Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17

Lecture 17:

Common Source/Gate/Drain Amplifiers

Prof. Niknejad

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

Lecture Outline

MOS Common Source Amp

Current Source Active Load

Common Gate Amp

Common Drain Amp

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

Common-Source Amplifier

Isolate DC level

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

Load-Line Analysis to find Q

Q

D

DD outR

D

V VI

R

−=

1

10kslope =

0V

10kDI =

5V

10kDI =

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

Small-Signal Analysis

inR = ∞

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

sv

sR

inRoutR LRm inG v

inv

+

−

Two-Port Parameters:

Find Rin, Rout, Gm

inR = ∞

m mG g= ||out o DR r R=

Generic Transconductance Amp

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

Two-Port CS Model

Reattach source and load one-ports:

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

Maximize Gain of CS Amp

Increase the gm (more current) Increase RD (free? Don’t need to dissipate extra

power) Limit: Must keep the device in saturation

For a fixed current, the load resistor can only be chosen so large

To have good swing we’d also like to avoid getting to close to saturation

||v m D oA g R r= −

,DS DD D D DS satV V I R V= − >

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

Current Source Supply

Solution: Use a current source!

Current independent of voltage for ideal source

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

CS Amp with Current Source Supply

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

Load Line for DC Biasing

Both the I-source and the transistor are idealized for DC bias analysis

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

Two-Port Parameters

From currentsource supply

inR = ∞

||out o ocR r r=

m mG g=

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

P-Channel CS Amplifier

DC bias: VSG = VDD – VBIAS sets drain current –IDp = ISUP

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

Two-Port Model Parameters

Small-signal model for PMOS and for rest of circuit

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

Common Gate Amplifier

DC bias:

SUP BIAS DSI I I= =

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

CG as a Current Amplifier: Find Ai

out d ti i i= = −

1iA = −

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

CG Input Resistance

At input:

Output voltage:

t outt m gs mb t

o

v vi g v g v

r

−= − + +

( || ) ( || )out d oc L t oc Lv i r R i r R= − =

gs tv v= −

( )||t oc L tt m t mb t

o

v r R ii g v g v

r

−= + +

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

Approximations…

We have this messy result

But we don’t need that much precision. Let’s start approximating:

11

||1

m mbt o

oc Lin t

o

g gi r

r RR vr

+ += =

+

1m mb

o

g gr

+ >> ||oc L Lr R R≈ 0L

o

R

r≈

1in

m mb

Rg g

=+

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

CG Output Resistance

( ) 0s s tm gs mb s

S o

v v vg v g v

R r

−− − − + =

1 1 ts m mb

S o o

vv g g

R r r

+ + + =

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

CG Output Resistance

Substituting vs = itRS

1 1 tt S m mb

S o o

vi R g g

R r r

+ + + =

The output resistance is (vt / it)|| roc

|| 1oout oc S m o mb o

S

rR r R g r g r

R

= + + +

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

Approximating the CG Rout

The exact result is complicated, so let’s try tomake it simpler:

Sgm µ500≈ Sgmb µ50≈ Ω≈ kro 200

][|| SSombSomoocout RRrgRrgrrR +++=

][|| SSomoocout RRrgrrR ++≅

Assuming the source resistance is less than ro,

)]1([||][|| SmoocSomoocout RgrrRrgrrR +=+≈

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

CG Two-Port Model

Function: a current buffer

• Low Input Impedance• High Output Impedance

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

Common-Drain Amplifier

21( )

2DS ox GS T

WI C V V

Lµ= −

2 DSGS T

ox

IV V

WC

Lµ

= +

Weak IDS dependence

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

CD Voltage Gain

Note vgs = vt – vout||

outm gs mb out

oc o

vg v g v

r r= −

( )||

outm t out mb out

oc o

vg v v g v

r r= − −

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

CD Voltage Gain (Cont.)

KCL at source node:

Voltage gain (for vSB not zero):

( )||

outm t out mb out

oc o

vg v v g v

r r= − −

1

|| mb m out m toc o

g g v g vr r

+ + =

1||

out m

inmb m

oc o

v g

v g gr r

=+ +

1out m

in mb m

v g

v g g≈ ≈

+

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

CD Output Resistance

Sum currents at output (source) node:

|| || tout o oc

t

vR r r

i= t m t mb ti g v g v= +

1out

m mb

Rg g

≈+

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad

CD Output Resistance (Cont.)

ro || roc is much larger than the inverses of the transconductances ignore

1out

m mb

Rg g

≈+

Function: a voltage buffer

• High Input Impedance• Low Output Impedance

Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad