Download Lecture 11

EEL6935 Advanced MEMS (Spring 2005)
Instructor: Dr. Huikai Xie
Active Devices -- BJT & MOSFET
Lecture 11
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Agenda:
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Cross-section
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N/P/N Bipolar Junction
Transistor (BJT)
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n-channel Metal Oxide
Semiconductor Field
Effect Transistor
(n-channel MOSFET)
Amplifier Basics
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–
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Device models
Current mirrors
Single-stage amplifiers
Operational amplifier configurations
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MOSFET Small-signal Model
Active Devices -- BJT & MOSFET
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MOSFET Small-signal Equivalent Circuit
Cgd
Active
region
G
D
vgs
Cgs
gmvgs
rds
Cdb
S
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Common-emitter configuration
Ê
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iC vs. vCE DC characteristics with
base-emitter voltage (or base
current) as parameter
Other configurations are commonbase and common-collector
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Common-source configuration
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iD vs. vDS DC characteristics with
gate-to-source voltage as parameter
Other configurations are commongate and common-drain
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Device Model Summary
Device Model Summary
Johns and Martin, Analog Integrated Circuit Design, p.56-60
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Device Model Summary
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Device Model Summary
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Device Model Summary
Operating Point
RL
vDS
vD
Active
region
Operating point
iD = (vD-vDS)/RL
Johns and Martin, Analog Integrated Circuit Design, p.56-60
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Diode-connected Transistor
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Basic Current Mirror
vD
ID
RL
Active
region
Iin
RD
Active
region
vDS
Iout
Q2
Q1
Operating point
iD = (vD-vDS)/RL
iD = (vD-vDS)/RL
iD = (vD-vDS)/RL
vGS = vDS
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• ID stays almost constant for a
given VGS for a wide range of VDS
Current can be adjusted by RL
Current can tune VDS
Sizing transistor to adjust VDs,
with ID fixed
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•
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Q1 and Q2 are identical
Iin is set by RD
VGS1 (and VGS2) are set by RD
Thus Iout = Iin
• Iout can exactly match Iin or can be a
fraction of it by adjusting the ratio
between (W1/L1) and (W2/L2)
ID =
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µn Cox  W 
2
 L  (VGS − Vtn )
 
2
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Basic Single-Stage Amplifiers
Basic Single-Stage Amplifiers
‰ Common-source Amplifier
‰ Common-drain Amplifier
Q2
Q1
Vin
ID
Q3
Ibias
Ibias
Vin
Q3
ID =
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µn Cox  W 
2
 L  (VGS − Vtn )
 
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Q2
Q1
2
High input impedance
Active load
Passive load for low-gain, high frequency
Most popular
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Vout
Vout
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Source follower
Voltage buffer
Voltage gain ~ 1 but < 1
Current gain
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Basic Single-Stage Amplifiers
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High-Quality Current Mirrors
‰ Cascode Current Mirror
‰ Common-gate Amplifier
‰ Wilson Current Mirror
Iout
Q2
Q1
Ibias
Vout
Vbias
Q3
Q3
Q4
Q1
Q2
Iout
rout
Q3
Q4
Q1
Q2
rout
Vin
• High output impedance
• rout = (rds2gm4)rds4
• Impedance increased by
10x to 100x
• Reduce output swings
• Small input impedance
• Active load
• Current amplification
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•
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High output impedance
Shunt-series feedback
rout = rds4 (rds1gm1)/2
Reduce output swings
Q3 is not required
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Cascode Gain Stage
‰ Telescopic-cascode Amplifier
‰ Folded-cascode Amplifier
Vout
Q2
Vin
Q3
Q2
Q1
CL
Ibias2
+
Vin
-
Vbias
Vout
AV = gm1zout
zout = rout // 1/(sCL)
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rout = rds2 // rds4
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General Amplifiers
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dvO
dvI
At operating point (known also as Quiescent point)
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Operational Amplifiers
Amplifier Saturation
Ê hard limit at power supply limits
Ê Jargon: “rail” = power supply
– rail-to-rail
Amplifier Nonlinearity
AV =
CL
Q2
Ibias
• Allows same dc level for
input and output signals
• Slower due to p-channel
• Limit the voltage across the
input transistor
Q1
CL
• Common-source plus
common-gate
• High gain from single stage
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rout
Vout
Vin
Q1
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Q4
Ibias1
Ibias
Vbias
Differential Pair
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Operational Amplifiers
Ê
Basic building block for analog signal
processing circuits
– First integrated circuit operational
amplifier, µ709, made by Fairchild
Semiconductor in 1960s followed by the
µ741.
– Consists of active transistors, resistors,
and limited number of capacitors
– Approximated by single-pole frequency
response
vO (t ) = VO + vo (t )
Ê
Three stages
– Differential amplifier stage
Power supply
Inverting input
– Amplifies difference between two inputs
– High-gain amplifier stage
– Output amplifier stage
Non-inverting input
vI (t ) = VI + vi (t )
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Power supply
Ref. Senturia, Microsystem Design, p. 382.
Ref. Sedra and Smith, Microelectronic Circuits, p. 16.
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Output
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Operational Amplifiers
Operational Amplifiers
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Non-inverting Amplifier
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Vo
R
= 1+ 1
Vs
R2
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Integrator
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Differentiator
Transimpedance Amplifier
Inverting Amplifier
Vo
R
=− 2
Vs
R1
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vo = −
vo = − R1 I s
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1
Vs (t )dt
R1C ∫
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vo = − R1C
dvs
dt
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