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Common-Emitter Configuration
Problem — Two Sources
ECE 332
BJT
Bias Circuits
iC
iB
+
RC
VCE
+
RB
+
VBE
VCC
VBB
iE
Single-Power-Source BJT Biasing
l Linear amplifier — Q-point should be
near the center of the load line
VCEQ
VCC
2
Single Base-Resistor Biasing
Base bias resistor
VCC
l Biasing — establishing the Q point —
the dc operating “reference.” Types:
Coupling Capacitor:
acts as open circuit
to dc & very low
impedance to ac
RB
Single-Resistor Circuit Relationships
I CQ
Base bias resistor
=
VCC –VCEQ
RC
VCC
I CQ
RC
I BQ
+
VBE
VCEQ
VCEQ
VBE
vs
u Voltage divider biasing, also termed
“stabilized bias.”
+
+ CC2
+
Direct-Current Equivalent Circuit
I CQ
VO
CC1
u Single base-resistor biasing
RB
RC
RB =
=
ICQ
VCC –VBE(on)
I BQ
VCEQ = VCC – R C I CQ
Voltage-Divider Biasing
Direct-Current Equivalent
VCC
VCC
Voltage divider
R1
Voltage divider
I CQ
RC
R1
I CQ
RC
VO
+ CC2
CC1
CC1
+
VCEQ
VCEQ
vs
R2
R2
RE
RE
Stabilizing resistor
Stabilizing resistor
BJT Bias Stability
Thévenin Equivalent of Base
VTh = RTh IBQ + VBE(on) + IEQ RE
VCC
VCC
I CQ
RC
RTh
RTh
+
+
VCEQ
VCEQ
+
IBQ
+
VTh
VTh
I EQ
I CQ
RC
RE
VTh = RTh IBQ + VBE(on) + ( + 1)IB RE
I BQ
=
VTh VBE(on)
RTh + ( + 1) RE
ICQ
=
(VTh VBE(on) )
RTh + ( + 1) RE
IBQ
I EQ
RE
For Bias Stability: RTh << (1 + ) RE
For Bias Stability: RTh << (1 + ) RE
ICQ
If
Then
(VTh VBE(on) )
≈
( + 1)RE
>> 1, then /(1 + ) ≈ 1
Therefore,
ICQ
BJT Bias Stability
General Rule
≈
VTh
VBE(on)
RE
ICQ not a significant function of
For Bias Stable Circuit
RTh ≈ 0.1 (1 + ) RE
BJT Biasing — Resistor Tolerances
l Typical resistor tolerances:
u ± 5 % for carbon film or metal-oxide film
u ± 1 % for metal film (also ± 0.1 %)
l The Q-point is a function of resistor
values
l Therefore
u IB(min) @ R1(max), R2(min), R E(max)
u IB(max) @ R1(min), R 2(max), RE(min)
BJT Thermal Runaway
l Without RE
u Expect junction current increase to cause
temperature increase (I 2R)
u This T can cause further current increase
thereby further increasing temperature.
u Phenomenon -- thermal runaway.
u Result
BJT Thermal Runaway
l Without RE
u Expect junction current increase to cause
temperature increase (I 2R)
u This T can cause further current increase
thereby further increasing temperature.
u Phenomenon -- thermal runaway.
u Result -- device destruction:
Pfffft
BJT Thermal Runaway
l RE stabilizes Q-point with respect to
temperature.
u
T causes
ground)
I which results in
u RTh independent of
u Therefore,
u
VE (to
T
VE reduces VBE (junction)
VBE tends to stabilize transistor current