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 inhighpassRC 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 drivetocutoff,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 nonsinusoidalwaveforms 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.
(a) Low pass
(b) High pass
(c) Band pass
(d) Band reject filters.
(AUC MAY’10)
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)
Monostablemultivibrator circuit
(AUC NOV’10)
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 noninverting (+) 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.