JFET Amplifier

8/12/2019 JFET Amplifier

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JFET amplifiers

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2/19

2uestion1

2uestions

The circuit shown here is a precision 3C voltmeter"

(+)

Test lead

1 45

6 45

1 45

6 k5

1 k5

F.S. = % 7A

Zero & 8

1 k5

Test lead

(-)

Span

E9plain wh! this circuit desi$n re:uires the use of a field;effect transistor, and not a bipolar unctiontransistor/


3/19

2uestion

A student builds this transistoramplifier circuiton a solderless breadboard"

+V

8in

D8out

G

S

-V

8outSGD

8in

Regulated DC power supply

V

The purpose of the potentiometer is to provide an adustable 3C bias volta$e for the transistor,so it ma! be operated in Class;A mode. After some adustment of this potentiometer, the student isable to obtain $ood amplification from the transistor /si$nal $enerators and oscilloscopes have beenomitted from the illustration for simplicit!0.

Later, the student accidentl! adusts the power suppl! volta$e to a level be!ond the JFET@s ratin$,destro!in$the transistor. -e;settin$thepower suppl! volta$eback where the student be$an the e9perimentand replacin$ the transistor, the student discovers that thebiasin$ potentiometer must be re;adusted toachieve $ood Class;A operation.

>ntri$ued b! this discover!, the student decides to replace this transistor with a third /of the same partnumber, of course0, ust to see if the biasin$ potentiometer needs to be adusted a$ain for $ood Class;Aoperation. >t does.

E9plain wh! this is so. (h! mustthe $atebiasin$ potentiometerbe re;adustedever! timethe transistor

is replaced, even if the replacementtransistor/s0 are of the e9act same t!pe?


4/19

file 116

2uestion+

The simple JFET amplifier circuitshown here /built withsurface;mountcomponents0emplo!s a biasin$techni:ue known as self;biasin$"

8in

C-

G- D S -

C

8338out

Ground

)elf;biasin$ provides much $reater 2;point stabilit! than $ate;biasin$. 3raw a schematic dia$ramof this circuit,and then e9plain how self;biasin$ works.

file 1161


5/19

e

2uestion*

The volta$e$ain for a b!passed common;emittert is difficult to keep the volta$e $ain stable in either t!pe ofamplifier,due to chan$in$ factors within the transistors themselves which cannot be ti$htl! controlled /rE

and $m,


6/19

respectivel!0. Dne solution to this dilemma is to swamp thoseuncontrollable factors b! notb!passin$ the

emitter /or source0 resistor. The resultis $reaterA8 stabilit!at the e9pense of A8 ma$nitude"


7/19

"Swamped" common-emitter (and common-source)single-transistor amplifier configurations

+V +V

8in

-C

8out

-E

8in

-3

8out

-)

(rite the volta$e $ain e:uations for both swamped


8/19

2uestionG

8 8

This is a schematic of an -F amplifier usin$ a JFET as the active element"

-V +Vin out

L1 C1 C L

L+ S D L*

C C+ G C* C%

(hat confi$uration of JFET amplifier is this /common drain, common $ate, or common source0? Also,e9plain the purpose of the two iron;core inductors in this circuit. Hint" inductors L1 and L are often

referred to as -F chokes.file 11G6

2uestion6

Calculatethe appro9imate input impedance of this JFET amplifier circuit"

833

1% k5

k5 8out

8in

1% k5

+.+ k5

8))

E9plain wh! itis easier to calculatethe Bin of a JFET circuitlike this than itis to calculatethe Bin of a

similar bipolar transistoramplifier circuit. Also, e9plain how calculationof this amplifier@s output impedancecompares withthat of a similar


9/19

2uestion&

>dentif!whatt!peof amplifier circuitthis is, and also whatwould happen to the output volta$eif 8inwere to become more positive"

+V

8out

8in1 8in

-V

file 11G


10/19

2uestion1

The followin$ circuitis a multicouplerfor audio si$nals" one audio si$nal source /such as a microphone0

is distributedto threedifferentoutputs"

+V

Input 2

21

2+

2*

ut!

ut"

ut#

)uppose an audio si$nal is $ettin$throu$hfrom the input to outputs and +, but not throu$hto output1. >dentif! possible failures in the circuitthat could cause this.


11/19

2uestion11

1

This rela9ation oscillator circuit uses a resistor;capacitor combination /-1 ; C1 0 to establish the

time dela! between outputpulses"

1 k5- -1

*G k5

T$!

utput

C1G 5 -+

1 7F

The volta$emeasured between TJ1 and $round looks like this on the oscilloscope displa!"

SCI%%SC$&

'ert()al

*

Vd('DC G,D C

tr(gger

t(e/ase

0

sd('DC G,D C

A sli$htl!different version of this circuit adds a JFET to the capacitor@s char$e currentpath"

1 k5-

utput G 5 -+

-11 k5

T$!

C1 1 7F

'ow, the volta$eat TJ1 looks like this"


12/19

SCI%%SC$&

'ert()al

*

Vd('DC G,D C

tr(gger

t(e/ase

0

sd('DC G,D C

(hat function does the JFET perform in this circuit, based on !our anal!sis of the new TJ1 si$nalwaveform? The strai$ht;line char$in$ volta$e pattern shown on the second oscilloscope displa! indicateswhatthe JFET is doin$ in this circuit.

Hint" !ou don@tneed to know an!thin$about the function of the uniunction transistor/at the circuit@s

output0 other than it acts as an on#off switch to periodicall! dischar$e the capacitor when the TJ1 volta$ereaches a certainthreshold level.

Challen$e :uestion" writea formula predictin$ the slope of the rampin$ volta$ewaveform measured at

T1.

file 116

2uestion1

3efine what a common;source transistor amplifier circuit is. (hat distin$uishes this amplifierconfi$uration from the other sin$le;FET amplifier confi$urations, namel! common;drain and common;$ate?(hat confi$uration of


13/19


14/19

1+

Answer 1

Answers

The volta$e ran$es for this meter are as follows"

= .1 volts

= . volts

= 1. volts

= . volts

= 1 volts

= volts

The JFET is bein$ used in the common drain confi$uration. A reasonable value for the capacitorwouldbe .1 7F.

Answer

This amplifier circuituses $ate bias, which is a notoriousl!unstablemethod of biasin$ a JFET amplifiercircuit.

Answer +

8dd

8out

8in

)elf;biasin$ uses the ne$ativefeedback createdb! a source resistorto establisha natural 2;pointfor

the amplifier circuit,rather than havin$ to suppl! an e9

ternalvol

ta$eas is done wi

th$a

tebiasin$.


15/19

Answer *

C-

C-

-CA8

E

-3A8

)

Common;emitter@ll let !ou e9plain wh! these two volta$e $ain appro9imations share the same form. Hint" ithas somethin$ to do withthe ma$nitudesof the currents throu$heach transistorterminalK

Follow;up :uestion" e9plain mathematicall!wh! the emitter#sourceresistancessucceed in swampin$

rE Ee and $m, respectivel!, in thesemore precise formulae. Mou should provide t!pical values for re and $m as

part of !our ar$ument"

A8 N-

-C

O rECommon;emitter


16/19

Answer 1

The common;source amplifier confi$urationis defined b! havin$ the input and output si$nals referencedto the $ate and drain terminals /respectivel!0, with the source terminalof the transistort!picall!havin$ a

low AC impedance to $round and thus bein$ common to one pole ofboththe input and output volta$es.The common;source amplifier confi$uration most resembles the common;emitter


17/19

e

'otes 1

'otes

This relativel! simple 3C volta$e amplifier circuit provides a wealth of educational value, bothfor understandin$the function of the JFET, and also for review on past electrical#electronics concepts.

'ote" John 4arkus@Puide b o ok of Electronic Circuits,firstedition,pa$e *&, provided theinspiration for this circuit.

'otes

Ask !our students to e9plain e9actl! whatitis that causes the 2pointof this amplifier circuitto chan$ewith each new transistor. >s itsomethin$ in the transistor itself,or in some otherpart of the circuit?

Piven the instabilit!of $ate biasin$, should this method be used in mass;produced amplifier circuits?Ask !our students to elaborateon wh! or wh! not.

'otes +

The concept of ne$ativefeedback is e9tremel!importantin electronic circuits,but itis not easil! $raspedb! all. )elf;biasin$ of JFET transistors is a relativel!eas!;to;understandapplication of ne$ative feedback,

so be sure to takeadvanta$eof this opportunit!to e9plore the concept with !our students.Ask !our students to e9plain wh! 2;point stabilit! is a desirable feature for mass;produced amplifier

circuits,as well as circuits subect to component;levelrepair.

'otes *

)wampin$ is a common en$ineerin$ practice, and one that students would do well to understand.

>t is unfortunate that parameters such as d!namic emitter resistance /r 0 and transconductance /$m0

are so variable, but this does not have to be the end of the stor!. To be able to work around practical

limitations such as theseis the essence of en$ineerin$ practice, in m! opinion.

'otes %

There are several other:uestions !ou could ask about this amplifier circuit. For e9ample"

= How is the 2;pointbias establishedfor the JFET?

= How is the 2;pointbias establishedfor the s thereanotherpossible location for the potentiometerthat would perform the same function?

'ote" the schematic dia$ram for this circuit was derived from one found on pa$e + of John4arkus@ Puide b o ok of Electronic Circuits, first edition. Apparentl!, the desi$n ori$inated from a

4otorolapublica

tionon usin$ field effect

transis

tors /Tips on usin$ FET

@s,H4A;++, 1&G10.

'otes

The purpose of this :uestion is to $et students thinkin$ in an e9perimentalmode. >t is ver! importantthat students learn to set up and run their own e9periments, so the! will be able to verif! /or perhapsdiscoverK0 electronic principles afterthe!have $raduated from school. There will be times when the answersthe!seek are not tobe found in a book, and the!will have to let the electrons teachthemwhatthe!needto know.

-emind !our students thatproper scientifice9periments include bothe9perimental and control subects,

so that results are based upon a comparison of measurements.

'otes G


18/19

'otes 6

Ask !our students to e9plain wh! input impedance is an important factor in amplifier desi$n. (h!should we care how much input impedance an amplifier has?

Also, ask !our students to e9plain wh! such hi$h;value bias resistors /1% k5 and k50 wouldprobabl! not be practical in a


19/19

'otes 1%

>t has been m! e9perience that students re:uire much practice withcircuitanal!sis tobecomeproficient.To this end, instructors usuall! provide their students with lots of practice problems to work throu$h, and

provide answers for students to check their work a$ainst. (hile this approach makes studentsproficient incircuittheor!, itfails to full! educatethem.

)tudents don@tust need mathematicalpractice. The! also need real, hands;on practice buildin$ circuitsand usin$ test e:uipment. )o, > su$$est the followin$ alternative approach" students should build their

own practice problems with real components, and tr! to mathematicall! predict the various volta$e and

current values. This wa!, the mathematical theor! comes alive, and students $ain practical

proficienc!the!wouldn@

t $ain merel! b! solvin$ e:ua

tions.Another reason for followin$ this method of practice is to teachstudents scientific method" the process

of testin$ a h!pothesis /in this case, mathematicalpredictions0 b! performin$ a real e9periment. )tudentswill also develop real troubleshootin$skills as the! occasionall! make circuitconstructionerrors.

)pend a few moments of timewith !our class to review some of the rules for buildin$ circuitsbeforethe!be$in. 3iscuss theseissues with!our students in the same )ocraticmanner !ou would normall! discussthe worksheet :uestions, rather than simpl! tellin$ them what the! should and should not do. > nevercease tobe amaed at how poorl! students $rasp instructionswhen presentedin a t!picallecture /instructormonolo$ue0 formatK

A noteto thoseinstructors who ma! complain about the wastedtimere:uired to have studentsbuildreal circuits insteadof ust mathematicall!anal!in$ theoreticalcircuits"

(hat is the purpose of students takin$ !our course?

>f !our students will be workin$ with real circuits, then the! should learn on real circuits wheneverpossible. >f !our $oal is to educate theoretical ph!sicists, then stick with abstract anal!sis, b! allmeansK n most sciences, realistic e9periments are much more difficult and e9pensive to set up than electricalcircuits. 'uclear ph!sics, biolo$!, $eolo$!, and chemistr! professors would ust love to be able to have theirstudents appl! advanced mathematics to real e9periments posin$ no safet! haard and costin$ less than ate9tbook. The! can@t,but !ou can. E9ploitthe convenience inherentto !our science, and $et those studentsof !ours practicin$ their math on lots of real circuitsK

JFET Amplifier

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