Two-Port Model: CE AmpliÞer
■
Use transconductance ampliÞer form for model (not mandatory)
■
Rin = rπ, Rout = ro || RC, Gm = gm by inspection
iout
+
vin
_
Rin = rπ
Gmvin
Rout = ro RC
+
vout
−
(a)
iout
RS
+
vs
+
−
vin
_
+
Rin = rπ
Gmvin
Rout = ro RC
vout
RL
−
(b)
■
Loaded ratio of iout to vin
 R out 
 ro RC 
G m  ------------------------ = g m  ------------------------------
 R out + R L
 r o R C + R L
Increasing RC seems desirable for increasing iout / vin ... but note that the DC
collector current decreases for a Þxed VCC --> gm decreases
EE 105 Spring 1997
Lecture 23
Common-Source AmpliÞer
■
ConÞguration is similar to common-emitter
VDD
VDD
RD
RD
iOUT = IOUT + iout
RS
vs
VBIAS
+
+
−
vOUT = VOUT + vout
RL
−
+
−
(a)
■
VBIAS
+
VOUT
_
+
−
(b)
Bias: remove source and load resistances (b)
EE 105 Spring 1997
Lecture 23
Graphical Load-Line Analysis
■
Load line is given by:
I D = ( V DD Ð V OUT ) ⁄ R D
ID
(mA)
1.0
4
0.9
VBIAS
(V)
0.5
4
3
3
0.25
2.5
2
0.10
0
VBIAS ≤ VTn = 1 V
2
1
1
2
3
4
5
VDS = VOUT
(V)
(a)
VOUT
HighGain
Region
5V 1
2
3
4
2.5 V
VBIAS
(b)
EE 105 Spring 1997
Lecture 23
Small-Signal Model of CS AmpliÞer
■
Substitute parameters at operating point selected so that V OUT ≈ V DD ⁄ 2
iout
+
+
vgs
gmvgs
ro
RD
vout
−
−
(a)
iout
RS
+
vs
+
−
vgs
gmvgs
ro
RD
RL
+
vout
−
−
(b)
Transconductance is proportional to ID1/2 unlike bipolar transistor
EE 105 Spring 1997
Lecture 23
Two-Port Model of Common-Source AmpliÞer
■
Use transconductance ampliÞer form for model (most natural choice)
Rin = inÞnty, Rout = ro || RD, Gm = gm by inspection
iout
+
vin
+
Gmvin
Rout = ro RD
vout
−
−
(a)
iout
RS
+
vs
+
−
vin
+
Gmvin
Rout = ro RD vout
−
RL
−
(b)
InÞnite input resistance is ideal for a voltage input
Output resistance increases with RD increasing, but DC drain current ID will
decrease and gm will decrease with ID1/2
EE 105 Spring 1997
Lecture 23
Current-Source Supplies
■
A current source to supply current, rather than a resistor, allows a high DC
current for the device with a large incremental (small-signal) resistance
iSUP
+
+
vSUP
vSUP
iSUP
−
roc
ISUP
−
(a)
iSUP
1
roc
ISUP
roc
vSUP
(b)
(c)
EE 105 Spring 1997
Lecture 23
Common-Source with Current Source Supply
■
RD is replaced with idealized current source with internal resistance
VDD
VDD
iSUP
ISUP
iOUT = IOUT + iout
+
RS
vs
VBIAS
vOUT = VOUT + vout
+
−
−
+
−
(a)
■
+
VOUT
RL
VBIAS
+
−
−
(b)
For DC bias analysis, the small-signal source (with RS) and the load resistor RL
are eliminated, along with the internal resistance roc of the current source
EE 105 Spring 1997
Lecture 23
Graphical Analysis of CS AmpliÞer with
Current-Source Supply
ID
(mA)
1.0
4V
0.9
VBIAS
0.5
3V
0.25
2
3
4
2.5 V
2V
0.10
0
VBIAS ≤ VTn = 1 V
1
1
2
3
4
5
VDS = VOUT
(V)
(a)
HighGain
Region
VDD 1
2
3
4
0
0
2.5 V
VBIAS
(b)
■
The region of input bias voltage VBIAS for which the current source and the
MOSFET are in their constant-current regions is extremely small ....
EE 105 Spring 1997
Lecture 23
Common-Source/Current-Source Supply Models
■
The small-signal model is identical to the resistor supply, except that the current
sourceÕs internal resistance roc replaces RD
iout
+
+
vgs
gmvgs
ro
roc vout
−
■
−
Two-port model in both transconductance and voltage ampliÞer forms
(the latter by direct conversion from the former ... by applying procedure for
Þnding Av)
iout
+
vin
Gmvin
−
(a)
Rout
iout
Rout
+
vin
−
+
−
Avvin = −GmRoutvin
+
vout
−
(b)
EE 105 Spring 1997
Lecture 23
p-Channel Common-Source AmpliÞer
■
Source of p-channel is tied to positive supply; current supply sinks ISUP to
ground or to lower supply
VDD
VDD
RS
vs
+
−
VBIAS
+
−
iOUT = IOUT + iout
VBIAS
+
vOUT = VOUT + vout
iSUP
+
−
RL
_
(a)
■
ISUP
+
VOUT
_
(b)
Small-signal model: substitute p-channel model directly
EE 105 Spring 1997
Lecture 23
p-Channel CS Small-Signal Model
■
Source is at top, but circuit can be inverted to show correspondence with nchannel common-source ampliÞer
s
+
vsg
gmvsg
ro
−
−
roc vout
+
g
d
iout
(a)
iout
g
d
−
vsg
gmvsg
ro
+
+
roc vout
−
s
(b)
iout
g
d
+
vgs
gmvgs
−
ro
+
roc vout
−
s
(c)
EE 105 Spring 1997
Lecture 23
EE 105 Spring 1997
Lecture 23