ekt 204 analogue electronics circuits 1 power amplifiers syllabus power amplifier classification;...
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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.TRANSCRIPT
EKT 204ANALOGUE ELECTRONICS CIRCUITS 1
Power AmplifiersSyllabusPower amplifier classification; class A, class B, class AB and class C, amplifier distortion, transistor power dissipation, thermal management.
Part IPower 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:
ParameterSmall-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 – 70fT (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 TRANSISTORThe 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 TRANSISTOREXAMPLE 8.1
Determine the required ratings (current, voltage and power) of the BJT.
– BJTPOWER TRANSISTOREXAMPLE 8.1 – Solution
For the maximum collector current;
0CEV
A 3824
max L
CCC R
VI
For the maximum collector-emitter voltage;
0CI
V 24max CCCE VV
– BJTPOWER TRANSISTOREXAMPLE 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 TRANSISTOREXAMPLE 8.1 – Solution
The maximum power occurs when
02 LCCC RIV
0C
T
dIdP
i.e. when
or when A 5.1CI
At this point; V 12 LCCCCE RIVV
and; W18 CCET IVP
Differentiating
– BJTPOWER TRANSISTOREXAMPLE 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.
Top view
Cross-sectional view