Post on 20-Jan-2018
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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