Chapter 28

Basic Transistor Theory

Transistor Construction

Bipolar Junction Transistor (BJT)

3 layers of doped semiconductor

2 p-n junctions

Layers are: Emitter, Base, and Collector

Can be NPN or PNP

Emitter and Collector both P or both N type

Transistor Construction

Structure and Electronic Symbol

Transistor Operation

Amplifier

B-E junction forward biased

V_BE ≈ 0.7 V for Si

C-B junction reverse biased

KCL: I_E = I_C + I_B

Transistor Operation

Transistor Bias Circuits

Transistor Operation

dc Beta (β_dc)

I_E = I_C + I_B

I_B << I_E

I_C ≈ I_E

40 < β_dc < 400

Transistor Operation

2N3904 NPN transistor spec

100 < β_dc < 300

β_dc dependent on

Operating point

Temperature

Transistor Operation

dc Alpha (α_dc)

α < 1

α-β Relationship

Transistor Specifications

Maximum voltage between C & E with Base open, V_CEO

Maximum reverse voltage between C & B with Emitter open, V_CBO

Maximum reverse voltage between E & B with Collector open, V_EBO

Transistor Specifications

Maximum collector current, I_C

Maximum power dissipated, P_D

P_D = I_C * V_CE

Transistor Specifications

Minimum C-E voltage for breakdown, V_(BR)CEO

Carefully examine absolute max ratings

dc current gain

variable

β = h_FE in specs

Collector Characteristic Curves

Saturation region

I_C increases rapidly for small values of V_CE

BJT behaves like closed switch

Collector Characteristic Curves

Active region

BJT acts as a signal amplifier

B-E junction is forward biased & C-B junction is reverse biased

Collector Characteristic Curves

β_dc not constant

β_dc dependent on dc operating point

Quiescent point = operating point

Active region limited by

Maximum forward current, I_C(MAX)

Maximum power dissipation, P_D

dc Load Line

Drawn on characteristic curves

Component values in a bias circuit

Determine quiescent point, Q

Q is between saturation and cutoff

Best Q for a linear amplifier

Midway between saturation and cutoff

DC Load Line

Characteristic curve with Load Line

Q-point, and current gain

Transistor Biasing

Fixed-Bias Circuit

Single power supply

Coupling capacitors

Transistor Biasing

Equations for Fixed-Bias circuit

Transistor Biasing

Fixed Bias Circuit highly dependent on β_dc

Emitter-Stabilized Bias Circuit

Add emitter resistor

Greatly reduces effect of change of β

Equations

Transistor Biasing

Transistor Biasing

Universal-Bias circuit

Sometimes referred to as voltage divider bias

Most stable

Equations:

Transistor Biasing

Universal-Bias circuit

Need I_B << I_C

Make

Simple Voltage divider between V_CC, Base, and ground

Transistor Biasing

Transistor Biasing

Common Collector Circuit

Less common than CE circuit

Collector connected to ground

Similar analysis

Voltage gain < 1

Transistor Biasing

Common Base Circuit

Least common

High frequency applications

Current gain < 1

The Transistor Switch

BJT less used as amplifiers

IC amplifiers available

Switching is a principal application of BJT’s

Current amplifier turn on LED’s

Power amplifier to turn on small motors

The Transistor Switch

A buffer has high input impedance and low output impedance

The Transistor Switch

BJT as a buffer between digital input and LED

Testing a Transistor with a Multimeter

Ohmmeter

dc voltage generates small current

Test CB and BE junctions

Forward bias = small resistance

Reverse bias = large resistance

Testing a Transistor with a Multimeter

Fail test

BJT will not operate correctly

Pass test

Not a guarantee that BJT is good

Testing a Transistor with a Multimeter

Six measurements required

An O.C. between two terminals (both directions) means other terminal is B

Only two low Ω readings if BJT is good

Testing a Transistor with a Multimeter

Lower of the two low Ω readings is C

Other one of low Ω readings is E

Junction Field Effect Transistor Construction and Operation

Construction and symbols

Junction Field Effect Transistor Construction and Operation

BJT

Current amplification

BE junction forward biased

Input impedance (Common Emitter) low

Junction Field Effect Transistor Construction and Operation

JFET

Voltage amplification

GS junction reverse biased

Input impedance very high

Junction Field Effect Transistor Construction and Operation

Basic operation of an n-channel JFET

Junction Field Effect Transistor Construction and Operation

I_S = I_D

Decrease V_GS from 0 to –4

Decrease current flowing

Pinchoff voltage reached

Junction Field Effect Transistor Construction and Operation

I_D vs V_GS (Transconductance curve) described by Shockley’s equation

Junction Field Effect Transistor Construction and Operation

Self-bias circuit

Junction Field Effect Transistor Construction and Operation

Load line

Junction Field Effect Transistor Construction and Operation

Another biasing circuit: similar to universal bias circuit for BJT’s

Voltage divider

Resistor from from V_DDto the Gate

Resistor from Gate to ground

Junction Field Effect Transistor Construction and Operation

Basic JFET circuit analysis: use

KVL and KCL

I_G = 0

I_D= I_S

MOSFETs

Metal Oxide Semiconductor Field Effect Transistors

Small

Low power

Higher current capability, I_DS

Do not have to reverse bias the gate

Depletion or Enhancement types

MOSFETs

Construction and symbols

MOSFETs

Depletion MOSFETs

Have a channel

Shockley’s equation still valid

Depletion mode

Enhancement mode

MOSFETs

Enhancement MOSFETs

No channel

Positive V_GS required prior to current

Enhancement mode only

No depletion mode

Shockley’s equation no longer valid

MOSFETs

Biasing

Voltage Divider

Drain-feedback circuit shown here

MOSFETs

MOSFETs

Handling precautions

Subject to damage by electrostatic charges

Packaged in static resistant bags

Handle at static safe workstation

Use grounded wrist strap

Troubleshooting a Transistor Circuit

Ensure correct biasing

Measure V_BE

Determine V_CEQ and I_CQ

Determine I_BQ

Calculate β


	Bipolar Junction Transistor (BJT)
		3 layers of doped semiconductor
		2 p-n junctions
		Layers are: Emitter, Base, and Collector
		Can be NPN or PNP
		Emitter and Collector both P or both N type


Amplifier
	B-E junction forward biased
		V_BE ≈ 0.7 V for Si
	C-B junction reverse biased
	KCL: I_E = I_C + I_B


	dc Beta (β_dc)
		I_E = I_C + I_B
		I_B << I_E
		I_C ≈ I_E
		40 < β_dc < 400


	2N3904 NPN transistor spec
		100 < β_dc < 300
	β_dc dependent on
		Operating point
		Temperature


	Maximum voltage between C & E with Base open, V_CEO
	Maximum reverse voltage between C & B with Emitter open, V_CBO
	Maximum reverse voltage between E & B with Collector open, V_EBO


	Maximum collector current, I_C
	Maximum power dissipated, P_D
	P_D = I_C * V_CE


	Minimum C-E voltage for breakdown, V_(BR)CEO
	Carefully examine absolute max ratings
	dc current gain
		variable
		β = h_FE in specs


	Saturation region
		I_C increases rapidly for small values of V_CE
		BJT behaves like closed switch


	Active region
		BJT acts as a signal amplifier
		B-E junction is forward biased & C-B junction is reverse biased


	β_dc not constant
	β_dc dependent on dc operating point
	Quiescent point = operating point
	Active region limited by
		Maximum forward current, I_C(MAX)
		Maximum power dissipation, P_D


	Drawn on characteristic curves
	Component values in a bias circuit
		Determine quiescent point, Q
		Q is between saturation and cutoff
	Best Q for a linear amplifier
		Midway between saturation and cutoff


	Characteristic curve with Load Line
	Q-point, and current gain


	Fixed Bias Circuit highly dependent on β_dc
	Emitter-Stabilized Bias Circuit
		Add emitter resistor
		Greatly reduces effect of change of β
		Equations


	Universal-Bias circuit
		Sometimes referred to as voltage divider bias
		Most stable
		Equations:


	Universal-Bias circuit
		Need I_B << I_C
		Make
		Simple Voltage divider between V_CC, Base, and ground


	Common Collector Circuit
		Less common than CE circuit
		Collector connected to ground
		Similar analysis
		Voltage gain < 1


	Common Base Circuit
		Least common
		High frequency applications
		Current gain < 1


	BJT less used as amplifiers
		IC amplifiers available
	Switching is a principal application of BJT’s
		Current amplifier turn on LED’s
		Power amplifier to turn on small motors


	Ohmmeter
		dc voltage generates small current
	Test CB and BE junctions
		Forward bias = small resistance
		Reverse bias = large resistance


	Fail test
		BJT will not operate correctly
	Pass test
		Not a guarantee that BJT is good


	Six measurements required
	An O.C. between two terminals (both directions) means other terminal is B
	Only two low Ω readings if BJT is good


	Lower of the two low Ω readings is C
	Other one of low Ω readings is E


	BJT
		Current amplification
		BE junction forward biased
		Input impedance (Common Emitter) low


	JFET
		Voltage amplification
		GS junction reverse biased
		Input impedance very high


	I_S = I_D
	Decrease V_GS from 0 to –4
		Decrease current flowing
		Pinchoff voltage reached


	I_D vs V_GS (Transconductance curve) described by Shockley’s equation


	Another biasing circuit: similar to universal bias circuit for BJT’s
		Voltage divider
		Resistor from from V_DDto the Gate
		Resistor from Gate to ground


	Metal Oxide Semiconductor Field Effect Transistors
		Small
		Low power
		Higher current capability, I_DS
		Do not have to reverse bias the gate
		Depletion or Enhancement types


	Depletion MOSFETs
		Have a channel
		Shockley’s equation still valid
		Depletion mode
		Enhancement mode


	Enhancement MOSFETs
		No channel
		Positive V_GS required prior to current
		Enhancement mode only
		No depletion mode
		Shockley’s equation no longer valid