ee105 fall 2007lecture 19, slide 1prof. liu, uc berkeley lecture 19 outline common-gate stage source...
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EE105 Fall 2007 Lecture 19, Slide 1 Prof. Liu, UC Berkeley
Lecture 19
OUTLINE• Common-gate stage• Source follower
Reading: Chapter 7.3-7.4
ANNOUNCEMENTS• For Problem 4 of HW10, use VDD = 1.8V and VTH = 0.4V
• Note: Midterm #2 will be held on Thursday 11/15
EE105 Fall 2007 Lecture 19, Slide 2 Prof. Liu, UC Berkeley
Diode-Connected MOSFETs
• Note that the small-signal model of a PMOSFET is identical to that of an NMOSFET
11
1o
mX r
gR 2
2
1o
mY r
gR
Diode-connected NMOSFET
Small-signal analysis circuit Small-signal analysis circuit
Diode-connected PMOSFET
EE105 Fall 2007 Lecture 19, Slide 3 Prof. Liu, UC Berkeley
Summary of MOSFET Impedances• Looking into
the drain, the impedance is ro if the gate and source are (ac) grounded.
• Looking into the gate, the impedance is infinite (∞).
• Looking into the source, the impedance is 1/gm in parallel with ro if the gate and drain are (ac) grounded.
EE105 Fall 2007 Lecture 19, Slide 4 Prof. Liu, UC Berkeley
Common-Gate Amplifier Stage• An increase in Vin decreases VGS and hence decreases ID.
The voltage drop across RD decreases Vout increases
The small-signal voltage gain (Av) is positive.
Dmv RgA
EE105 Fall 2007 Lecture 19, Slide 5 Prof. Liu, UC Berkeley
Operation in Saturation Region• For M1 to operate in saturation, Vout cannot fall below Vb-VTH.
Trade-off between headroom and voltage gain.
EE105 Fall 2007 Lecture 19, Slide 6 Prof. Liu, UC Berkeley
I/O Impedances of CG Stage ( = 0)
Dout RR m
in gR
1
Small-signal analysis circuit fordetermining output resistance, Rout
Small-signal analysis circuit fordetermining input resistance, Rin
EE105 Fall 2007 Lecture 19, Slide 7 Prof. Liu, UC Berkeley
CG Stage with Source Resistance
S
m
Dv
Rg
RA
1
in
mS
mX v
gR
gv
1
1
1
1
SmDm
in
X
X
out
in
out
RgRg
v
v
v
v
v
v
Small-signal equivalent circuit seen at input
For= 0:
EE105 Fall 2007 Lecture 19, Slide 8 Prof. Liu, UC Berkeley
OSOmSSmOout rRrgRRgrR 11
• The output impedance of a CG stage with source resistance is identical to that of CS stage with degeneration.
Small-signal analysis circuit fordetermining output resistance, Rout
EE105 Fall 2007 Lecture 19, Slide 9 Prof. Liu, UC Berkeley
CG Stage with Biasing• R1 and R2 establish the gate bias voltage.
• R3 provides a path for the bias current of M1 to flow.
Dm
Sm
m
in
out RgRgR
gR
v
v
/1||
/1||
3
3
EE105 Fall 2007 Lecture 19, Slide 10 Prof. Liu, UC Berkeley
CG Stage with Gate Resistance• For low signal frequencies, the gate conducts no current. Gate resistance does not affect the gain or I/O impedances.
EE105 Fall 2007 Lecture 19, Slide 11 Prof. Liu, UC Berkeley
CG Stage Example
DOS
m
Omout RrRg
rgR ||||1
1
2
11
Smm
Dm
in
X
X
outv Rgg
Rg
v
v
v
vA
21
1
1
Small-signal equivalent circuit seen at input
Small-signal equivalent circuit seen at output
inSmm
in
Smm
mmX v
Rggv
Rgg
ggv
21
21
21
1
1
11
11
12
111
1O
mSOmout rg
RrgR
EE105 Fall 2007 Lecture 19, Slide 12 Prof. Liu, UC Berkeley
Source Follower Stage
Small-signal analysis circuit fordetermining voltage gain, Av
1||
1||
LO
m
LO
in
outv
Rrg
Rr
v
vA
Equivalent circuit
Looutinm
Lomout
Rrvvg
Rrvgv
1outin vvv 1
EE105 Fall 2007 Lecture 19, Slide 13 Prof. Liu, UC Berkeley
Source Follower Example• In this example, M2 acts as a current source.
21
1
21
||1
||
OO
m
OOv
rrg
rrA
EE105 Fall 2007 Lecture 19, Slide 14 Prof. Liu, UC Berkeley
Rout of Source Follower• The output impedance of a source follower is relatively low,
whereas the input impedance is infinite (at low frequencies); thus, it is useful as a voltage buffer.
L
m
LO
m
out Rg
Rrg
R ||1
||||1
Small-signal analysis circuit fordetermining output resistance, Rout
EE105 Fall 2007 Lecture 19, Slide 15 Prof. Liu, UC Berkeley
Source Follower with Biasing• RG sets the gate voltage to VDD; RS sets the drain current.
(Solve the quadratic equation to obtain the value of ID.)
221
THSDDDoxnD VRIVLW
CI
Assuming = 0:
EE105 Fall 2007 Lecture 19, Slide 16 Prof. Liu, UC Berkeley
Supply-Independent Biasing• If Rs is replaced by a current source, the drain current ID
becomes independent of the supply voltage VDD.