Lecture17 Ee474 Folded Cascode Ota

7/27/2019 Lecture17 Ee474 Folded Cascode Ota

1/17

Sam Palermo

Analog & Mixed-Signal Center

Texas A&M University

ECEN474: (Analog) VLSI Circuit DesignFall 2010

Lecture 17: Folded Cascode OTA


2/17

Announcements

HW4 Due Friday 10/22 Exam 2 Wednesday Nov 3 9:10-10:10AM

2


3/17

Agenda

Single-Stage Cascode OTA Folded Cascode OTA

3


4/17

Simple OTA

4

M1 M2Vi+ Vi-

VDD

VSS

Rbias

Itail

M5 M6

M3 M4

CL

Vo

( )621 || oomoutmv rrgRGA ==GainDC

Gain is limited by single-transistor outputresistance


5/17

Single-Stage Cascode OTA

Gain is larger by a gmro factor

Output swing range is limiteddue to large compliancevoltage of cascode currentsource load

5

( )6864241 || oomoommoutmv rrgrrggRGA =GainDC

[Razavi]


6/17

Single-Stage Cascode OTAUnity Gain Feedback Voltage Range

6

( )!!!athanLess

RangeInput)(&Output

conditionsatM2intopluggingandAs

saturationM4bysetVMinimum

saturationM2bysetVMaximum

out

out

TH

THGSTH

xGSxb

THbout

THxout

VVVV

VVVV

VVV

VVV

244

4

4

2

=

+=

+

Cascode configuration constrains output & unity-gain swing

[Razavi]


7/17

Folded Cascode Circuits

7

Folding about the cascode node will increase input and

output swing range

PMOS Input & NMOS Cascode

NMOS Input & PMOS Cascode

[Razavi]


8/17

Folded Cascode OTA

8

[Razavi]


9/17

Folded Cascode OTAUnity Gain Feedback Voltage Range

9

With proper (high-value) choice of Vb2, a decent output andinput swing range can be achieved

1

2 ||

GSDSATIoutDSATNDSATNCout

THPbout

VVVVVV

VVV

MP

Tail++

+

OR

saturationsourcecurrenttailorcascodeNMOSoutputbysetVMinimum

saturationbysetVMaximum

out

out

MP

MNC


10/17

TAMU-ELEN-474 2009 Jose Silva-Martinez

- 1 0 -

Folded-Cascode OTA: gm, rout and poles?

VB1 and VB2 must keep M1

- M5 in saturation region

VB2 > Vsat,4 + VGS3

VB1 < VDD - Vsat,5 VSG2

Notice that ID5 biases both M2 and M1

( )( ) ( )43351221 ; dsmdsdsdsmdsoutmm rgrrrgrrgG =

(for M4 sat)

(for M5 sat)


11/17

TAMU-ELEN-474 2009 Jose Silva-Martinez

- 11 -

Example: Folded-Cascode OPAMP

Find the gain and the phase from input to output and from input to node 2.

The low frequency gain is 77 dB and the unity gain frequency is around 80 MHz.

The behavior of the gain from the input to node 2 is interesting: above the dominant pole.

51

1

oo

m

gg

g

+

2

1

m

m

g

g

+ Loutoo

m

z Crgg

g 1

51

2

2


12/17

TAMU-Elen-474 Jose Silva-Martinez-08

- 1 2 -

There is a limitation due the

non- dominant pole

IBIAS ADC

Wcasc VDSAT

+

+

=

o

L

mc

po

md

in

o

g

Cs1g

Cs1

1

g

g

v

v

TYPICAL FOLDED CASCODE (One of the most popular circuit in ADCs)


13/17


- 1 3 -

FOLDED-CASCODE OTA

Frequency response:

Righ hand side

Righ hand side: 2 poles (at VW and Vout)

Left hand side: 4 poles !!! (at VX, VY, VZ and

Vout)

The poles at Vy and Vz are associated to

N-type transistors higher frequencies

( )

mp

PC

out

outout1mV

g

sC1

1

g

sC1

1RgsA+

+

vin-

2ID1

M1

MP

vout

VB2

VB1vin+

2ID1

VB2 iOUT1

2ID1

VDD

-VSSMN

VB3

MN

VB1

VW

VX

VY

VZ


14/17


- 1 4 -

Output referred noise

M1 produces an output current given by

Each transistor M2 generates a differential output

current

Similarly, for each transistor M5

At low and medium frequencies, noise contribution

of the cascode transistors can be neglected (M3 and

M4)

Vn4

ro5

i04

M4054n4n

054m4m04

r

vvrg1

gi +=

For cascode transistors

1n1m01 vgi =

2n2m02 vgi =

5n5m05 vgi =

vin-

2ID1

M1

M2

vout

VB2

VB1vin+

2ID1

VB2iOUT1

2ID1

VDD

-VSSMN

VB3

MN

VB1

VWVX

VY

VZM4

M5

M3

iout2 = 2(ieq1

2 + ieq22 + ieqn

2 )meq kTgi3

82 =Remember


15/17


- 1 5 -

Noise density and noise level

The output referred current spectral density is

And the input referred noise density becomes

The noise level is

25n

25m

22n

22m

21n

21m

2T0 vgvgvg2i ++=

21m

25n

25m

22n

22m

21n

21m2

in,eqg

vgvgvg2v

++=

++==

BW2

1m

25n

25m

21m

22n

22m2

1nBW

2in,eqnoise df

g

vg

g

vgv2dfvv

FOLDED-CASCODE OTA

vin-

2ID

1

M1

M2

vout

VB2

VB1vin+

2ID

1

VB2iOUT1

2ID

VDD

-VSSM5

VB3

M5

VB1

VWVX

VY

VZ


16/17


- 1 6 -

Noise level for the folded-cascode OTA

Lets consider thermal noise (vn2 =(8/3)KT/(gm))

Low-noise is associated with large gm1

and relatively small gm2 and gm5

++=

BW2

1m

25n

25m

21m

22n

22m2

1nnoise dfg

vg

g

vgv2v

++=

BW m

m

m

m

m

noise dfg

g

g

g

gkTv

2

1

52

1

2

1

1

3

16

( )

++

1

5

1

2

1

18

m

m

m

m

m

noiseg

g

g

gBW

g

kTv

Or for a dominant (single) pole system with NBW = (/2)BW

vin-

2ID1

M1

M2

vout

VB2

VB1vin+

2ID1

VB2iOUT1

2ID1

VDD

-VSSM5

VB3

M5

VB1

VWVX

VY

VZ

Noise of diff pair Noise Factor

(due to other transistors)


17/17

Next Time

Two Stage Miller OTA

17

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Lecture17 Ee474 Folded Cascode Ota