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. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. ! ! 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). 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. - 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!! Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Table 5.2 Regions of Operation of the Enhancement PMOS Transistor Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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)| Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. ATTENZIONE: Il circuito equivalente per piccolo segnale è lo stesso per n-Mos e p-MOS Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Figure E5.7 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Figure 5.25 Circuit for Example 5.7 p.388 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Example 5.7 p.388 5.25 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Example 5.7 p.388 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Figure E5.14 Circuit for Exercise D5.14 p. 389 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 5.8.2 The Common-Source (CS) amplifier Soffre dell’effetto di substrato in DC Non soffre dell’effetto di substrato in AC Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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 ] Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 5.8.4 The Common-Gate (CG) Amplifier Soffre dell’effetto di substrato in DC Soffre dell’effetto di substrato in AC Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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 Microelectronic Circuits, International Sixth Edition Sedra/Smith Copyright © 2011 by Oxford University Press, Inc. 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) Microelectronic Circuits, International Sixth Edition Sedra/Smith Copyright © 2011 by Oxford University Press, Inc. 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. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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. Microelectronic Circuits, International Sixth Edition Sedra/Smith Copyright © 2011 by Oxford University Press, Inc. CS Stage (nMOS) with Current Source Load (pMOS) Av = −g m1 ( rO1 || rO2 ) Rout = rO1 || rO 2 no body effect € Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. From the Common-source amplifier with CMOS to CMOS inverter/amplifier VDD G S PMOS D VOUT VIN D G Microelectronic Circuits, International Sixth Edition Sedra/Smith NMOS S Copyright © 2011 by Oxford University Press, Inc. 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 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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= Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. CMOS common-source amplifier with current mirror as active load current mirror with pMOS Microelectronic Circuits, International Sixth Edition Sedra/Smith Copyright © 2011 by Oxford University Press, Inc. current mirror with nMOS Microelectronic Circuits, International Sixth Edition current mirror With pMOS Sedra/Smith Copyright © 2011 by Oxford University Press, Inc. 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. Microelectronic Circuits, International Sixth Edition Sedra/Smith Copyright © 2011 by Oxford University Press, Inc. Figure 6.23 Basic MOSFET current mirror (current sink). Microelectronic Circuits, International Sixth Edition Sedra/Smith Copyright © 2011 by Oxford University Press, Inc. 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. Microelectronic Circuits, International Sixth Edition Sedra/Smith Copyright © 2011 by Oxford University Press, Inc. 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 Microelectronic Circuits, International Sixth Edition Sedra/Smith Copyright © 2011 by Oxford University Press, Inc. 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 Microelectronic Circuits, International Sixth Edition Sedra/Smith Copyright © 2011 by Oxford University Press, Inc. 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. Microelectronic Circuits, International Sixth Edition Sedra/Smith Copyright © 2011 by Oxford University Press, Inc. A current source Common drain or source follower common-source A current sink Figure 6.25 & Figure 6.26 Sedra/Smith Copyright © 2011 by Oxford University Press, Inc. Figure 6.27 (a) A current source; and (b) a current sink. Microelectronic Circuits, International Sixth Edition Sedra/Smith Copyright © 2011 by Oxford University Press, Inc. Esercizio Microelectronic Circuits, International Sixth Edition Sedra/Smith Copyright © 2011 by Oxford University Press, Inc. CMOS common-source amplifier with current mirror as active load (1) Load curve active load Figure 6.4 The CMOS common-source amplifier Microelectronic Circuits, International Sixth Edition Sedra/Smith Copyright © 2011 by Oxford University Press, Inc. 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=-∞ Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. CMOS common-source amplifier with current mirror as active load (3) Small-Signal Equivalent Circuit in Q point Region III Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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). Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. 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 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
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