high-gain differential amplifier design

7/30/2019 High-Gain Differential Amplifier Design

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Lecture 13

High-Gain Differential Amplifier Design

Woodward Yang

School of Engineering and Applied Sciences

Harvard University

[email protected]


2/21

2ES154 - Lecture 13

Overview

Background

This lecture investigates different topologies (and their

characteristics) that can be used to implement differential amplifiers

with extremely high gain. We will again be using cascoding.


3/21

3ES154 - Lecture 13

Review of Amplifier Characteristics

Lets review some of the characteristics of the different (single-ended) amplifier topologies that weve looked at so far.

We will augment this table when we look at the frequency

response characteristics of these amplifiers

Amplifier Type Rin Rout Av Ai

Common-

source/emitterHigh High High High

Common-

gate/base Low High High 1

Common-

drain/collectorHigh Low < 1 High


4/21

4ES154 - Lecture 13

Multi-Stage Amplifiers (Cascading)

We can cascade different types of amplifiers to get desiredoverall characteristics. Often want:

High input impedance

High gain

Low output impedance

Mix and match cascades of different types of amplifiers to get

desired result


5/21

5ES154 - Lecture 13

Common-Emitter Emitter-Follower Cascade

A common configuration (for discrete BJT amplifier design) is a common-emitteremitter-follower (common-collector) cascade

CE stage has high voltage gain and high input impedance

CC stage has low output impedance to drive various load conditions

CC stage also presents a high impedance load to the CE amplifier whichenables high voltage gain for the CE stage

R1

R2

Rs

vS

RC

REA

REB CE

RE2

RLD

vO

Q1

Q2

Cout

Cin


6/21

6ES154 - Lecture 13

Common-Source Source-Follower Cascade

Similarly, cascade a common-source amplifier with a source-follower.

Rs

vS

RD

IS1

CS R

LD

vO

M1

M2

Cout

IS2


7/21

7ES154 - Lecture 13

Building Op Amps

Op amps are an important component of modern CMOS ICs. They used to designed asgeneral purpose amplifiers that can meet a variety of requirements. The main target was

extremely high gain (>1e5), high input impedance and low output impedance (like an ideal

amplifier). This was done (to some extent) at the expense of different aspects of

performance (e.g., speed, output voltage range, power, etc.). Designs these days are much

more tailored to have (good enough) performance w.r.t. the specific needs of particular

applications. Within an IC, often use Operational Transconductance Amplifiers (OTA).

Some performance parameters of op amps

Gain and Bandwidth

Want as large as possible

Output Swing

Maximize w.r.t. power supply (but supply shrinking in modern processes)

Linearity

Combat non-linearity with feedback

Noise and Offset

Can minimize by trading off other parameters

Supply Rejection

Strong dependence on current source output resistance


8/21

8ES154 - Lecture 13

Simple One-Stage Op Amps

Two differential pair amplifiers that we have already seen can be used as op

amps. The low-frequency, small-signal gain of both is gmN(roN||roP). The

capacitive loads (CL) usually determine their bandwidth.

CL

Vout

Vin

CL

Vout

Vin

CL

Vb


9/21

9ES154 - Lecture 13

Cascoded Amplifier

Use cascoding to increase load resistance Cascode both the active loads and the

differential pair

Higher effective load resistance

Higherro for the differential pair

Reduces Miller effect (will see later)

However, there are some limitations

Reduced output swing (must keep all

devices in saturation)

What is the output dynamic range?

How might one increase the output swing

range for vo?

vo

I

M1 M2

vid

Vbias

M4M3

M6M5

M8M7


10/21

10ES154 - Lecture 13

Use High-Swing Cascodes

We can use the high-swing cascode circuit as a load to achieve higher outputrange in a single-ended output telescopic amp

CL

Vout

Vin

Vb

CL

Vout

Vin

Vb1

Vb2


11/21


Cascode Op Amps

Amplifiers that use cascoding are often called telescopic cascode amps. Whilegain increases, the output range of these devices are limited.

Connecting in unity-gain feedback configuration results in significantreduction of output range

CL

Vout

Vin

CL

Vout

Vin

CL

Vb3

Vb

Vb2

Vb1


12/21


DC Biasing for High-Gain Amplifiers

One of the challenges of using cascodes for high gain is appropriately settingthe DC biasing for the circuit. Lets look at an example

What is the raitio of ILOAD vs. ITAIL?

vd

vOUT

ITAIL

ILOAD

ILOAD

IREF

VBN

VBNC

VBPC

VBP


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DC Biasing Contd

Strategy for setting up DC bias All transistors should be saturation

Set VBNC so that differential input pair in saturation

Want to set it to the edge with sufficient saturation margin(~300mV)

Set VBP so that ILOAD = ITAIL/2 Set VBPC so that pMOS currnet source loads are close to

edge of saturation

Need to set VBP and VBPC carefully to keep devices insaturation and the DC common mode of the output nodes tobe in the middle of the output swing range

This can be challenging to do due to the high output resistanceat the output.

Would be nice if there was a way to automatically set thebiasing


14/21


Common-Mode Feedback Biasing

Use an amplifier to set the pMOS current source with respect to somedesired output common-mode voltage (VREF).

vd

vOUT

ITAIL

ILOAD

IREF

VBN

VBNC

VBPC

VBP

VREF


15/21


CM FB Biasing

Heres how it works: Use large resistors to find the average (common-mode)

output voltage

An amplifier compares VREF to VOUT,CM and sets VBP such

that VOUT,CM = VREF

Lets understand how it works

What happens to VBP if VREF increases?

What happens to VBP if VOUT,CM increases?


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Folded Cascode Circuit

In order to alleviate some of thedrawbacks of telescopic op amps (limited

output range), a folded cascode can be

used

M1 is common-source

transconductance amp and M2 is

common-gate transimpedance amp Advantage is M2 no longer stacks on

top of M1

Possible for either pMOS or nMOS

cascodes

The output resistance for cascode andfolded cascode are roughly equivalent

(gmro2)

Vout

Vin

Vb

M1

M2

VoutVinV

b

M1

M2

Vout

Vb

M1

M2

Vout

Vin

Vb

M1

M2

Vin


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Folded Cascode Amplifier

Turn a differential telescopic cascode amplifier into a folded cascode amplifier

Vb

Vin

Vout

Vb

Vin

Vout


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Full circuit Implementation

of Folded Cascode Amplifier

Reference current sources are set: A version with nMOS differential pair inputs also possible (flip upside down)

What sets output common mode?

Depends on relative output resistances looking up and down

Can vary with process and reference current mismatches

2123 REFREFREF III

Vbn2

Vin

Vout

Vbp2

IREF1

IREF2

IREF3


19/21


Gain of a Folded-Cascode Amplifier

Calculate gain using the differential half-circuit. Gaincan be calculated as GmRoutwhere Gm is the short-

circuit transconductance of the overall circuit and Rout

is the output resistance.

Short out Vout to ground and solve for Iout/Vin = Gm

Solve for the output resistance

Vin

-Vx

ro45

ro3

-gm3Vx

gm1Vin

ro1||r

o2

Vout

M4

Vbn2

Vin

Vout

Vbp2

Vbp1

Vbn1

M1

M2

M3

M5

ro45


20/21


Common-Mode Feedback

Use feedback to set the output common mode of a folded cascode amplifier,called common-mode feedback

Sense the average (common-mode) voltage at the output, compare to a

desired reference voltage (Vref), and use it to set the current source

ForVin=0, feedback sets IFB=IREF2+IREF1/2 and common-mode voltage = Vref

VbV

in

Vout

CM

Sense

Vref

IREF1

IREF2

IREF2

IFB


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Two-Stage Op Amps

In order to implement amplifiers with high gain and high swing, we must resort totwo-stage amplifier designs

First stage used to generate high gain

Second stage to generate high swing

Use any high-gain first stage and high-swing second stage

two simple examples (differential and single-ended output amplifiers)

High-Gain

Stage

High-Swing

StageV

inV

out

Vin

Vbp

Vbn

Vin

Vbp

Vout

Vout2

Vout1

high-gain differential amplifier design

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