analog electronics

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Electronic Devices, 9th editionThomas L. Floyd

Analog Electronics

Lecture 4:Transistors



Semiconductor material and pn-junction diode

P-type semiconductor

N-type semiconductor

PN- Junction

Diode



Diode and its resistive behavior

R = 0

R = inf



Transformative Resistor

Signal controlled transformative resistor

R

Control signal


Controlled Resistor

Vsup



It leads to an electronic switch


R

Control signal


Controlled Resistor

Vsup

How does it switch?



It leads to signal amplification


R

Control signal


Controlled Resistor

Vsup

How does it amplify?



Constructing amplifying device

A semiconductor is doped with penta and tri valent impurities to divide it into three regions.

Three regions make 2 PN junctions.


A region called ‘Emitter’ is heavily doped as an n-type material. It has an excess of electrons in conduction band.

Next to Emitter a lightly doped p-region with fewer holes is called ‘Base’

The 3rd region is n-type and is called ‘Collector’



The heavily doped n-type emitter region has a very high density of conduction-band (free) electrons.

The base has a low density of holes, which are the majority carriers.

These free electrons easily diffuse through the forward biased BE junction into the lightly doped and very thin p-type base region.




A very little free electrons recombine with holes in base and move as valence electrons through the base region and into the emitter region as hole current.

The valence electrons leave the crystalline structure of the base and become free electrons in the metallic base lead and produce the external base current.

Majority of free electrons move toward the reverse-biased BC junction and swept across into the collector region by the attraction of the positive collector supply voltage.

The free electrons move through the collector region, into the external circuit, and then return into the emitter region along with the base current.


How does it amplify?Note the junction biases for this discussion.



Transistor Currents

IE IE

IC

IB

IC

IBn

p

n

p

n

p

+

– +

–

–+

IE

IC

IB

+

–

+

IE

IC

IB

+

–

–

npn pnp

The conventional current flows in the direction of the arrow on the emitter terminal. The emitter current IE is the sum of the collector current IC and the small base current IB.

That is, IE = IC + IB.

The emitter current is slightly more that collector current.


The voltage drop between base and emitter is VBE whereas the voltage drop between collector and base is called VCE.



The collector current is directly proportional to the base current.

IC ∝ IB

βDC as constant of proportionality.IC = βDC IB

This eq explains amplification of current.

Ratio of DC collector current and DC base current.

βDC == IC/IB

Ratio of DC collector current to the DC emitter current.Collector and emitter currents are ca

IC = αDC IE,

αDC is always less than 1

Current Relationships




This is Bipolar Junction Transistor

This semiconductor device with three doped regions:

i. Emitterii. Base iii. Collector

And two pn junctions:B

(base)

C (collector)

n

p

n

Base-Collectorjunction

Base-Emitterjunction

E (emitter)

B

C

p

n

E

p

npn pnp

i. Base-emitter junction.ii. Base-collector junction

The device is named as ‘Bipolar Junction Transistor’ or BJT

Bipolar Junction Transistors



BJT Biasing

In order for a BJT to operate properly , the two pn junctions must be correctly biased with external dc voltages.

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.

More about BJTs



BJT Circuit Analysis

Currents and voltages in BJT

IB : dc base currentIE: dc emitter currentIC: dc source current

VBE: dc voltage at base wrt. emitterVCE: dc voltage at collector wrt. emitter.VCB: dc voltage at collector wrt. Base.

VBE = 0.7V

VCE = VCC – IC RC

IB = (VBB – VBE ) / RB

VCB = VCE – VBE



The collector characteristic curves shows three mode of operations of transistor with the variation of collector current IC w.r.t VCE for a specified value of base current IB.

VBB is set to produce a certain value of IB and VCC is zero and VCE is zero.

BJT Collector Characteristics Curve

IC

B

C

A

0 0.7 V VCE(max)

VCE

Saturation region

As VCE is increased, IC increases until B.

Both BE and BC junctions are forward biased and the transistor is in Saturation region.



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

–

+

– +IC(SAT) =VCC –VCE(SAT) /RC

BJT Collector Characteristics Curve - Saturation

At this point, the transistor current is maximum and voltage across collector is minimum, for a given load.



IC

B

C

A

0 0.7 V VCE(max)

VCE

Active region

As VCE is increased furthers and exceeds 0.7V the base-collector junction becomes reverse-biased and the transistor goes into the active, or linear, region of its operation.

IC levels off and remains essentially constant for agiven value of IB as VCE continues

to increase.

the value of IC is determinedonly by the relationship expressed as

BJT Collector Characteristics Curve - Linear



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.

It can be read from the curves. The value of bDC is nearly the same wherever it is read in active region.

CDC

B

I

I

BJT Collector Characteristics Curve family



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.

BJT Collector Characteristics Curve –Cut off



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)



DC Load Line

0

IC

VCE

IB = 0 Cutoff

VCE(sat) VCC

IC(sat)

Saturation

Here IB = 0 and VCE = VCC

Here VCE = 0 andIC = IC-Sat = VCC -VCE(Sat) / RC



The DC Operating Point

Bias establishes the operating point (Q-point) of a transistor amplifier; the ac signal moves above and below this point.

Improper biasing can cause distortion in the output signal as the transistor may go into the saturation and cutoff region.




Point A,Q,B represents the Q-point for IB 400mA. 300 mA and 200 mA respectively.

0VCE (V)

400 A

IC (mA)

300 A = IBQ

200 A

A

B

Q

1.2 3.4 5.6VCEQ

ICQ

Vce

IbIc

20

30

40 µ

µ

µ

Load line

The point at which the load line intersects a characteristic curve represents the Q-point for that particular value of IB.

Assume a sinusoidal Ib is superimposed on VBB varying between 200uA to 400uA. It makes the collector current varies between 20 mA and 40 mA.




A signal that swings outside the active region will be clipped.

For example, the bias has established a low Q- point. As a result, the signal is will be clipped because it is too close to cutoff.

VCC

VCE

Cutoff

Q

IC

ICQ

Cutoff 0

Vce

VCEQ

Inputsignal



Voltage-Divider Bias

A practical way to establish a Q-point is to form a voltage-divider from VCC.

Summary

R1 and R2 are selected to establish VB. If the divider is stiff, IB is small compared to I2. Then,

+VCC

RCR1

RER2

2B CC

1 2

RV V

R R

+VCC

RCR1

RER2

βDC = 200

27 kW

12 kW

+15 V

680 W

1.2 kW

Determine the base voltage for the circuit.

2B CC

1 2

12 k15 V

27 k 12 k

RV V

R R

W W W 4.62 V

I2

IB



Voltage-Divider Bias

A practical biasing technique that utilize single biasing sources instead of separate VCC and VBB.

A dc bias voltage at the base of the transistor can be developed by a resistive voltage divider that consists of R1 and R2



DC Load Line

What is the saturation current and the cutoff voltage for the circuit? Assume VCE = 0.2 V in saturation. –

+ –

+VCC15 V

VBB

3 V

RC

RB

βDC = 200220 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

W

IC = b IB = 200 (10.45 mA) =

2.09 mA Since IC < ISAT, it is not saturated.



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.



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.



The JFET

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

The n-channel is connected with two leads i.e. Drain and Source

Two p-type regions are diffused in n-type material and both connected to gate.



n

p p

n

The JFET Construction

For the n-channel device, 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



The JFET-Symbol and biasing

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



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.

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 W1.0 MW

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

VGS = 0 – 0.99 V = - 0.99 V



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 E-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

Inducedchanneln

n

SiO2

Source

p substrateGate

DrainE-MOSFET



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.

The D-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



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.

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



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