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1. Darlington Connection

A very popular connection of two bipolar junction transistors for operation as one “superbeta” transistor is the Darlington connection shown below

C

E

BThe main feature of Darlington connection is that the composite transistor acts as a single unit with a current gain that is the product of the current gains of the individual transistors

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If connection is made using two separate transistors having current gains of β1 and β2, the Darlington connection provides a current gain of

βT = β1β2

If the two transistors are matched so that β1 = β2= β,the Darlington connection provides a current gain of

βT = β2

A Darlinton transistor connection provides a transistor having a very large current gain, typically a few thousand

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2. DC Bias of Darlington Circuit

A basic Darlington circuit is shown in figure

VCC

RE

RB

C

E

B

The base current may be calculate from

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Bias Bias Bias Bias dengandengandengandengan sistemsistemsistemsistem pembagipembagipembagipembagitegangantegangantegangantegangan

EDB

BEB RR

VVccI

β+−

=

Besar arus base adalah

Besar arus emitor adalah

IE = (βD + 1)IB ≈≈≈≈βDIB

Tegangan DC

VE = IEREVB = VE + VBE-

βD = Penguatan arus total dari kedua transistor

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RangkaiaRangkaiaRangkaiaRangkaia SetaraSetaraSetaraSetara acacacac

Pada rangkaian ini tahanan transistor digantikan olehsebuah tahanan pengganti yaitu ri

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Tahanan Masukan ac (Ri)

i

ob r

VViI

−=

EDib

i RrI

V β+=

Vi = Ib(r i +(1+βD)RE) ≈≈≈≈ Ib(r i + βDRE)

Ri = RB //(r i + βDRE)

Arus base yang melalui ri adalah

Vo = (Ib +βDIb) RE

Dari kedua persamaan diperoleh

Ibr i = Vi – Vo = Vi – Ib(1+βD)RE

Pemecahan untuk Vi

Dengan demikian tahanan masukan pada transistor adalah (Rit)

Tahanan masukan penguat

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Penguatan Arus ac (Ai)

Db

o

I

I β=i

b

b

o

i

oi I

I

I

I

I

IA ==

iBED

Bi

BEDi

Bb I

RR

RI

RRr

RI

+≈

++=

ββ )(

BED

BD

BED

BDi RR

R

RR

RA

+=

+=

ββ

ββ

I o = Ib + βDIb = (1 + βD)Ib = βDIb

Besar arus keluaran Io yang melalui RE adalah

Penguatan arus transistor Penguatan arus rangkaian

Dengan mengunakan rangkaian pembagi arus

Dapat diperoleh penguatan arus rangkaian

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Tahanan Keluaran ac (Ro)

−+=−+=

i

oD

i

o

E

obD

i

o

E

oo r

V

r

V

R

VI

r

V

R

VI ββ

oi

D

iE

VrrR

++=

β11

i

D

iE

o

oo

rrRI

VR β++

==11

1

D

i

D

iiEo

rrrRR

ββ≈

= ////

Perhitungan tahanan keluaran ac didekati dengan mamasangsebuah sumber tegangan Vo kemuadian mengukur arus Io

Maka besar tahanan Ro adalah

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Penguatan Tegangan ac (Kv)

)()( EDE

EDEi

io RR

RRr

VV β

β+

++=

1)(

)( ≈++

+==EDEi

EDE

i

ov RRr

RR

V

VK

ββ

Vi= I b (r i + RE + βDIb) RE

Penguatan tegangan dapat ditentukan dari rangkaian setara ac

Vo = (Ib +βDIb) RE = Ib (RE + βD RE) Vi = Ibr i + (Ib+ βDIb) RE

Atau Vi dapat ditulis

Dan Vo

Sehingga penguatan tegangan adalah

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TaskTaskTaskTaskBerdasarkan gambar tentukan :

a. IB, IE, dan VE

b. Tahanan masukan jika ri = 5 k

c. Penguatan arus (Ai)

d. Penguatan tegangan (KV