transistor bjt : dc biasing. transistor currents ■ emitter current (i e ) is the sum of the...
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TRANSISTOR
BJT :DC BIASING
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Transistor Currents
Emitter current (IE) is the sum of the collector current (IC) and the base current (IB) .
Kirchhoff’s current law;
BCE III …(Eq. 3.1)
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Collector Current (IC) Collector current (IC) comprises two components;
majority carriers (electrons) from the emitter
minority carriers (holes) from reverse-biased BC junction → leakage current, ICBO
Total collector current (IC);
Since leakage current ICBO is usually so small that it can be ignored.
EC IImajority
CBOC II minority
CBOEC III …(Eq. 3.2)
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Collector Current (IC) Then;
The ratio of IC to IE is called alpha (α), values typically range from 0.95 to 0.99.
E
C
I
I …(Eq. 3.3)
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Base Current (IB) IB is very small compared to IC;
The ratio of IC to IB is the dc current gain of a transistor, called beta (β)
The level of beta typically ranges from about 50 to over 400
B
C
I
I …(Eq. 3.4)
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Current & Voltage Analysis Consider below figure. Three dc currents and three dc voltages can be
identified
IB: dc base current
IE: dc emitter current
IC: dc collector current
VBE: dc voltage across base-emitter junction
VCB: dc voltage across collector-base junction
VCE: dc voltage from collector to emitter Transistor bias circuit.
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Current & Voltage Analysis When the BE junction is forward-biased, it is like a forward-
biased diode. Thus; (Si = 0.7, Ge = 0.3)
From HVK, the voltage across RB is
By Ohm’s law;
Solving for IB
V7.0VBE
BEBBR VVVB
BBR RIVB
B
BEBBB R
VVI
…(Eq. 3.5)
…(Eq. 3.6)
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Current & Voltage Analysis The voltage at the collector is;
The voltage drop across RC is
VCE can be rewritten as
The voltage across the reverse-biased CB junction is
CCR RIVC
CRCCCE VVV
CCCCCE RIVV
BECECB VVV …(Eq. 3.8)
…(Eq. 3.7)
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Transistor as Amplifier Transistor is capable to amplify
AC signal : (output signal > input signal)
Eg: Audio amplifier that amplify the sound of a radio
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Transistor Amplifier Circuit Analysis There are 2 analysis;
DC Analysis AC Analysis
Transistor will operate when DC voltage source is applied to the amplifier circuit
Q-point must be determined so that the transistor will operate in active region (can operate as an amplifier)
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Transistor Amplifier Circuit Analysis Q-Point
Operating point of an amplifier to state the values of collector current (ICQ) and collector-emitter voltage (VCEQ).
Determined by using transistor output characteristic and DC load line
Q-Point
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DC LOAD LINE DC Load Line
A straight line intersecting the vertical axis at approximately IC(sat) and the horizontal axis at VCE(off).
IC(sat) occurs when transistor operating in saturation region
VCE(off) occurs when transistor operating in cut-off region
DC Load Line
Cutoff Region
Saturation Region
Q-Point
0
CE
sat
VC
CCC R
VI
0)(
Coff ICCCCCE RIVV
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DC LOAD LINE (Example)VCC = 8V
RB = 360 kΩRC = 2 kΩ
Draw DC Load Line and Find Q-point.Answers;
mAk
V
R
VI
CE
sat
VC
CCC 4
2
8
0
0)(
Coff ICCCCCE RIVV
VVV CCCE off8
)(
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DC LOAD LINE (Example)
Draw DC Load Line and Find Q-point.Answers;
Q-point can be obtained by calculate the half values of maximum IC and VCE
4V
2 mA
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DC Analysis of Amplifier Circuit
Amplifier Circuit Amplifier Circuit w/o capacitor
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DC Analysis of Amplifier Circuit Refer to the figure, for DC analysis:
Replace capacitor with an open-circuit
R1 and R2 create a voltage-divider circuit that connect to the base
Therefore, from DC analysis, you can find: IC VCE
Amplifier Circuit w/o capacitor
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DC Analysis of Amplifier Circuit
Amplifier Circuit w/o capacitor Simplified Circuit
Thevenin Theorem;
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DC Analysis of Amplifier Circuit Important equation for DC Analysis
1
2
21
2121 //
RR
RRRRRTH
CCTH VRR
RV
21
2
BCETH
BETHB II
RR
VVI
;)1(
)( ECCCCCE RRIVV
1
2
From HVK;
From Thevenin Theorem;
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TRANSISTOR
BJT BIASING CIRCUIT
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BJT BIASING CIRCUIT Fixed Base Bias Circuit
(Litar Pincangan Tetap)
Fixed Bias with Emitter Resistor Circuit(Litar Pincangan Pemancar Terstabil)
Voltage-Divider Bias Circuit(Litar Pincangan Pembahagi Voltan)
Feedback Bias Circuit(Litar Pincangan Suap-Balik Voltan)
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FIXED BASE BIAS CIRCUIT
This is common emitter (CE) configuration
Solve the circuit using HVK 1st step: Locate capacitors
and replace them with an open circuit
2nd step: Locate 2 main loops which; BE loop CE loop
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FIXED BASE BIAS CIRCUIT 1st step: Locate capacitors and replace them with an open circuit
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FIXED BASE BIAS CIRCUIT 2nd step: Locate 2 main loops.
12
1
2
BE Loop CE Loop
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FIXED BASE BIAS CIRCUIT BE Loop Analysis
1
From HVK;
IBB
BECCB
BEBBCC
R
VVI
0VRIV
A
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FIXED BASE BIAS CIRCUIT CE Loop Analysis
From HVK;
As we known;
Subtituting with
BC II 2
IC CCCCCE
CECCCC
RIVV
0VRIV
B
A B
B
BECCDCC R
VVI
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FIXED BASE BIAS CIRCUIT DISADVANTAGE
Unstable – because it is too dependent on β and produce width change of Q-point
For improved bias stability , add emitter resistor to dc bias.
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FIXED BASE BIAS CIRCUIT Example 1 Find IC, IB, VCE, VB,
VC, VBC? (Silikon transistor);
Answers;IC = 2.35 mAIB = 47.08 μAVCE = 6.83VVB = 0.7VVC = 6.83VVBC = -6.13V
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FIXED BIAS WITH EMITTER RESISTOR An emitter resistor, RE is added
to improve stability Solve the circuit using HVK 1st step: Locate capacitors and
replace them with an open circuit
2nd step: Locate 2 main loops which; BE loop CE loop
Resistor, RE added
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FIXED BIAS WITH EMITTER RESISTOR 1st step: Locate capacitors and replace them with an open circuit
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FIXED BIAS WITH EMITTER RESISTOR 2nd step: Locate 2 main loops.
12
2
BE Loop CE Loop
1
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FIXED BIAS WITH EMITTER RESISTOR BE Loop Analysis
From HVK;
Recall;
Subtitute for IE
0 EEBEBBCC RIVRIV
EB
BECCB
EBBEBBCC
RR
VVI
RIVRIV
)1(
0)1(
BE II )1(
1
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FIXED BIAS WITH EMITTER RESISTOR CE Loop Analysis
From HVK;
Assume;
Therefore;
CE II
0 EECECCCC RIVRIV
2
)( ECCCCCE RRIVV
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FIXED BIAS WITH EMITTER RESISTOR Example 2 Find IC, IB, VCE, VB,
VC, VE & VBC? (Silikon transistor);
Answers;IC = 2.01 mAIB = 40.1 μAVCE = 13.97VVB = 2.71VVE = 2.01VVC = 15.98VVBC = -13.27V
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VOLTAGE DIVIDER BIAS CIRCUIT Provides good Q-point stability
with a single polarity supply voltage Solve the circuit using HVK 1st step: Locate capacitors and
replace them with an open circuit 2nd step: Simplified circuit using
Thevenin Theorem 3rd step: Locate 2 main loops
which; BE loop CE loop
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VOLTAGE DIVIDER BIAS CIRCUIT 1st step: Locate capacitors and replace them with an open circuit
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VOLTAGE DIVIDER BIAS CIRCUIT
Simplified Circuit
Thevenin Theorem;
2nd step: : Simplified circuit using Thevenin Theorem
21
2121 //
RR
RRRRRTH
CCTH VRR
RV
21
2
From Thevenin Theorem;
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VOLTAGE DIVIDER BIAS CIRCUIT 2nd step: Locate 2 main loops.
1
2
BE Loop CE Loop
1
2
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VOLTAGE DIVIDER BIAS CIRCUIT BE Loop Analysis
From HVK;
Recall;
Subtitute for IE
0 EEBETHBTH RIVRIV
ERTH
BETHB
EBBETHBTH
RR
VVI
RIVRIV
)1(
0)1(
BE II )1(
1
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VOLTAGE DIVIDER BIAS CIRCUIT CE Loop Analysis
From HVK;
Assume;
Therefore;
CE II
0 EECECCCC RIVRIV
)( ECCCCCE RRIVV
2
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VOLTAGE DIVIDER BIAS CIRCUIT Example 3 Find RTH, VTH, IC, IB, VCE,
VB, VC, VE & VBC? (Silikon transistor);
Answers;RTH = 3.55 kΩVTH = 2VIC = 0.85 mAIB = 6.05 μAVCE = 12.22VVB = 1.978VVE = 1.275VVC = 13.5VVBC = -11.522V