FET Amplifiers-Electronics-Lecture Slides, Slides of Electronics

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Basic FET Amplifiers

Ch.

Department of Electrical Engineering

THE MOSFET AMPLIFIER

 What is a Linear amplifier

A linear amplifier magnifies an input signal and produces an output signal whose magnitude is larger and

directly proportional to the input signal.

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Understanding

Graphical Analysis, Load Lines,

and Small-Signal Parameters

 Consider an NMOS common-source circuit with a time- varying voltage source in series with the dc source.

 Assume the time-varying input signal is sinusoidal Remember !!!!! This is Enhancement Type MOS FET

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For Negative Vi

 VGS

 Smaller VGS Smaller ID and Larger VDS

• Vi VGS=^ VGSQ-Vi

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 Here Vgs = vi

where Vgs is the time varying component of the gate-to-source voltage.

Important Point to REMEMBER !!!!!

 For the FET to operate as a linear amplifier, the transistor must be biased in the saturation region, and the ID and VDS must also be confined to the saturation region

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Transistor Parameters

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Applying (a+b)^2 =a^2 +b^2 +2ab

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Amplification occurs in saturation region

 Which means that the third term in Equation will be much smaller than the second term if we have very less distortion.

 Important Point to REMEMBER !!!!

Equation

represents the small-signal condition that must be satisfied

for linear amplifiers.

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Transconductance parameter gm

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What is gm on VI Curve

• Smaller the vgs , the VGSQ will become more and more close to VGS and

gm will be more constant.

16 •^ gm α^ Kn , So gm will depend upon Width and Length^ parameters

Small-Signal

Equivalent Circuit

of Common Source

MOSFET

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Small-Signal Equivalent Circuit

NMOS

AC

Equi cct.

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 Now we have

Remember from BJT Lecture!!!!!! ro= 1/gm

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