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1Dept. of Electrical and Electronic Engineering, KUET 1
EE 2109 Electronics-I
Dr. Mostafa Zaman Chowdhury
Chapter 4:
DC Biasing–BJTs
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Biasing
Biasing: The DC voltages applied to a transistor in order to turn
it on so that it can amplify the AC signal.
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Operating Point
The DC input establishes an
operating or quiescent point
called the Q-point.
Which point is best?
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The Three States of Operation
• 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
DC Biasing Circuits
Fixed-bias circuit
Emitter-stabilized bias circuit
Collector-emitter loop
Voltage divider bias circuit
DC bias with voltage feedback
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Fixed Bias
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The Base-Emitter Loop
From Kirchhoff’s voltage law:
Solving for base current:
+VCC – IBRB – VBE = 0
B
BECCB
R
VVI
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Collector-Emitter Loop
Collector current:
From Kirchhoff’s voltage law:
BIIC
CCCCCE RIVV
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Saturation
When the transistor is operating in saturation, current through the transistor
is at its maximum possible value.
CR
CCV
CsatI
V 0CEV
Load Line Analysis
ICsat
IC = VCC / RC
VCE = 0 V
VCEcutoff
VCE = VCC
IC = 0 mA
where the value of RB sets the value of
IB
that sets the values of VCE and IC
The Q-point is the operating point:
The end points of the load line are:
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Circuit Values Affect the Q-Point
more …
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Circuit Values Affect the Q-Point
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Circuit Values Affect 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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Base-Emitter Loop
From Kirchhoff’s voltage law:
0R1)I(-RI-V EBBBCC
0 RI-V-RI-V EEBEEECC
EB
BECCB
1)R(R
V-VI
Since IE = ( + 1)IB:
Solving for IB:
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Collector-Emitter Loop
From Kirchhoff’s voltage law:
0 CC
VC
RC
I CE
V E
RE
I
Since IE IC:
)R (RI– V V ECCCCCE
Also:
EBEBRCCB
CCCCECEC
EEE
V V RI– V V
RI - V V V V
RI V
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Improved Biased Stability
Stability refers to a circuit condition in which the currents and voltages
will 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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Saturation Level
VCEcutoff: ICsat:
The endpoints can be determined from the load line.
mA 0 I
V V
C
CCCE
ERCR
CCV
CI
CE V 0V
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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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Approximate Analysis
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
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Voltage Divider Bias Analysis
Transistor Saturation Level
EC
CCCmaxCsat
RR
VII
Load Line Analysis
Cutoff: Saturation:
mA0I
VV
C
CCCE
V0VCE
ERCR
CCV
CI
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Exact 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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Base-Emitter Loop
)R(RR
VVI
ECB
BECCB
From Kirchhoff’s voltage law:
0RI–V–RI–RI– V EEBEBBCCCC
Where IB << IC:
CI
BI
CI
CI'
Knowing IC = IB and IE IC, the loop
equation becomes:
0RIVRIRI– V EBBEBBCBCC
Solving for IB:
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Collector-Emitter Loop
Applying Kirchoff’s voltage law:
RE 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)
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Base-Emitter Bias Analysis
Transistor Saturation Level
EC
CCCmaxCsat
RR
VII
Load Line Analysis
Cutoff: Saturation:
mA 0I
VV
C
CCCE
V 0VCE
ER
CR
CCV
CI
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Transistor Switching Networks
Transistors with only the DC source applied can be used as electronic
switches.
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Switching Circuit Calculations
C
CCCsat
R
VI
dc
CsatB
II
Csat
CEsatsat
I
VR
CEO
CCcutoff
I
VR
Saturation current:
To ensure saturation:
Emitter-collector resistance
at saturation and cutoff:
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Switching Time
Transistor switching times:
dron ttt
fsoff ttt
tr=rise time
td=delay time
ts=storage time
tf=forward time
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PNP Transistors
The analysis for pnp transistor biasing circuits is the same
as that for npn transistor circuits. The only difference is that
the currents are flowing in the opposite direction.
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