5.1.7 The p-Channel MOSFET
- The p-Channel MOSFET is
fabricated on an n-type substrate with
p+ regions for the drain and source.
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! 
! 
In an n-channel MOSFET, the channel is
made of n-type semiconductor, so the
charges free to move along the channel are
negatively charged (electrons).
In a p-channel device the free charges which
move from end-to-end are positively charged
(holes).
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5.1.7 The p-Channel MOSFET
- The p-Channel MOSFET is fabricated on an n-type substrate
with p+ regions for the drain and source.
- vGS, vDS, and Vt are negative. The current flows from the source to the drain.
- PMOS technology originally dominated MOS manufacturing.
- NMOS has virtually replaced because it is smaller, faster, and needs lower
supply voltage.
- But you have to be familiar with PMOS because:
there are many discrete PMOSFETs and
there are complementary MOS, CMOS!!
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5.1.8 Complementary MOS or CMOS
body terminal
for the pchannel
device
p-type body
Figure 5.10 Cross-section of a CMOS integrated circuit. Note that the
PMOS transistor is formed in a separate n-type region, known as an n
well. Another arrangement is also possible in which an n-type body is
used and the n device is formed in a p well. Not shown are the
connections made to the p-type body and to the n well; the latter
functions as the body terminal for the p-channel device.
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5.2.5 Characteristics of the p-channel MOSFET
Figure 5.19 (a) Circuit symbol for the p-channel
enhancement-type MOSFET. (b) Modified symbol with an
arrowhead on the source lead. (c) Simplified circuit symbol
for the case where the source is connected to the body.
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Table 5.2
Regions of Operation of the Enhancement PMOS Transistor
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MOSFET canale p ad arricchimento
tensione di soglia Vt < 0
k = (1/2) µp Cox (W/L)
Transistor ON se VGS < Vt (ovvero VSG > | Vt |)
vDS < 0
(ovvero vSD > 0 )
iD > 0 (uscente dal drain)
In regione di triodo vDS ≥ vGS – Vt
iD = k [2(vGS – Vt) vDS -vDS2]
In regione di saturazione vDS ≤ vGS – Vt
iD = k (vGS – Vt)2 (1+λvDS)
Per il punto di lavoro, spesso si approssima:
ID = k (VGS – Vt) 2
λ = 1/VA
λ, VA<0
Per piccolo segnale: ro=⎢VA⎢/ ID gm = 2 k |(vGS – Vt)|
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Figure 5.20 The relative levels of the terminal voltages of
the enhancement-type PMOS transistor for operation in
the triode region and in the saturation region.
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ATTENZIONE: Il circuito equivalente per piccolo
segnale è lo stesso per n-Mos e p-MOS
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Figure E5.7
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Figure 5.25 Circuit for Example 5.7 p.388
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Example 5.7 p.388
5.25
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Example 5.7 p.388
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Figure E5.14 Circuit for Exercise D5.14 p. 389
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5.9.1 The Role of the Substrate-The Body Effect
- Usually, the source terminal is connected to the substrate (or body) terminal.
- In integrated circuit, many MOS transistors are fabricated on a single substrate.
- In order to maintain the cutoff condition for all the substrate-to-channel junctions,
the substrate is usually connected to the most negative power supply in an NMOS
circuit (the positive in a PMOS circuit).
- The reverse bias will widen the depletion region.
- The channel depth is reduced.
- To return the channel to its former status, vGS
has to be increased.
The body effect can cause considerable
degradation in circuit performance
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5.9.2 MODELING the Body Effect
gmb
For small signal
Figure 5.62 Small-signal, equivalent-circuit model of a
MOSFET in which the source is not connected to the body.
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Per stabilire se un circuito soffre dell’effetto di substrato:
Si considera prima il circuito in DC e si valuta se il
Source di un NMOS è connesso alla tensione continua
più bassa del circuito. Se è così, allora il transistor
NMOS considerato non soffre dell’effetto di substrato in
DC poichè VSB=0.
Si valuta inoltre se il Source di un PMOS è connesso alla
tensione continua più alta del circuito. Se è così, allora il
transistor PMOS considerato non soffre dell’effetto di
substrato in DC poichè VSB=0.
Si considera il circuito in AC e si valuta se i terminali di
Source dei MOSFET sono a massa per il segnale. Se si,
il MOSFET considerato non soffre dell’effetto di substrato
in AC poichè vbs=0.
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5.8.2 The Common-Source (CS) amplifier
Soffre dell’effetto di substrato in DC
Non soffre dell’effetto di substrato in AC
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5.8.3 The Common-Source Amplifier with a Source Resistance
Soffre dell’effetto di substrato in DC
Soffre dell’effetto di substrato in AC
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Body Effect in the Common-Source Amplifier with a
Source Resistance (1)
i= gmvgs + g v =(gm + g )vgs
mb bs
mb
= gm(1+ χ )vgs
€
g = χgm
mb
⎛
⎞
CS
gm R
gm
⎜
⎟⎟
=
R
+r
1+
g
(1+
χ
)R
Rout S o⎜⎝
m
S
⎠
S
€ v
vs =
vg € io =
g
1+ g m (1+ χ )R
1+ g m (1+ χ )R
S
S
€
gm + g = gm ⎛⎜⎝1+ χ ⎞⎟⎠
mb
v o -i o RL € g m RL
=
=−
(ro=∞, RD=∞.€ Sarebbe in // a RL)
vg
vg
1+ g m (1+ χ )R
S
€
Body effect in the Common-Source Amplifier with a
Source Resistance (2) (schema con RD, Rsig)
Per tenere conto dell’effetto di substrato (“body”):
1) Al denominatore del guadagno metto gm + gmb al posto di gm
2) Nella resistenza di uscita metto gm + gmb al posto di gm
gm + g = gm ⎛⎜⎝1+ χ ⎞⎟⎠
mb
vo
Rin
AV =
≅−
v sig
Rsig + Rin
€
Rout = RD R’
Microelectronic Circuits, Sixth Edition
(
g m ro RD RL
€
)
⎡
⎛
⎞⎤
ro
⎢
⎜
⎟⎥
1+ g m (1+ χ ) RS ⎜
⎟⎥
⎢⎣
r
+
R
R
(
)
D
L ⎠⎦
⎝ o
R’ = ro + RS [1+ g m (1 + χ ) ro ]
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5.8.4 The Common-Gate (CG) Amplifier
Soffre dell’effetto di substrato in DC
Soffre dell’effetto di substrato in AC
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Common-Gate Amplifier with Body Effect (1)
G
B
v0
RD
RL
S
Rin
Rsig
vsig
The body terminal is not connected to the source terminal, but rather is connected
to the lowest voltage in the circuit (ground). Because the gate and body are both
grounded, then Vgs=Vbs
Per tenere conto dell’effetto di substrato
(“body”), nel guadagno, nella resistenza
di ingresso e nella resistenza di uscita
metto gm + gmb al posto di gm
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Common-Gate Amplifier with Body Effect (2)
vo
vs
=+gm(1+ χ )R
L
(ro=∞; RD=∞, sarebbe in // a RL)
1
CG
=
Rin
€
g m (1+ χ )
⎛
⎞
CG
⎜1+ g (1+ χ )(R R )⎟
=
r
R
o⎜⎝
m
€ out
I S ⎟⎠
€
5.8.5 The Common-Drain (CD) Amplifier or Source Follower
Soffre dell’effetto di substrato in DC
Soffre dell’effetto di substrato in AC
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Common-Drain Amplifier with Body Effect
io =
€
€
gm
1+ g m (1+ χ )R
vg
L
gm R
v o +i o RL
L
=
=
vg
vg
1+ g m (1+ χ )R
L
(ro=∞)
1
CD
=
Rout
g m (1+ χ )
€
Per tenere conto dell’effetto di substrato
(“body”):
1) Al denominatore del guadagno metto gm + gmb al posto di gm
2) Nella resistenza di uscita metto gm + gmb al posto di gm
Results of Body Effect
•  Gain of source follower is degraded.
•  Input resistance of C-G and output resistance of C-D amplifier is
lowered.
•  Output resistance of both C-S and C-G amplifiers is raised.
•  Body effect increases input signal range.
CHAPTER 6
Building Blocks of Integrated-Circuit Amplifiers
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6.2 The basic gain cells of IC amplifiers:
active-loaded common-source amplifier
The current source
as active load
Bias must ensure saturation for Q1
Intrinsic gain= max gain
A0 = −g m rO = −
ID
VOV
VA
VA
=−
/ 2 ID
VOV / 2
Figure 6.1 The basic gain cells of IC amplifiers: (a) current-source- or activeloaded common-source amplifier; (c) small-signal equivalent circuit of (a)
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Good Example of Current Source
"  As long as a MOS transistor is in saturation region and λ=0, the
current is independent of the drain voltage and it behaves as an
ideal current source seen from the drain terminal.
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Bad Example of Current Source
"  Since the variation of the source voltage directly affects the
current of a MOS transistor, it does not operate as a good
current source if seen from the source terminal
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Common-source amplifier with CMOS
active load
large-signal model
no body effect
small-signal
equivalent
circuit
Figure 6.3 (a) The CS amplifier with the current-source load implemented
with a p-channel MOSFET Q2 ; (b) the circuit with Q2 replaced with its largesignal model; and (c) small-signal equivalent circuit of the amplifier.
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CS Stage (nMOS) with Current Source Load (pMOS)
Av = −g m1 ( rO1 || rO2 )
Rout = rO1 || rO 2
no body effect
€
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From the Common-source amplifier with CMOS
to CMOS inverter/amplifier
VDD
G
S
PMOS
D
VOUT
VIN
D
G
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NMOS
S
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EXAMPLE 5.8
Vt= ±1 V, k’(W/L) 1 mA/V2 for NMOS and PMOS
- Find iDN, iDP, υO, for υI =0 V, +2.5 V, and -2.5 V.
For υI =0 V,
- QN and QP are perfectly matched
- Equal |VGS| (2.5 V)
- The circuit is symmetrical.
(upper and lower part)
- Thus |VDG| = 0 V.
- Thus in saturation region !
Figure 5.26 Circuits for Example 5.8.
Non scorre corrente in R
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Vt= ±1 V, k’(W/L) 1 mA/V2 for NMOS and PMOS
For υI =+2.5 V
- for QP, VGS = 0 V, cutoff !
υO should be negative for IDN.
υGD will be greater than Vt.
for QN, triode !
For υI = -2.5 V
- Exact complement of +2.5 V
- QN will be off.
for Qp, triode !
Figure 5.26 Circuits for Example 5.8.
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37
Guadagno di piccolo segnale
Vt= ±1 V, k’(W/L) 1 mA/V2 for NMOS and PMOS
- Find small signal gain
For υI small signal with 0 DC component
NO BODY EFFECT
Draw equivalent circuit
R resistenza di carico
gm2vgs2
R
vgs2=
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CMOS common-source amplifier
with current mirror as active load
current mirror
with pMOS
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current mirror
with nMOS
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current mirror
With pMOS
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6.4 IC Biasing
current mirror
with nMOS
IP: Q2 operates in saturation
Figure 6.22 Circuit for a basic MOSFET constantcurrent source. For proper operation, the output
terminal, that is, the drain of Q2, must be connected to
a circuit that ensures that Q2 operates in saturation.
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Figure 6.23 Basic MOSFET current mirror (current sink).
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Figure 6.24 Output characteristic of the current
source in Fig. 6.22 and the current mirror of Fig. 6.23
for the case of Q2 matched to Q1.
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Example 6.5, p. 505: R?
Iref = 100µA, VDD = 3 V, Vt = 0.7 V, kʹ′n(W/L) = 2 mA/V2
Figure 6.22
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Example 6.5, p. 505: R?
Iref = 100µA, VDD = 3 V, Vt = 0.7 V, kʹ′n(W/L) = (200 µA/V2)(10)
Figure 6.22
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Figure 6.25 A current-steering circuit.
Figure 6.26 Application of the constant currents I2 and
I5 generated in the current-steering circuit of Fig. 6.25.
Constant-current I2 is the bias current for the source
follower Q6, and constant-current I5 is the load current
for the common-source amplifier Q7.
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A current source
Common drain or
source follower
common-source
A current sink
Figure 6.25 & Figure 6.26
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Figure 6.27 (a) A current source; and (b) a current sink.
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Esercizio
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CMOS common-source amplifier
with current mirror as active load (1)
Load curve
active load
Figure 6.4 The CMOS common-source amplifier
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CMOS common-source amplifier
with current mirror as active load (1)
Voltage transfer characteristic
VTC
active load
no body effect
Figure 6.4 The CMOS
common-source amplifier
(a) circuit; (d) transfer
characteristic.
CMOS common-source amplifier
with current mirror as active load (2)
Pendenza p=Av
Se trascuro effetto
Early in Q p=-∞
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CMOS common-source amplifier
with current mirror as active load (3)
Small-Signal Equivalent Circuit in Q point
Region III
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CMOS common-gate amplifier
with current mirror as active load
(serve in combinazione con altri stadi)
small-signal equivalent circuit
MOS in saturation
active load
pb: body effect of Q1
The CMOS CG amplifier:(a) circuit;(b) small-signal equivalent circuit
CMOS common-gate amplifier
with current mirror as active load
(serve in combinazione con altri stadi)
pb: body effect of Q1
active load
vo
≅ ( g m1 + g mb1 ) (ro1 ro2 ) =
vi
= g m1(1+ χ ) (ro1 ro2 )
The CMOS common-gate amplifier: (a) circuit; (b) small-signal
equivalent circuit; and (c) simplified version of the circuit in (b).
€
Source Follower (Common drain)
with Current Source as active load (1)
IF MOS in saturation
pb: body effect of M1
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Source Follower (Common drain) with Current Source (2)
Current mirror as active load
small-signal equivalent circuit
MOS in saturation
active load
pb: body effect of Q1
The source follower: (a) circuit; (b) small-signal equivalent circuit;
Appendix D:
D.3 Source-Absorption Theorem
Figure D.4 The source-absorption theorem.
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Source Follower (Common drain) with Current Source (2)
Current mirror as active load
pb: body effect of Q1
Use source-absorption theorem
g m1) (ro1 ro2 )
(
vo
=
v i 1+ ( g m1 + g mb1 ) (ro1 ro2 )
active load
€
The source follower: (a) circuit; (b) small-signal equivalent
circuit; and (c) simplified version of the equivalent circuit.
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5.9.6 The n-channel Depletion-Type MOSFET
Figure 5.63 The n-channel Depletion-Type MOSFET (a) transistor with current and
voltage polarities indicated; (b) the iD–vGS characteristic in saturation.
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Figure 5.63 The current-voltage characteristics of a depletion-type n-channel
MOSFET for which Vt = –4 V and kʹ′n(W/L) = 2 mA/V2: (b) the iD–vDS
characteristics; (c) the iD–vGS characteristic in saturation.
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The relative levels of terminal voltages of a depletion-type
NMOS transistor for operation in the triode and the
saturation regions. The case shown is for operation in the
enhancement mode (vGS is positive).
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nMOS amplifier with depletion load
Voltage transfer characteristic
VTC
Small-signal equivalent circuit of the
depletion-load amplifier
(if Q1 and Q2 in saturation – region III of VTC)
With the body effect of Q2
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Small-signal equivalent circuit of the
depletion-load amplifier
(if Q1 and Q2 in saturation – region III of VTC)
With the body effect of Q2
Use source-absorption theorem: 1/gmb2
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