Lecture 16-1

IC implementations• So far, we talked mostly about discrete circuits...

• In IC:

• Very rare use of capacitors, inductors practically never (only in some RF circuits)

• - you typically do not have resistors > several K Ohm,

• - the larger resistance, the more expensive it is,

• - a resistor is much more expensive than a bunch of transistors!

9V

10kΩ

10kΩ

9V

1kΩ

1kΩ

9V

270kΩ

100kΩ

3.3V

Lecture 16-2

Opamps

• Previously we looked at the block-level internal structure of an operational amplifier

• We are now starting to look at the transistor level implementations of these blocks

• What sort of amplifier circuits would you propose for the blocks below based on what you’ve seen so far?

vo

vid

1

2+

_

+

_

Gm

C (compensation capacitor)

+1µ–

DifferentialInput Trans-conductanceAmplifier

VoltageAmplifier

Unity-GainBuffer

Lecture 16-3

Opamps

• We macromodeled these blocks in the following way:

vo

1

2

+

_

C

Gmvid Ro1 Ri2

+

_

µvi2–vi2

• What do the R’s represent?

vid+

_

Lecture 16-4

IC implementations

• There is one more thing you have when you design IC:

Device Matching

• This would be to some extent possible, but veeeeeeeeeeeeeery expensive in discrete design

For example:

- electrical parametes of device 0.1 %

- temperature of operation of many devices can be almost the same.

Lecture 16-5

Basic Differential Amplifier

• Emitter voltage becomes whatever value is necessary so that forward active transistor currents sum to current source value, I

VCC

RC

vB1

VCC

RC

vB2 vC1 vC2

I

vE iE1 iE2

VEE

Lecture 16-6

Basic Differential Amplifier

• You can express input signal in more convenient terms:

VCC

RC

vB1

VCC

RC

vB2 vC1 vC2

I

vE iE1 iE2

VEE

common mode signal:

differential mode signal:

then:

Lecture 16-7

Basic Differential Amplifier

• Differential output rejects common mode inputs

VCC

RC

vCM

VCC

RC

vCMvC1 vC2

I

vE iE1 iE2

VEE

+ vDiff -

Lecture 16-8

Differential Amplifier: Qualitative View (1)

• Think about seesaw...

VCC

RC

VCC

RC

vCM

vC1 vC2

I

vE iE1 iE2

VEE

- vDiff +

-vD/2 vD/2

vCM

-vD/2

vD/2

vCMvB1 vB2

• Is vE constant for a given value of vCM and varying vD?

Lecture 16-9

Differential Amplifier: Qualitative View(2)

• Is vE constant for a given value of vCM and varying vD?

I

vE

VEE

-vD/2

vD/2

vCM

vB1 vB2

vC1 vC2

I

vE iE1 iE2

VEE

vB1 vB2

Lecture 16-10

Mismatch of elements• Differential output rejects common mode inputs, but what if T1 and T2 are

not identical or “Rc in not identical to Rc”?

VCC

RC

vCM

VCC

RC

vCM vC1 vC2

I

vE iE1 iE2

VEE

+ vDiff -

β=100 β=101

Lecture 16-11

What if we have only single ended output?

• Using transistors as loads we can do some tricks not to lose gain for asymmetric output - you will see this later.

VCC

RC

VCC

RC

vB2

I

vE iE1 iE2

VEE

vB1

Lecture 16-12

ECL

• Basic component of an emitter-coupled logic (ECL) gate

VCC

RC

vin

VCC

RC

vref

vo1 vo2

I

vE iE1 iE2

VEE

Lecture 16-13

Switch Example

RC110E3Ω

+ 10VVCC

+

-

VO2

10.000 V

+ 0VVREF

RC210E3Ω

IC1

967.584 µA

+ -10VVEE

+

-

VIN

1000.000 mV

1E-3AI

IC2

19.775 pA

+

-

VO1

324.164 mV

+ 1VVIN1

Lecture 16-14

dc Characteristic

-1 0 1

0

2

4

6

8

10

V

VO2 VO1

Vin

Lecture 16-15

Small Signal Differential Amplifier

• For analog applications we use the differential amplifier in a small signal sense

VCC

RC

vcm+vi

VCC

RC

vcm vo1 vo2

I

VEE

+ vdiff -

I2--- ∆I+ I

2--- ∆I–

Lecture 16-16

PNP Differential Amplifier

• Works the same way, but VEE is more positive than VCC

VCC

RC

vcm+vi

VCC

RC

vcm

vo1 vo2

I

VEE

- vdiff +

Lecture 16-17

Small Signal Differential Amplifier

• Assume that the common mode signal has been used for biasing and the input is a small signal differential input

VCC

RC

vd

VCC

RC vo1 vo2

I

VEE

R

+

_

Q1 Q2

What is the added resistor modeling?

Lecture 16-18

Small Signal Model of Diff Amp

• Establish small signal model the same way as we did for other amplifiers

• Fabricated very carefully for perfect matching of parameters

vd/2

vo1

RC -vd/2

gmvπ2

vo2

RC gmvπ1

R

rπ rπ

Lecture 16-19

Calculate Gain

vd/2

vo1

RC -vd/2

gmvπ2

vo2

RC gmvπ1

+ vdiff -

rπ rπ

Lecture 16-20

Calculate Gain

vid/2

vo1

RC -vid/2

gmvπ2

vo2

RC gmvπ1

+ vdiff -

• What is the impact of ro?

rπ rπ

Lecture 16-21

Differential Input Resistance

• The differential input resistance is huge

vo1

RC gmvπ2

vo2

RC gmvπ1

+ vdiff -

Rid

rπ rπ

Lecture 16-22

Differential Input Resistance

vo1

RC gmvπ2

vo2

RC gmvπ1

+ vdiff -

Rid

rπ rπ

Lecture 16-23

Common Mode Gain

• Just consider the common component of the input signal

• By symmetry arguments we only have to look at half of the ckt

vcm

vo1

RC gmvπ1

R

i1 i2

rπ

Lecture 16-24

Common Mode Gain

• “Looks” like a common emitter amplifier with an emitter resistor

vcm

vo1

RC gmvπ1

2R

rπ

Lecture 16-25

Common Mode Gain

vcm

vo1

RC gmvπ1

2R

rπ

Lecture 16-26

Common Mode Rejction Ratio

• If output is taken differentially, then the CMRR is apparently infinite

• But for a single sided output response is has a finite value

CMRRAd

Acm----------=

Lecture 16-27

Common Mode Rejction Ratio

• More importantly, due to mismatch in parameters even the differential output CMRR is not infinite

• Assume one side has a variation in RC

vcm

vo1

gmvπ1

2R

rπRC ∆RC+

Lecture 16-28

Common Mode and Differential Gain

• Process is controlled as tightly as possible to minimize Acm

vo Ad v1 v2–( ) Acm

v1 v2+

2-----------------

+=

• Mismatch in transistors Q1 and Q2 creates a dc offset voltage

VCC

RC

VCC

RC

I

VEE

+ Vo -

Lecture 16-29

Common Mode Characteristics

• Differential inputs (like opamps) have a common mode input resistance too --- see derivation in the book

• Also, there is an input bias current to both inputs

• When the transistors are not perfectly matched there will be a slight offset in these values