FET AND ITS APPLICATIONS UNIT - III

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

FET small signal model Transconductance The relationship of VGS (input) to ID (output) is called transconductance. The transconductance is denoted gm.

FET small signal model Graphical Determination of gm

FET small signal model Mathematical Definition of gm gm for VGS =0V:

FET small signal model 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.amplifierfield-effect transistorvoltage or Tran conductance amplifiercommon draincommon gatebipolar junction transistorcommon-emitter 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.amplifierfield effect transistorvoltagebufferbipolar junction transistorcommon-collector amplifier

COMMON DRAIN AMPLIFIER

Freq. Response CS amplifier

Summary of CS, CG, and CD Amplifiers

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