en2042102...
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EN2042102 วงจรไฟฟ้าและอเิลก็ทรอนิกส์Circuits and Electronics
บทที ่8 การไบอสัทรานซิสเตอร์Transistor Biasing
สาขาวชิาวศิวกรรมคอมพวิเตอร์คณะวศิวกรรมศาสตร์ มหาวิทยาลยัเทคโนโลยรีาชมงคลพระนคร
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Biasing
Biasing: Applying DC voltages to a transistor in order to turn it on so that it can amplify AC signals.
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Operating Point
The DC input establishes an operating or quiescent point called the Q-point.
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The Three Operating Regions
• Active or Linear Region Operation
• Base–Emitter junction is forward biased• Base–Collector junction is reverse biased
• Cutoff Region Operation
• Base–Emitter junction is reverse biased
• Saturation Region Operation
• Base–Emitter junction is forward biased• Base–Collector junction is forward biased
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DC Biasing Circuits
Fixed-bias circuitEmitter-stabilized bias circuitCollector-emitter loopVoltage divider bias circuitDC bias with voltage feedback
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Fixed Bias
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From Kirchhoff’s voltage law:
Solving for base current:
+VCC – IBRB – VBE = 0
B
BECCB
R
VVI
The Base-Emitter Loop
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Collector current:
From Kirchhoff’s voltage law:
BC II
CCCCCE RIVV
Collector-Emitter Loop
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CR
CCV
CsatI
VCE
V 0
Saturation
When the transistor is operating in saturation, current through the transistor is at its maximum possible value.
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ICsat
IC = VCC / RC
VCE = 0 V
VCEcutoff
VCE = VCC
IC = 0 mA
The Q-point is the operating point where the value of RB sets the
value of IB that controls the values of VCE and IC .
The load line end points are:
Load Line Analysis
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The Effect of VCC on the Q-Point
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The Effect of RC on the Q-Point
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The Effect of IB on the Q-Point
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Emitter-Stabilized Bias Circuit
Adding a resistor (RE) to the emitter circuit stabilizes the bias circuit.
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From Kirchhoff’s voltage law:
01 EBBEBBCC R)I(βVRIV
0 RIVRIV EEBEBBCC
EB
BECCB
)R(βR
VVI
1
Since IE = ( + 1)IB:
Solving for IB:
Base-Emitter Loop
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From Kirchhoff’s voltage law:0 VR I V R I CCCCCEEE
Since IE IC:
) R (R – I V V ECCCCCE
Also:
EBEBRCCB
CCCCECEC
EEE
VV R – I V V
RIVV V V
R I V
Collector-Emitter Loop
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Improved Biased Stability
Stability refers to a condition in which the currents and voltages remain fairly constant over a wide range of temperatures and transistor Beta ( ) values.
Adding RE to the emitter improves the stability of a transistor.
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VCEcutoff: ICsat:
The endpoints can be determined from the load line.
mA0 I
V V
C
CCCE
ERCR
CCV
CI
VCE
V0
Saturation Level
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Voltage Divider Bias
This is a very stable bias circuit.The currents and voltages are nearly independent of any variations in .
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Where IB << I1 and I1 I2 :
Where RE > 10R2:
From Kirchhoff’s voltage law:
21
CC2B
RR
VRV
E
EE
R
VI
BEBE VVV
EECCCCCE RI RI V V
)R (RIV V
II
ECCCCCE
CE
Approximate Analysis
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Transistor Saturation Level
EC
CCCmaxCsat
RR
VII
Cutoff: Saturation:
mA0 I
VV
C
CCCE
V0 VCE
ER
CR
CCV
CI
Load Line Analysis
Voltage Divider Bias Analysis
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DC Bias With Voltage Feedback
Another way to improve the stability of a bias circuit is to add a feedback path from collector to base.
In this bias circuit the Q-point is only slightly dependent on the transistor beta, .
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)Rβ(RR
VVI
ECB
BECCB
From Kirchhoff’s voltage law:
0EEBEBBCCCC R–I–VR–IRI – V
Where IB << IC: CBCC IIII'
Knowing IC = IB and IE IC, the
loop equation becomes:
0 EBBEBBCBCC RβIVRIR – β–V
Solving for IB:
Base-Emitter Loop
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Applying Kirchoff’s voltage law:
IE + VCE + I’CRC – VCC = 0
Since IC IC and IC = IB:
IC(RC + RE) + VCE – VCC =0
Solving for VCE:
VCE = VCC – IC(RC + RE)
Collector-Emitter Loop
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Base-Emitter Bias Analysis
Transistor Saturation Level
EC
CCCmaxCsat
RR
VII
Cutoff Saturation
mA0 I
VV
C
CCCE
V0 VCE
ER
CR
CCV
CI
Load Line Analysis
Base-Emitter Bias Analysis
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Transistors with only the DC source applied can
be used as electronic switches.
Transistor Switching Networks
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Transistor switching times:
dron ttt
fsoff ttt
Switching Time
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C
CCCsat
R
VI
dc
CsatB
β
II
Csat
CEsatsat
I
VR
CEO
CCcutoff
I
VR
Saturation current:
To ensure saturation:
Emitter-collector
resistance at
saturation and cutoff:
Switching Circuit Calculations
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Troubleshooting Hints
Approximate voltages
– VBE .7 V for silicon transistors– VCE 25% to 75% of VCC
Test for opens and shorts with an ohmmeter. Test the solder joints.Test the transistor with a transistor tester or a curve tracer.Note that the load or the next stage affects the transistor operation.
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PNP Transistors
The analysis for pnp transistor biasing circuits is the same as that for npntransistor circuits. The only difference is that the currents are flowing in the opposite direction.
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