FET AND ITS APPLICATIONS UNIT - III 7/2/2015.

FET AND ITS APPLICATIONS

UNIT - III

04/19/23 www.noteshit.com 1

Field Effect Transistor Family


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


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


Drain characteristics for an ideal representative N-channel JFET.


MOSFETs

• Depletion-Mode MOSFET

• Enhancement-Mode MOSFET


MOSFETs

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


MOSFETs

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


MOSFETs

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


MOSFETs

FET Switches

• JFET Resistance in Ohmic Region

• N-Channel JFET Switch

• N-Channel MOSFET Switch


N-channel JFET switch.


Switch using an N-channel enhancement-mode MOSFET.


FET small signal model

FETs provide

• excellent voltage gain

• high input impedance

• low-power consumption

• good frequency range

• small in size and weight



• Transconductance

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

The transconductance is denoted gm.

GS

Dm

V

Ig



Graphical Determination of gm



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



FET AC Equivalent Circuit


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.


FET Cascade Amplifier


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.


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)


Simplified Capacitance Model


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.


http://en.wikipedia.org/wiki/Electronic_amplifier

http://en.wikipedia.org/wiki/Field-effect_transistor

http://en.wikipedia.org/wiki/Electronic_amplifier#Input_and_output_variables

http://en.wikipedia.org/wiki/Amplifier

Common source Amplifier


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.


COMMON DRAIN AMPLIFIER


Freq. Response CS amplifier


Freq. Response CS amplifier


Summary of CS, CG, and CD Amplifiers


Summary of CS, CG, and CD Amplifiers


THANK U


FET AND ITS APPLICATIONS UNIT - III 7/2/2015.

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