Slide điện tử từ trường lecture 6 bjt circuits
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Lecture 06
BJT Circuits
1
Microelectronic Circuit by
meiling CHEN
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topics
• Small-signal operation
– Hybrid π model
– T-model
– Early Effect
• Single-stage BJT amplifiers
– Amplifier basic structure
– CE Amplifier
– CB Amplifier
– CC Amplifier
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Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
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DC analysis
Small signal
IC = I S e
DC analysis
IE =
IC
IB =
IC
VBE
VT
α
β
VC = VCE = VCC − I C RC
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Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
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iC = I S e
vBE
VT
= ISe
(
VBE +vbe
)
VT
vbe
iC ≈ I C (1 + ) ← if
VT
VBE
VT
= ISe e
vbe
VT
= ICe
vbe
VT
vbe << VT
IC
ic = vbe = g m vbe
VT
IC
gm =
VT
transconductance
4
Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
∂iC
gm =
∂vBE
iC = I C
ic
+
vbe
−
ic
g m vbe
5
Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
iC
IC
1 IC
iB = =
vbe
+
β β β VT
ic
iB = I B + ib
gm
1 IC
ib =
vbe =
vbe
β VT
β
vbe
β
rπ ≡
⇒ rπ =
ib
gm
IC
VT
⇒ rπ =
∴ gm =
VT
IB
Small-signal input resistance
6
Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
iE =
iC
α
=
IC
α
+
ic
α
iE = I E + ie i
c
IC
IE
ie =
vbe = vbe
VT
αVT
vbe
VT
⇒ re =
re ≡
ie
IE
IC
1
α
⇒ re =
≈
∴ gm =
VT
gm gm
vbe = ib rπ = ie re
Emitter resistance
ie
rπ = re ⇒ rπ = ( β + 1)re
ib
Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
7
Voltage gain
vc
Av ≡
vbe
vC = VCC − iC RC
= VCC − ( I C + ic ) RC
= (VCC − I C RC ) − ic RC
= VC − ic RC
vc = −ic RC = − g m vbe RC
⇒ Av = − g m RC = − I CVRT C
8
Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Hybird-π Model
vbe
vbe
+ g m vbe =
(1 + g m rπ )
ie =
rπ
rπ
vbe
vbe
v
(1 + β ) =
= be
rπ
rπ
re
(1 + β )
g m vbe = g m (ib rπ ) = β ib
=
9
Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
T- Model
vbe
vbe
ib =
+ g m vbe =
(1 + g m re )
re
re
=
vbe
v
v
β
(1 + α ) = be (1 −
) = be
re
re
β + 1 rπ
g m vbe = g m (ie re ) = αie
10
Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Example 5.14
DC analysis
β = 100
DC
IB =
analysis
VBB − VBE 3 − 0.7
=
= 0.023mA
RBB
100k
I C = βI B = 100 × 0.023 = 2.3mA
IC
2.3
= 2.32mA
α 0.99
VC = VCC − I C RC = 10 − 2.3 × 3 = 3.1V
IE =
=
Confirm transistor
in active mode
11
Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Small-signal analysis
25mV
VT
re =
=
= 10.8Ω
I E 2.32mA
gm =
I C 2.3mA
=
= 92 mA
V
VT 25mV
β
100
=
= 1.09kΩ
rπ =
g m 92
rπ
1.09
vbe = vi
= vi
= 0.011vi
rπ + RBB
101.09
vo = − g m vbe RC = −92 × 0.011vi × 3 = −3.04vi
vo
Av = = −3.04
vi
12
Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
13
Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Example 5.16
DC analysis
10 − 0.7
IE =
= 0.93mA
10k
I C= αI E = 0.99 × 0.93 = 0.92mA
VC = I C RC − 10V = −5.4V
Confirm transistor in
active mode
14
Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Small-signal analysis
re =
VT
25mV
=
= 27Ω
I E 0.93mA
vo = −αie RC
vi
ie = −
re
RC
⇒ vo = α
vi
re
vo
RC
5k
= 0.99
= 183.3
Av = = α
27
vi
re
15
Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
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Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Small-signal model with Early Effect
VA + VCE VA
ro =
≈
IE
IE
VA → Early voltage
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Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Exercise 5.40
I C = αI E = 0.99 ×1mA = 0.99mA
0.99m
×10k = −0.099V
100
VE = VB − 0.7V = −0.799V
Active mode
VB = − I B ×10k = −
VC = I C RC − 10V = −0.99m × 8k + 10V = 2.08V
ro =
VA 100mV
=
= 100Ω
IE
1mA
gm =
rπ =
given
I C 0.99mA
=
= 0.0396 mA
V
VT
25mV
β
gm
=
100
= 2.525kΩ
0.0396
I E = 1mA
β = 100
VA = 100V
18
Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Hybrid-πModel
T Model
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Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Signal-stage BJT amplifiers
• Common-Emitter (CE) Amplifier
• Common-Base (CB) Amplifier
• Common-Collector (CC) Amplifier
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Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Amplifier Categories
•
•
•
•
Voltage Amplifier
Current Amplifier
Transconductance Amplifier
Transresistance Amplifier
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Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
ii
vi
output
gain ≡
input
io
Amplifier
Circuit
vo
powergain
AP ≡
vo io
vi ii
voltagegain
Av ≡
vo
vi
currentgain
Ai ≡
io
ii
transconduc tan ce Gm ≡
io
vi
vo
transresis tan ce Rm ≡
ii
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Microelectronic Circuit by
meiling CHEN
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Voltage Amplifier
Open-Circuit voltage Gain :
vo
Avo ≡
vi
Ideal voltage amplifier :
Ri = ∞
io = 0
Ro = 0
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Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Current Amplifier
Short-Circuit current Gain :
io
Ais ≡
ii
Ideal voltage amplifier :
Ri = 0
vo = 0
Ro = ∞
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Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Transconductance Amplifier
Short-Circuit Transconductance :
io
Gm ≡
vi
vo = 0
Ideal transconductance amplifier :
Ri = ∞
Ro = ∞
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Microelectronic Circuit by
meiling CHEN
CuuDuongThanCong.com
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