V
POWER TRANSISTOR – BJT
EXAMPLE 8.1 – Solution
The load line equation
is;
VCE  VCC  I C RL
The load line must lie
within the SOA
The transistor power
dissipation;
PT  VCE I C  VCC  I C RL I C  VCC I C  I C2 RL
POWER TRANSISTOR – BJT
EXAMPLE 8.1 – Solution
dPT
0
The maximum power occurs when
dI C
i.e. when VCC  2 I C RL  0
Differentiating
or when I C  1.5 A
At this point; VCE  VCC  I C RL  12 V
and;
PT  VCE I C  18 W
POWER TRANSISTOR – BJT
EXAMPLE 8.1 – Solution
Thus the transistor ratings are;
I C max   3 A
VCE max   24 V
PT  18 W
In practice, to find a suitable transistor for a given
application, safety factors are normally used. The
transistor with I C max   3 A, VCE max   24 V, PT  18 W
will be required.
POWER TRANSISTOR – BJT
Physical structure;
• Large emitter area to
handle large current
densities
• Narrow emitter width to
minimize parasitic base
resistance
Crosssectional
view
• May include small
resistors (ballast resistor)
in emitter leg to help
maintain equal currents
in each B–E junction.
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