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Lab 5 - JFET Circuits II

University of California at BerkeleyDonald A. Glaser Physics 111AInstrumentation LaboratoryLab 5 JFET Circuits II

©2016 by the Regents of the University of California. All rights reserved.

References: [1]

Art of Electronics Student Manual, [2] Hayes & Horowitz Chapter 3

The Art of Electronics, [2] Horowitz & Hill Chapter 3, 7.1.1-7.1.3

Other References [3]

Physics 111-Lab Library Reference SiteReprints and other information can be found on the Physics 111 Library Site. [4]

Lab 3 Appendices: [5] Data sheets and Curve Tracer operation. [6]

NOTE: You can check out and keep the portable breadboards, VB-106 or VB-108, from the 111-Lab for yourself ( Onlyone each please)

In this lab you will investigate some more sophisticated JFET circuits, such as voltage amplifiers, differential amplifiers,attenuators, and modulators.

Before coming to class complete this list of tasks:

· Completely read the Lab Write-up

· Answer the pre-lab questions utilizing the references and the write-up

· Perform any circuit calculations use Matlab [7] or anything that can be done outside of lab use RStudio [8](freeware).

· Plan out how to perform Lab tasks.

All part spec sheets are located here the Physics111 Library site [1]

Pre-lab

1. Explain in detail how a signal applied to the gate of one transistor in a differential amplifier can produce an output on the drainof the other transistor.

2. What are parasitic oscillations? How do you minimize them?

Do not forward bias the JFET gates. Forward gatecurrents larger than 50mA will burn out the JFETs!

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The Laboratory Staff will not help debug any circuitwhose power supplies have not been properlydecoupled!

BackgroundVoltage Amplifiers

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Adding a drain resistor to asource-follower turns it into a voltageamplifier, as shown to the right. Thistopology amplifier is known as acommon source amplifier

The equivalent small signal circuitfor the amplifier is shown to theextreme righnt and is most easilyunderstood by remembering that thecurrent in a source follower is givenby

.

where . Since this currentis unchanged by the addition of thedrain resistor, the output voltage willbe

.

Thus the gain of the amplifier is

, (1)

where the last equality assumes thatthe transconductance is high.

As with the source follower, the input current is given by the gate leakage current, so the amplifier’s input impedance is extremelyhigh. The output impedance of the amplifier equals the drain resistance , and unlike the output impedance in the followercircuit, it is not low.

Differential Amplifiers

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Differential amplifiers have two inputs, V+ and V-, and one or two outputs.

In an ideal differential amplifier, the amp’s output depends solely on thedifference between the two inputs,

, namely . Unfortunately, the output of any realdifferential amplifier also depends weakly on the average of the two inputs. Thisaverage, , is called the common mode of the amp.

Differential amplifiers are one of the most common building blocks in analog circuit design. The front end of every op amp, forexample, consists of a differential amplifier. Differential amplifiers are used whenever a desired signal is the difference betweentwo signals, particularly when this difference is masked by common mode noise.

Electrocardiograms are an example of the use of differential amplifiers. The heart generates electrical signals, which can bedetected by electrodes placed against the skin, but the signal from a single electrode would be swamped by background pickup; asyou know from touching the input to an oscilloscope, the body is an excellent antenna for noise in frequencies ranging from

to . Fortunately these undesired signals are nearly equal everywhere on the body. By using two pickups, placedso that the signal from the heart has the opposite sign on the pickups, and amplifying the difference between the two pickupswith a differential amplifier, the desired signal from the heart can be preferentially amplified over the unwanted noise.

Differential amplifiersare constructed from amatched pair oftransistors as shown tothe right. The twoinputs are on the gatesof the transistors. Thedrain of either transistorcan be used as theoutput; in some casesboth JFET drains areused to provide adifferential output, i.eoutputs of oppositephase.

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When driven with acommon mode signalonly, symmetry tells usthat both sides of thecircuit will behaveidentically. In this case,we can pretend to splitthe circuit down themiddle, as shown atright. The two

resistors combineto make the original . Symmetry requires thatthe two upper ends ofthe resistors be atthe same potential; thusno current would flow inthe dashed line thatconnects the two ends. Since there is nocurrent, we canpretend that theresistors aredisconnected.

We can use Eq. (1) tocalculate the gain of thetwo halves of the circuitindividually. Thiscommon mode gain isminus one times thefollowing:

(2)

In practice, the commonresistor is alwaysmade much greaterthan the sourceresistors, so the gainreduces to .

Further, isalways chosen to bemuch larger than thedrain resistor .Consequently, the gainfor common modesignals is low and thesesignals are stronglyattenuated.

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The response of the twotransistors to a smalldifferential signal , onthe other hand, will beequal and opposite. Ifthe current throughthe left JFET increases (

), the currentthrough the right JFETwill decrease by anequal and oppositeamount. The net currentflow through, andvoltage drop across, thecommon resistor will notchange with

. Consequently thecommon resistor can bereplaced with aconceptual voltagesource of strength

, as depicted atright.

Since the two sides ofthe circuit are onlyconnected at place heldat constant voltageby the conceptualbattery, the two sideseffectively decouple. Then Eq. (1) yields thedifferential gain (twofactors of -1 result in a+1 here)

(3)

If we choose the drainresistors, , to belarge compared to thesource resistors ,then the differential gainwill be large.

Thus, we haveaccomplished our goal: a large differential gain(Eq. (3)) and a smallcommon mode gain(Eq. (2)).

The circuit is linear. Consequently for any particular and , the output will be the sum of the outputs for the corresponding and . A not uncommon case occurs with an input signal is applied to just one input, say , and the other input ( ) is

grounded. Then, and as well. (Note how ) The gain for this single input will be halfthe differential gain: .

JFET Linearized Resistor

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JFET's can be used as variableresistors. IMPORTANT NOTE: Thelower resistor R in the diagram atright is in the wrong location; thecorrect location is shown in Problem5.8. In this application, the drainsource resistance can becontrolled by a voltage, ,applied to the gate. With just a JFET,however, this resistance isdependent on the , and the"resistor" is nonlinear. It can beshown that the linear-regimeresistance between a JFET’sdrain and source is given by

,

where is a parameter that dependson the individual JFET, is thevoltage where the JFET firstconducts (the pinch-off, cutoff, orthreshold voltage), and is thevoltage between the drain and thesource.

The linearity between and can be restored by adding a

signal equal to to the gate.This can be done by adding tworesistors to the gate circuit as shownto the right. This circuit functionsas an excellent variable resistor.

Note that this is an application of aJFET being used in the somewhatunusual linear regime as opposed tothe more common saturated regime.

Parasitic Oscillations

Parasitic oscillations are unwanted, high fre quency oscillations usuallycaused by unintended positive-feedback loops. The loops are closed byunintended capacitative coupling between two neighboring wires. Becausethe oscillations normally occur at very high frequencies (1 to 100 MHz),capacitances of only a few picofarads are sufficient to cause oscillations. Parasitic oscillations often first appear as “hair” on an otherwise undistortedsignal. For example, in the scope trace on the right, the small oscillations atthe top of the original sine wave are due to parasitic oscillations.

Minor Parasitic Oscillations

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Larger parasiticoscillations maycover a substantialfraction of the signal,as shown to the right,or they maycompletely dominatethe signal, as shownat the extreme right.

Touching the circuit,or even waving yourhand near the circuit,can eitheraccentuate orsuppress theoscillations.

Parasitic oscillationsgenerally, but notalways, requirecircuits with gain.The followers that webuilt in last week’slab were relativelyimmune fromoscillating, but theamplifiers that youare building thisweek will likelyexhibit paras

Significant Parasitic OscillationsDisasterous Parasitic Oscillations

There are no hard and fast rules for eliminating parasitic oscillations, but clean circuit layout goes a long way. Keep yourleads short and un-jumbled. Lay your signals out from left to right, and keep large output signals away from small inputsignals. Believe it or not, beautifully wired circuits work better!

Unfortunately not even clean circuit layouts will always suppress the oscillations. At very high frequencies, the internalcapacitances inside components, aided by the inductances formed by the component leads, can cause oscillations. An additionalfeedback route is through the power supplies themselves. Frequently parasitic oscillations can be tamed by liberally adding“decoupling” capacitors across the power supply. The decoupling capacitors function by providing a low impedance path toground at high frequency, thereby shorting out the high frequency oscillations. To be effective, decoupling capacitors shouldplaced close to the circuit components. Try placing 0.1 capacitors between +12V and ground, –12V and ground, and between+12V and –12V. Vary the locations of the capacitors, and use more than one, until the oscillations go away. Don’t be greedy; a 1or 10 capacitor will not work better than a 0.1 capacitor. In fact, the larger internal inductances of large capacitors may wellmake them work worse than smaller capacitors. This is particularly true of electrolytic capacitors.

When power supply decoupling capacitors are insufficient, placing a small capacitor across the input of a circuit will sometimeswork.

Packaging and Leads

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Transistors are manufactured inmany different packages and sizes.Our 2N4392 JFETs come in a metalcan. (Devices starting with 1N arealways diodes; devices starting with2N are always transistors.)

As with many devices, the leaddiagram depends on the view. Abottom view (immediate right) looksat the device from the lead side. Atop view (center right) looks at thedevice from the non-leaded side.

The 2N4392 leads are arranged in atriangle; the gate lead is the firstlead clockwise from the tab whenlooked at in a top view.

When inserting the JFET into thebreadboard, there is no need tosquash the leads out horizontally. Infact, doing so will risks accidentallyshorting the JFET leads to the metalcase.

Instead of squashing the leads,just bend them out gently so thatthey form a triangular pattern,and are in different columns onthe breadboard. It may be usefulto have one of the leads span thebreadboard's narrow socketlessrow region.

2N4392

Bottom View

Top View

Many JFETs, including the 2N4392, are symmetrically constructed. The source and drain can be exchangedwithout changing the device behavior. But for simplicity, use should generally use the correct source and drainleads.

Asymmetric JFETs, in which the source and drain cannot be exchanged, are normally drawn with an offset gatelead as shown at right.

In the lab

Problem 5.1 - Common Source JFET Amplifier

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Construct theamplifier Shown atright. What are theequilibrium voltagesand currents in thecircuit ( , , ,

) when ,i.e. when the input isgrounded?

Drive the amplifier witha ,

sine wave,and look at its outputon the scope. What isthe amp’s gain? Doesit agree with thepredicted value? Whatis the maximumundistorted outputamplitude? (Note: Varythe input voltage tosearch for thisquantity.) What limitsthe amplitude?

Cool the JFET withcircuit cooler. Howmuch does the gainchange? Measureand record the gainfor four other JFETs. (Keep all these JFETSfor 5.3)

Problem 5.2 - Increased Gain Common Source JFET Amplifier-Large Drain Resistor

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The gain of thecircuit in 5.1 is nothigh. A naïveapplication of thegain formula[Eq. (1)] wouldimply that thegain shouldincreasesubstantially if thedrain resistor ischanged to

, as shownat right.

Build thiscircuit. Whatactually happensto the gain? Why? (Hint:Remeasure ).

Problem 5.3 - Increased Gain Common Source JFET Amplifier-Small Source Resistor

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Equation (1) alsosuggests thatdecreasing the sourceresistor will increasethe gain. Build thecircuit at right. (The

capacitorsuppresses feedbackoscillations, but doesnot otherwise affectthe circuit. Ignore itwhen calculating thegain.)

What is the gain now? Change the inputamplitude; what is themaximum undistortedoutput amplitude?

Cool the JFET withcircuit cooler. Howmuch does the gainchange? Measure andrecord the gain for theother four JFETs. Whyis the fractionalvariation in the gainlarger for this circuitthan for the circuit in5.1?

The behavior of a well-designed circuit shouldnot depend ontemperature or on theparameters of itsparticular components.Consequently, thecircuit in 5.3 is not veryuseful. The point ofthis section is to showyou that circuit designis not as simple asplugging in to aformula and getting auseful circuit.

Problem 5.4 - Increased Gain Common Source JFET Amplifier-Source Resistor Bypass

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Bypassing the sourceresistor with a capacitorwill also increase thegain. Build the circuit atright, which uses a

bypasss capacitor. Use a regular capacitor,not a polarizedelectrolytic capacitor.

Measure the gain of thecircuit. Is the gainfrequency dependent?(Particularly explore lowfrequencies; the highfrequency behavior is setby parasitic capacitanceswhich are not interestingin this context.)

Why does the capacitorincrease the gain? Whatis the highfrequency predicted gain?

Problem 5.5 - Differential Amplifier

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Build the differentialamplifier shown atright using yourmatched pair of JFETsfrom Lab 4. The

resistorsestablish the inputsnear ground, and donot otherwise affectthe circuitperformance. Notethat the powersupplies are atpositive and negativevoltages centeredaround the ground;this is a very commonsetup with differentialamplifiers.

Leaving the inputattached only to its

resistor, drivethe input with a

, sinewave. Measure theamplitude and phaseof the output signal

. Also look at ; what is the

signal there? How doyour measurementscompare to thevalues predicted byEqs (2) and (3)?

Reverse the drivesetup; drive the

input and let be grounded

through its resistor. What

are the amplitude andphase at ? Whatabout at ?

Now drive and with identical

signals (short themtogether). What is thecommon mode gain?

If your circuit doesnot work, particularlyif only one branch ofthe circuit worksslightly and theother not at all, it islikely thatyour JFETs are notsufficiently wellmatched.

Explore this effect bytemporarilyreplacing one of theJFETs with anunmatched JFET.Does the circuit stillwork? If it doesn’t,why not?

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Hint�Measure thevoltage drop acrossboth drain resistors.

Problem 5.6 - Improved Diffential Amplifier

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Differentialamplifierperformance isvastlyimproved byreplacing thecommonsource resistorwith a currentsource. Makeall themeasurementsin 5.5 with thecircuit at right.

Note thatExercise5.13 uses thematchedJFETs again,so make surethat you keepthem.

Problem 5.7 - JFET Attenuator

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Build the JFET attenuator shown at right. This circuit looks like a voltage divider. The "top"resistor in the divider is the resistor between and , while the base resistor is theJFET. As discussed in the background information, the JFET can be controlled by thevoltage on the gate, here adjusted with the potentiometer.

Drive with a , triangle wave. Show that you can vary the output amplitude with the potentiometer. Is the circuit linear (i.e. is the output still a good triangle wave)? With

the potentiometer set to produce an output signal about half as large as the input signal, what isthe largest input signal passed relatively undistorted?

Hint for 5.7 and 5.8: If you find that your circuit always attenuates, i.e. you cannot make , try replacing the resistor with a smaller resistor.

Problem 5.8 Linearized JFET Attenuator

You should have found in 5.7 that the attenuator becamenonlinear at higher amplitudes linear. Build thecircuit at right, which contains two resistors, andwhich should linearize the attenuator.

With the potentiometer set to produce an output signalabout half as large as the input signal, find the largestinput signal passed relatively undistorted. Is theamplitude larger than in 5.7?

Now set the potentiometer for the greatest attainableattenuation. Measure the resulting attenuation, and fromthis measurement and the standard voltage dividerformula, calculate the JFET drain-source resistance

. This is the lowest possible for the JFET, aparameter that is often given on the JFET spec sheet.

Problem 5.9 - JFET Modulator

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The circuit in 5.8 uses a fixed control voltageon the JFET gate, and, consequently, yieldsa constant attenuation. However, this not arequirement; if the signal on the JFET gateis driven with a time dependent signal, theattenuation will then be time dependent aswell.

Build the circuit at right. Drive with a , sine wave from your signal

generator. In this setup, this wave iscalled the carrier wave.

Drive the input with an audiofrequency ( ), sine wave. Since we only have one function generatorper station, the distribution boxes with BNCoutputs labeled "T1" and "T2" have been setup to provide you with the extra signal. Specifically, T1 should be a 1 kHz sinewave, and T2 should be an audio signal. Ifthey are not outputting any signal, a GSImay need to set up the amplifier thatdistributes them to the room.

Setup the scope with the signal onchannel 1, the modulation signal

on channel 2, and the outputsignal on channel 3. Trigger the scopeon .

Adjust the potentiometer until you see amodulated carrier wave on : theenvelope (local amplitude) of the highfrequency carrier wave should be related tothe instantaneous amplitude of the audiosignal. The output of the circuit shouldthen be proportional to

,

where is the radial frequency of the carrier wave, is the radial

frequency of the audio wave, and is called the amplitude of the

oscillation. The signals should look like thelower right scope traces. When you attain aproperly modulated signal, record the scopetraces..

This type of circuit is called a modulator, andhas many uses.

Modulated Carrier

Modulated Carrier

Problem 5.10 - JFET AM Transmitter_www.youtube.com/playlist?list=PL23YUdVBPT-_DviqMS2wJ-MSjU4uLAWGs [9]

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Attach a 2-meter long wire to the output of the modulator you built in 5.9. Obtain an AM radio, place the antenna very near theradio, and tune it to a quiet frequency in the AM band. Adjust the high frequency carrier signal until you can hear the 1kHz tonecoming from the radio. Then replace the sine modulating signal from T1 with the audio signal from T2. Listen to the radio. Youhave built a low power AM transmitter! Note that you can extend the range of your transmitter by turning up the amplitude ofyour carrier wave.

Problem 5.11 - Surprise Circuit

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Obtain a printed circuit board(PCB) marked "SurpriseCircuit Physics 111a: Rev 2.0,as pictured at right. The schematic for the circuitbelow the dashed line is alsoshown at right. (The circuitryabove the dashed line on thePCB cleans anddecouples the power andneed not concern you exceptwhile confirming the power.

Use the breadboard boxpower suppliesto generate approximately 24-27V, and use a set of leads(preferably twisted)to connect this power to"PWR Input Block". Wires canbe inserted into this Block in amanner similar to the way youinserted wires to the block onthe Thevenin circuit. Insertstripped wires into the roundholes while simultaneouslydepressing the orange tabs. Ittakes some force to depressthese tabs, but treat themgently nonetheless. Makesure that you use the properpolarity.

Remove any components thatmight be inserted into the"...Cap" connector blocks.

Power the circuitto confirm the red power LEDlights. Further confirm, bymeasuring the voltage acrossthe red and black test pointsat the top, that the power youare applying to the circuit iswhat you would expect. Notethat black test points arealways grounded. In allcases making measurementswith the scope, connect thescope ground to one of theblack testpoints.

Inject a 100mV, 3kHz sinewave into the "1st State In"orange test point. Attach ascope (on AC) to the "1stStage Out" yellow testpoint. Measure the approximategain. Is the output signalinverted or non-inverted? Now move the injected sinewave to the "2nd Stage In"purple test point (the purple

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and black test points aredifficult to distinguish) andthe scope to the "2nd StageOut" green testpoint. Measure theapproximate gain. Is thesignal inverted or non-inverted? Finally, move theinjected sine wave back tothe "1st State In" orange testpoint while leaving the scopeon the green test point. Is thegain what you would expectgiven that the two stages arecoupled through the 0.001capacitor? (The value of thecapacitor does notsignificantly affect the gain at3kHz.) More importantly,is the signal inverted or non-inverted?

Now remove the injected sinewave while keeping the scopeon the "2nd Stage Out" greentestpoint. Insert a100pF capacitor across the"2nd to 1st Stage Cap"terminal block (see bottom atright). What do you observeon the scope? Concentrate onthe frequency and shape ofthe waveform, not itsamplitude. Play with differentvalues of the "2nd to 1stStage Cap" capacitor, goingdown to 10pF, and up to10nf. Try inserting acapacitor across the "1st to2nd Stage Cap" (this insertedcapacitor will be in parallel tothe 0.001uF capacitor C3). Can you makequalitative sense of thechanges to the output that youobserve?

Problem 5.12 - Phase Splitter

Design and build a unity gain phase splitter: a circuit that splits an input signal into two signals of equal magnitude and oppositephase. Maximize the undistorted output amplitude of your circuit. Hint: Your circuit should be fairly simple; you do not needa differential amplifier. A modified source follower will work.

Problem 5.13 - High Gain Amplifier

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With a careful designemploying feedback, itis possible to make ahigh gain amplifier withconstant gain across awide frequency spanand with a gain thatis componentindependent. Duringthis exercise, work the5.14 questions intandem.

Obtain a printed circuitboard (PCB) marked"High Gain JFET AmpPhysics 111a: Rev 2.0as shown at right Apartial schematic forthe circuit is alsoshown at right (notethat this circuit is notentirely identical to thecircuit in the "old"version.) A useful skillis the ability toreconstruct a circuit ona board by tracing the"wires". ( Wires arecalled traces on aPCB.). The two-layerboard used in thisexercise is particularlysimple. All the tracesexcept the groundconnections are on thetop surface. The traceare easily visible,though, except wherethe components aresoldered, thetraces are coveredwith a green,transparentinsulator. The entirebottom surface of thePCB is a sheet ofcopper called a groundplane, andcomponents that needto be grounded aresoldered directly tothis ground plane.

You can almost alwaysreconstruct the circuitfor a two-layer boardlike this; it becomesmuch harder, oftenimpossible, for a boardthat has tracessandwiched into themiddle of the board;such a board is calleda multilayer board.

Visually, perhaps withthe assistance of themultimeter,reconstruct themissing pieces of the

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circuit diagram. Ignoreall the power supplycomponents above thedashed line.

Connect power to the"PWR Input" Block,much as you did inexercise 5.11, but useapproximately 31V, not26V. Confirm that thepower is properlyattached as in exercise5.11. Note that blacktest points are alwaysgrounded. In all casesmaking measurementswith the scope,connect the scopeground to one of theblack testpoints.

Insert your matchedJFETs into the twoblack sockets Q1 andQ2. Make sure thatyou get the orientationcorrect. The tab onthe socket must matchthe metal tab on theJFET.

Calculate the value ofthe resistor that youneed to attach to the"Source Resistor"Input Block to makethe voltage midway inthe circuit, at TestPointGRN, approximatelyhalf the total voltage,i.e. about 15.5V. Obtain and insert thisresistor. (When lookingfor the proper resistor,don't forget theprecision (1%) resistorstash by the labentrance door.) Leavethe "FeedbackResistor" Input Blockopen.

Turn the poweron. Check to makesure that the DCvoltage at thegreen testpoint markedOut is near 15.5V.

Inject a 10mV, 3kHzsine signal into thecircuit at theorange testpoint marked In. Connect thegreen Out testpoint toa scope. The outputsignal will be centeredaround 15.5V, so useAC coupling on theoutput scope channel.

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Measure andrecord the gain at3kHz. Then measureand record the gainfrom 3kHz down to1Hz and then from3kHz up to 1MHz.(You will find it easierto scan up and downfrom 3kHz, where thesignal is initially large,than from 1Hz to1MHz in onepass.) You need takeonly approximatelytwo points everydecade, thoughconsider taking slightlymore dense pointswhen the gain ischanging quickly. Useaveraging whenneccessary, and auxtriggering on Normal,not Auto. (Note that atlow frequencies, theAC coupling will affectthe signal magnitude. Compensate for thisby measuring the sizeof the input signal aswell as the outputsignal on AC coupling. Since the frequencyresponse of bothscope channels will bethe same, thisresponse will cancelout of the gaincalculation.)

The gain should bequite high (over 100) inthe neighborhood of

3kHz. Find the approximatemaximum gainfrequency, as well asthe span around thisfrequency for whichthe gain is 80% ormore and 50% ormore of the maxgain.

Now set the frequencyto 3kHz, and calculatean appropriatefeedback resistorvalue to make the totalgain about 15. Insertthis resistor into the"Feedback Resistor"Block. If the value isoff, adjust the resistorvalue to be close to15; aim to besomewhere between13 and 17.

Now redo thefrequency scan

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between 1Hz and1MHz. Find theapproximate maximumgain frequency, as wellas the span aroundthis frequency forwhich the gain is 80%or more and 50% ormore of the maxgain. Plot the gainwith and withoutfeedback. Though thegain is much lowerwith feedback, youshould find that itvaries much less overthe frequency rangethan the gain withoutfeedback.

Set the frequencyback to 3kHz. Insertand measure the gainfor several differentJFETs swapped in forthe lower JFET (Q2)with the feedbackresistor removed. Calculate the averagegain, and theproportional standarddeviation (the standarddeviation divided bytheaverage). Put backthe feedback resistor.and try the samesample of differentJFETs. Calculate theaverage gain and theproportional standarddeviation again. Is theproportional standarddeviation much lowerwith feedback?

Finally, change theinput to a 1kHz squarewave. With yourmatched JFET,compare the outputwaveforms with andwithout feedback. Isthe waveform cleaner,with sharper rises andfalls (i.e. closer to anideal square wave)with feedback? Takescope images withboth conditions.

Problem 5.14 - High Gain Amplifier Explanation

In the circuit of 5.14, identify the components that perform the following functions, and explain how each function is acco mplished:

1. Sets the gain through feedback.

2. Acts as a current source to increase the gain.28

3. Increases the open-loop gain by bypassing the source resistor.

4. Sets the current through the JFETs.

5. Assures that the drain source voltage across both JFETs is approximately 12V, independent of the particular parameters ofeach JFET.

6. Increases the stiffness of the current source by providing a bypass for AC signals.

Problem 5.15 - Improved Differential Amplifier Gain

What are the expected common mode and differential gains for a differential amplifier built with a current source? How do theseexpected gains compare to the gains you measured in 5.6? Hint�You will have to assume a stiffness (or Zout) for the source.

Please fill out the

Student Evaluation of Lab Report [10]

Source URL: http://instrumentationlab.berkeley.edu/Lab5

Links[1] http://physics111.lib.berkeley.edu/Physics111/BSC/indexbsc.html[2] http://physics111.lib.berkeley.edu/Physics111/BSC/Readings/Horowitz&Hills%20Books/indexHorowitz.html[3] http://physics111.lib.berkeley.edu/Physics111/BSC/Readings/indexreadings.html[4] http://physics111.lib.berkeley.edu/Physics111/[5] http://instrumentationlab.berkeley.edu/lab4#curve_tracer[6] http://instrumentationlab.berkeley.edu/sites/default/files/BSC03/Curve%20Tracer%20Manual.pdf[7] http://instrumentationlab.berkeley.edu/matlabintro[8] https://www.rstudio.com/products/rstudio/features/[9] http://www.youtube.com/playlist?list=PL23YUdVBPT-_DviqMS2wJ-MSjU4uLAWGs[10] http://instrumentationlab.berkeley.edu/StudentEvaluation

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