Transistors

1
Chapter 7

• Transistors

2
Objectives

• Describe the basic properties of a bipolar
  junction transistor (BJT), including the names of
  the terminals and the classifications as NPN or
  PNP.
• Construct equivalent circuits of the BJT for both
  cutoff and saturation.
• Describe the nature of both the base and
  collector characteristic curves for a BJT.

3
Objectives

• Describe the three regions of operation for a
  BJT.
• Define dc current gain for a BJT and discuss its
  significance.
• Design a BJT switch using worst-case conditions.
• Discuss the family tree for the field effect
  transistor (FET) and show the various
  classifications.

4
Objectives

• Describe the general properties of the junction
  field effect transistor (JFET).
• Describe the three regions of operation for a
  JFET.
• Describe the general properties of the
  metal-oxide semiconductor field effect transistor
  (MOSFET).
• Explain the differences between the
  depletion-mode MOSFET and enhancement-mode MOSFET.

5
Objectives

• Discuss how FETs can be used as switches.
• Provide a qualitative explanation of how BJTs and
  FETs can function as amplifiers.
• Provide a general comparison between BJTs and
  FETs.

6
7-1 Introduction

• Why Study Transistors?

7
7-2 Bipolar Junction Transistor

• NPN and PNP
• NPN Layout
• PNP Layout
• Analogy to Two Diodes
• Basic Operation
• Relationship for Currents

8
Layout of an NPN transistor and its schematic
symbol.
9
Layout of a PNP transistor and its schematic
symbol.
10
Models to assist in visualizing BJT as two diodes.
11
7-3 BJT Operating Regions

• Cutoff Region
• Saturation Region
• Active Region

12
Ideal model of BJT in cutoff region.
13
Ideal model of BJT in saturation region.
14
Better models for BJTs in saturation region.
15
7-4 BJT Characteristic Curves

• Common-Emitter Form
• Measurement Circuit
• Base Characteristics
• Collector Characteristics
• Operating Regions
• DC Current Gain
• Polarities and Current Directions
• Variations in the Current Gain

16
Circuit that can be used to measure transistor
characteristics.
17
Base characteristic with collector-to-emitter
voltage constant.
18
Collector characteristics of an ideal
representative BJT.
19
Three regions of operation for a BJT.
20
Reference positive directions for both NPN and
PNP transistors.
21
7-5 Brief Look at a Classical BJT Amplifier
Circuit
22
Classical BJT amplifier circuit that operates in
active region.
23
7-6 BJT Switches

• Saturation and Cutoff in Switches
• NPN Switch
• PNP Switch
• Increase of Base Current After Saturation

24
BJT switch using an NPN transistor.
25
BJT switch using a PNP transistor.
26
7-7 Field Effect Transistor Family

• Family Tree
• JFETs
• MOSFETs

27
Classification scheme for field effect
transistors.
28
7-8 Junction Field Effect Transistors

• N-Channel Layout
• Three JFET Operating Regions
• Obtaining JFET Characteristics
• Gate-to-Source Cutoff Voltage and the Cutoff
  Region
• Drain Characteristics
• Ohmic Region
• Beyond Pinchoff Region
• Formula for JFET Drain Current

29
Construction of an N-channel JFET and the
schematic symbol.
30
Circuit that can be used to measure N-channel
JFET characteristics.
31
Ideal model of the JFET in the cutoff region.
32
Idealized drain characteristics of N-channel JFET
in linear portion of the ohmic region.
33
Drain characteristics for an ideal representative
N-channel JFET.
34
7-9 Brief Look at Classical JFET Amplifier
Circuits
35
Classical JFET amplifier circuit that operates in
beyond pinchoff region.
36
7-10 MOSFETs

• Depletion-Mode MOSFET
• Enhancement-Mode MOSFET
• Threshold Voltage
• Drain Current Equation

37
Construction of an N-channel depletion-mode
MOSFET with external connection for substrate and
the schematic symbol.
38
Construction of an N-channel enhancement-mode
MOSFET with external connection for substrate and
the schematic symbol.
39
Drain characteristics for an ideal representative
N-channel enhancement-mode MOSFET.
40
7-11 FET Switches

• JFET Resistance in Ohmic Region
• N-Channel JFET Switch
• N-Channel MOSFET Switch
• Analog Multiplexer

41
N-channel JFET switch.
42
Switch using an N-channel enhancement-mode MOSFET.
43
Implementation of a four-channel multiplexer
using P-channel JFET switches and an operational
amplifier.
44
7-12 FET Schematic Summary
45
Schematic diagrams for different types of field
effect transistors.