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CMOS Design Note

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    MOSFET Circuit Symbol & I-V Characteristics

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    Whites, EE 320 Lecture 26 Page 1 of 8 Lecture 26: MOSFET Circuit Symbols, iD-vDS Characteristics. There are two circuit symbols you may encounter for the enhancement type MOSFET. For the n-channel, one symbol is (Fig. 4.10b) Referring to this circuit symbol: 9 The arrowed terminal indicates the source, 9 This arrow direction indicates n-type (direction of current) 9 The gap at the gate indicates the oxide layer. However, the body is often connected to the source. This leads to a more common circuit symbol: (Fig. 4.10c) Similar circuit symbols are used for p-channel enhancement type MOSFETS: © 2009 Keith W. Whites Whites, EE 320 Lecture 26 Page 2 of 8 (Fig. 4.18b,c) MOSFET iD–vDS Characteristics Similar to a BJT, we can generate a set of iD–vDS characteristic curves for a MOSFET by setting vGS and varying vDS. This is shown in Fig. 4.11 for an n-type MOSFET: (Fig. 4.11) There are three regions of operation: (1) Cutoff. To operate an enhancement type MOSFET, we first must induce the channel. For NMOS, this means that vGS ≥ Vt (induce) (4.8),(1) Whites, EE 320 Lecture 26 Page 3 of 8 If vGS < Vt there is no channel and the device is cutoff, which we see in Fig. 4.11. When the MOSFET is cutoff, iD = iS = 0. (2) Triode. To operate in this mode, we first must induce the channel as in (1) above. We must also keep vDS small enough so the channel is continuous (not pinched off): vGD > Vt (continuous) (4.9),(2) [Note how similar this last criterion is to vGS > Vt for the channel to be induced. Here in (2), we have vGD > Vt for a continuous channel at the drain end. This observation can help us to remember these criterion.] Another way of writing this criterion in (2) is in terms of vDS. Referring to this circuit element vGD vDS we see that vDS = vGS + vDG (3) For a continuous channel, as required by (2), (3) becomes Whites, EE 320 Lecture 26 Page 4 of 8 vDS − vGS = vDG < −Vt Therefore, vDS < vGS −Vt (continuous) (4.10),(4) We can use either (2) or (4) to check for triode operation of the MOSFET. As given in the last lecture, in the triode region iD = kn′ W L ⎡⎢⎣( vGS − Vt ) vDS − 1 2 vD2 S ⎤ ⎥⎦ (4.11),(5) If vD2S 2  (vGS − Vt ) vDS then iD ≈ kn′ WL (vG S − V t ) vDS ≡ r v −1 DS DS (4.12),(6) ≡ r −1 DS where rDS is defined as the (linear) resistance between the drain and source terminals. The value of rDS is controlled by vGS. (See Fig. 4.4). (3) Saturation. To operate in this mode we need to first induce the channel vGS ≥ Vt (induce) (4.16),(7) then ensure that the channel is pinched off at the drain end vGD ≤ Vt (pinch off) (4.17),(8) or equivalently vDS ≥ vGS −Vt (pinch off) (4.18),(9) As we saw in the previous lecture, the drain current in this region is Whites, EE 320 Lecture 26 iD = 1 2 kn′ W L ( vGS − Vt )2 and is not dependent on vDS. Page 5 of 8 (4.20),(10) A plot of iD versus vGS for an enhancement type NMOS device in saturation is shown in Fig. 4.12: (Fig. 4.12) In the saturation mode, this device behaves as an ideal current source controlled by vGS: (Fig. 4.13) In reality, though, there is a finite output resistance (ro) that should be added to this model: Whites, EE 320 iG = 0 vGS Lecture 26 kn′ 2 W L ( vGS − Vt )2 iD vDS Page 6 of 8 where ro = VA ID (4.26),(11) This finite output resistance gives a slope to the iD–vDS characteristic curves: (Fig. 4.16) Example N26.1 (similar to text exercise 4.4). Given an enhancement type NMOS with Vt = 2 V. Whites, EE 320 Lecture 26 VD D G 3V S Page 7 of 8 Determine the region of operation of this device for the following VD. Use these criteria for the region of operation: • Cutoff: vGS < Vt • Triode: vGS ≥ Vt and vDS < vGS −Vt • Saturation: vGS ≥ Vt and vDS ≥ vGS −Vt (a) VD = 0.5 V. VGS = 3 V > Vt = 2 V ⇒ not cutoff. Then VDS = 0.5 V < VGS −Vt (= 3 − 2 = 1 V) . ∴ triode mode. (b) VD = 1 V. VGS = 3 V > Vt = 2 V ⇒ not cutoff. Then VDS = 1 V = VGS −Vt (= 1 V). ∴ saturation (or triode) mode. (c) VD = 5 V. VGS = 3 V > Vt = 2 V ⇒ not cutoff. Then VDS = 5 V > VGS −Vt (= 1 V). ∴ saturation mode. Example N26.2 (similar to text problem 4.16). An NMOS enhancement type MOSFET has Vt = 2 V. If VGS ranges from 2.5 to 5 V what is the largest VDS for which the channel remains continuous? Whites, EE 320 vGD vDS Lecture 26 Page 8 of 8 9 VGS > Vt, ∀VGS so the channel is always present. 9 Then for the channel to remain open at the drain end, VDS < VGS −Vt (triode) Which VGS to use here? The smallest. Therefore, VDS max < 2.5 − 2 = 0.5 V

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