8/19/2019 Lecture 2 Biasing

1/33

KEEE2225 Professor Dr. Mahmoud 1

Low frequency Amplifiers

8/19/2019 Lecture 2 Biasing

2/33

Best Biasing techniques for stable operation

• Amplifiers are used for signal amplifications.

• The condition of course is that the signal beamplified without changing the shape of the

input signal (no distortion).

• How can we get maximum amplification withoutdistortion and maing sure our amplifier is not

easil! influenced b! other changes in thecircuits such as heat" change of beta" etc.#

8/19/2019 Lecture 2 Biasing

3/33

KEEE2225 Professor Dr. Mahmoud$

Which one of these configuration is suited for Amplifier?

Including RE for T.Stability

Including E for !igh"ain and Thermal

Stability

8/19/2019 Lecture 2 Biasing

4/33

KEEE2225 Professor Dr. Mahmoud %

&uiescent 'oint (& 'oint) or operating point is a pointestablished on the load line for best operation

• Quiescent current is the current that flows

in an electrical circuit when no load is present. 

• *nce the operating point is established"

when input signal is applied" the Q-pointshould not move either to saturation or cut+off region. Howe,er this unwanted shift mightoccur due to ,arious reasons.

8/19/2019 Lecture 2 Biasing

5/33

8/19/2019 Lecture 2 Biasing

6/33

KEEE2225 Professor Dr. Mahmoud -

Why Q-point shift?

• The shifting of operating point is due tothree maor reasons/

• 1. 'arameters of transistor depend ontemperature. As it increases" leaage currentdue to minorit! charge carriers (0B*)

increases. As 0B* increases" 0* alsoincreases" causing increase in collectorcurrent 0. This produces heat at thecollector 2base unction . This process repeat"and finall! &+pt ma! shift into saturation

region. 3ometimes the excess heat producedat the unction ma! e,en burn the transistor.This is nown as thermal runaway.

8/19/2019 Lecture 2 Biasing

7/33

KEEE2225 Professor Dr. Mahmoud 4

• 5. changes in current gain (β can shiftthe &+point. 6 ,aries with temperature

and in some case the ,ariation can bedrastic.

• $. !hanges of "#$%(change with temperature 

To a,oid shift of &+pt" bias+stabili7ation is necessar!. 8ariousbiasing techniques can be used for this purpose.

8/19/2019 Lecture 2 Biasing

8/33

KEEE2225 Professor Dr. Mahmoud9

&o' what are the equirements of proper #iasing?

• &+pt is established in the center of active region of

transistor characteristic. 0t should not shift tosaturation region or cut+off region" when input isapplied.

• &+pt should be independent of transistor parameters

ie. should not shift if transistor is replaced byanother transistor of the same type%

• ollector current should be stabili7ed against changesin temperature.

• The circuit must be practical in its implementation" andcost+effecti,e.

8/19/2019 Lecture 2 Biasing

9/33

KEEE2225 Professor Dr. Mahmoud :

)ypes of )ransistor #iasing !ircuits

• There are fi,e main t!pes of biasing circuitsused with Bipolar transistors.

• ;ixed bias

• ollector+to+base bias

• ;ixed bias with emitter resistor

• 8oltage di,ider bias

• mitter bias

8/19/2019 Lecture 2 Biasing

10/33

KEEE2225 Professor Dr. Mahmoud 1<

*i+ed bias

8/19/2019 Lecture 2 Biasing

11/33

KEEE2225 Professor Dr. Mahmoud 11

;ixed Bias

• This form of biasing is also called BaseBias. 0n the gi,en circuit"• 8 = 0B>B ? 8B• Therefore"• 0B = (8 + 8B)@>B• ;or a gi,en transistor" 8B does not ,ar!

significantl! during use. 8!! has fixed

,alue" so b! selecting >B" the basecurrent 0B is fixed. Therefore this t!peis called fixed bias t!pe of circuit.

8/19/2019 Lecture 2 Biasing

12/33

KEEE2225 Professor Dr. Mahmoud 15

• Also for the gi,en circuit"• 8 = 0> ? 8c

 Therefore" 8 is gi,en as • 8 = 8 2 0>!

• 3ince 0 = 60B" we can obtain 0 as well. 0nthis manner" operating point gi,en as(8"0) can be set for gi,en transistor.

ote that both 0 and 8 are 6 dependent

8/19/2019 Lecture 2 Biasing

13/33

KEEE2225 Professor Dr. Mahmoud 1$

• Advantage,• 0t is simple to shift the operating pointan!where in the acti,e region b! merel!

changing the base resistor (>B).• 8er! few number of components are

required.

8/19/2019 Lecture 2 Biasing

14/33

8/19/2019 Lecture 2 Biasing

15/33

KEEE2225 Professor Dr. Mahmoud 1C

!ollector-to-#ase bias

8/19/2019 Lecture 2 Biasing

16/33

KEEE2225 Professor Dr. Mahmoud 1-

ollector to base bias• 0n this form of biasing" the base resistor >B

is connected to the collector instead ofconnecting it to the batter! 8 as shown.

• 8 = 8 + 0> (3ince 0B DD 0)

• 0n case of an increase in temperature"collector current tends to increase" causingthe ,oltage drop across resistor > toincrease. Hence 8 decreases. Thereforebase current reduces" thereb! compensatingfor the increase in collector current.

• 0t can be noted that for the gi,en circuit"• 0B = (8)@(>B?6>)

8/19/2019 Lecture 2 Biasing

17/33

KEEE2225 Professor Dr. Mahmoud 14

• Advantage,

• ircuit has a tendenc! to stabili7e theoperating point against ,ariations intemperature and 6 (ie. replacement oftransistor)

• /isadvantage,• The resistor >B causes an ac feedbac"

reducing the ,oltage gain of the amplifier. Thisis a mostl! undesirable effect.

• .sage,• ue to the maor drawbac of feedbac" this

biasing form is rarel! used.

8/19/2019 Lecture 2 Biasing

18/33

KEEE2225 Professor Dr. Mahmoud 19

*i+ed bias with emitter resistor

8/19/2019 Lecture 2 Biasing

19/33

KEEE2225 Professor Dr. Mahmoud 1:

• 3ince 8be is ,er! small" we get

• 0b = (8+8B + 0>)@>B

• hen the temperature increases" the leaagecurrent increases. Therefore there is anincrease in 0 and 0. This increases the

emitter ,oltage" which in turn reduces the,oltage across the base resistor. This reducesthe base current which results in lesscollector current. Thus collector current is

not allowed to increase" and operating point isept stable.

8/19/2019 Lecture 2 Biasing

20/33

KEEE2225 Professor Dr. Mahmoud 5<

• 3imilarl!" if the transistor is replacedb! another" there ma! be a change in 0

corresponding to change in 6+,alue. B!similar process as abo,e" the change isnegated and operating point ept stable.

• 8 = 8 + (>?>)0 (since 0 roughl!equals 0 as 0B is ,er! small).

• Advantage,

• The circuit has the tendenc! tostabili7e operating point againstchanges in temperature and 6+,alue.

/i d nt

8/19/2019 Lecture 2 Biasing

21/33

KEEE2225 Professor Dr. Mahmoud 51

/isadvantage

• 0n this circuit" for properfunctioning" the following conditionmust be met/

  > EE >B@6• As 6+,alue is fixed for a gi,en

transistor" this relation can besatisfied either b! eeping > ,er!large" or maing >B ,er! low.

• 0f > is of large ,alue" high 8 isnecessar!. This increases the cost.

8/19/2019 Lecture 2 Biasing

22/33

KEEE2225 Professor Dr. Mahmoud 55

• 0f >B is low" a separate low ,oltagesuppl! should be used in the base

circuit. Fsing two supplies of different,oltages is impractical.

• 0n addition to the abo,e" > causes ac

feedbac which reduces the ,oltagegain of the amplifier.

• Fsage/

• ue to the abo,e disad,antages" thist!pe of biasing circuit is generall! notused.

8/19/2019 Lecture 2 Biasing

23/33

KEEE2225 Professor Dr. Mahmoud 5$

"oltage divider bias

8/19/2019 Lecture 2 Biasing

24/33

KEEE2225 Professor Dr. Mahmoud 5%

8oltage i,ider• The ,oltage di,ider is formed using

external resistors >1 and >5.

• The ,oltage across >5 forward biases theemitter unction.

• B! proper selection of resistors >1 and>5" the operating point of the transistorcan be made independent of 6.

• 0n this circuit we get"

• 8B = 8oltage across >5 = (>5G8)@(>1?>5)• Also 8B = 8B ? 0>

8/19/2019 Lecture 2 Biasing

25/33

KEEE2225 Professor Dr. Mahmoud 5C

8. i,ider

• hen temperature increases" 0 increases

and so does 0. hen 0 increases" 8Bdecreases. Therefore 0 decreases and theoperating point remains stable.

• Also" 8 = 8 + 0>

• 3ince 0 is approximatel! equal to 0"• 8c = 8 + (>?>)0

• 0ote that β is absent from all the above

equations. Therefore" if the transistor isreplaced b! another ha,ing a different ,alueof 6" the operating point is largel! unchanged.

8/19/2019 Lecture 2 Biasing

26/33

KEEE2225 Professor Dr. Mahmoud 5-

,. di,ider

• Advantage,• onl! one dc suppl! is necessar!.• *perating point is almost independent of 6 ,ariations.• *perating point stabili7ed against shift in

temperature.• /isadvantage,

• Ac feedbac is caused b! >" which reduces the,oltage gain of the amplifier. A solution to a,oid thiswill follow.

• .sage,

• The circuits stabilit! and merits as abo,e mae itthe most widel! for linear circuits.

8/19/2019 Lecture 2 Biasing

27/33

KEEE2225 Professor Dr. Mahmoud 54

"oltage divider with capacitor

8

8/19/2019 Lecture 2 Biasing

28/33

KEEE2225 Professor Dr. Mahmoud 59

8• The standard ,oltage di,ider circuit faces one

critical drawbac + ac feedbac caused b!resistor >. This can be a,oided using acapacitor () in parallel with >" as shown incircuit diagram.

• The impedance of the capacitor (I) is gi,enb! the equation"• I = 1@(5GJG;G)• where+

• ; is the frequenc! of input signal• the ,alue of capacitance.

• J is pi

http://en.wikipedia.org/wiki/Pihttp://en.wikipedia.org/wiki/Pi

8/19/2019 Lecture 2 Biasing

29/33

KEEE2225 Professor Dr. Mahmoud 5:

8

• ith the capacitor the emitter isplaced at ground potential for ac

input. *nl! dc feedbac is pro,idedfor stabili7ation of operating point.

8/19/2019 Lecture 2 Biasing

30/33

KEEE2225 Professor Dr. Mahmoud $<

$mitter bias

8/19/2019 Lecture 2 Biasing

31/33

KEEE2225 Professor Dr. Mahmoud $1

• this biasing circuit is the most effecti,ewhen a split suppl! (dual power suppl!) isa,ailable".

• The negati,e suppl! 8 is used to forward+bias the emitter unction through >. The

positi,e suppl! 8 is used to re,erse+biasthe collector unction.• *nl! three resistors are necessar!.

e now that"• 8B + 8 = 8B

8/19/2019 Lecture 2 Biasing

32/33

KEEE2225 Professor Dr. Mahmoud $5

• 0f >B is small enough" base ,oltage will beapproximatel! 7ero. Therefore emittercurrent is"

• 0 = (8 + 8B)@>• The operating point is independent of 6 if

> EE >B@6

• Advantage,• Kood stabilit! of operating point similar to

,oltage di,ider bias.• /isadvantage,

• This t!pe can onl! be used when split (dual)power suppl! is a,ailable.

8/19/2019 Lecture 2 Biasing

33/33

KEEE2225 Professor Dr Mahmoud $$