Transistor Bias Circuits

8/18/2019 Transistor Bias Circuits

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TRANSISTOR BIAS CIRCUITS&

BJT AMPLIFIERS

BIAS

The DC Operating Point

VoltageDi!i"er Bia#Other Bia# Metho"#

A transistor must be properly biased with a dc voltage in order to operate as a linear amplifier. A dc operating point must be set so that signal variations at the input terminal are

amplified and accurately reproduced at the output terminal. As you learned, when you bias a

transistor, you establish the dc voltage and current values. This means, for example, that at

the dc operating point, IC and VCE have specified values. The dc operating point is oftenreferred to as the Qpoint !"uiescent point#.

Bias establishes the dc operating point !$point% or proper linear operation o an

a'pliier( If an amplifier is not biased with correct dc voltages on the input and output, it can

go into saturation or cutoff when an input signal is applied

effects of proper and improper dc biasing

Analisa

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The transistor in $igure %&'!a# is biased with VCC and VBB to obtain certain values of IB,

IC, IE, and VCE

(itentu)an tiga!*# nilai IB untu) menganalisa yang ter+adi pada I dan --BB diatur untu) mendapat)an nilai IB/ '001A. 2arena IC = ßDC IB, ma)a a)an

didapat)an I / '0mA.

3ntu) mendapat)an nilai -V = V - I R = 10 V - (20 m!(220"! = 10 V - #$# V

= %$& V

(engan cara yang sama

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Q-point


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Sehingga bisa digambar)an

Linear Operation

The region along the load line including all points between saturation and cutoff is generally)nown as the linear region of the transistor4s operation. As long as the transistor is operated

in this region, the output voltage is ideally a linear reproduction of the input

ontoh transistor yang beroperasi pada daerah linear

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(ari circuit di atas tegangan sinusoidal -in ditumpang)an pada -BB menyebab)an arus base bervariasi secara sinusoidal 5001A di bawah dan di atas Qpoint yang bernilai *001A. 6al

ini +uga menyebab)an nilai arus collector bervariasi 50 mA di bawah dan di atas Qpoint

*0mA. Sebagai a)ibat dari bervariasinya arus collector, tegangan collector to emitter +uga

bervariasi '.' - di bawah dan diatas Qpoint *.7 -. Titi) A pada garis beban terhubungdengan punca) positif tegangan input sinusoidal, sedang)an titi) B terhubung dengan dengan

punca) negatif tegangan input sinusoidal. Titi) Q terhubung dengan nilai nol !0# dari

gelombang sinusoidal.

)*%VOLTA+EDIVIDER BIAS

8ou will now study a method of biasing a transistor for linear operation using a single source

resistive voltage divider. This is the most widely used biasing method

A more practical bias method is to use VCC as the sin'e bias so)*ce, as sho+n beo+

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ontoh

Tentu)an nilai - dan I untu) 9( / 500 pada circuit beri)ut:

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Tegangan base:

Maka

)*%VOLTA+E

DIVIDER BIAS),%E'itter Bia#

)-%Ba#e Bia#

).%E'itterFee"/a01

Bia#

!%# Colle0torFee"/a01 Bia#


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B;T AI$I?S

)*% A'pliier Operation

),% Tran#i#tor AC Mo"el#

)-% The Co''onE'itter A'pliier

).% The Co''onColle0tor A'pliier

)2% The Co''onBa#e A'pliier)3% M4lti#tage A'pliier#

)5% The Dierential A'pliier

)6% AC $4antitie#

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TRANSISTOR AC MODELS

Co'pari#on o the AC Beta ) 7a0% to the DC Beta ) 7DC%

COMMONEMITTER AMPLIFIER

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AC Anal8#i#

To analy@e the ac signal operation of an amplifier, an ac e"uivalent circuit is developed as

follows:

The 0apa0itor# C1, C2, and C3 are replaced by effective shorts because their values are

selected so that C is ne'i'ibe at the si'na f*e)enc. and can be conside*ed to be 0

The "0 #o4r0e i# repla0e" /8 gro4n"

Input Resistance at the Base

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Input resistnace pada base

Tegangan base

2arena Ie Ic ma)a

Subtitusi -b dan Ib


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ontoh

Tentu)an nilai minimum ' pada rang)aian beri)ut +i)a amplifier harus beroperasi pada range

fre)uensi antara '00 6@ s.d 50 26@Tentu)an gain tegangan basecollector amplifier dengan dan tanpa ' +i)a td) terdapat resistor

beban !load resistor# pada rang)aian

re4 / D.%E

Solusi

• Tanpa ', nilai gain adalah

•

• (engan ' nilai gain adalah

•

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• Effect of a Load on the !olta"e #ain

•

•

The collector resistance at the signal fre"uency is effectively ? in parallel with ?>. ?emember,

the upper end of ? is effectively at ac groundThe total ac collector resistance is

ontoh

=ada rang)aian di atas, +i)a resistor beban ?> / % ) dihubung)an dengan output hitung voltage

gain basecollector pada amplifier. (i)etahui re4 / D.%E

SolusiThe ac collector resistance is


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C4rrent +ain

Po9er +ain

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Transistor Bias Circuits