ESE319 Introduction to Microelectronics 1 Kenneth R. Laker update KRL 03Oct14 Differential Amplifier Common & Differential Modes ● Common & Differential Modes ● BJT Differential Amplifier ● Diff. Amp Voltage Gain and Input Impedance ● Small Signal Analysis – Differential Mode ● Small Signal Analysis – Common Mode
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Differential Amplifier Common & Differential Modes
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ESE319 Introduction to Microelectronics
1Kenneth R. Laker update KRL 03Oct14
Differential AmplifierCommon & Differential Modes
● Common & Differential Modes● BJT Differential Amplifier● Diff. Amp Voltage Gain and Input Impedance● Small Signal Analysis – Differential Mode● Small Signal Analysis – Common Mode
ESE319 Introduction to Microelectronics
2Kenneth R. Laker update KRL 03Oct14
+-
Vo = A (V
1 - V
2)A
Most “Famous” Differential AmplifierV
CC
VEE
V1 & V
2 are single-ended input voltages defined w.r.t. ground.
ESE319 Introduction to Microelectronics
3Kenneth R. Laker update KRL 03Oct14
Common and Differential ModesConsider the two-input conceptual circuit shown below.
Analyze Circuits by SuperpositionBalanced Circuit Differential Mode (V
C = 0)
Z1
Z3
Z2
I1 = Id
Vd/2
- Vd/2
I2 = -Id
Id
Z 1=Z 2=Z
V x
I c=0⇒V x=0
+
+
±.
±.
I c= I 1 I 2=
V d
2−V x
Z1
−V d
2−V x
Z 2=−2V x
Z
balanced circuit =>
+ -VZ1
+- VZ2
Z i n−dm=V d
I d=2 Z
NOTE: If , then => Vx ≠ 0.Z 2=Z 1Z Ic ≠ 0
Ic = 0
I c=I 1I 2=I d−I d=0
I d=I 1− I 22
=
V d
2−V x
2Z 1−
−V d
2−V x
2 Z 2=
V d
2Z
ESE319 Introduction to Microelectronics
11Kenneth R. Laker update KRL 03Oct14
Analyze Circuits by SuperpositionBalanced Circuit Common Mode (V
d = 0)
Z1
Z3
Z2
Ic/2
Vc
IcIc/2
Id = 0
Z 1=Z 2=Z
I c=V c
Z 1∣∣Z 2Z 3=
V c
Z2Z 3
+ ±.
balanced circuit =>
I1
I2
Z i n−cm=V c
I c=
Z2Z 3
I 1= I c /2I 2=I c /2
=> I d=0
ESE319 Introduction to Microelectronics
12Kenneth R. Laker update KRL 03Oct14
Summary
<=>
Definitions:
V c=V 1V 2
2V d=V 1−V 2
I c= I 1 I 2 I d=I 1−I 22
V 1=V cV d /2V 2=V c−V d /2I 1= I c /2 I d
I 2= I c /2−I d
I c
I 1
I 2
All V's & I's have differential & com-mon mode com-ponents
Z 1
Z 2
Z 3
I 2
Id
ESE319 Introduction to Microelectronics
13Kenneth R. Laker update KRL 03Oct14
Summary cont.In a balanced differential circuit (Z1 = Z2 = Z): 1. Differential mode voltages result in differential mode currents.2. Common mode voltages result in common mode currents.
The differential mode input impedance of a balanced circuit is: Z i n−dm=Z 1Z 2=2ZThe common mode input impedance of a balanced circuit is: Z i n−cm=Z 1∥Z 2Z 3=
Z2Z 3
Id Id = 0
ESE319 Introduction to Microelectronics
14Kenneth R. Laker update KRL 03Oct14
BJT Differential Amplifier – DC Bias View
I C= I cm /2 I cm /2
I BI B
I B=I cm
21
RC1=RC2=RC
RB1=RB2=RB
Collector bias path inherentlycommon mode.
I C=I cm
2≈
I cm
2
Choose Icm and RC for approximate½ VCC drop across RC.
balanced circuit
Icm
Q1=Q2
IE
IE
I E=I cm
2
RB1 RB2
RC2RC1
I 1=I c /2 I d
I 2= I c/2−I d
Recall00
for dc bias
ESE319 Introduction to Microelectronics
15Kenneth R. Laker update KRL 03Oct14
vid/2
-vid/2
vic
BJT Differential Amplifier – AC Signal View
RB1 RB2
RC1 RC2
vo1
vo2+-v
od
vod=vo2−vo1
vi1
vi2
v ic=v i1v i2
2v id=v i1−v i2
ro
ESE319 Introduction to Microelectronics
16Kenneth R. Laker update KRL 03Oct14
vid /2
rin-dm
+-
Z in−dm=v id
ib1d
Avdm=vodm
v idvo1d
Single-ended DM outputs: vo1d, vo2d
Differential DM output: vodm
=vo1d
- vo2d
vid /2
RC1
Avdm1 ,2=vo1 ,2d
vid
(diff Adm
)
(s-e Adm
)+
-
ro
NOTE: ib1d=ib1
; ib2d
=-ib2
ib1d i
b2d
RC2
RB1RB2
vodm vo2d
BJT Differential Amplifier – AC Diff Mode View
ro
ie1d
ie2d
DM input impedance
vidm=vid
ESE319 Introduction to Microelectronics
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ro
vic
ib1c
rin-cm
+-vocm
Avcm=vocm
vicm
Single-ended CM outputs: vo1c
, vo2c
Differential CM output: vocm
= vo2c
- vo1c
vo1c vo2c
RB2Avcm1 ,2=
vo1 ,2c
vic
(diff Acm
)
(s-e Acm
)
NOTE: ib1c
=ib1
; ib2c
=ib2
BJT Differential Amplifier – AC Cmn Mode View
RB1
RC1RC2
ICM
ib2c
Z in−cm=v ic
ib1c
CM input impedance
ESE319 Introduction to Microelectronics
18Kenneth R. Laker update KRL 03Oct14
BJT Differential Amplifier – Small-signal View
ICM current source output impedance
ib1 ib2ie1
ic1ic2
ie2
vic
vid /2 vid /2
NOTE: 1. ro for amplifier Q1 & Q2 is ignored.2. vid /2 and vic are ac small signals