Advantages of Using CMOS

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Advantages of Using CMOS. Compact (shared diffusion regions) Very low static power dissipation High noise margin (nearly ideal inverter voltage transfer characteristic) Very well modeled and characterized Mechanically robust Lends itself very well to high integration levels - PowerPoint PPT Presentation

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EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 1

Advantages of Using CMOS

• Compact (shared diffusion regions)

• Very low static power dissipation

• High noise margin (nearly ideal inverter voltage transfer characteristic)

• Very well modeled and characterized

• Mechanically robust

• Lends itself very well to high integration levels

• “Analog” CMOS process usually includes non-salicided poly layer for linear resistors.

• SiGe BiCMOS is very useful but is a generation behind currently available standard CMOS

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 2

fT is the frequency at which becomes 1.

idig

=gm

2πfΤCgs

gm =μCoxWL

VGS −Vt( )

Cgs =γWLCox

fT gives a fundamental speed measure of a technology.

0.25 µm CMOS: fT ~ 23GHz (VDD = 2.5V)0.18 µmCMOS: fT ~ 57GHz (VDD = 1.8V)

ωT =2πfT =μγL2

VGS −Vt( )

VDD

Cgs

VGS

vgs

igid

Transistor fT Calculation

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 3

Static CMOS propagation delay:

τ fall ≈CL

μnWn

Ln

VDD −Vt( )

τ rise ≈CL

μp

Wp

Lp

VDD −Vt( )

Assume: Wp = 3Wn for optimum noise margin.

Lp = Ln = Lmin

τ rise =τ fall =γLmin(Wp +Wn)Cox

μnCoxWn

Lmin

(VDD −Vt)

=γLmin

2

μn

1+Wp

Wn

⎝ ⎜ ⎜

⎠ ⎟ ⎟

1VDD −Vt

=4ωT

Operation is 4X slower than theoretical maximum due to n-channel & p-channel gates connected in parallel.

(Actual τ values will be higher due to high diffusion capacitances present in sub-micron transistors.)

VinVout

Wn

Ln

Wn

Ln

Wp

Lp

Wp

Lp

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 4

IG

ID

fT = 57GHz

VoutVin

τ =18ps≈6.4ωT

n-channel ac simulation to determine fT:

CMOS inverter transient simulation:

Verifying with simulation:

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 5

Single-Ended Signaling in CMOS

Series R & L cause supply/ground bounce.Resulting modulation of transistor Vt’s result in pattern-dependent jitter.

Vin Vout

sub

IDD

ISS

VDD

VSS

Vin

Vout

ISS

IDD

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 6

Rs = 0Ls = 0

clock out

Rs = 5Ls = 5nH

clock out

data in clock in clock out data out

′ VDD

′ VSS

′ VDD

′ VSS

data out

Rs = 5 Ls = 5nH

Effect of Supply/Ground Bounce on Jitter

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 7

Summary of CMOS Gate Performance

1. Simple & straightforward design.

2. Robust operation.

3. Nearly zero static power dissipation.

Advantages of static CMOS gates:

1. Full speed of transistors not exploited due to n-channel & p-channel gate in parallel at load.

2. Single-ended operation causes current spikes leading to VDD/VSS bounce.

3. Single-ended operation also highly sensitive to VDD/VSS bounce leading to jitter.

Disdvantages of static CMOS gates:

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 8

Current-Mode Logic (CML)

•Based on conventional differential pair

•Differential operation

•Inherent common-mode rejection

•Very robust in the presence of common- mode disturbances (e.g., VDD / VSS bounce)

CML inverter:VDD

Vin+ Vin-

Vout-Vout+

ISS

R R

CL CL

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 9

Vin(DC ) =Vout(DC ) =VDD −12

ISSR

To keep current source transistor in saturation:

VS >Vbias−Vt

VS =Vin(DC ) −VGS

Vin(DC ) >Vbias + VGS −Vt( )

12

ISSR+

_

12

ISSR

+

_

VDD

RR

WL

WL

ISS

VIN(DC )

VOUT(DC )

VOUT(DC )

VS

VGS

+_ VGS

+_

VBIAS

DC Biasing of CML Inverter

VIN(DC )

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 10

Vhigh =VDD

Vlow =VDD −ISSR

Vswing=ISSR

To achieve full current switching:

Vswing≥VGS −Vt( )|ID=ISS

=2ISS

μnCoxWL

≡Vmin

Vswing

Vmin

>1 for correct operation

Vswing

Vmin

=R12μCox

WL

ISS

Logic Swing & Gain of CML Inverter

VDD

Vin+ Vin-

Vout-Vout+

ISS

R R

CL CL

WL

WL

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 11

Av =gmR =R μCoxWL

ISS

Small-signal voltage gain:

Note: rising & falling time constants are the same

Recall

Vswing

Vmin

=R12μCox

WL

ISS >1

for full switching

Vswing

Vmin

=Av

2

Av ≥ 2

(Assuming fanout of 1)

rise/fall time constant:

τ =RCL

CL =γCoxWL

τ =R(γWLCox)

Small-Signal Behavior of CML Inverter

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 12

Speed vs. Gain in Logic Circuits

Largest possibleLargest possible gain-bandwidth product is desirable.is desirable.

fast input transition:step response determined by τ

slow input transition:step response determined by Av

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 13

Av =R μCoxWL

ISS

Av2 =μCox

WL

ISSR2

=μγL2

⋅RγWLCox( ) ⋅ ISSR( )

τ =R(γWLCox)

Vswing=ISSR

Av2 =

μγL2

⋅τ ⋅Vswing

“large-signal” gain-bandwidth product

Av2

τ=

μγL2

Vswing

Relationship between Av , τ , and Vswing

Larger logic swing preferred for higher gain-bandwidth product

Larger Vswing Larger Vmin smaller W/L larger current density

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 14

Suppose we decrease current density by increasing W/L:

WL

×2 ⇒ Vmin×1

2, CL ×2

R ×1

2

τ =RC × 2

Slower!

Thought Experiment

R R

WL

WL

R R

WL

WL

ISS ISS

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 15

Note that the load is only one gate capacitance:

CML speed ~ 2.5 times faster than static CMOS

IG

ID

fT = 57GHz

n-channel ac simulation to determine fT:

CML buffer transient simulation:

τ =RCL =Rgm

ωT

=Av

ωT

≈2

ωT

τ =8ps≈2.9ωT

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 16

Typical Vswing:

• Should be large enough to allow sufficient gain-bandwidth product.• Should be small enough to prevent transistors from going into triode. * CML will still work in triode (unlike BJT), but there is no additional speed benefit.

Vswing=ISSR

Once Vswing has been chosen, designer can trade off between gain &

bandwidth by parameterizing between R & ISS:

Higher speed: ISS R

Higher gain: ISS R

Av =R μCoxWL

ISS

τ =R(γWLCox)

0.3 ⋅VDD

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 17

Other Benefits of CML Gates

1. Constant current bias VDD/VSS bounce greatly reduced

KCL sets this current to be nearly constant.

ISS

ISS

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 18

data in clock in

Rs = 0Ls = 0

clock out

clock out

Rs = 5Ls = 5nH

clock out

data out

′ VDD

′ VSS

′ VDD

′ VSS

data out

Rs = 5 Ls = 5nH

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 19

2. Non-inverting buffer available without additional delay:

CMOS:

inverter buffer

CML:

inverter buffer

Vin+

Vin−

Vout+

Vout+

Vin+

Vin−

Vout−

Vout+

tp 2tp

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 20

Fanout & Scaling of CML Gates

1x

=

nx

=All voltages unchanged from unit-sized buffer.Currents & power increase by factor of n.

ISS

Vin+ Vin-

Vout- Vout+

R R

WL

WL

nISS

Vin+ Vin-

Vout- Vout+

R/n R/n

n×WL

n×WL

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 21

For fanout of n:

τ =nCLR

Av2

τ=

2μnγL2

Vswing

τincreases linearly with fanout.

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 22

From interconnect, etc.; assumed not to scale with buffer sizes

τ = nCL +Cp( ) ⋅R /n( ) =γWLCoxR ⋅1+Cp

nCL

⎝ ⎜ ⎜

⎠ ⎟ ⎟

Av2 =2μCox

nWL

nISS( ) ⋅R /n( )2=2μCox

WL

ISSR2

Av2

τ=2μγL2

Vswing⋅1+Cp

nCL

⎝ ⎜ ⎜

⎠ ⎟ ⎟

−1

Should set

n ≈0.1⋅Cp /CL( )degradation due to interconnect capacitance

to minimize

Power (proportional to n) determined primarily by interconnect capacitance!

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 23

Sub-micron MOSFETs obey square-law characteristics only in a limited region!

CML buffer design procedure:

1. Determine largest allowable ISS (usually limited by electromigration constraints)

2. Choose “unit-sized” n-channel transistor (typically W/L=20)

3. Run a series of simulations to determine optimum value of R:

R too small: full current switching not achieved

R too large: slower than necessary

4. Choose minimum scaling factor after laying out some test buffers of various sizes and determining approximate value of interconnect capacitance Cp.

VGS

ID

Mobility reduction (linear)

Square-law behavior

Weak inversion (exponential)

+_VGS

ID

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 24

1. Determine largest allowable ISS

ID ≤Imax

standard layout shared drain (1/2 diffusion capacitance)

Imax independent of Wdetermined by electromigration limits

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 25

R = 1200ISSR = 480mV

tp = 12ps*R optimum*

R = 900ISSR = 360mV

tp = 10psR too small

R = 1500ISSR = 600mV

tp = 14psR too large

CML Design Procedure Example

ISS =400μA

WL

=4 μm

0.18 μm

Choose:

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 26

Parameterizing Between Gain & Bandwidth

ISS = 100 µAR = 4.8 kAv = 9.3 dBBW = 2.6 GHz

ISS = 200 µAR = 2.4 kAv = 7.1 dBBW = 5.5 GHz

ISS = 400 µAR = 1.2 kAv = 3.9 dBBW = 11.5 GHz

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 27

R •GBW

GSCALE • MSCALE

ISS •GSCALE • MSCALE

GBW

W

L• GSCALE • MSCALE

GSCALE: Global scaling parameter (depends on Cp)MSCALE: Local scaling parameter (depends on fanout or bit rate)GBW: Gain-bandwidth parameter

Parameterized CML Buffer

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 28

CML with p-channel Active Load

p-channel load transistors operates in triode region:Increased capacitance and mismatch result

Can be used if linear resistors are not available.

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 29

Capacitance Comparison (1)

Poly resistor:

p-channel MOSFET:

C ≈1

2Cpoly −sub

C ≈Cdepletion+1

2Cchannel −gate + Cchannel −sub( )

gate

sub

channel

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 30

Cpoly-sub Cchannel-sub : 0.13 fF/m2

Cdepletion : 1.20 fF/m2

Cchannel-gate : 7.80 fF/m2

Poly resistor:

p-channel MOSFET:

C ≈1

2Cpoly −sub

C ≈Cdepletion+1

2Cchannel −gate + Cchannel −sub( )

Wchannel = Wdiff = 2.5 µm Lchannel = 0.18 µm

Ldiff = 0.3 µm

Wpoly = 0.6 Lpoly = 2.5

= 0.1 fF

= 0.9 fF + 1.8 fF + .03 fF = 2.8 fF

Capacitance Comparison (2)

(Numbers based on TSMC 180nm CMOS process)

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 31

R = 1.2 ks = 235

Wr = 0.6 µmLr = 2.5 µmCres = 0.1 fF

Wp = 2.5 µm Ldiff = 0.3 µm

Cd2 = 2.8 fF

M2 M2

M1 M1

M1 M1

Cd1 = 3.7 fFCg1 = 5.8 fF

Capacitance Comparison (3)

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 32

Pulse Response ComparisonPWin = 100ps

resistor loadR = 1.2 k

td = 16 ps; PWout = 100 ps

p-channel load(W/L)p = 2.5 µm / 0.18 µmtd = 20 ps; PWout = 98 ps

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 33

Eye Diagram Comparisonincluding mismatch effects

resistor load

= 1.5% mismatch

σR

R160mV gate-referred mismatch

σ ID

ID= 4% mismatch

p-channel load

ISI

DCD

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 34

MA MA

MBMB

MA MA

Series-Gated CML Topology

XOR gate:

Common-mode voltage of BP/N critical:

• Too low current source transistor biased in triode• Too high Transistors MB biased in triode

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 35

VBP −VBN

VS

I1 I2

I1−I2

-ISS

ISS

Slope = gm

Transistors should be biased in saturation to realize maximum gm .

Especially important when gate voltages exhibitslow slew rates

BP BN

VBP −VBN

Series-Gated CML (2)

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 36

VB(cm) = 1.6

VB(cm) = 1.3VB(cm) = 1.0

VBP −VBN

t

VB(cm) = 1.6

VB(cm) = 1.3 VB(cm) = 1.0

DC current:

Transient response:

(400mV amplitude sinewave applied to BP/BN)

IBP −IBN

IBP −IBN

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 37

Level-Shifting CML Buffer

VDD

Rcm

ISS

R R

+

_

ISSRcm

Output levels:

VDD −ISSRcm( )

VDD −ISSRcm( )−ISSR

Used to drive clock inputs of series-gated CML gates

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 38

Be reassigning the inputs, the XOR can be transformed into a Select circuit. Used in a 2:1 multiplexer.

CML Select Circuit

ISS

R R

AP AN BP BN

SELA SELB

OUTPOUTN

SELA

AP/N

BP/N

OUTP/N

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 39

CML Latch

By setting BP/N = OUTP/N, we can construct a CML latch:

ISS

DP DN

OUTPOUTN

CKP CKN

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 40

CML D Flip-Flop

Output OUTP/N is synchronized withCKP/N falling edge.

DP DN

CKP CKN

QINQIP

QIP QIN

OUTNOUTP

CKPCKN

CKP/N

DP/N

OUTP/N

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 41

CML Latch Design Considerations

IGG

VGG

dc operating points

Necessary criterion for bistability:

rgg =2

1/R −gm

=2R

1−gmR< 0 at middle operating point

(Equivalent to loop gain = gmR > 1)

slope=1/rgg

IGGVGG

12

ISS

R R

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 42

gmR >1

gmR ≈1

gmR ≤1

“transparent” latch

Avoiding Latch Transparency

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 43

R=1000

R=800

R=600

GBW parametercan be increased toensure bistability.

gmR >1

gmR ≈1

gmR <1

DP DN

CKP CKN

QINQIP

QIP QIN

OUTNOUTP

CKPCKN

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 44

Buffering Clock Signals (1)

Clock signals (generated from VCO or clock divider) often drive large capacitive loads.

1x

Fanout = n

τ =nRC → f−3dB =1

2π τ

C

C

C

For a large fanout, attenuation of clock amplitude will occur.

1x

1x

n

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 45

…1x k x k2 xn x

m stages

Now is increased by k << n less attenuation at each stage

Delay = mktp

ktp

Power = P1(1 + k + k2 + … + n)

As fclock 1/tp then k 1; number of stages and total power become very large.

Power dissipated by first stage

Buffering Clock Signals (2)

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 46

Since clock signal is made up of a single frequency (+ harmonics), resonance can be used to increase gain with greatly reduced power dissipation.

Y =1R

+ jωC +1

jωL=(1−ω2LC) + jω(L /R)

jωL

Resonant frequency:

ωr =1

LC

Y =1R

at resonance

If lossless inductors were available, we could achieve high gain at any frequencysimply by choosing the correct inductor value.

Buffering Clock Signals (3)

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 47

On-Chip Passive Elements

R =ρt⋅w

Resistor:

C ≈ ⋅w⋅εd

Capacitor:

dw

substrate

(+ fringing)

Inductor:

t

t

w

l

w

Inductance calculation much more complicated!

l

l

l

l

L≈0.2 ln

2t+w

⎝ ⎜

⎠ ⎟+0.50049 +

t+w3

⎣ ⎢

⎦ ⎥ pH/m

ll

l

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 48

tw

Special case of Greenhouse result

Note for l >> W, L is a weak function of w

To increase effective inductance per unit length, we make use of mutual inductance via spiral structure:

L≈0.2 ln

2t+w

⎝ ⎜

⎠ ⎟+0.50049 +

t+w3

⎣ ⎢

⎦ ⎥ pH/m

ll

ll

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 49

Modeling of Spiral Inductor

Design of inductor requires: • inductor simulation package (e.g., asitic) • trial and error• conversion to lumped

element model

number of turns n = 2

1

2

Accurate lumped model should include:• Series inductance (self + mutual) & resistance • Skin effect (frequency dependent series resistance)• Interwinding capacitance• Capacitance to substrate• Substrate capacitance & loss

Procedure for constructing lumped model:1. 2-port s-parameters over frequency range of interest

(this comes from the inductor simulator)2. Choose lumped circuit topology.3. Run simulations to find the optimal lumped circuit

element values such that the the circuit s-parametersare sufficiently close to the inductor’s s-parameters(can use .net and .optimize in HSPICE)

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 50

Parameters most relevant to circuit designers:• Inductance• Series resistance• Self-resonant frequency

http://rfic.eecs.berkeley.edu/~niknejad/asitic.html

Link to “asitic” web pages:

Modeling of Spiral Inductor (cont.)

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 51

1

2

Cint

L Rs

Cox2Cox1

Csub1 Csub2Rsub1Rsub2

1 2

L:Rs:

Cint:Cox:

Csub/Rsub:

Self/mutual inductanceSeries resistanceInterwinding capacitanceOxide capacitanceSubstrate capacitance/resistance

Values of L and Rs in lumped model should correlate with physical parameters.Values of other lumped model elements need not necessarily correlate with physical parameters.

Modeling of Spiral Inductor (cont.)

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 52

Parasitic capacitances usually combine with load capacitance L decreases slightly

Series Rs has more important effect:

Y =1R

+ jωC +1

Rs + jωL

′ Y =1′ R+ jωC +

1jω ′ L

L

Rs

C R R’CL’

At resonance, Im [Y(jr)] = 0:

ωr2=

1LC

−Rs

L

⎝ ⎜

⎠ ⎟2

ωr2=

1′ L C

Y jωr( ) =1R

+CRs

L

′ Y jωr( ) =1′ R

′ L =L

1−CRs2

L

′ R =R ||L

CRs

⎝ ⎜

⎠ ⎟

Slight increase in effective inductance Very important effect!

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 53

Differential-mode groundSets common-mode output voltage

Gain at resonant frequency = gm R’

CL resonates out with L

CML Tuned Amplifiers (1)

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 54

Symmetric inductor structure can be used:

Single structure allows more inductance tobe realized from mutual coupling less series resistance

CML Tuned Amplifiers (2)

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 55

Gain is much higher at resonance, but depends completely on Rs.Variation in gain correlates with variation in metal (not resistor) sheet resistance.

CML Tuned Amplifiers (3)

Higher-gain topology:

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 56

R’CL’Vswing

+

_

Iin

IL

Let Vswing = 500mV, L=0.5nH, f =10GHz:

Spiral inductor should be wide enoughto meet ac electromigration specs.

Iin =Vswing

′ R

IL =Vswing

ω ′ L

IL =16mA

Watch out for ac current amplitude in inductors!

CML Tuned Amplifiers (4)

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 57

Inductors in Broadband Circuits

R

R

Vin

Vout

+

_

LC lossless transmission line (Z0)R

RCVin

Vout

+

H(s) =12⋅

1

1+sCR2

ω

ω

|H( jω) |

∠H( jω)

−90 o

2CR

H(s) =12⋅e−sTd

Td = LC

forR =Z0

|H( jω) |

∠H( jω)

0.5 0.5

ω

ω

slope = -Td

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 58

Series Peaking (1)

CdCg

CdCg

Lser

With direct connection of 2 buffers,output & input capacitances are in parallel:

By connecting an inductor between the capacitors, the bandwidth and delay increase:

“Series peaking”

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine

59

Series Peaking (2)

Cd = Cg = 16 fFR = 400

Lser

Cd Cg

R

Vin+ Vin-

Vx+

Vx-

R ≈Lser

Cd

Using

Lser ≈CdR2set

Vx

Vin

∠Vx

Vin

⎝ ⎜

⎠ ⎟

Lser = 0BW = 6.3 GHz

Lser = 3.5 nHBW = 8.3 GHz

Lser = 3.5 nH

Lser = 0

Frequency response:

Vin

Vx (Lser = 0)

Transient response:

Vx (Lser = 3.5 nH)

Vin

Vx (Lser = 0)

Series peaking provides speed at the expense of extra delay.

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 60

Shunt-Peaking (1)

By connecting an inductor in series with the load resistor (series connection in shunt with output), more current is used, for a longer time, to charge the load capacitance.

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 61

Properties of Shunt-Peaking

ωr2=

1LCL

1−CLR

2

L

⎝ ⎜

⎠ ⎟

Resonant frequency:

No resonance for

LCLR

2<1

CL

X Re s

Im s

L = 0:pole at s = −1/RC

X O

L ≠ 0:zero at s = −R/Ladditional pole at s ≈ −(1/CR + R/L)

Z(s) =R ⋅1+ s

LR

1+ sCLR + s2LCL

Z( jω) =R ⋅1+ jω L

R1−ω2LCL( ) + jωCLR

Frequency response:

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 62

L =0BW = 6.3 GHz

LCLR

2=0.3

L = 1.8 nHBW = 9.4 GHz

LCLR

2=0.6

L = 3.7 nHBW = 14.3 GHz

Shunt-Peaking -- AC Response

Use of shunt-peakingincreases small-signal bandwidth

CL =38 fF

R =400

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 63

Shunt Peaking − Transient Response (1)

Step Response:

L =0td = 13.4 ps

L = 1.8 nHtd = 8.5 ps

L = 3.7 nHtd = 6.7 ps

Pulse Response (tin = 50 ps):

L = 3.7 nHtout = 50.8 psISI = 16 mUI

L = 1.8 nHtout = 50.0 psISI = 0 mUI

L = 0tout = 48.7 psISI = 26 mUI

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 64

Other Advantages of Shunt-Peaking

• CML load is passive & linear

• Can be shown to be very robust in the presence of parasitic series resistance and shunt capacitance inductors can be placed far away from other CML circuit elements.

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 65

long metal lines

Effect of Shunt-Peaking Inductor Parasitics (1)

• Series resistance RP simply adds to R

• Shunt capacitance CP resonates with L …

L L

R R

CL CL

CL CL

L L

R R

RP RP

CPCP

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 66

LCLR

2=0

LCLR

2=0.3

LCLR

2=0.6

LCLR

2=0

LCLR

2=0.3

LCLR

2=0.6

Moderate amount of parasitic capacitancehas similar effect to slightly larger inductor.

Disadvantages of using passive inductors:• Consume huge die area• Difficult to design & model

Effect of Shunt-Peaking Inductor Parasitics (2)

CP =0

CP =0.2CL

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 67

Multi-layer Inductors (1)

metal 6

metal 5d

Distance d between two metal layers is much smaller than lateral distances (e.g., w, l, s)

metal 6

metal 5

d

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 68

φ1

φ2

⎝ ⎜

⎠ ⎟=

L1 MM L2

⎣ ⎢

⎦ ⎥i1i2

⎝ ⎜

⎠ ⎟

L1 L21 2

+

_

+

_

i1 i2

2-port representation of coupled inductors:

M =k L1L2

Passivity constraint: 1≤k

For metal geometries close to each other, k is close to unity.

series connection of coupled inductors:

L1 L2

i1

i2

M

1

+

_2

+

_

φseries=φ1 +φ2 =(L1 +M)i1 +(L2 +M)i2

iseries=i1 =i2

Lseries=φseries

iseries

=L1 +L2 +2M

For L1 = L2 = L, we have:

Lseries=2L+2M =2L(1+k) ≈4L

In general, for n layers we have:

Lseries ≈n2L

Multi-layer inductors are more appropriate for shunt-peaking than resonant structures due to additional contact resistance.

Multi-layer Inductors (2)

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 69

Ci

Cj

For more details, see:

A. Zolfaghari, A. Chan & B. Razavi, “Stacked inductors and transformers in CMOS technology,”IEEE Journal of Solid-State Circuits, vol. 36, April 2001, pp. 620-628.

Leffective≈4L

Ceffective≈13

Ci +112

C j

Effective Capacitance:

Multi-layer Inductors (3)

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 70

metal 6 only100 x 100w = 4; s = 2; n = 4L=2.0 nHR=6.9

metal 6 over metal 446 x 46w = 4; s = 2; n = 2.5L=2.0 nHR=12.5

+

Multi-layer Inductors (4)

Area comparison:

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 71

Active Inductors (1)

Rgyr

+

_

v1

i1+

_

v2

i2

Ideal gyrator:

v2 = R gyr i1

v1 = −R gyr i2

Impedance inversion:

Rgyr

+

_

vin

iin

C

Zin =Rgyr2 sC( )

Port 1 exhibits inductance when port 2 is connected to a capacitance.

v1

v 2

⎝ ⎜

⎠ ⎟=

0 −R gyr

R gyr 0

⎣ ⎢

⎦ ⎥i1i2

⎝ ⎜

⎠ ⎟

Matrix representation (Z-parameters):

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 72

Active Inductors (2)

RG

+

_

v1

i1

+

_v2

i2

i1 applied with port 2 open-circuited:

i2 applied with port 1 open-circuited:

v2 =1

gm

i1

v1 = − RG −1

gm

⎝ ⎜

⎠ ⎟i2

(Assume RG gm > 1)

v1

v 2

⎝ ⎜

⎠ ⎟=

1 gm − RG −1 gm( )

1 gm 1 gm

⎣ ⎢ ⎢

⎦ ⎥ ⎥

i1i2

⎝ ⎜

⎠ ⎟

Complete Z-parameters (lossy/active gyrator):

Consider common-drain configuration:

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 73

v1

v 2

⎝ ⎜

⎠ ⎟=

1 gm 1 gm − RG

1 gm 1 gm

⎣ ⎢

⎦ ⎥i1i2

⎝ ⎜

⎠ ⎟

+

_

vin

Active Inductors (3)

Interpretation of non-ideal matrix entries:

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 74

Zsource =1

gm

1+ sC gs RG

1+ s Cgs gm

⎣ ⎢

⎦ ⎥

At low frequencies (Cgs open) Zsource = 1/gm

At high frequencies (Cgs short) Zsource = RG

Active Inductors (4)

Impedance at port 1 with port 2 terminated with transistor Cgs:

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 75

ω

gm

Cgs

1

Cgs RG

1

gm

RG

Zsource

Leff ≈CgsRG

gm

=RG

ωT

gmRG >1

Equivalent circuit:

+

_

vin

1

gm

RG −1

gm

Cgs ⋅RG

gm

Active Inductors (5)

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 76

Av =gm1

gm2

=W1

W2

For shunt peaking:

L ≈0.3CLR2

CgsRG

gm2

=0.3CL

gm22

gm2RG =0.3CL

Cgs

Low-frequency gain:

CML Buffer with Active Inductor Load

WL

⎝ ⎜

⎠ ⎟

1

=4

0.18

WL

⎝ ⎜

⎠ ⎟2

=2.50.18

ISS =400 μA

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 77

Active Inductor AC Response

RG = 4k

RG = 2k

RG = 0

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 78

Active Inductor Transient Response (1)

RG = 0PW = 97ps

RG = 5kPW = 100 ps

RG = 10kPW = 104 ps

Differential signals:

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 79

Active Inductor Transient Response (2)

Problem: n-channel load shifts output by Vt.Vsb > 0; body effects exacerbates this effect..

Single-endedinput

Single-endedoutputs

Single-ended signals:

EECS 270C / Spring 2009 Prof. M. Green / U.C. Irvine 80

Alternate topology:p-channel load exhibits lower Vt (Vbs = 0)

Active Inductor Alternate Topology

differential

single-ended

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