1The Flipped Voltage Follower (FVF)

A useful cell for low-voltage, low-power circuit design

part of this material was provided by Profs. A.Torralba1 J. Ramrez-Angulo2, R.G.Carvajal1, A. Lpez-Martn3,

ELEN 607 (ESS)

2OUTLINE

1. Introduction.2. The Basic FVF.3. FVF Structures.4. Applications.5. Conclusions.

3The Basic Flipped Voltage Follower (FVF)

gm1 gm2 ro1

gm1

gout

= 1IbHIGHb) FVF< 1HIGHIba) ConventionalAvSinkingSourcing

a) Conventional

Voltage Follower

[Ramirez-Angulo92] J.Ramrez-Angulo, R.G.Carvajal, A.Torralba, J.Galn, A.P.VegaLeal, and J.Tombs. The Flipped Voltage Follower: a useful cell for low-voltage low power circuit design, Proc. ISCAS02, vol. 3, pp. 615-618, 2002.

b) (FVF)Ib

M1

M2vo

vi

4FVF transistors voltage limits to keep them in saturation

5The Basic FVF Analysis

I b

M1

VoX

rb

M 2

Cc

Vi

Vr

Y Compensation

Dc gainAOL = Vr / Vi = - gm2 ROLYROLY = rb || gm1 ro1 ro2 ,ROLX (1+rb/ro1)/gm1 || ro2

Dominant PoleAt Y: wpY = 1 / CY ROLY

Non-Dominant PoleAt X: wpX = 1 / CX ROLX

Gain-Bandwidth ProductGB = gm2 / CY

Open Loop Analysis

CX parasitics at node X (incl. LOAD)CY parasitics at node Y (incl. Cc if any)

6Stability Criterion: wpX > 2 GB

For Ib a simple current mirror (rb ro1)

For Ib a cascode current mirror(rb gm1 ro1 ro2)

usually requires compensation

2. The Basic FVF

I b

M1

VoX

rb

M 2

Cc

Vi

Vr

Y

2

1

4gmgm

CC

Y

X