Http:// Budapest University of Technology and Economics Department of Electron Devices...

http://www.eet.bme.hu

Budapest University of Technology and EconomicsDepartment of Electron Devices

Microelectronics, BSc course

Bipolar transistors 3

http://www.eet.bme.hu/~poppe/miel/en/08-bipolar3.pptx

Budapest University of Technology and Economics

Department of Electron Devices

16-10-2014 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2014 2

Characteristics of the ideal BJT




Common base setup

Also called grounded base setup

input output




Common base setup

Input characteristic: Output characteristic:

IE

normal active

saturationclosed

normal active

closed

inverse active

saturation

input output




The process of amplificationcommon base setup

6,210

26

mA

mV

I

Ur

E

Td

20010

2000

in

outu u

uA

mVmAriu dbe 4,106,24 in

mVmARiu tki 20005004 out




Common emitter setup

Also called as grounded emitter setup





0CBENC IIAI

CBE III

N

CBB

N

NC A

II

A

AI

110

0)( CBCBNC IIIAI

N

NN A

AB

1

N

CBCE A

II

1

00

0CEBNC IIBI

B : common emitter, large signal current gain





No current flows. Part of the base current is spent on accumulating the base chrage. UBE voltage increases, the emitter current starts to flow. The other part of the base current is spent on recombination with some part of the emitter current.

The charge in the base is not increased any longer. Any increase of the base current recombines with a given part of the emitter current, thus, the emitter current will also increase.

charge increases

charge is constant

is constant

increasing





Input characteristic: Output characteristic:

normal active

saturation

closed

inverse active

saturation

normal active




1)/exp(1)/exp(

1)/exp(1)/exp(

TBCCSTBEESNC

TBCCSITBEESE

UUIUUIAI

UUIAUUII

XUCE

ITCES AUU

1ln0

normal active

saturation

closed

inverse active

saturationThreshold of saturation:UBC = 0

UBE = UCEnormal active




Characteristics of real BJTs: secondary effects

► Parasitic CB diode► Series resistances► Early effect► Operating point dependence of the gain




Effect of the parasitic CB diode

No emitter region opposite to it, thus, in inverse oparation the electrons injected from the collector into the base will be lost: inverse active current gain is worsened.

ITCES AUU

1ln0

inner transistor parasitic junction




Effect of series resistancesBase contact

E C

Where is it exactly?

RBB'

The "inner base" – good approximation: RBB'

B'

B




Effect of series resistancesCollector contact

ICRCC' adds to UCE characteristics can be only on the right hand side of the 1/RCC' line

reduction of RCC' in case of discrete transistors: epitaxial structure (like in case of diodes)

n+ emitter

p basecollector

chip carrier (collector lead)




The Early effect

Backlash: The output voltage influences the input characteristic




The Early effect




The Early effectThe Early voltage

C

E

C

CEki I

U

dI

dUr out




Early effect at common base setup




Early effect: the backlash

~exp(UBE/UT)

CB setup CE setup




The Early effect Problem

What is the output resistance of the transistor in common emitter setup is the Early voltage is 80V and the collector current in the operating point is 5mA?

C

Eki I

Ur k

mA

Vrki 16

5

80out out




Op.p. dependence of current gainVoltage dependence: due to the Early effect

BC

B

B

N

BC

N

dU

dw

dw

dA

dU

dA

EE

BB

n

pe Nw

Nw

D

D1

2

2

11

n

Btr L

w

CBBMB Uconstww

CB

BCtre

BC

N

U

wS

dU

dA /)1(2)1(

2

1 '




Op.p. dependence of current gainVoltage dependence: due to the Early effect

Current dependence:

high level

Http:// Budapest University of Technology and Economics Department of Electron Devices...

Documents

Transcript of Http:// Budapest University of Technology and Economics Department of Electron Devices...