Analog Circuit Design with Submicron Transistors - IEEE · Analog Circuit Design with Submicron Transistors © 2004 B. Boser 2 Analog Circuit Design •Objective: Translate circuit

Analog Circuit Design with Submicron Transistors © 2004 B. Boser 1

Analog Circuit Design with Submicron Transistors

Bernhard BoserUC Berkeley

Department of Electrical Engineering and Computer [email protected]


Analog Circuit Design

• Objective:Translate circuit specifications(gain, bandwidth, dynamic range, …)into transistor sizes and bias currents

• Challenge:Accurate device models for deep submicron transistors– “Square-law model”– Simulation models (BSIM, EVK, …)– “Model” for analog design


Device Model Objectives• Device Physics

• Simulation / Verification– Accuracy, efficiency

• (Analog) Circuit Design– Relate device characteristics to circuit specifications– E.g.

• Bandwidth• Gain• Power dissipation• Dynamic range (noise)

– Accurate, simple


Device Parameters for Analog Design

• Large signal– Current ID power dissipation– Minimum VDS available signal swing

• Small signal– Transconductance gm speed / voltage gain– Capacitances CGS, CGD, … speed– Output impedance ro voltage gain


Metrics for Design

• Transistor Objectives: High transconductance– Without large ID– Without large CGS

• Figures of Merit – Current efficiency

– Transit frequencyD

m

Ig

gs

m

Cg


Current Efficiency

• “Square-law transistor”:

• High efficiency low overdrive voltage

( )

satd

D

THGS

Dm

THGSoxD

VI

VVIg

VVLWCI

2

221 2

=

−=

−= µ

Overdrive voltage


Current Efficiency gm/ID• High efficiency is good

for low power

• Higher gm/ID at low VGS

• Approaches BJT for VGS < VTH

gm/IC = 1/Vt ~ 40 V-1

• Weak dependence on transistor type and process


Transit Frequency ωT

• Unity current-gain bandwidth

model) law-(square 2LV

CCg

satd

gdgs

mT

µ

ω

≈

+=


Efficiency gm/ID versus fT

Speed-Efficiency Tradeoff

NMOS faster than PMOS


Device Scaling

0

10

20

30

40

50

60

-0.1 0.0 0.1 0.2 0.3 0.4 0.5VGS-VTH [V]

f T [G

Hz]

0.18µm

0.25µm

0.35µm

0.5µm

Short channel devices are significantly faster!


Current Efficiency vs Transient frequency

• Tradeoff:

• What about:

satdT

satdD

m

V

VIg

∝

∝

ω

1

2LIg

TD

m µω ∝×


Device Figure-of-Merit

Peak performance for low VGS-VTH

0

50

100

150

200

250

300

350

400

-0.1 0.0 0.1 0.2 0.3 0.4 0.5

VGS-VTH [V]

f T⋅g

m/I D

[GH

z/V]

0.18µm

0.25µm

0.35µm

0.5µm


Device Scaling for Analog Circuits

• “Moore’s Law”– Lmin decreases ~ 2x every 5 years– Lmin = 10µm in 1970, 90nm in 2004

• Benefits (for analog circuits):– Higher speed: increase gm/Cgs while keeping gm/ID

constant– Lower power: increase gm/ID while keeping

bandwidth (gm/Cgs) constant• In both cases, reducing L is advantageous!


Short Channel Devices

• Short channel effects– Velocity saturation– Mobility degradation (thin oxide)

• Prior considerations assume “square law”models and ignore these effects

Significant discrepancies

• Let’s fix this …


Efficiency gm/ID• Important design parameter … but a little unusual: units 1/V

• Let’s define

e.g. V* = 200mV gm/ID = 10 V-1

• Square-law devices only: V* = VGS-VTH = Vdsat

*22 :law SquareVI

VVIg D

THGS

Dm =

−=

*2 2*VI

ggIV

D

m

m

D =⇔=


Saturation Voltage versus V*• Saturation voltage

– Minimum VDS for “high” output resistance– Poorly defined: transition is smooth in

practical devices

• “Long channel” (square law) devices:– VGS – VTH = Vdsat = Vov = V*– Significance:

• Channel pinch-off• ID ~ Vdsat

2

• Boundary between triode and saturation


Design ExampleExample: Common-source amp av0 > 100, fu = 100MHz for CL = 5pF

• av0 > 100 L =0.5µm

•

• High fT (small CGS): V* = 250mV

•

mS14.32 =≈ Lum Cfg π

µA3932

*==

VgI mD

M138 / 0.5 Vgs

dc = 820mVac = 1V

I1dc = 393uA

Vi

Vo

DC Analysis

sweep from 800m to 900m (1001 steps)Device Vgs

DC1

AC Analysislog sweep from 1k to 10G (101 steps)

AC1

C15pF


Device Sizing• Pick L 0.5µm• Pick V* 250mV• Determine gm 3.14mS

• ID = 0.5 gm V* 393µA

• W from graph (generate with SPICE)

W = 10µm (393µA /103µA)= 38µm

• Create such graphs for several device length’ for design reference

NMOSW / L = 10 / 0.5


Common Source Example

Dead on!


Device Sizing ChartW = 10µm for all devicesVDS = VGS VSB = 0V


Device Sizing Chart

W = 10µm for all devicesVDS = VGS VSB = 0V


Device Parameters for Analog Design

• Large signal– Current ID power dissipation– Minimum VDS available signal swing

• Small signal– Transconductance gm speed / voltage gain– Capacitances CGS, CGD, … speed– Output impedance ro voltage gain


Output Resistance ro

Hopeless to model this with a simple equation(e.g. gds = λ ID)


Process Variations for ro

L = 0.35µm


Open-loop Gain av0

More useful than ro


Gain, av0 = gm ro (gm/ID = 10/V)

• Strong tradeoff:av0 versus VDS range

• Create such plots for several device length’for design reference

L = 0.35µm


Gain, av0 = gm ro (gm/ID = 10/V)

L = 0. 5µm

L av0

like long channel device


Technology Trend

0

10

20

30

40

50

60

70

80

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

VDS [V]

g m⋅r o

0.18µm

0.25µm

0.35µm

0.5µm

Short channel devices suffer from reduced per transistor gain


Transistor Gain Detail

For practical VDS the effect the “short-channel” gain penalty is less severe(remember: worst case VDS is what matters!)

0

5

10

15

20

25

30

35

40

45

0.0 0.1 0.2 0.3 0.4

VDS [V]

gm⋅r

o

0.18µm

0.25µm

0.35µm

0.5µm


Current Sources (Biasing)


Cascoding

How choose Vbias2?


Output Resistance


Rout = f(k)

*11 kVVDS =

How choose k? Issues:

• Swing versus Ro• Large k useful only for large

Vmin simultaneously• Note: small or no penalty for

large k and small Vmin• typically choose k>1


High-Swing Bias Example• M5 … M10 replace

quarter size device

• All devices same size

• Less sensitive to body-effect

• Lcurrent-source = Lcascode


Noise

• M2 (and Iref!) can add noise– Choose small M (power penalty), or– Filter at gate of M1

• Current source FOMs– Output resistance Ro– Noise resistance RN– Active sources boost Ro, not RN

( )( )

oov

o

mN

NB

mB

mmB

ddon

rRMa

r

MgR

fR

Tk

fMgTkfgMgTk

iMii

=<<+

=

+=

∆=

∆+=∆+=

+=

−

−

1

11

14

144

1

0

1

1

1

22

1

22

221

2

γ

γ

γγ


Vmin versus Noise

MKIV

MgR

kVkV

D

mN

+=

+=

=×=

−

−

12

11

2...1 typ.*

1min

1

1

min

γ

γ

• Voltage required for large Ro(saturation): Vmin ~ V* (based on intuition from square-law model)

• Minimizing noise (for given ID):large RNlarge Vmin (k >> 1)

• At odds with signal swing(to maximize the dynamic range)


Bipolar’s, GaAs, …

• BJT and RE contribute noise• Increasing RE lowers overall noise• BJT and MOS exhibit essentially

same noise / Vmin tradeoff• Lowest possible noise source is a

resistor (and large Vmin, VDD)

Q1Q2

RERE

io

Iref

Cbig

fRgRgi

Rgii

Em

EmRn

Emcnon ∆

++

+=

44344214434421ER

22

BJT

222

111

0 a) =EmRg

1 b) >>EmRg

fTgki mBon ∆= 22

fR

TkiE

Bon ∆=142

CC

t

mN I

IV

gR by set 22

==

min

minmin

VVV

IVRR

satce

CEN

−==

KIVRD

MOSN 2 compare

1min

,

−

=γ

( )02 =bi


Small Signal Design Summary• Determine gm (from design objectives)

• Pick L– Short channel high fT– Long channel high ro, av0

• Pick V* = 2ID/gm– Small V* large signal swing– High V* high fT– Dynamic range: Psig ~ 1/V*, Pnoise ~ V*

• Determine ID (from gm and V*)

• Determine W (SPICE / plot)

• Accurate for short channel devices key for design


Device Parameter Summary

• Obtain from L, ID• Self loading (CGS, CDB, …)

W

• Cutoff frequency, fT phase margin• Intrinsic transistor gain (av0)

L

• Current efficiency, gm/ID• Power dissipation (ID)• Speed (gm)• Cutoff frequency, fT phase margin• Headroom, VDS,min

V*

Circuit ImplicationsDevice Parameter

Analog Circuit Design with Submicron Transistors - IEEE · Analog Circuit Design with Submicron Transistors © 2004 B. Boser 2 Analog Circuit Design •Objective: Translate circuit

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