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EEE235
Field Effect Transistors (FET)-2
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EEE235
Typically L = 1 to 10 m, W = 2 to 500 m, and the thickness of the oxide layer is in the range of 0.02 to 0.1 m.
Field Effect (MOS) Transistor
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The enhancement-type NMOS transistor with a positive voltage applied to the gate.
An n channel is induced at the top of the substrate beneath the gate.
Operation
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EEE235vGS > Vt ,small vDS applied.(0.1-0.2V)
The channel conductance is proportional to
vGS - Vt, and
Drain current also proportional to
(vGS - Vt) ;
of course to vDS.
Triode Region(applying a small vds)
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Saturation Region (as vds increased)
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Enhancement-type NMOS transistor operated with vGS > Vt.
Drain current iD versus vDS
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Derivation of the iD - vDS characteristic of the NMOS transistor.
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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.
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•n-channel MOSFET in saturation, incorporating the output resistance ro.
•The output resistance ro VA/ID.
Large-signal equivalent circuit model
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The current-voltage characteristics of a depletion-type n-channel MOSFET for which Vt = -4 V and k’n(W/L) = 2 mA/V2
iD - vDS characteristicsiD - vGS saturation
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EEE235MOSFET as an amplifier.
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Instantaneous voltages vGS and vD
Small Signal
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Models for MOSFET
neglecting the dependence of iD on vDS in saturation (channel-length modulation effect)
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Model with Output Resistance
Including the effect of channel-length modulation modeled by output resistance ro = |VA|/ID.
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T model of the MOSFET augmented with the drain-to-source resistance ro.
T model of the MOSFET
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MOSFET current mirror.
Sample Circuit Output characteristic of the current
current mirror Q2 is matched to Q1.
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The CMOS common-source amplifier
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EEE235The CMOS common-gate amplifier
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(a) circuit;
(b) small-signal equivalent circuit
(c) simplified version of the equivalent circuit.
The source follower