Analog Electronics - Miunapachepersonal.miun.se/~amiyou/AE/Lecture4.pdf · Electronic Devices, 9th...

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.

All rights reserved. Electronic Devices, 9th edition

Thomas L. Floyd

Analog Electronics



Thomas L. Floyd

BJT Structure

The BJT has three regions called the emitter, base, and

collector. Between the regions are junctions as indicated.

B

(base)

C (collector)

n

p

n

Base-Collector

junction

Base-Emitter

junction

E (emitter)

B

C

p

n

E

p

npn pnp The base is a thin lightly

doped region compared to the

heavily doped emitter and

moderately doped collector

regions.

The pn junction joining the base region

and the emitter region is called the base-

emitter junction.

The pn junction joining the base region

and the collector region is called the base-

collector junction



Thomas L. Floyd

BJT Operation

In normal operation, the base-emitter is forward-biased

and the base-collector is reverse-biased.

npn

For the pnp type, the voltages

are reversed to maintain the

forward-reverse bias. –

+ –

+

–

+

+

BC reverse-

biased

–

BE forward-

biased

–

+

+

–

–

BC reverse-

biased

+

BE forward-

biased

–

+

pnp

For the npn type shown, the

collector is more positive

than the base, which is more

positive than the emitter.



Thomas L. Floyd

BJT Currents

IE IE

IC

IB

IC

IBn

p

n

p

n

p

+

– +

–

–+

IE

IC

IB

+

–

+

IE

IC

IB

+

–

–

npn pnp

The direction of conventional current is in the direction of the arrow

on the emitter terminal. The emitter current is the sum of the

collector current and the small base current. That is, IE = IC + IB.



Thomas L. Floyd

BJT Parameters

Two important parameters, βDC (dc current gain) and

αDC are introduced and used to analyze a BJT circuit.

DC Beta (βDC )

Ratio of DC collector current and

DC base current.

βDC == IC/IB

DC Alpha (αDC )

Ratio of DC collector current to the

DC emitter current.

αDC == IC/IE

VBB forward bias the base-

emitte jumction and VCC reverse

bias the base-collector junction



Thomas L. Floyd

Transistor DC Model

BJT in un-saturation mode is a current controlled current

source.

Input circuit is a forward-biased

diode through which there is base

current.

The output circuit is dependent

current source. The value of o/p

current is dependent on base

current.



Thomas L. Floyd

BJT Circuit Analysis

Currents and voltages in BJT

IB : dc base current

IE: dc emitter current

IC: dc source current

VBE: dc voltage at base wrt. emitter

VCE: dc voltage at collector wrt.

emitter.

VCB: dc voltage at collector wrt.

Base.



Thomas L. Floyd

The collector characteristic curves show the relationship

of the three transistor currents.

The curve shown is for a fixed

based current. The first region is

the saturation region.

BJT Characteristics

IC

B

C

A

0 0.7 V VCE(max)

VCE

Saturation

region

Active region

Breakdown

region

As VCE is increased, IC increases

until B. Then it flattens in region

between points B and C, which

is the active region.

After C, is the breakdown

region.



Thomas L. Floyd

The collector characteristic curves illustrate the relationship of the

three transistor currents.

0

IC

VCE

IB6

IB5

IB4

IB3

IB2

IB1

IB = 0Cutoff region

By setting up other values of

base current, a family of

collector curves is developed.

bDC is the ratio of collector

current to base current.

BJT Characteristics

It can be read from the curves.

The value of bDC is nearly the

same wherever it is read.

CDC

B

I

Ib



Thomas L. Floyd

What is the bDC for the transistor shown?

Choose a base current near the

center of the range – in this

case IB3 which is 30 mA.

IC

VCE

IB6

IB5

IB4

B3I

IB2

B1I

IB = 0

= 10 Am

= 20 Am

= 30 Am

= 0

= 60 Am

= 40 Am

= 50 Am

10.0

8.0

6.0

4.0

2.0

0

(mA)

Read the corresponding

collector current – in this case,

5.0 mA. Calculate the ratio:

CDC

B

5.0 mA

30 A

I

Ib

m 167

BJT Characteristics



Thomas L. Floyd

Cutoff

In a BJT, cutoff is the condition in which there is no base

current, which results in only an extremely small leakage

current (ICEO) in the collector circuit. For practical work, this

current is assumed to be zero.

IB = 0 –

+

–

+ICEO

RC

VCCVCE ≅VCC

RB

In cutoff, neither the base-emitter

junction, nor the base-collector

junction are forward-biased.



Thomas L. Floyd

Saturation

In a BJT, saturation is the condition in which there is

maximum collector current. The saturation current is

determined by the external circuit (VCC and RC in this case)

because the collector-emitter voltage is minimum (≈ 0.2 V)

In saturation, an increase of base

current has no effect on the

collector circuit and the relation

IC = bDCIB is no longer valid.

–

+ –

+

VCC

VBB

VCE = VCC – IC RC

RB

RC

IB

IC

–

+

– +When the base-emitter junction

is forward biased and there is

enough base current to produce

maximum collector current, the

transistor is saturated

IC(SAT) =VCC –VCE(SAT) /RC

IB(min) = IC(SAT) βDC



Thomas L. Floyd

DC Load Line

the transistor. It is drawn by

connecting the saturation

and cutoff points.

The transistor characteristic

curves are shown superimposed

on the load line. The region

between the saturation and

cutoff points is called the

active region.

The DC load line represents the circuit that is external to

0

IC

VCE

IB = 0 Cutoff

VCE(sat) VCC

IC(sat)

Saturation



Thomas L. Floyd

DC Load Line

What is the saturation current and

the cutoff voltage for the circuit?

Assume VCE = 0.2 V in saturation. –

+ –

+VCC

15 V

VBB

3 V

RC

RB

βDC = 200

220 kW

3.3 kW

CCSAT

C

0.2 V 15 V 0.2 V

3.3 k

VI

R

W4.48 mA CO CCV V 15 V

Is the transistor saturated? B

3.0 V 0.7 V10.45 A

220 kI m

W

IC = b IB = 200 (10.45 mA) = 2.09 mA Since IC < ISAT, it is not saturated.



Thomas L. Floyd

Data Sheets

Data sheets give manufacturer’s specifications for maximum operating

conditions, thermal, and electrical characteristics. For example, an

electrical characteristic is bDC, which is given as hFE. The 2N3904 shows

a range of b’s on the data sheet from 100 to 300 for IC = 10 mA.

ON Characteristics

DC current gain ( IC = 0.1 mA dc, VCE = 1.0 V dc)

( IC = 1.0 mA dc, VCE = 1.0 V dc)

( IC = 10 mA dc, VCE = 1.0 V dc)

( IC = 50 mA dc, VCE = 1.0 V dc)

( IC = 100 mA dc, VCE = 1.0 V dc)

2N39032N3904

2N39032N3904

2N39032N3904

2N39032N3904

2N39032N3904

hFE2040

3570

50100

3060

1530

––

––

150300

––

––

–

Characteristic Symbol Max UnitMin



Thomas L. Floyd

Data Sheets



Thomas L. Floyd

DC and AC Quantities

The text uses capital letters for both AC and DC currents and voltages

with rms values assumed unless stated otherwise.

DC Quantities use upper case roman subscripts. Example: VCE.

(The second letter in the subscript indicates the reference point.)

AC Quantities and time varying signals use lower case italic

subscripts. Example: Vce.

Internal transistor resistances are indicated as lower case

quantities with a prime and an appropriate subscript. Example: re’.

External resistances are indicated as capital R with either a

capital or lower case subscript depending on if it is a DC or ac

resistance. Examples: RC and Rc.



Thomas L. Floyd

BJT Amplifiers

A BJT amplifies AC signals by converting some of the DC power from

the power supplies to AC signal power. An ac signal at the input is

superimposed in the dc bias by the capacitive coupling. The output ac

signal is inverted and rides on a dc level of VCE.

–

+

–

+

VCC

VBB

RB

RC

Vb

Vc

r ′e

Vin

VCE

Vc

VBB

Vin

0

0



Thomas L. Floyd

BJT Switches

A BJT can be used as a switching device in logic circuits to turn on or

off current to a load. As a switch, the transistor is normally in either

cutoff (load is OFF) or saturation (load is ON).

In cutoff, the transistor

looks like an open switch.

In saturation, the transistor

looks like a closed switch.

RB

0 V

RC IC = 0

+VCC

RC

C

E

+VCC

IB = 0–

+RB

RC IC(sat)

+VCC

RC

C

E

+VCC

IB

+VBB

IC(sat)



Thomas L. Floyd

The FET

The idea for a field-effect transistor (FET) was first

proposed by Julius Lilienthal, a physicist and inventor. In

1930 he was granted a U.S. patent for the device.

His ideas were later refined and

developed into the FET. Materials

were not available at the time to

build his device. A practical FET

was not constructed until the

1950’s. Today FETs are the most

widely used components in

integrated circuits.



Thomas L. Floyd

n

p p

n

The JFET

The JFET (or Junction Field Effect Transistor) is a normally

ON device. For the n-channel device illustrated, when the

drain is positive with respect to the source and there is no

gate-source voltage, there is current in the channel.

When a negative gate voltage is

applied to the FET, the electric

field causes the channel to

narrow, which in turn causes

current to decrease. +

–

–

+

RD

D

S

GVDD

VGG

p p



Thomas L. Floyd

The JFET

As in the base of bipolar transistors, there are two types of

JFETs: n-channel and p-channel. The dc voltages are

opposite polarities for each type.

The symbol for an n-channel JFET is

shown, along with the proper polarities of

the applied dc voltages. For an n-channel

device, the gate is always operated with a

negative (or zero) voltage with respect to

the source.

RD

VDD

VGG

+

–

+

–

Gate

Source

Drain



Thomas L. Floyd

The JFET

There are three regions in the characteristic curve for a JFET

as illustrated for the case when VGS = 0 V.

Between A and B is the Ohmic

region, where current and voltage

are related by Ohm’s law.

From B to C is the active (or

constant-current) region where

current is essentially independent

of VDS.

Beyond C is the breakdown

region. Operation here can

damage the FET. VP

0A

B C

ID

IDSS

VDS

Ohmic region

Active region

(constant current)

(pinch-off voltage)

Breakdown

VGS = 0



Thomas L. Floyd

The JFET

When VGS is set to different values, the relationship between

VDS and ID develops a family of characteristic curves for the

device.

VGS = 0

ID

IDSS

VDS

VGS = –1 V

VGS = –2 V

VGS = –4 VVGS = VGS(off) = –5 V

VGS = –3 V

VP = +5 V

Notice that Vp is

positive and has the

same magnitude as

VGS(off).

Pinch-off Voltage

IDSS

VGS(off).



Thomas L. Floyd

ID

IDSS

0VGS(off)

–VGS

The JFET

A plot of VGS to ID is called the transfer or transconductance

characteristic curve. The transfer curve is a is a plot of the

output current (ID) to the input voltage (VGS).

The transfer curve is based on the

equation 2

GSD DSS

GS(off)

1V

I IV

By substitution, you can find other

points on the curve for plotting the

universal curve.

IDSS

2

0.3 VGS(off) 0.5 VGS(off)

IDSS

4



Thomas L. Floyd

The input resistance of a JFET is given by:

Summary

JFET Input Resistance

GSN

GSS

I

VR

I

Compare the input resistance of a 2N5485 at 25 oC and at 100 oC.

The specification sheet shows that for VGS = 20 V, IGSS – 1 nA at 25 oC and 0.2 mA at 100 oC.

At 25 oC,

where IGSS is the current into the reverse biased gate.

GSN

GSS

20 V

1 nAI

VR

I 20 GW!

JFETs have very high input resistance, but it drops when the temperature

increases.

At 100 oC, GSN

GSS

20 V

0.2 μAI

VR

I 100 MW



Thomas L. Floyd

JFET Biasing

Self-bias is simple and effective, so it is the most common

biasing method for JFETs. With self bias, the gate is

essentially at 0 V.

Summary

RD

IS+

–

RSRG

VG = 0 V

+VDD

An n-channel JFET is illustrated. The current

in RS develops the necessary reverse bias that

forces the gate to be less than the source.

Assume the resistors are as shown and the

drain current is 3.0 mA. What is VGS?

= +12 V

1.5 kW

330 W

1.0 MW

VG = 0 V; VS = (3.0 mA)(330 W) = 0.99 V

VGS = 0 – 0.99 V = 0.99 V



Thomas L. Floyd

You can use the transfer curve to obtain a reasonable value

for the source resistor in a self-biased circuit.

Summary

ID

0–VGS

10 mA

8.0

6.0

4.0

2.0

(mA)

1234

What value of RS should you use

to set the Q point as shown?

375 W

JFET Biasing

Q The Q point is approximately at

ID = 4.0 mA and VGS = 1.25 V.

GSS

D

1.25 V

3.0 mA

VR

I



Thomas L. Floyd

Voltage-divider biasing is a combination of a voltage-divider

and a source resistor to keep the source more positive than

the gate.

Summary

JFET Biasing

RD

RSR2

+VDD

R1

VG

VS

ID

IS

VG is set by the voltage-divider and is independent

of VS. VS must be larger than VG in order to

maintain the gate at a negative voltage with

respect to the source.

Voltage-divider bias helps stabilize the bias for

variations between transistors.



Thomas L. Floyd

The metal oxide semiconductor FET uses an insulated gate

to isolate the gate from the channel. Two types are the

enhancement mode (E-MOSFET) and the depletion mode

(D-MOSFET).

The MOSFET

An E-MOSFET has no

channel until it is induced by

a voltage applied to the gate,

so it operates only in

enhancement mode. An n-

channel type is illustrated

here; a positive gate voltage

induces the channel.

VGG–

+

RD

–

+VDD

n

n

++++

––––

ID

Induced

channeln

n

SiO2

Source

p substrateGate

Drain

E-MOSFET



Thomas L. Floyd

The D-MOSFET has a channel that can is controlled by the

gate voltage. For an n-channel type, a negative voltage

depletes the channel; and a positive voltage enhances the

channel.

Summary

The MOSFET

A D-MOSFET can

operate in either

mode, depending on

the gate voltage.

D-MOSFET

––––––

n

nVGG

+

–

RD

–

+VDDp

++++++

n

nVGG–

+

RD

–

+VDDp

––––––

++++++

operating in D-mode operating in E-mode



Thomas L. Floyd

MOSFET symbols are shown. Notice the broken line

representing the E-MOSFET that has an induced channel.

The n channel has an inward pointing arrow.

Summary

The MOSFET

n channel p channel

D D

G G

S S

E-MOSFETs

n channel p channel

D D

G G

S S

D-MOSFETs



Thomas L. Floyd

The transfer curve for a MOSFET is has the same parabolic

shape as the JFET but the position is shifted along the x-axis.

The transfer curve for an n-channel E-MOSFET is entirely in

the first quadrant as shown.

Summary

The MOSFET

ID

0 VGS(th) +VGS

The curve starts at VGS(th), which is a

nonzero voltage that is required to have

channel conduction. The equation for

the drain current is

2

D GS GS(th)I K V V



Thomas L. Floyd

The D-MOSFET can be operated in either mode. For the n-

channel device illustrated, operation to the left of the y-axis

means it is in depletion mode; operation to the right means is

in enhancement mode.

The MOSFET

As with the JFET, ID is zero at VGS(off).

When VGS is 0, the drain current is

IDSS, which for this device is not the

maximum current. The equation for

drain current is 2

GSD DSS

GS(off)

1V

I IV

ID

0V

I

GS(off)

DSS

–VGS



Thomas L. Floyd

E-MOSFETs can be biased using bias methods like the BJT

methods studied earlier. Voltage-divider bias and drain-

feedback bias are illustrated for n-channel devices.

MOSFET Biasing

+VDD

R2

RDR1

+VDD

RDRG

Voltage-divider bias Drain-feedback bias



Thomas L. Floyd

The simplest way to bias a D-MOSFET is with zero bias. This

works because the device can operate in either depletion or

enhancement mode, so the gate can go above or below 0 V.

MOSFET Biasing

Zero bias, which can only be used for the D-MOSFET

+VDD

RG

VG = 0 V

VGS = 0

RD

IDSS

+VDD

RG

RD

Cac

input

Analog Electronics - Miunapachepersonal.miun.se/~amiyou/AE/Lecture4.pdf · Electronic Devices, 9th...

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