chptr 4 - power amp (i).pdf
TRANSCRIPT
EKT 204
ANALOGUE ELECTRONICS CIRCUITS 1
Power Amplifiers
SyllabusPower 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)
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 0.8 1
POWER TRANSISTOR
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.
– BJTPOWER TRANSISTOR
Typical dc beta
characteristics
( hFE versus IC)
for 2N3055
– BJTPOWER 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.
– BJTPOWER TRANSISTOR
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
– BJTPOWER TRANSISTOR
– BJTPOWER TRANSISTOR
IC–VCE characteristics
showing breakdown
effect
Figure 1
– BJTPOWER TRANSISTOR
BBECCEQ ivivp
The second term is usually small, hence;
CCEQ ivp
The average power over ONE CYCLE of the signal:
T
CCEQ dtivT
P0
1
The total instantaneous power dissipation in transistor
– BJTPOWER 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, PT
CCET IVP
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.
– BJTPOWER TRANSISTOR
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
– BJTPOWER TRANSISTOR
SOA of a BJT
(log scale)
Figure 3
– BJTPOWER TRANSISTOR
EXAMPLE 8.1
Determine the required ratings
(current, voltage and power) of
the BJT.
– BJTPOWER TRANSISTOR
EXAMPLE 8.1 – Solution
For the maximum
collector current;0CEV
A 38
24max
L
CCC
R
VI
For the maximum collector-
emitter voltage;0CI
V 24max CCCE VV
– BJTPOWER TRANSISTOR
EXAMPLE 8.1 – Solution
The load line equation
is;
The load line must lie
within the SOA
LCCCCE RIVV
The transistor power
dissipation;
LCCCCCLCCCCCET RIIVIRIVIVP 2
– BJTPOWER TRANSISTOR
EXAMPLE 8.1 – Solution
The maximum power occurs when
02 LCCC RIV
0C
T
dI
dP
i.e. when
or when A 5.1CI
At this point; V 12 LCCCCE RIVV
and; W18 CCET IVP
Differentiating
– BJTPOWER TRANSISTOR
EXAMPLE 8.1 – Solution
Thus the transistor ratings are;
W18
V 24
A 3
max
max
T
CE
C
P
V
I
In practice, to find a suitable transistor for a given
application, safety factors are normally used. The
transistor with
will be required.
W18 V, 24 A, 3 maxmax TCEC PVI
– BJTPOWER 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.
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