ECE 546 Lecture 12 Integrated Circuitsemlab.illinois.edu/ece546/Lect_12.pdfLecture 12 Integrated...

ECE 546 – Jose Schutt‐Aine 1

ECE 546Lecture 12

Integrated CircuitsSpring 2020

Jose E. Schutt-AineElectrical & Computer Engineering

University of [email protected]


Integrated Circuits

• IC Requirements– Biasing of ICs is based on the use of constant

current sources– Use current mirrors– Source circuits are used as loads


• Analog Design Requirements– Analog ICs may need resistors and capacitors for

the design of amplifiers– Resistors and capacitors occupy the space of

tens or hundreds of MOS devices– It is important to minimize their use

Integrated Circuits


Transistor Biasing


Transistor Biasing


Current Mirrors

A current mirror will reproduce a reference current to the output while allowing the output voltage to assume any value within a specified range. Io=KIin where K is a factor that can be less than or equal or greater than 1


MOS Current Mirror

2'1

1

12D n GS Tn

WI k V VL

1DD GS

D REFV VI I

R

2'2

2

12o D n GS Tn

WI I k V VL

R is usually external to IC


MOS Current Mirror

2 12

1 21

//

o

REF

W LI W LI W L W L

Assuming that the transistors are using the same process

• Can be limited by– Channel length modulation ()– Threshold voltage mismatch– Imperfect geometrical matching


MOS Current Mirror

2 22 1

1 2 1 1

11

DS DSPo

REF DS DSP

V VI W LI W L V V

• Some Properties1. MOS current mirrors draw zero control

currentbetter than BJT’s2. Matching of threshold voltages harder than

in BJT’s


ExampleA matched pair of MOSFETs are used in a current mirror witl = 0.032 V-1, Cox=70 A/V2, W/2L =10, and VT = 0.9 V. Find the value of R to create an input current of 100 A. Calculate the output current when Vo = 3 V.

21 11

2ox

D GS T DSC WI V V V

L

Use drain current equation in active region to calculate

21 100 700 0.9 1 0.032*D GS GSI V V

We can now solve for the value of VGS


ExampleMOS Current Mirror


Example

1

1

5 5 1.272 37.20.1

DS

D

VR kI

The resistance needed is:

The output current is calculated from:

22 700 1.272 0.9 1 0.032 3 106DI A

VGS = 1.272 V

2 106DI A

22 11

2ox

D GS T DSC WI V V V

L


Ideal MOS Common Source CKT

, ,i vo m ds o dsR A g r R r

m dsIntrinsic gain is g r


PMOS Implementation of Active Load

1 1 1ds oLet r r for Q

2 2 2ds or r for Q

1 2, ||out o othen R r r

1MB m outA g R

1 11

1o ds

o

Let g gr

2 22

1o ds

o

g gr


PMOS Implementation of Active Load

01 022

1 2 1 22odb db gd gd

g gf

C C C C

Can show that the 3dB point is given:

Large incremental load leads to high gain while maintaining acceptable DC current (resistor would not work)


Background• Differential Amplifiers

– The input stage of every op amp is a differential amplifier

– Immunity to temperature effects– Ability to amplify dc signals– Well-suited for IC fabrication because

– (a) they depend on matching of elements– (b) they use more components

– Less sensitive to noise and interference– Enable to bias amplifier and connect to other

stage without the use of coupling capacitors


Differential Amplifiers

• Practical Considerations– Both inputs to a differential amplifier may have

different voltages applied to them– In the ideal situation with perfectly symmetric

stages, the common-mode input would lead to zero output

– Temperature drifts in each stage are often common-mode signals

– Power supply noise is a common-mode signal and has little effect on the output signal

MOS Differential Pair

Assume current source is ideal

Transistors should not enter triode region

Common-Mode Operation

Input voltage vcm to both gates

Difference in voltage between the two drains is zero

Differential Input Voltage

Differential pair responds to differntial input signals by providing corresponding differential output signal between the two drains.



Assume current source is idealvID=vgs1-vgs2Output is collected as vD2-vD1


- If vID is positive, vD2-vD1 is positive

vID>0 vgs1>vgs2 ID1 > ID2vD1 lower voltage point than vD2


For proper operation, MOSFETS should not enter triode region


DC Analysis

1 2D

D DDIRV V 2 2

DD DD

IRV V

2

2ox

D GS TC WI V V

L

2DII

GS Tox

LIV VC W

SQ Tox

LIV VC W


Incremental Analysis

'1 2

ino m D

vv g R Neglecting the body effect

'2 2

ino m D

vv g R

' ||D D outR R r '2 1o oD m D

in

v vA g Rv

112

g cm idv v v 212

g cm idv v v


'

12high

out D

fC R

Frequency Response

When driven by a low-impedance signal source, the upper corner frequency is determined by the output circuit


1 2

1 2o o D

icm icmSS

m

v v Rv v R

g

Assume RSS >> 1/gm

Common-Mode Rejection Ratio


1 2

2o o D

icm icm SS

v v Rv v R

1,2 2

Dcm d m D

SS

RA A g RR

(a) For single-ended output:

dm SS

cm

ACMRR g RA




(b) For differential output:

2 1 0o ocm

icm

v vAv

2 1o od m D

id

v vA g Rv

CMRR


Differential Amplifiers - Observations

• Observations– The differential pair attenuates the input signal of

each stage by a factor of one-half cutting the gain of each stage by one-half

– The double-ended output causes the two single-ended gains to be additive

– Thus, the voltage gain of a perfectly matched differential stage is equal to that of a single stage


Replacing drain resistances with current sources, results in much higher voltage gain and savings in chip area in diff amp

MOS Differential Amp with Active Load

MOS Differential Amp - Equilibrium


Current mirror action makes it possible to convert the signal to single-ended form without loss of gain.


2 4||od m o o

id

vA g r rv

2 4o o oIf r r r

12d m oA g r

The differential gain is:


The active-loaded MOS differential amplifier has a low common-mode gain high CMRR


4

3 3

12 1

o ocm

icm SS m o

v rAv R g r

3 3 3 4, 1m o o oUsually g r and r r

The common-mode gain is:

3

12cm

m SS

Ag R

RSS is internal impedanceof current source



2 4 3|| 2dm o o m SS

cm

ACMRR g r r g R

A

Since RSS is large, Acm will be small

2 4 3o o o m mIf r r r and g g

m o m SSCMRR g r g R


In the differential amplifier shown, Q1 and Q2 form the differential pair while the current source transistors Q4 and Q5 form the active loads for Q1 and Q2 respectively. The dc bias circuit that establishes an appropriate dc voltage at the drains of Q1 and Q2 is not shown. The following specifications are desired: differential gain Ad = 80V/V, IREF = 100 A, the dc voltage at the gates of Q6 and Q3 is +1.5V; the dc voltage at the gates of Q7, Q4 and Q5 is –1.5V.

CMOS OP Amp Example

The technology available is specified as follows: nCox=3pCox = 90A/V2; Vtn=|Vtp|=0.7V, VAn=|VAp| = 20V. Specify the required value of R and the W/L ratios for all transistors. Also, specify IDand VGS at which each transistor is operating. For dc bias calculations, you may neglect channel-length modulation. Fill in the entries in the table provided to show your results.


CMOS OP Amp Example


1.5 ( 1.5) 3100 300.1REF

VI A R kR mA

Drain currents are determined by symmetry and inspectionVGS values are also determined by inspection for all transistors except Q1 and Q2. To determine VGS for Q1 and Q2, we do the following: the equivalent load resistance will consist of ro1 in parallel with ro4 for Q1 and ro2 in parallel with ro5 for Q5. Since the ro’s are equal, this corresponds to ro/2. We have:

2 2 80 0.4 /2 400o d

m d mo

r Ag A g mA Vr k

CMOS OP Amp Example


Take polarity into account for PMOS

1,2 0.25 0.95GS TV V

To find W/L ratios, use

22

2( )2 ( )

DD ox GS T

ox GS T

IW WI C V VL L C V V

taking into account PMOS and NMOS devices separately

2 2 2 0.05 0.250.4

D Dm ov

ov m

I Ig VV g

CMOS OP Amp Example


Q1 Q2 Q3 Q4 Q5 Q6 Q7 Units

Cox 30 30 30 90 90 30 90 A/V2

ID 50 50 100 50 50 100 100 A

VGS -.95 -.95 -1 +1 +1 -1 +1 V

W/L 57.3 57.3 74 1. 12.3 12.3 73.1 24.7

CMOS OP-AMP DESIGN TABLE

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