MAHALAKSHMI

ENGINEERING COLLEGE

TIRUCHIRAPALLI-621213.

QUESTION BANK

DEPARTMENT: EEE SEMESTER :III

SUBJECT CODE: EE2203 SUBJECT NAME:ELECTRONIC

DEVICES &CIRCUITS

UNIT 5 -PULSE CIRCUITS

PART A (2 Marks)

1.Definepulseandpulsecircuits.

Theword“pulse”isappliedtowaveformsthat exist

foraveryshortperiod.Theword“pulsecircuits”refertotheactiveandpassivecircuitsintendedto

handle,generate, shape andsotrepulse signals.

2.Defineswitchingcircuit.

AcircuitwhichcanturnONorOFFthecurrentintheelectroniccircuitsiscalled switching circuit

3.Definewaveshapingandwaveshapingcircuits.

Theprocessofgeneratingnewwaveshapesfromolderwaveformsusingsome

networkiscalledwaveshaping.Thecircuitswhichperformwaveshapingarecalledwave

shaping circuits.

Eg:Clippers, Clampers, Integrator, Multipliers, etc.

4.Givesomeexamplesoflinearandnon-linearwaveshapingcircuits.

Linear wave shaping circuits –use R,L,C.

Examples:RC,RL, RLC circuits,Integrator, Summer, etc. Non-linear wave shaping

circuits –uses R,L,Cdiodes, Examples:Clippers,Clampers, etc.

5.WhythecapacitorinahighpassRCcircuitiscalledblockingcapacitor?

BecauseoftheblockingpropertyofthecapacitorforDCorlowfrequencyinput

signals,thecapacitoractslikeanopencircuitandblocksthesignal.Sothecapacitor inhigh-

passRC circuits is called “blockingcapacitor”.

6.Whyahigh-passRCcircuitiscalleddifferentiator?

Becauseitgivestheoutputvoltageproportionaltothedifferentiationofinput voltage.

7.WhataretheconditionsforaseriesRCcircuittoactasadifferentiator?i.RCTime

constant<<Time period ofinput signal

RC<<Tii.XC>10R

8.Listtheapplicationsofhigh-passRCcircuits.

Togenerate a stepfrom rampinput.

Togenerate a square wavefrom atriangular wave.

Togenerateaseriesofnarrowpulsescalled“pips”fromrectangularorsquare waves.

UsedinR-Ccouplingofamplifierswheredistortionanddifferentiationof waveform isto

be avoided.

9.Whyalow-passRCcircuitiscalledanintegrator?

Because itgivesthe output voltage proportionalto the integral ofinputvoltage.

10.WhataretheconditionsforaseriesRCcircuittoactasanintegrator?i.RC>>T

ii.R>10XC

11.Listtheapplicationsoflow-passRCcircuits.

Usedas bypass capacitors.

Toperformmathematicalintegrationin analogcomputers.

Togenerate triangular and rampwaveforms.

Usedto discriminatepulses of differentlengths.

12.Whatarethecharacteristicsofpulsewaveforms?Rise time,falltimeandtilt.

13.DefineClamping.

Clampingistheprocessofshiftingtheinputsignalaboveorbelowthezerolevel.By

clampingtheinputsignalsuitably,wecanintroduce(insert)anyrequiredDClevelinto the

signal. So clapmers are also calledDClevel restorers.

14.WhatisaClamper?

Thecircuitwithwhichthewaveformcanbeshifted,suchthat,aparticularpartofit(saypositiveorn

egativepeak)ismaintainedataspecifiedlevel,iscalleda “clamping circuit or simply,clamper”.

15.ListthetypesofClampers.

Positive Clamper

Negative Clamper

BiasedClamper

16.Whatisthefunctionofapositiveclamper

Itshiftsthesignaltowardsthepositivesidesuchthatthenegativesideofthesignal reduces to

zero.

17.Whatisthefunctionofanegativeclamper?

Itshiftsthesignaltowardstonegativesidesuchthatthepositivesideofthesignal reduces to

zero.

18.Whatisbiasedclamper?

Abiasedclampermeansthatclampingcanbedoneatanyvoltagelevelotherthan zero.

19.ListtheapplicationsofClampers.

TheyareusedinT.V.receiverstorestoretheoriginalDCreferencesignal(correspondiongtoth

e brightnesslevel ofthe picture)tothe video signal.

TheyareusedtoproduceaDCvoltageisamultipleofpeakACinputvoltagei.e., they are used

aas voltage multipliers.

Theyareusedtosupplypowertohighvoltage/lowcurrentdeviceslikeCRTsusedinT.Vreceive

rs, CROs andcomputer displays.

20.Whatisaclipper?

Thecircuitwithwhichthewaveformisshapedbyremoving(orclipping)acertain

portionoftheinputsignalvoltageaboveorbelowapresentleveliscalledclipping circuit

orsimply, clipper. They are usedto limitthe amplitude ofthe inputsignal.

21.Listthetypesofclipers. Based on limitingaction:

Positive Clipper

Negative Clipper

BiasedClipper

Combination Clipper

22.Whatisthedifferencebetweentheoutputfromaclipperandaclamper?Theoutputofaclipper

appearsasifaportionoftheinputsignalwereclippedoff, buta clamper simply shifts the

inputtoa different DClevel.

23.Whatisthedifferencebetweenpositiveandnegativeclippers?

Thepositiveclippersremovesthepositivehalfcycles,whilethenegativeclipper removes the

negative halfcycles, of theinputwaveform.

24.Whatisthedifferencebetweenpositiveandnegativeclampers?

Apositiveclamperpushesthesignalonthepositivesideorupwardwhileanegative clamper

pushes thesignal onthe negative side ordownward

27.Listtheapplicationsofclippers.

Theyareusedtoremoveunwantedportionslikenoiseaccumulatedonpeaks of

waveforms.

TheyareusedinT.Vreceiverstoseparatesincepulsesfromthecomposite videosignal.

Twolevel clippers areusedassquare wave generators.

They are usedin PPM modulators.

28.Whatisamultivibrator?Listthedifferenttypesofmultivibrators.

AMultivibratorisbasicallyatwostageamplifierwith100%feedbackbetweenthe two stages

such that output of one isfedback to the other.

Thefeedbackfromonestagetotheotherissoarrangedthatwhenonetransistoris drivetocut-

off,theotherisdriventosaturation.Thusatanyparticularinstantof time,one transistoris ON

andthe other isOFF.

Types

Therearethreebasictypesofmultivibratorsdependingonthetypeofcoupling network

used.Theyare:

Astablemultivibrators(AMV) or freerunninggenerator.

Monostablemultivibrator(MMV) orone-shotmultivibratoror

univibration. Bistablemultivibrator(BMV) or flip-flop.

An AMV uses capacitive coupling. An MMV uses RC coupling.

AnBMVuses resitivecoupling.

29.WhatisAMV?Whyisitcalledasquarewavegenerator?

AnAMVisessentiallyatwo-stageRCcoupledamplifierwithoutputofonestag suppliedback to

theinput of another stage.

AnAMVgeneratessquarewaveofknownfrequency(orperiod).So,itiscalleda “square wave

generator”.

30.HowdoesaMMVcircuitbeconstructedfromaAMV?By replacingone R-Ctimingcircuitby

aDC voltage divider.

31. Whatis the function of commutatingcapacitors inmultivibrator?Toimprove the

switching characteristics of thecircuit.

32.WhyaremnostableMultivibratorscalledone-shortMultivibrators?

Theygenerateoneoutputpulseforeverytriggerpulseandhencethename“one

shotMultivibrators or univibrators”.

33.WheytheBMViscalledaflip-flop?

InaBMV,onetriggerpulsecausestheMultivibratortoflipfromonestatetothe

otherstateandthenextpulsecausesittoflopbacktoitsoriginalstate.So,itis calledthe flip – flop.

33.WhataretheapplicationsofAMVs? AMVsareused as

Square wave generators.

Voltage to frequency converters.

Pulse synchronization circuits

Clock for binary logic signals

34.WhataretheapplicationsofMMVs? MMVsareusedfor

Generation of welldefinedpulses

Logicdesignof pulsedelay

Variable pulse width

35.WhataretheapplicationsofBMVs? BMVsare usedas

Memory elements in shift registers, counters, etc.,

36.WritetheapplicationofBistablemultivibrator.

1. The bistablemultivibratoris used asmemory elementin shift registers counters, and so

on.

2.Itisusedtogeneratesquarewavesofsymmetricalshapebysendingregular

triggeringpulsestotheinput.Byadjustingthefrequencyoftheinputtriggerpulse, the width of

the squarewave canbe altered

37.WhataretheapplicationsofSchmitttrigger?

Amplitude comparator

Squaring circuit Flipflop

38.WhyanAMViscalledfreerunningrelaxationoscillator?Because itrunsandrelaxes

alternately.

39.WhichportionoftheUJTcharacteristicisusedtomakeUJTtogeneratesawtoothwaves?

Negative resistanceregion.

40.GivetheformulaforpulserepetitionfrequencyofUJTbasedsawtoothgenerator?()

41.WhatisthefunctionofClampercircuit?

Clamper circuitintroduces a d.clevelto an a.csignal.Hence, the damper circuit or network

is also known as d.crestorer. These circuitsfind applicationsin television receivers to

restorethe d.c reference signalto the videosignal.

42.Definerisetime.

The time duringwhich the voltage orcurrent reaches to a maximum positiveor negative

value is calledthe rise time. Therise time iszerofor the square wave.

43.Definesinusoidalwaveform.

A sine wave variescontinuouslyinamplitude inproportion to the sine of an angle which

varies from oto 360°.

44.Definemarketpips.

Circuits make use of the transientproperties of R, C andltorealizethemathematical

operations ofdifferentiations,integration andsummation. The differentiating

circuitisextensively usedto convertsummation.The differentcircuitisextensively usedto

convertpulse trains to a series of timingpipsknown as marketpips.

45.Mentiontheclassificationofswitches.

1. Mechanicalswitch.

2. Electronic switch.

46.Definerampfunctiongenerator.

The shape of the pulse resemble the teethor a saw,thereforethe name sawtooth

waveform. A sawtoothgenerator isalsocalled ramp function generator.

47.DefineMultivibrators.

Thereis aclassof RC coupledoscillators calledMultivibratorswhichgenerate non-

sinusoidalwaveforms such as triangularsquare andsawtooth.

48.Definefreerunningmultivibrators.

A Multivibratoris aformof relaxationoscillators. They may needno external excitation, i.e.,

theyareself-excited;ifso they are termed asfreerunningMultivibrator.

49.MentionthetypesofMultivibrators.

1. Monostablemultivibrator.

2. Bistablemultivibrator.

3. Astablemultivibrator.

50.DefineAstablemultivibrator.

The change inthe output state is regenerative.The outputneverremains permanentlyat

aparticular state. Hence,thismultivibratoris calledastablemultivibrator.

51.Definebistablemultivibrator.

Since in the absenceof anexternalsignal,either transistor can continue indefinitely inON

or OFFstate, they areequallystable in bothstaes.Therefore, the name

Bistablemultivibrator.

52.DefineMonostablemultivibrator.

Itisalso calledthe one shotmultivibrator. The one shotmultivibratoris driven, itis not

freerunning unlike the bistabletrigger, themonostable hasone stable state to which

itreturnsafter the eternaldrivingpulse hascauseditto execute its cycle.

53.DefineSchmittTrigger.

When the inputis sinusoidal signal and convertedto squarewaveform in outputis called

Schmitttrigger. Generation of pulses when the voltage level rises to a certain value is

achievedbyusing a circuit.

54.Definehighpassfilter.

At very high frequencies the capacitor actsas a virtualshort circuit andthe output falls to

zero. Hence this circuitpasses the low frequencies ofthe inputand attenuatesthe high

frequencyiscalledlowpass filter.

56.Definedutycycle.

Itisthe rationof the ONperiodto the totalperiod. (T=TON+TOFF)

Therefore, Dutycycle=TON /T

57.WritetheapplicationofAstableMultivibrator.

1. The astablemultivibratoris used assquare wave generator, voltage to frequency

convertorandinpulse synchronization,as clock for binary logicsignalsand so on.2. Since

itproducessquare waves itis a sourceof production of harmonic frequencies of

higherorder,

58.IfanastablemultivibratorhasC1=C2=1000PFandR1=R2=20KΩ.Calculatethefrequency

ofoscillation.

The frequency of a symmetricalastablemultivibrator

59.Writetheapplicationsofmonostablemultivibrator.

1. The monostablemultivibratorisusedto functionas an adjustable pulse width generator.

2. Itisusedto generate uniform width pulses from avariable width inputpulsetrain.

1. Draw the differentiator & integrator circuit.(AUC MAY 11)

2. Sketch the output waveform for the clipper shown in the figure below. Neglect the drop

across the diode.

3. Sketch the idealized characteristics for the filter types. (AUC MAY’10)

(a) Low pass

(b) High pass

(c) Band pass

(d) Band reject filters.

4. Define intrinsic standoff ratio of UJT and draw its equivalent circuit. (AUC MAY’10,

NOV11)

5. Write the frequency equation of an Astablemultivibrator. (AUC NOV’10)

T=1.88RC

F=1/T=1/1.38 RC

F=0.7246/RC

6. What is Schmitt Trigger? (AUC NOV’10)

The emitter of two transistors are connected to each other and grounded through the

resistance RE. if the feedback is obtained through the resistance RE. this circuit is called

Schmitt trigger circuit. There exists two stable states of the output of this circuit.

7. Mention some applications of UJT. (AUC APR’09)

UJT is mainly used in the triggering of other deviceslike SCR

UJT is used in sawtooth wave generation and some timing circuits.

Most popular application of UJT is relaxation oscillator to obtain short pulses for

triggering of SCR

8. What is a multivibrator? (AUC APR’09)

The electronic circuits used for generation of non sinusoidal waveforms are called

multivibrators. It is a two stage amplifier operating in two modes.

PART B(16 Marks)

1. 1. Explain the construction, equivalent circuit and operation of UJT. Draw the

characteristics of UJT.

2. Explain how UJT is used to generate saw tooth waveform.(AUC DEC’11)

The UJT as the name implies, is characterized by a single pn junction. It exhibits

negative resistance characteristic that makes it useful in oscillator circuits.

The symbol for UJT is shown in fig. 1. The UJT is having three terminals base1 (B1),

base2 (B2) and emitter (E). The UJT is made up of an N-type silicon bar which acts as

the base as shown in fig. 2. It is very lightly doped. A P-type impurity is introduced into

the base, producing a single PN junction called emitter. The PN junction exhibits the

properties of a conventional diode.

A complementary UJT is formed by a P-type base and N-type emitter. Except for the

polarity of voltage and current the characteristic is similar to those of a conventional UJT.

A simplified equivalent circuit for the UJT is shown in fig. 3. VBB is a source of biasing

voltage connected between B2 and B1. When the emitter is open, the total resistance

from B2 to B1 is simply the resistance of the silicon bar, this is known as the inter base

resistance RBB. Since the N-channel is lightly doped, therefore RBB is relatively high,

typically 5 to 10K ohm. RB2 is the resistance between B2 and point ‗a', while RB1 is the

resistance from point ‗a' to B1, therefore the interbase resistance RBB is

RBB = RB1 + RB2

2. i) Sketch the response of RC high pass filter for the following inputs and explain.

(1)Ramp

(2) Pulse. (AUC MAY’10)

(ii) Explain the operation of a bistable multi-vibrator circuit with neat sketch.

Basic mode of operation

The circuit keeps one transistor switched on and the other switched off. Suppose that

initially, Q1 is switched on and Q2 is switched off.

State 1:

of C1 (and the b

pulling the base of Q1 up, but its base-emitter diode prevents the voltage from rising

Because R4 is less than R2, C2 charges faster than C1.

When the base of Q2 reaches 0.6 V, Q2 turns on, and the following positive feedback

loop occurs:

a capacitor cannot suddenly change, this causes the left side of C2 to suddenly fall to

base voltage.

R1 and R2 work to pull both ends of C1 toward +V, completing Q2's turn on. The

process is stopped by the B-E diode of Q2, which will not let the right side of C1 rise very

far.

This now takes us to State 2, the mirror image of the initial state, where Q1 is switched

off and Q2 is switched on. Then R1 rapidly pulls C1's left side toward +V, while R3 more

slowly pulls C2's left side toward +0.6 V. When C2's left side reaches 0.6 V, the cycle

repeats.

Bistablemultivibrator circuit

Suggested values:

This latch circuit is similar to an astablemultivibrator, except that there is no charge or

discharge time, due to the absence of capacitors. Hence, when the circuit is switched

on, if Q1 is on, its collector is at 0 V. As a result, Q2 gets switched off. This results in

more than half +V volts being applied to R4 causing current into the base of Q1, thus

keeping it on. Thus, the circuit remains stable in a single state continuously. Similarly,

Q2 remains on continuously, if it happens to get switched on first.

Switching of state can be done via Set and Reset terminals connected to the bases. For

example, if Q2 is on and Set is grounded momentarily, this switches Q2 off, and makes

Q1 on. Thus, Set is used to "set" Q1 on, and Reset is used to "reset" it to off state.

3. i) Draw the circuit of a monostablemultivibrator and explain. (14) (AUC NOV’10)

Monostablemultivibrator circuit

When triggered by an input pulse, a monostablemultivibrator will switch to its unstable

position for a period of time, and then return to its stable state. The time period

monostablemultivibrator remains in unstable state is given by t = ln(2)R2C1. If repeated

application of the input pulse maintains the circuit in the unstable state, it is called a

retriggerablemonostable. If further trigger pulses do not affect the period, the circuit is a

non-retriggerablemultivibrator.

Basic mode of operation

The circuit keeps one transistor switched on and the other switched off. Suppose that

initially, Q1 is switched on and Q2 is switched off.

State 1:

s

pulling the base of Q1 up, but its base-emitter diode prevents the voltage from rising

Because R4 is less than R2, C2 charges faster than C1.

When the base of Q2 reaches 0.6 V, Q2 turns on, and the following positive feedback

loop occurs:

a capacitor cannot suddenly change, this causes the left side of C2 to suddenly fall to

almost

base voltage.

R1 and R2 work to pull both ends of C1 toward +V, completing Q2's turn on. The

process is stopped by the B-E diode of Q2, which will not let the right side of C1 rise very

far.

This now takes us to State 2, the mirror image of the initial state, where Q1 is switched

off and Q2 is switched on. Then R1 rapidly pulls C1's left side toward +V, while R3 more

slowly pulls C2's left side toward +0.6 V. When C2's left side reaches 0.6 V, the cycle

repeats.

(ii) What are the applications of monostablemultivibrator? (AUC NOV’10)

used to produce rectangular waveform and hence can be used as gating circuit.

Used to introduce time delays as gate width is adjustable.

Used to generate uniform width from a variable width input pulse train.

4. What are a clipper and clamper? Explain the concept of a positive clipper and a clamper.

(AUC APR’09)

Clippers:

Clipping circuits are used to select that portion of the input wave which lies above or

below some reference level.

For a clipping circuit at least two components—an ideal diode and resistor are required

and sometimes a dc battery is also employed for fixing the clipping level. The diode acts

as a closed switch when forward biased and an open switch when reverse biased. The

input waveform can be clipped at different levels by simply changing the voltage of the

battery and by interchanging the positions of the various elements.

Depending on the orientation of the diode, the positive or negative region of the input

signal is ―clipped‖ off and accordingly the diode clippers may be positive or negative

clippers.

There are two general categories of clippers: series and parallel (or shunt). The series

configuration is defined as one where diode is in series with the load, while the shunt

clipper has the diode in a branch parallel to the load.

1. Positive Clipper

The clipper which removes the positive half cycles of the input voltage is called the

positive clipper. The circuit arrangements for a positive clipper are illustrated in the figure

given below.

The figure illustrates the positive series clipper circuit (that is, diode in series with the

load). From the figure (a) it is seen that while the input is positive, diode D is reverse

biased and so the output remains at zero that is, positive half cycle is clipped off. During

the negative half cycle of the input, the diode is forward biased and so the negative half

cycle appears across the output.

Figure (b) illustrates the positive shunt clipper circuit (that is, diode in parallel with the

load). From the figure (b) it is seen that while input side is positive, the diode D is

forward biased and conducts heavily (that is, diode acts as a closed switch). So the

voltage drop across the diode or across the load resistance RL is zero. Thus output

voltage during the positive half cycles is zero, as shown in the output waveform. During

the negative half cycles of the input signal voltage, the diode D is reverse biased and

behaves as an open switch. Consequently the entire input voltage appears across the

diode or across the load resistance RL if R is much smaller than RL

Actually the circuit behaves as a voltage divider with an output voltage of [RL / R+ RL]

Vmax = - Vmax when RL >> R

Clamper:

Clamper circuits are used to change the DC level of the signal

Positive Clamper:

The positive clamper circuit is shown in fig. 1, which introduces positive dc voltage equal

to the peak of input signal. The operation of the circuit is same as of negative clamper.

Let the input signal swings form +10 V to -10 V. During first negative half cycle as Vi

rises from 0 to -10 V, the diode conducts. Assuming an ideal diode, its voltage, which is

also the output must be zero during the time from 0 to t1. The capacitor charges during

this period to 10 V, with the polarity shown.

After that Vi starts to drop which means the anode of D is negative relative to cathode,

(VD= vi - vC) thus reverse biasing the diode and preventing the capacitor from

discharging. Fig. 2. Since the capacitor is holding its charge it behaves as a DC voltage

source while the diode appears as an open circuit, therefore the equivalent circuit

becomes an input supply in series with +10 V dc voltage and the resultant output voltage

is the sum of instantaneous input voltage and dc voltage (+10 V).

To clamp the input signal by a voltage other than peak value, a dc source is required. As

shown in fig. 3, the dc source is reverse biasing the diode

The input voltage swings from +10 V to -10 V. In the negative half cycle when the

voltage exceed 5V then D conduct. During input voltage variation from –5 V to -10 V, the

capacitor charges to 5 V with the polarity shown in fig. 3. After that D becomes reverse

biased and open circuited. Then complete ac signal is shifted upward by 5 V. The output

waveform is shown in fig. 4.

5. Distinguish between Astable and Bistablemultivibrators. Mention some applications.

(AUCAPR ’09)

6. What is a Schmitt Trigger? Discuss any two applications of Schmitt Trigger.

(AUC APR’09)

Schmitt Trigger:

Comparator implementation

Schmitt triggers are commonly implemented using a comparator[nb 1] connected to have

positive feedback (i.e., instead of the usual negative feedback used in operational amplifier

circuits). For this circuit, the switching occurs near ground, with the amount of hysteresis

controlled by the resistances of R1 and R2:

The comparator extracts the sign of the difference between its two inputs. When the non-

inverting (+) input is at a higher voltage than the inverting (−) input, the comparator output

switches to +VS, which is its high supply voltage. When the non-inverting (+) input is at a lower

voltage than the inverting (−) input, the comparator output switches to -VS, which is its low

supply voltage. In this case, the inverting (−) input is grounded, and so the comparator

implements the sign function – its 2-state output (i.e., either high or low) always has the same

sign as the continuous input at its non-inverting (+) terminal.

Because of the resistor network connecting the Schmitt trigger input, the non-inverting (+)

terminal of the comparator, and the comparator output, the Schmitt trigger acts like a

comparator that switches at a different point depending on whether the output of the comparator

is high or low. For very negative inputs, the output will be low, and for very positive inputs, the

output will be high, and so this is an implementation of a "non-inverting" Schmitt trigger.

However, for intermediate inputs, the state of the output depends on both the input and the

output. For instance, if the Schmitt trigger is currently in the high state, the output will be at the

positive power supply rail (+VS). V+ is then a voltage divider between Vin and +VS. The

comparator will switch when V+=0 (ground). Current conservation shows that this requires

Applications

Schmitt triggers are typically used in open loop configurations for noise immunity and closed

loop positive feedback configurations to implement multivibrators.

Noise immunity

One application of a Schmitt trigger is to increase the noise immunity in a circuit with only a

single input threshold. With only one input threshold, a noisy input signal near that threshold

could cause the output to switch rapidly back and forth from noise alone. A noisy Schmitt

Trigger input signal near one threshold can cause only one switch in output value, after which it

would have to move beyond the other threshold in order to cause another switch.

For example, in Fairchild Semiconductor's QSE15x family of infrared photosensors[3], an

amplified infrared photodiode generates an electric signal that switches frequently between its

absolute lowest value and its absolute highest value. This signal is then low-pass filtered to form

a smooth signal that rises and falls corresponding to the relative amount of time the switching

signal is on and off. That filtered output passes to the input of a Schmitt trigger. The net effect is

that the output of the Schmitt trigger only passes from low to high after a received infrared signal

excites the photodiode for longer than some known delay, and once the Schmitt trigger is high,

it only moves low after the infrared signal ceases to excite the photodiode for longer than a

similar known delay. Whereas the photodiode is prone to spurious switching due to noise from

theenvironment, the delay added by the filter and Schmitt trigger ensures that the output only

switches when there is certainly an input stimulating the device.

Schmitt trigger with two transistors

In the positive-feedback configuration used in the implementation of a Schmitt trigger, most of

the complexity of the comparator's own implementation is unused. Hence, it can be replaced

with two cross-coupled transistors (i.e., the transistors that would otherwise implement the input

stage of the comparator). An example of such a 2-transistor-based configuration is shown

below. The chain RK1 R1 R2 sets the base voltage for transistor T2. This divider, however, is

affected by transistor T1, providing higher voltage if T1 is open. Hence the threshold voltage for

switching between the states depends on the present state of the trigger.

For NPN transistors as shown, when the input voltage is well below the shared emitter voltage,

T1 does not conduct. The base voltage of transistor T2 is determined by the mentioned divider.

Due to negative feedback, the voltage at the shared emitters must be almost as high as that set

by the divider so that T2 is conducting, and the trigger output is in the low state. T1 will conduct

when the input voltage (T1 base voltage) rises slightly above the voltage across resistor RE

(emitter voltage). When T1 begins to conduct, T2 ceases to conduct, because the voltage

divider now provides lower T2 base voltage while the emitter voltage does not drop because T1

is now drawing current across RE. With T2 now not conducting the trigger has transitioned to

the high state.

With the trigger now in the high state, if the input voltage lowers enough, the current through T1

reduces, lowering the shared emitter voltage and raising the base voltage for T2. As T2 begins

to conduct, the voltage across RE rises, further reducing the T1 base-emitter potential and T1

ceases to conduct.

In the high state, the output voltage is close to V+, but in the low state it is still well above V−.

This may not be low enough to be a "logical zero " for digital circuits. This may require additional

amplifiers following the trigger circuit.

The circuit can be simplified: R1 can be replaced with a short circuit connection, connecting the

T2 base directly to the T1 collector, and R2 can be taken out and replaced with an open circuit.

The key to its operation is that less current flows through RE when T1 is switched on (as a

result of input current into its base) than when T1 is off, because turning T1 on turns T2 off, and

T2, when on, provides more current through RE than does T1. With less current entering RE,

the voltage across it will be lower, and so once current gets going into T1, the input voltage

must go lower to turn T1 back off as now its emitter voltage has been lowered. This Schmitt

trigger buffer can also be turned into a Schmitt trigger inverter and another resistor saved in the

process, by replacing RK2 with a short connection, and connecting Vout to the emitter of T2

instead of its collector. In this case however, a larger value of resistance should be used for RE

as it now serves as the pull-down resistor on the output, lowering the voltage on the output

when the output should be low, instead of a serving as only a small resistance which is only

intended to develop a small voltage across it that actually adds to the output voltage when it

should be at a digital low.