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Field Effect Transistors
Slides taken from: A.R. Hambley, Electronics, © Prentice Hall, 2/e, 2000 A. Sedra and K.C. Smith, Microelectronic Circuits, © Oxford University Press, 4/e, 1999
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Overview (1) Types of FET p-channel JFET n-channel enhancement
MOSFET p-channel n-channel enhancement depletion
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Overview (2) FET characteristics and modes of operation
Analysis and Design of FET Amplifiers bias operating point (DC analysis) small signal model (AC analysis)
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Enhancement mode n-MOSFET
Figure 5.1 n-Channel enhancement MOSFET showing channel length L and channel width W.
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Figure 5.2 Circuit symbol for an enhancement-mode n-channel MOSFET.
Enhancement mode n-MOSFET Figure 5.2 Circuit symbol for an enhancement-mode n-channel MOSFET.
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nMOS operation modes (1)
Fig An NMOS transistor with vGS < Vt The device acts as an open circuit.
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nMOS operation modes (2)
Fig An NMOS transistor with vGS > Vt and with a small vDS applied. The device acts as a conductance whose value is determined by vGS. Specifically, the channel conductance is proportional to vGS - Vt, and this iD is proportional to (vGS - Vt) vDS. Note that the depletion region is not shown (for simplicity).
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nMOS operation modes (3)
Fig Operation of the enhancement NMOS transistor as vDS is increased. The induced channel acquires a tapered shape and its resistance increases as vDS is increased. Here, vGS is kept constant at a value > Vt.
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Derivation of the iD vs. vDS characteristic
Fig Derivation of the iD - vDS characteristic of the NMOS transistor.
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Derivation of the iD vs. vDS characteristic
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nMOS equations (1)
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nMOS equations (2)
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nMOS characteristics Source: Kang, Leblebici, CMOS Digital Integrated Circuits, 3/e, McGraw-Hill
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nMOS output characteristics
Fig (a) An n-channel enhancement-type MOSFET with vGS and vDS applied and with the normal directions of current flow indicated. (b) The iD - vDS characteristics for a device with Vt = 1 V and k’n(W/L) = 0.5 mA/V2.
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Zooming in the output characteristics
Fig The drain current iD versus the drain-to-source voltage vDS for an enhancement-type NMOS transistor operated with vGS > Vt.
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nMOS in saturation: iD vs. vGS
Fig The iD - vGS characteristic for an enhancement-type NMOS transistor in saturation (Vt = 1 V and k’n(W/L) = 0.5 mA/V2).
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channel length modulation
Fig Increasing vDS beyond vDSsat causes the channel pinch-off point to move slightly away from the drain, thus reducing the effective channel length (by L).
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Effect of channel length modulation
Fig Effect of vDS on iD in the saturation region. The MOSFET parameter VA is typically in the range of 30 to 200 V.
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The device’s figure of merit
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The MOSFET as an amplifier
Fig Conceptual circuit utilized to study the operation of the MOSFET as an amplifier.
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Small signal analysis (AC)
Fig Small-signal operation of the enhancement MOSFET amplifier.
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Total component analysis (DC + AC)
Fig Total instantaneous voltages vGS and vD.
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Total component analysis
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Total component analysis
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Small signal models Fig Small-signal models for the MOSFET: (a) neglecting the dependence of iD on vDS in saturation (channel-length modulation effect); and (b) including the effect of channel-length modulation modeled by output resistance ro = |VA|/ID.
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Figure 5.24 Determination of gm and ro
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Load Line Analysis Example (1)
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Load Line Analysis Example (2)
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Load Line Analysis Example (3)
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Practical Bias Circuits (1) (DC operating point)
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Practical Bias Circuits (2) (DC operating point)
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Practical Bias Circuits (3): Load Line
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Practical Bias Circuits (4): Load Line
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Practical Bias Circuits (4) (DC operating point)
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Practical Bias Circuits (5) (DC operating point)
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Practical Bias Circuits (5) (DC operating point)
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An Alternative Bias Circuit (6) (DC operating point)
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Common Source (CS) Amplifier with Degeneration (1)
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Common Source (CS) Amplifier with Degeneration (2)
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Common Source (CS) Amplifier w/o degeneration (1)
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Common Source (CS) Amplifier w/o degeneration (2)
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Common Drain (CD) Amplifier
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Common Drain (CD) Amplifier (2)
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Common Gate (CG) Amplifier (1)
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Common Gate (CG) Amplifier (2)
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Common Gate (CG) Amplifier (3)
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n-MOSFET current mirror
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MOSFET current mirror characteristic
Fig Output characteristic of the current source in Fig and the current mirror for the case Q2 is matched to Q1.
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p-MOSFET current mirror
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Common source amplifier
Fig The CMOS common-source amplifier: (a) circuit; (b) i-v characteristic of the active-load Q2; (c) graphical construction to determine the transfer characteristic; and transfer characteristic.
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CS amplifier (small signal analysis)
Q2 is modeled as usual by a current source I2 with in parallel ro2. However I2 is a DC current source so from a small signal pespective It is an open !!!
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Body effect (1)
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Body effect (2)
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Body effect (3)
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Body effect (4)
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Common gate amplifier Fig The CMOS common-gate amplifier: (a) circuit; (b) small-signal equivalent circuit; and (c) simplified version of the circuit in (b).
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CG amplifier (small signal analysis)
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CG amplifier (small signal analysis)
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Common drain amplifier
Fig The source follower: (a) circuit; (b) small-signal equivalent circuit; and (c) simplified version of the equivalent circuit.
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CD amplifier (small signal analysis)
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CD amplifier (small signal analysis)
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Junction FET (n-channel)
Figure 5.38 n-Channel JFET.
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(Note: The two gate regions of each FET are connected to each other.)
JFET for vDS=0 (n-channel) Figure 5.39 The nonconductive depletion region becomes thicker with increased reverse bias. (Note: The two gate regions of each FET are connected to each other.)
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Figure 5.42 n-Channel FET for vGS = 0.
JFET for vGS=0 (n-channel) Figure 5.42 n-Channel FET for vGS = 0.
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JFET for vGS=0 (n-channel)
Figure 5.41 Drain current versus drain-to-source voltage for zero gate-to-source voltage.
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Figure 5.43 Typical drain characteristics of an n-channel JFET.
n-channel JFET: output characteristics Figure 5.43 Typical drain characteristics of an n-channel JFET.
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n channel JFET: iD vs. vGS
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Breakdown Figure 5.44 If vDG exceeds the breakdown voltage VB, drain current increases rapidly.
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Figure 5.46 n-Channel depletion MOSFET.
Depletion mode n-MOSFET Figure 5.46 n-Channel depletion MOSFET.
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Depletion n-MOSFET characteristics
Fig The current-voltage characteristics of a depletion-type n-channel MOSFET for which Vt = -4 V and k’n(W/L) = 2 mA/V2: (a) transistor with current and voltage polarities indicated; (b) the iD - vDS characteristics; (c) the iD - vGS characteristic in saturation.
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n-channel FET Figure 5.47 Drain current versus vGS in the saturation region for n-channel devices.
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p-channel FET Figure 5.48 p-Channel FET circuit symbols. These are the same as the circuit symbols for n-channel devices, except for the directions of the arrowheads.
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iD vs. vGS for the various FET
Figure 5.49 Drain current versus vGS for several types of FETs. iD is referenced into the drain terminal for n-channel devices and out of the drain for p-channel devices.
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p-FET equations
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p-FET output characteristics
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