ee3950 class notes chapter 12 hambley 3-12

8/10/2019 EE3950 Class Notes Chapter 12 Hambley 3-12

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Objectives

Understand MOSFET operation.

Use the graphical load-line technique to

analyze basic FET amplifiers.Analyze bias circuits.

Use small-signal equivalent circuits to

analyze FET amplifiers.


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Objectives

Compute the performance parametersof several FET amplifier configurations.

Select a FET amplifier configuration thatis appropriate for a given applications.

Understand the basic operation of

CMOS logic gates.


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Transistors

These are threeterminal devices,

where the current orvoltage at oneterminal, the inputterminal, controls

the flow of currentbetween the tworemaining terminals.


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Transistors

Can be classified as:

FETField Effect Transistor;

Majority carrier device; Unipolar device;

BJTBipolar Junction Transistor;

Minority carrier device;

Bipolar device.


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FETs

Two primary types: MOSFET, Metal-Oxide-Semiconductor FET. Also

known as IGFETInsulated Gate FET;

JFET, Junction FET.

MOS transistors can be: n-Channel;

Enhancement mode;

Depletion mode;

p-Channel;

Enhancement mode;

Depletion mode;


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MOSFET Structure

Figure 12.1 n-channel enhancement MOSFET

showing channel length Land channel width W.


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NMOS Symbol

Figure 12.2 Circuit symbol for an enhancement-

mode n-channel MOSFET.


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Threshold VoltageVto

The value of VGSwhere the drain

current just beginsto flow.

Typical values:

0.3 to 1.0 volts.


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Operation in the Cutoff Region

Figure 12.3 For vGS< Vto, the pnjunction between

drain and body is reverse biased and iD= 0.

for


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Operation in the Triode(Ohmic) Region

Figure 12.4 For vGS> Vto, a channel of n-type material is induced in

the region under the gate. As vGSincreases, the channel becomes

thicker. For small values of vDS, iDis proportional to vDS. The device

behaves as a resistance whose value depends on vGS.


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Operation in the Triode(Ohmic) Region

or< and


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Conduction Parameter K

or where nis the mobility of the electrons in the inversion layer.

Coxis the oxide capacitance per unit area.

KP is determined by the fabrication process.


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Operation in the Saturation Region

Figure 12.5 As vDSincreases, the channel pinches

down at the drain end and iDincreases more slowly.

Finally, for vDS> vGS

Vto, iDbecomes constant.


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Operation in the Saturation Region

or> and


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NMOS Characteristic Curves

Figure 12.6 Characteristic curves for an NMOS

transistor.


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PMOS Symbol

Figure 12.8 Circuit symbol for PMOS transistor.


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PMOS Characteristic Curves

Figure 12.9Characteristic curves for a PMOS

transistor.


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MOSFET Summary


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Depletion Mode MOSFETs

n-Channel is built in.

VGSvaries from

negative values topositive values,where negativevalues of VGS

depletes the channelwhile positive valuesenhance it further.


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JFETs

Depletion-mode FET with a different structure thanthat of the MOSFET.

Not generally used for switching elements of digitalcircuits.

Used in special applications such as analog circuits

where very high input impedance is required.


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JFETs

Every p-njunction has a depletion regiondevoid of carriers, and the width of thedepletion region can be controlled by theapplied voltage across the junction.


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JFETs

Note the highestvalue of VGS.

What happens if youmake VGSpositivewith respect toground.


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Load Line Analysis

Figure 12.10 Simple NMOS amplifier circuit.

sin 2000 4

20 1k

Solve for: 0 and 0


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Load Line Analysis

Figure 12.11 Drain characteristics and load line for

the circuit of Figure 12.10.

20 1k


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Load Line Analysis


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Bias Circuits

Figure 12.13 Fixed-plus self-bias circuit.

+ and +


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Bias Circuits

Figure 12.14 Graphical solution of Equations

12.12 and 12.13.


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Exercise 12.5

Determine IDQ and VDSQfor the circuit shown inFigure 12.16. The

transistor has: KP = 50A/V2

Vto = 1V

L= 10m

W= 200m

Figure 12.16 Circuit for

Exercise 12.5.


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Solution

Compute:

A/V 0.5mA/V

+ 20V K.M+K 7VGiven:

Then:


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Solution

2

0

Substitute known values results in: 6 0

The roots are: 2V 3V (correct root)


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Solution

Therefore:

2mA

16V


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Small-Signal Equivalent Circuits

Figure 12.18 Illustration of the terms in Equation 12.15.


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FETs Small-signal Equivalent Circuit

Figure 12.19 Small-signal equivalent circuit for

FETs.


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Given:

Substituting: We get:


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Expanding expression:

2

Reducing expression:

Q-points current cancels:

2

makes last term negligible: 2 (12.20)


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Defining transconductance of the FET as: 2

Allows the current equation to be written as:

0


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Transconductance Dependencyon Q-Point

Solving for andsubstituting it into 2 :

2 Transconductance is proportional to the square rootof the Q-point drain current.

Substitute into 2 and verifythe relationship between transconductance and thewidth-to-length ratio of the FET.


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Dependence of iDon vDS

Figure 12.20 FET small-signal equivalent circuit

that accounts for the dependence of iDon vDS.


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Transconductance and DrainResistance as Partial Derivatives

=

The condition 0is equivalent to remainingconstant at the Q-point, or . Therefore we canwrite: =and

Similarly:

=and


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Transconductance and DrainResistance as Partial Derivatives

=

The condition 0is equivalent to remainingconstant at the Q-point, or . Therefore we canwrite: =and

Similarly:

=and


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Exercise 12.7

Find gm and rdfor the characteristics of shown inFigure 12.21 at the Qpoint of VGSQ= 2.5V and VDSQ

= 6V.


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Solution

= ..mA

v 3.3mSSimilarly:

=

.9.mAV 0.05mS

. 20k


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Figure 12.18 Illustration of the terms in Equation 12.15.


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Common-source Amplifier.

Figure 12.22 Common-source amplifier.


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+ +

and


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Figure 12.24 Circuit used to find Ro.

+


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Source Follower

Figure 12.26 Source follower.


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Source Follower

+ +

and

+

+


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Source Follower

Figure 12.28 Equivalent circuit used to find the

output resistance of the source follower..

+ +

+ +


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Common-gate Amplifier

Figure 12.29 Common-gate amplifier.


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Logic Gates

AND gate

OR Gate

Inverter


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Logic Gates

NAND Gate

NOR Gate


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CMOS Inverter

Figure 12.31 CMOS inverter.


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Two-input CMOS NAND Gate

Figure 12.32 Two-input CMOS NAND gate.


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Three-input CMOS NOR Gate

Figure 12.35 Three-input CMOS NOR gate.

(Answer for Exercise 12.15.)

ee3950 class notes chapter 12 hambley 3-12

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