5.1.7 the p-channel mosfet - unipv · pdf filecmos common-gate amplifier with current mirror...

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

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.


!  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).


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!!


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.

5.1.8 Complementary MOS or CMOS

p-type body

body terminal for the p-channel device


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.

5.2.5 Characteristics of the p-channel MOSFET


Table 5.2 Regions of Operation of the Enhancement PMOS Transistor


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 -vDS

2]

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)|


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.


ATTENZIONE: Il circuito equivalente per piccolo segnale è lo stesso per n-Mos e p-MOS


Figure E5.7


Figure 5.25 Circuit for Example 5.7 p.388


Example 5.7 p.388 5.25


Example 5.7 p.388


Figure E5.14 Circuit for Exercise D5.14 p. 389


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.

The body effect can cause considerable degradation in circuit performance

- To return the channel to its former status, vGS has to be increased.


Figure 5.62 Small-signal, equivalent-circuit model of a MOSFET in which the source is not connected to the body.

5.9.2 MODELING the Body Effect For small signal

gmb


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.


5.8.2 The Common-Source (CS) amplifier

Soffre dell’effetto di substrato in DC Non soffre dell’effetto di substrato in AC


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

€

vs=gmRS

1+gm(1+χ)RSvg €

gmb=χgm

€

io=gm

1+gm(1+χ)RSvg

€

vovg

=-ioRL

vg=−

gmRL1+gm(1+χ)RS

€

i=gmvgs+gmbvbs=(gm+gmb)vgs=gm(1+χ)vgs

€

outCSR =RS+ro1+gm(1+χ)RS

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

Body Effect in the Common-Source Amplifier with a Source Resistance (1)

(ro=∞, RD=∞. Sarebbe in // a RL) €

gm+gmb=gm1+χ⎛

⎝ ⎜

⎞

⎠ ⎟


€

AV =vovsig

≅ −Rin

Rsig + Rin

gm ro RD RL( )

1+ gm 1+ χ( )RSro

ro + RD RL( )⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

⎡

⎣

⎢ ⎢

⎤

⎦

⎥ ⎥

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

Body effect in the Common-Source Amplifier with a Source Resistance (2) (schema con RD, Rsig)

€

Rout = RD R’ R’= ro + RS 1+ gm 1 +χ( )ro[ ]

€

gm+gmb=gm1+χ⎛

⎝ ⎜

⎞

⎠ ⎟


5.8.4 The Common-Gate (CG) Amplifier



RD

v0

RL

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

Rsig

vsig

Rin

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

G

S

B

Common-Gate Amplifier with Body Effect (1)

Common-Gate Amplifier with Body Effect (2)

€

vovs

=+gm(1+χ)RL

€

inCGR = 1

gm(1+χ)

€

outCGR =ro1+gm(1+χ)(RI RS )

⎛

⎝

⎜ ⎜

⎞

⎠

⎟ ⎟

(ro=∞; RD=∞, sarebbe in // a RL)


5.8.5 The Common-Drain (CD) Amplifier or Source Follower


Common-Drain Amplifier with Body Effect

€

vovg

=+ioRL

vg=

gmRL1+gm(1+χ)RL

€

outCDR = 1

gm(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

€

io=gm

1+gm(1+χ)RLvg

(ro=∞)

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.

Microelectronic Circuits, International Sixth Edition Sedra/Smith Copyright © 2011 by Oxford University Press, Inc.

C H A P T E R 6

Building Blocks of Integrated-Circuit Amplifiers


Figure 6.1 The basic gain cells of IC amplifiers: (a) current-source- or active-loaded common-source amplifier; (c) small-signal equivalent circuit of (a)

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 = −gmrO = −ID

VOV / 2VAID

= −VA

VOV / 2


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.


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


no body effect

Common-source amplifier with CMOS

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 large-

signal model; and (c) small-signal equivalent circuit of the amplifier.

active load large-signal model

small-signal equivalent circuit


CS Stage (nMOS) with Current Source Load (pMOS)

€

Av = −gm1 rO1 || rO2( )Rout = rO1 || rO2

no body effect


From the Common-source amplifier with CMOS to CMOS inverter/amplifier

VDD

VIN VOUT

S

D

G

G S

D

PMOS

NMOS


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.

- 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 !

For υI =0 V,

Non scorre corrente in R Figure 5.26 Circuits for Example 5.8.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 37

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

for Qp, triode !

- QN will be off.

Figure 5.26 Circuits for Example 5.8.


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

gm2vgs2

vgs2= R

R resistenza di carico


CMOS common-source amplifier with current mirror as active load

current mirror with pMOS


current mirror with nMOS

current mirror With pMOS


Figure 6.22 Circuit for a basic MOSFET constant-current 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.

6.4 IC Biasing

current mirror with nMOS

IP: Q2 operates in saturation


Figure 6.23 Basic MOSFET current mirror (current sink).


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.


Figure 6.22

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

Example 6.5, p. 505: R? Iref = 100µA, VDD = 3 V, Vt = 0.7 V, kʹ′n(W/L) = (200 µA/V2)(10)

Figure 6.25 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.


Figure 6.27 (a) A current source; and (b) a current sink.


Esercizio


CMOS common-source amplifier with current mirror as active load (1)

active load

Figure 6.4 The CMOS common-source amplifier

Load curve


no body effect

Figure 6.4 The CMOS common-source amplifier (a) circuit; (d) transfer characteristic.

active load

Voltage transfer characteristic VTC



Pendenza p=Av Se trascuro effetto Early in Q p=-∞


Small-Signal Equivalent Circuit in Q point Region III


CMOS common-gate amplifier with current mirror as active load

(serve in combinazione con altri stadi)

The CMOS CG amplifier:(a) circuit;(b) small-signal equivalent circuit

pb: body effect of Q1

active load


MOS in saturation

CMOS common-gate amplifier with current mirror as active load

(serve in combinazione con altri stadi)

The CMOS common-gate amplifier: (a) circuit; (b) small-signal equivalent circuit; and (c) simplified version of the circuit in (b).


active load

€

vovi≅ gm1 + gmb1( )(ro1 ro2) =

= gm1 1+ χ( )(ro1 ro2)


Source Follower (Common drain) with Current Source as active load (1)

pb: body effect of M1

IF MOS in saturation

The source follower: (a) circuit; (b) small-signal equivalent circuit;


Source Follower (Common drain) with Current Source (2) Current mirror as active load

active load


MOS in saturation


Figure D.4 The source-absorption theorem.

Appendix D: D.3 Source-Absorption Theorem


The source follower: (a) circuit; (b) small-signal equivalent circuit; and (c) simplified version of the equivalent circuit.


Use source-absorption theorem

Source Follower (Common drain) with Current Source (2) Current mirror as active load

active load

€

vovi

=gm1( )(ro1 ro2)

1+ gm1 + gmb1( )(ro1 ro2)


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.


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.


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).

nMOS amplifier with depletion load Voltage transfer characteristic

VTC


With the body effect of Q2

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

Small-signal equivalent circuit of the depletion-load amplifier

(if Q1 and Q2 in saturation – region III of VTC)

Use source-absorption theorem: 1/gmb2

5.1.7 the p-channel mosfet - unipv · pdf filecmos common-gate amplifier with current mirror...

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