fet and its applications unit - iii 7/2/2015

FET AND ITS APPLICATIONS

UNIT - III

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Field Effect Transistor Family

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Construction of an N-channel JFET and the schematic symbol.

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Circuit that can be used to measure N-channel JFET characteristics.

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Drain characteristics for an ideal representative N-channel JFET.

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MOSFETs

• Depletion-Mode MOSFET

• Enhancement-Mode MOSFET

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MOSFETs

Construction of an N-channel depletion-mode MOSFET with external connection for substrate and the schematic symbol.

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MOSFETs

Construction of an N-channel enhancement-mode MOSFET with external connection for substrate and the schematic symbol.

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MOSFETs

Drain characteristics for an ideal representative N-channel enhancement-mode MOSFET.

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MOSFETs

FET Switches

• JFET Resistance in Ohmic Region

• N-Channel JFET Switch

• N-Channel MOSFET Switch

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N-channel JFET switch.

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Switch using an N-channel enhancement-mode MOSFET.

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FET small signal model

FETs provide

• excellent voltage gain

• high input impedance

• low-power consumption

• good frequency range

• small in size and weight

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FET small signal model

• Transconductance

The relationship of VGS (input) to ID (output) is called transconductance.

The transconductance is denoted gm.

GS

Dm

V

Ig

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FET small signal model

Graphical Determination of gm

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FET small signal model

Mathematical Definition of gm

GS

Dm

V

Ig

P

GS

P

DSSm

V

V1

V

2Ig

P

DSSm0

V

2Ig

gm for VGS =0V:

P

GSm0m

V

V1gg

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FET small signal model

FET AC Equivalent Circuit

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Cascade Connection

• The output of one amplifier is the input to the next amplifier.

• The overall gain:

Av total : Av1 * Av2

• Note the DC bias circuits are isolated from each other by the coupling capacitors.

• The DC calculations are independent of the cascading. • The AC calculations for gain and impedance are

interdependent.

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FET Cascade Amplifier

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Darlington Connection

This combination provides large current gain, typically a few thousand.It has a voltage gain of near 1, a low output impedance and a high input impedance.

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Frequency response

• Internal and external capacitances

• High-frequency equivalent circuit model for the MOSFET (a)

• High-frequency equivalent circuit model for the MOSFET when

• Source is connected to Body (b)

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Simplified Capacitance Model

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Common source Amplifier

common-source amplifier is one of three basic single-

stage field-effect transistor (FET) amplifier topologies, typically used as a voltage or Tran conductance amplifier. The easiest way to tell if a FET is common source, common drain, or common gate is to examine where the signal enters, and leaves. The remaining terminal is what is known as "common". In this example, the signal enters the gate, and exits the drain. The only terminal remaining is the source. This is a common-source FET circuit. The analogous bipolar junction transistor circuit is the common-emitter amplifier.

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Common source Amplifier

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COMMON DRAIN AMPLIFIER

Common-drain amplifier, also known as a source follower, is one of three basic single-stage field effect transistor (FET) amplifier topologies, typically used as a voltage buffer. In this circuit the gate terminal of the transistor serves as the input, the source is the output, and the drain is common to both (input and output), hence its name. The analogous bipolar junction transistor circuit is the common-collector amplifier.

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COMMON DRAIN AMPLIFIER

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Freq. Response CS amplifier

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Freq. Response CS amplifier

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Summary of CS, CG, and CD Amplifiers

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Summary of CS, CG, and CD Amplifiers

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THANK U

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