Application Note 74 LM139/LM239/LM339 A Quad Of Independently Functioning Comparators AN 0074

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LM139/LM239/LM339 A

Quad of Independently

Functioning Comparators

INTRODUCTION

The LM139/LM239/LM339 family of devices is a monolithic

quad of independently functioning comparators designed to

meet the needs for a medium speed, TTL compatible com-

parator for industrial applications. Since no antisaturation

clamps are used on the output such as a Baker clamp or

other active circuitry, the output leakage current in the OFF

state is typically 0.5 nA. This makes the device ideal for sys-

tem applications where it is desired to switch a node to

ground while leaving it totally unaffected in the OFF state.

Other features include single supply, low voltage operation

with an input common mode range from ground up to ap-

proximately one volt below V

. The output is an uncommit-

ted collector so it may be used with a pull-up resistor and a

separate output supply to give switching levels from any volt-

age up to 36V down to a V

CE SAT

above ground (approx. 100

mV), sinking currents up to 15 mA. In addition it may be used

as a single pole switch to ground, leaving the switched node

unaffected while in the OFF state. Power dissipation with all

four comparators in the OFF state is typically 4 mW from a

single 5V supply (1 mW/comparator).

CIRCUIT DESCRIPTION

Figure 1

shows the basic input stage of one of the four com-

parators of the LM139. Transistors Q

through Q

make up a

PNP Darlington differential input stage with Q

and Q

serv-

ing to give single-ended output from differential input with no

loss in gain. Any differential input at Q

and Q

will be ampli-

fied causing Q

to switch OFF or ON depending on input sig-

nal polarity. It can easily be seen that operation with an input

common mode voltage of ground is possible. With both in-

puts at ground potential, the emitters of Q

and Q

will be at

one V

above ground and the emitters of Q

and Q

at 2

. For switching action the base of Q

and Q

need only

go to one V

above ground and since Q

and Q

can oper-

ate with zero volts collector to base, enough voltage is

present at a zero volt common mode input to insure com-

parator action. The bases should not be taken more than

several hundred millivolts below ground; however, to prevent

forward biasing a substrate diode which would stop all com-

parator action and possibly damage the device, if very large

input currents were provided.

Figure 2

shows the comparator with the output stage added.

Additional voltage gain is taken through Q

and Q

with the

collector of Q

left open to offer a wide variety of possible ap-

plications. The addition of a large pull-up resistor from the

collector of Q

to either +V

or any other supply up to 36V

both increases the LM139 gain and makes possible output

switching levels to match practically any application. Several

outputs may be tied together to provide an ORing function or

the pull-up resistor may be omitted entirely with the com-

parator then serving as a SPST switch to ground.

Output transistor Q

will sink up to 15 mA before the output

ON voltage rises above several hundred millivolts. The out-

put current sink capability may be boosted by the addition of

a discrete transistor at the output.

The complete circuit for one comparator of the LM139 is

shown in

Figure 3

. Current sources I

and I

are added to

help charge any parasitic capacitance at the emitters of Q

and Q

to improve the slew rate of the input stage. Diodes D

and D

are added to speed up the voltage swing at the emit-

ters of Q

and Q

for large input voltage swings.

AN007385-1

FIGURE 1. Basic LM139 Input Stage

AN007385-2

FIGURE 2. Basic LM139 Comparator

National Semiconductor

Application Note 74

January 1973

LM139/LM239/LM339—A Quad of Independently Functioning Comparators AN-74

Biasing for current sources I

through I

is shown in

Figure 4

When power is first applied to the circuit, current flows

through the JFET Q

to bias up diode D

. This biases tran-

sistor Q

which turns ON transistors Q

and Q

by allowing

a path to ground for their base and collector currents.

Current from the left hand collector of Q

flows through di-

odes D

and D

bringing up the base of Q

to2V

above

ground and the emitters of Q

and Q

to one V

will

then turn OFF because its base emitter voltage goes to zero.

This is the desired action because Q

and Q

are biased

ON through Q

and D

so Q

is no longer needed. The

“bias line” is now sitting at a V

below +V

which is the

voltage needed to bias the remaining current sources in the

LM139 which will have a constant bias regardless of +V

fluctuations. The upper input common mode voltage is V

minus the saturation voltage of the current sources (appoxi-

mately 100 mV) minus the 2 V

of the input devices Q

and

(or Q

and Q

COMPARATOR CIRCUITS

Figure 5

shows a basic comparator circuit for converting low

level analog signals to a high level digital output. The output

pull-up resistor should be chosen high enough so as to avoid

excessive power dissipation yet low enough to supply

enough drive to switch whatever load circuitry is used on the

comparator output. Resistors R

and R

are used to set the

input threshold trip voltage (V

REF

) at any value desired within

the input common mode range of the comparator.

COMPARATORS WITH HYSTERESIS

The circuit shown in

Figure 5

suffers from one basic draw-

back in that if the input signal is a slowly varying low level

signal, the comparator may be forced to stay within its linear

region between the output high and low states for an unde-

sireable length of time. If this happens, it runs the risk of os-

cillating since it is basically an uncompensated, high gain op

amp. To prevent this, a small amount of positive feedback or

hysteresis is added around the comparator.

Figure 6

shows

a comparator with a small amount of positive feedback. In or-

der to insure proper comparator action, the components

should be chosen as follows:

PULL-UP

LOAD

and

PULL-UP

This will insure that the comparator will always switch fully up

to +V

and not be pulled down by the load or feedback. The

amount of feedback is chosen arbitrarily to insure proper

switching with the particular type of input signal used. If the

output swing is 5V, for example, and it is desired to feedback

1%or 50 mV, then R

≈100 R

. To describe circuit operation,

assume that the inverting input goes above the reference in-

put (V

REF

). This will drive the output, V

, towards

ground which in turn pulls V

REF

down through R

. Since

REF

is actually the noninverting input to the comparator, it

too will drive the output towards ground insuring the fastest

possible switching time regardless of how slow the input

moves. If the input then travels down to V

REF

, the same pro-

cedure will occur only in the opposite direction insuring that

the output will be driven hard towards +V

AN007385-3

FIGURE 3. Complete LM139 Comparator Circuit

AN007385-4

FIGURE 4. Current Source Biasing Circuit

AN007385-5

FIGURE 5. Basic Comparator Circuit

AN007385-6

FIGURE 6. Comparator with Positive Feedback to

Improve Switching Time

www.national.com 2

Putting hysteresis in the feedback loop of the comparator

has far more use, however, than simply as an oscillation sup-

pressor. It can be made to function as a Schmitt trigger with

presettable trigger points. A typical circuit is shown in

Figure

. Again, the hysteresis is achieved by shifting the reference

voltage at the positive input when the output voltage V

changes state. This network requires only three resistors

and is referenced to the positive supply +V

of the com-

parator. This can be modeled as a resistive divider, R

and

, between +V

and ground with the third resistor, R

, al-

ternately connected to +V

or ground, paralleling either R

or R

. To analyze this circuit, assume that the input voltage,

, at the inverting input is less than V

. With V

≤V

the

output will be high (V

=+V

). The upper input trip voltage,

, is defined by:

(1)

When the input voltage V

, rises above the reference volt-

age (V

), voltage, V

, will go low (V

=GND). The

lower input trip voltage, V

, is now defined by:

(2)

When the input voltage, V

, decreases to V

or lower, the

output will again switch high. The total hysteresis, ∆V

, pro-

vided by this network is defined by:

∆V

−V

or, subtracting equation 2 from equation 1

(3)

To insure that V

will swing between +V

and ground,

choose:

PULL-UP

LOAD

and (4)

PULL-UP

(5)

Heavier loading on R

PULL-UP

(i.e. smaller values of R

LOAD

) simply reduces the value of the maximum output

voltage thereby reducing the amount of hysteresis by lower-

ing the value of V

. For simplicity, we have assumed in the

above equations that V

high switches all the way up to

To find the resistor values needed for a given set of trip

points, we first divide equation (3) by equation (2). This gives

us the ratio:

(6)

AN007385-7

FIGURE 7. Inverting Comparator with Hysteresis

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If we let R

=nR

, equation (6) becomes:

(7)

We can then obtain an expression for R

from equation (1)

which gives

(8)

The following design example is offered:

Given: V

=+15V

LOAD

=100 kΩ

=+10V

=+5V

To find: R

PULL-UP

Solution:

From equation (4) R

PULL-UP

LOAD

PULL-UP

<100 kΩ

so let R

PULL-UP

=3kΩ

From equation (5) R

LOAD

>100 kΩ

so let R

=1MΩ

and since R

=nR

this gives R

=1R

=1MΩ

These are the values shown in

Figure 7

The circuit shown in

Figure 8

is a non-inverting comparator

with hysteresis which is obtained with only two resistors, R

and R

. In contrast to the first method, however, this circuit

requires a separate reference voltage at the negative input.

The trip voltage, V

, at the positive input is shifted about

REF

as V

changes between +V

and ground.

Again for analysis, assume that the input voltage, V

,islow

so that the output, V

, is also low (V

=GND). For the out-

put to switch, V

must rise up to V

IN 1

where V

IN 1

is given

by:

(9)

As soon as V

switches to +V

will step to a value

greater than V

REF

which is given by:

(10)

To make the comparator switch back to its low state (V

GND) V

must go below V

REF

before V

will again equal

REF

. This lower trip point is now given by:

AN007385-8

FIGURE 8. Non-Inverting Comparator with Hysteresis

www.national.com 4

(11)

The hysteresis for this circuit, ∆V

, is the difference between

IN 1

and V

IN 2

and is given by:

(12)

As a design example consider the following:

Given: R

LOAD

=100 kΩ

IN 1

=10V

IN 2

=5V

=15V

To find: V

REF

and R

Solution:

Again choose R

PULL-UP

LOAD

to minimize loading, so let

PULL-UP

=3kΩ

From equation (12)

From equation (9)

To minimize output loading choose

PULL-UP

or R

>3kΩ

so let R

=1MΩ

The value of R

is now obtained from equation (12)

These are the values shown in

Figure 8

LIMIT COMPARATOR WITH LAMP DRIVER

The limit comparator shown in

Figure 9

provides a range of

input voltages between which the output devices of both

LM139 comparators will be OFF.

This will allow base current for Q

to flow through pull-up re-

sistor R

, turning ON Q

which lights the lamp. If the input

voltage, V

, changes to a value greater than V

or less than

, one of the comparators will switch ON, shorting the base

of Q

to ground, causing the lamp to go OFF. If a PNP tran-

sistor is substituted for Q

(with emitter tied to +V

) the

lamp will light when the input is above V

or below V

and V

are arbitrarily set by varying resistors R

and R

ZERO CROSSING DETECTOR

The LM139 can be used to symmetrically square up a sine

wave centered around zero volts by incorporating a small

amount of positive feedback to improve switching times and

centering the input threshold at ground (see

Figure 10

). Volt-

age divider R

and R

establishes a reference voltage, V

,at

the positive input. By making the series resistance, R

plus

equal to R

, the switching condition, V

, will be sat-

isfied when V

=0. The positive feedback resistor, R

,is

made very large with respect to R

=2000 R

). The re-

sultant hysteresis established by this network is very small

(∆V

<10 mV) but it is sufficient to insure rapid output volt-

age transitions. Diode D

is used to insure that the inverting

input terminal of the comparator never goes below approxi-

mately −100 mV. As the input terminal goes negative, D

will

forward bias, clamping the node between R

and R

to ap-

proximately −700 mV. This sets up a voltage divider with R

and R

preventing V

from going below ground. The maxi-

mum negative input overdrive is limited by the current han-

dling ability of D

AN007385-9

FIGURE 9. Limit Comparator with Lamp Driver

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COMPARING THE MAGNITUDE OF VOLTAGES OF

OPPOSITE POLARITY

The comparator circuit shown in

Figure 11

compares the

magnitude of two voltages, V

IN 1

and V

IN 2

which have oppo-

site polarities. The resultant input voltage at the minus input

terminal to the comparator, V

, is a function of the voltage di-

vider from V

IN 1

and V

IN 2

and the values of R

and R

. Diode

connected transistor Q

provides protection for the minus in-

put terminal by clamping it at several hundred millivolts be-

low ground. A 2N2222 was chosen over a 1N914 diode be-

cause of its lower diode voltage. If desired, a small amount of

hysteresis may be added using the techniques described

previously. Correct magnitude comparison can be seen as

follows: Let V

IN 1

be the input for the positive polarity input

voltage and V

IN 2

the input for the negative polarity. If the

magnitude of V

IN 1

is greater than that of V

IN 2

the output will

go low (V

OUT

=GND). If the magnitude of V

IN 1

is less than

that of V

IN 2

, however, the output will go high (V

OUT

MAGNETIC TRANSDUCER AMPLIFIER

A circuit that will detect the zero crossings in the output of a

magnetic transducer is shown in

Figure 12

. Resistor divider,

and R

, biases the positive input at +V

/2, which is well

within the common mode operating range. The minus input

is biased through the magnetic transducer. This allows large

signal swings to be handled without exceeding the input volt-

age limits. A symmetrical square wave output is insured

through the positive feedback resistor R

. Resistors R

and

can be used to set the DC bias voltage at the positive in-

put at any desired voltage within the input common mode

voltage range of the comparator.

OSCILLATORS USING THE LM139

The LM139 lends itself well to oscillator applications for fre-

quencies below several megacycles.

Figure 13

shows a

symmetrical square wave generator using a minimum of

components. The output frequency is set by the RC time

constant of R

and C

and the total hysteresis of the loop is

set by R

and R

. The maximum frequency is limited only

by the large signal propagation delay of the comparator in

addition to any capacitive loading at the output which would

degrade the output slew rate.

To analyze this circuit assume that the output is initially high.

For this to be true, the voltage at the negative input must be

less than the voltage at the positive input. Therefore, capaci-

tor C

is discharged. The voltage at the positive input, V

will then be given by:

(13)

where if R

then

(14)

Capacitor C

will charge up through R

so that when it has

charged up to a value equal to V

, the comparator output

will switch. With the output V

=GND, the value of V

is re-

duced by the hysteresis network to a value given by:

(15)

using the same resistor values as before. Capacitor C

must

now discharge through R

towards ground. The output will

return to its high state (V

=+V

) when the voltage across

the capacitor has discharged to a value equal to V

. For the

circuit shown, the period for one cycle of oscillation will be

twice the time it takes for a single RC circuit to charge up to

one half of its final value. The period can be calculated from:

AN007385-10

FIGURE 10. Zero Crossing Detector

AN007385-11

FIGURE 11. Comparing the Magnitude of Voltages of

Opposite Polarity

AN007385-12

FIGURE 12. Magnetic Transducer Amplifier

www.national.com 6

MAX

−t

/RC

(16)

where

(17)

and

(18)

One period will be given by:

(19)

or calculating the exponential gives

(20)

Resistors R

and R

must be at least 10 times larger than R

to insure that V

will go all the way up to +V

in the high

state. The frequency stability of this circuit should strictly be

a function of the external components.

PULSE GENERATOR WITH VARIABLE DUTY CYCLE

The basic square wave generator of

Figure 13

can be modi-

fied to obtain an adjustable duty cycle pulse generator, as

shown in

Figure 14

, by providing a separate charge and dis-

charge path for capacitor C

. One path, through R

and D

will charge the capacitor and set the pulse width (t

). The

other path, R

and D

, will discharge the capacitor and set

the time between pulses (t

). By varying resistor R

, the time

between pulses of the generator can be changed without

changing the pulse width. Similarly, by varying R

, the pulse

width will be altered without affecting the time between

pulses. Both controls will change the frequency of the gen-

erator, however. With the values given in

Figure 14

, the

pulse width and time between pulses can be found from:

MAX

(1−e

−t

1) risetime (21a)

MAX

−t

1falltime (21b)

where

(22)

and

(23)

which gives

(24)

is then given by:

(25)

These terms will have a slight error due to the fact that V

MAX

is not exactly equal to

⁄

but is actually reduced by the

diode drop to:

(26)

therefore

(27)

AN007385-13

FIGURE 13. Square Wave Generator

www.national.com7

and

(28)

CRYSTAL CONTROLLED OSCILLATOR

A simple yet very stable oscillator can be obtained by using

a quartz crystal resonator as the feedback element.

Figure

gives a typical circuit diagram of this. This value of R

and

are equal so that the comparator will switch symmetrically

about +V

/2. The RC time constant of R

and C

is set to

be several times greater than the period of the oscillating fre-

quency, insuring a 50%duty cycle by maintaining a DC volt-

age at the inverting input equal to the absolute average of

the output waveform.

When specifying the crystal, be sure to order series resonant

along with the desired temperature coefficient and load ca-

pacitance to be used.

MOS CLOCK DRIVER

The LM139 can be used to provide the oscillator and clock

delay timing for a two phase MOS clock driver (see

Figure

). The oscillator is a standard comparator square wave

generator similar to the one shown in

Figure 13

. Two other

comparators of the LM139 are used to establish the desired

phasing between the two outputs to the clock driver. A more

detailed explanation of the delay circuit is given in the sec-

tion under “Digital and Switching Circuits.”

WIDE RANGE VCO

A simple yet very stable voltage controlled oscillator using a

mimimum of external components can be realized using

three comparators of the LM139. The schematic is shown in

Figure 17a

. Comparator 1 is used closed loop as an integra-

tor (for further discussion of closed loop operation see sec-

tion on Operational Amplifiers) with comparator 2 used as a

triangle to square wave converter and comparator 3 as the

switch driving the integrator. To analyze the circuit, assume

that comparator 2 is its high state (V

=+V

) which drives

comparator 3 to its high state also. The output device of

comparator 3 will be OFF which prevents any current from

flowing through R

to ground. With a control voltage, V

,at

the input to comparator 1, a current l

will flow through R

and begin discharging capacitor C

, at a linear rate. This dis-

charge current is given by:

(29)

and the discharge time is given by:

(30)

∆V will be the maximum peak change in the voltage across

capacitor C

which will be set by the switch points of com-

parator 2. These trip points can be changed by simply alter-

ing the ratio of R

to R

, thereby increasing or decreasing

the amount of hysteresis around comparator 2. With R

100 kΩand R

=5kΩ, the amount of hysteresis is approxi-

mately ±5%which will give switch points of +V

/2 ±750

mV from a 30V supply. (See “Comparators with Hysteresis”).

As capacitor C

discharges, the output voltage of compara-

tor 1 will decrease until it reaches the lower trip point of com-

parator 2, which will then force the output of comparator 2 to

go to its low state (V

=GND).

AN007385-14

FIGURE 14. Pulse Generator with Variable Duty Cycle

AN007385-15

FIGURE 15. Crystal Controlled Oscillator

www.national.com 8

This in turn causes comparator 3 to go to its low state where

its output device will be in saturation. A current l

can now

flow through resistor R

to ground. If the value of R

is cho-

sen as R

/2 a current equal to the capacitor discharge cur-

rent can be made to flow out of C

charging it at the same

rate as it was discharged. By making R

/2, current l

will equal twice l

. This is the control circuitry which

guararantees a constant 50%duty cycle oscillation indepen-

dent of frequency or temperature. As capacitor C

charges,

the output of comparator 1 will ramp up until it trips compara-

tor 2 to its high state (V

=+V

) and the cycle will repeat.

The circuit shown in

Figure 17a

uses a +30V supply and

gives a triangle wave of 1.5V peak-to-peak. With a timing ca-

pacitor, C

equal to 500 pF, a frequency range from approxi-

mately 115 kHz down to approximately 670 Hz was obtained

with a control voltage ranging from 50V down to 250 mV. By

reducing the hysteresis around comparator 2 down to ±150

mV (R

=100 kΩ,R

=1kΩ) and reducing the compensat-

ing capacitor C

down to .001 µF, frequencies up to 1 MHz

may be obtained. For lower frequencies (f

≤1 Hz) the timing

capacitor, C

, should be increased up to approximately 1 µF

to insure that the charging currents, l

and l

, are much

larger than the input bias currents of comparator 1.

Figure 17b

shows another interesting approach to provide

the hysteresis for comparator 2. Two identical Zener diodes,

and Z

, are used to set the trip points of comparator 2.

When the triangle wave is less than the value required to Ze-

ner one of the diodes, the resistive network, R

and R

, pro-

AN007385-16

FIGURE 16. MOS Clock Driver

AN007385-17

(a)

AN007385-18

(b)

FIGURE 17. Voltage Controlled Oscillator

www.national.com9

vides enough feedback to keep the comparator in its proper

state, (the input would otherwise be floating). The advantage

of this circuit is that the trip points of comparator 2 will be

completely independent of supply voltage fluctuations. The

disadvantage is that Zeners with less than one volt break-

down voltage are not obtainable. This limits the maximum

upper frequency obtainable because of the larger amplitude

of the triangle wave. If a regulated supply is available,

Figure

17a

is preferable simply because of less parts count and

lower cost.

Both circuits provide good control over at least two decades

in frequency with a temperature coefficient largely depen-

dent on the TC of the external timing resistors and capaci-

tors. Remember that good circuit layout is essential along

with the 0.01 µF compensation capacitor at the output of

comparator 1 and the series 10Ωresistor and 0.1 µF capaci-

tor between its inputs, for proper operation. Comparator 1 is

a high gain amplifier used closed loop as an integrator so

long leads and loose layout should be avoided.

DIGITAL AND SWITCHING CIRCUITS

The LM139 lends itself well to low speed (<1 MHz) high

level logic circuits. They have the advantage of operating

with high signal levels, giving high noise immunity, which is

highly desirable for industrial applications. The output signal

level can be selected by setting the V

to which the pull-up

resistor is connected to any desired level.

AND/NAND GATES

A three input AND gate is shown in

Figure 18

. Operation of

this gate is as follows: resistor divider R

and R

establishes

a reference voltage at the inverting input to the comparator.

The non-inverting input is the sum of the voltages at the in-

puts divided by the voltage dividers comprised of R

and R

. The output will go high only when all three inputs are

high, causing the voltage at the non-inverting input to go

above that at inverting input. The circuit values shown work

for a “0” equal to ground and a “1” equal +15V. The resistor

values can be altered if different logic levels are desired. If

more inputs are required, diodes are recommended to im-

prove the voltage margin when all but one of the inputs are

the “1” state. This circuit with increased fan-in is shown in

Figure 19

To convert these AND gates to NAND gates simply inter-

change the inverting and non-inverting inputs to the com-

parator. Hysteresis can be added to speed up output transi-

tions if low speed input signals are used.

OR/NOR GATES

The three input OR gate (positive logic) shown in

Figure 20

is achieved from the basic AND gate simply by increasing R

thereby reducing the reference voltage. A logic “1” at any of

the inputs will produce a logic “1” at the output. Again a NOR

gate may be implemented by simply reversing the compara-

tor inputs. Resistor R

may be added for the OR or NOR

function at the expense of noise immunity if so desired.

AN007385-19

VOUT =A•B•C

FIGURE 18. Three Input AND Gate

AN007385-20

VOUT =A•B•C•D

FIGURE 19. AND Gate with Large Fan-In

www.national.com 10

OUTPUT STROBING

The output of the LM139 may be disabled by adding a clamp

transistor as shown in

Figure 21

. A strobe control voltage at

the base of Q

will clamp the comparator output to ground,

making it immune to any input changes.

If the LM139 is being used in a digital system the output may

be strobed using any other type of gate having an uncommit-

ted collector output (such as National’s DM5401/DM7401).

In addition another comparator of the LM139 could also be

used for output strobing, replacing Q

Figure 21

,ifde-

sired. (See

Figure 22

ONE SHOT MULTIVIBRATORS

A simple one shot multivibrator can be realized using one

comparator of the LM139 as shown in

Figure 23

. The output

pulse width is set by the values of C

and R

(with R

>10

to avoid loading the output). The magnitude of the input

trigger pulse required is determined by the resistive divider

and R

. Temperature stability can be achieved by balanc-

ing the temperature coefficients of R

and C

or by using

components with very low TC. In addition, the TC of resistors

and R

should be matched so as to maintain a fixed ref-

erence voltage of +V

/2. Diode D

provides a rapid dis-

charge path for capacitor C

to reset the one shot at the end

of its pulse. It also prevents the non-inverting input from be-

ing driven below ground. The output pulse width is relatively

independent of the magnitude of the supply voltage and will

change less than 2%for a five volt change in +V

The one shot multivibrator shown in

Figure 24

has several

characteristics which make it superior to that shown in

Fig-

ure 23

. First, the pulse width is independent of the magni-

tude of the power supply voltage because the charging volt-

age and the intercept voltage are a fixed percentage of

. In addition this one-shot is capable of 99%duty cycle

and exhibits input trigger lock-out to insure that the circuit will

not re-trigger before the output pulse has been completed.

The trigger level is the voltage required at the input to raise

the voltage at point A higher than the voltage at point B, and

is set by the resistive divider R

and R

and the network R

and R

. When the multivibrator has been triggered, the

output of comparator 2 is high causing the reference voltage

at the non-inverting input of comparator 1 to go to +V

. This

prevents any additional input pulses from disturbing the cir-

cuit until the output pulse has been completed.

The value of the timing capacitor, C

, must be kept small

enough to allow comparator 1 to completely discharge C

before the feedback signal from comparator 2 (through R

)

switches comparator 1 OFF and allows C

to start an expo-

nential charge. Proper circuit action depends on rapidly dis-

charging C

to a value set by R

and R

at which time com-

parator 2 latches comparator 1 OFF. Prior to the

establishment of this OFF state, C

will have been com-

pletely discharged by comparator 1 in the ON state. The time

delay, which sets the output pulse width, results from C

re-

charging to the reference voltage set by R

and R

. When

the voltage across C

charges beyond this reference, the

output pulse returns to ground and the input is again reset to

accept a trigger.

BISTABLE MULTIVIBRATOR

Figure 25

is the circuit of one comparator of the LM139 used

as a bistable multivibrator. A reference voltage is provided at

the inverting input by a voltage divider comprised of R

and

. A pulse applied to the SET terminal will switch the output

high. Resistor divider network R

, and R

now clamps

the non-inverting input to a voltage greater than the refer-

ence voltage. A pulse now applied to the RESET Input will

AN007385-21

VOUT =A+B+C

FIGURE 20. Three Input OR Gate

AN007385-22

FIGURE 21. Output Strobing Using a Discrete

Transistor

AN007385-23

FIGURE 22. Output Strobing with TTL Gate

AN007385-24

FIGURE 23. One Shot Multivibrator

www.national.com11

pull the output low. If both Q and Q outputs are needed, an-

other comparator can be added as shown dashed in

Figure 25

Figure 26

shows the output saturation voltage of the LM139

comparator versus the amount of current being passed to

ground. The end point of 1 mV at zero current along with an

SAT

of 60Ωshows why the LM139 so easily adapts itself to

oscillator and digital switching circuits by allowing the DC

output voltage to go practically to ground while in the ON

state.

AN007385-25

FIGURE 24. Multivibrator with Input Lock-Out

AN007385-26

FIGURE 25. Bistable Multivibrator

www.national.com 12

TIME DELAY GENERATOR

The final circuit to be presented “Digital and Switching Cir-

cuits” is a time delay generator (or sequence generator) as

shown in

Figure 27

This timer will provide output signals at prescribed time inter-

vals from a time reference t

and will automatically reset

when the input signal returns to ground. For circuit evalua-

tion, first consider the quiescent state (V

=O) where the

output of comparator 4 is ON which keeps the voltage across

at zero volts. This keeps the outputs of comparators 1, 2

and 3 in their ON state (V

OUT

=GND). When an input signal

is applied, comparator 4 turns OFF allowing C

to charge at

an exponential rate through R

. As this voltage rises past the

present trip points V

, and V

of comparators 1, 2 and 3

respectively, the output voltage of each of these comparators

will switch to the high state (V

OUT

=+V

). A small amount

of hysteresis has been provided to insure fast switching for

the case where the R

time constant has been chosen large

to give long delay times. It is not necessary that all compara-

tor outputs be low in the quiescent state. Several or all may

be reversed as desired simply by reversing the inverting and

non-inverting input connections. Hysteresis again is optional.

LOW FREQUENCY OPERATIONAL AMPLIFIERS

The LM139 comparator can be used as an operational am-

plifier in DC and very low frequency AC applications

(≤100 Hz). An interesting combination is to use one of the

comparators as an op amp to provide a DC reference volt-

age for the other three comparators in the same package.

Another useful application of an LM139 has the interesting

feature that the input common mode voltage range includes

ground even though the amplifier is biased from a single

supply and ground. These op amps are also low power drain

devices and will not drive large load currents unless current

is boosted with an external NPN transistor. The largest appli-

cation limitation comes from a relatively slow slew rate which

restricts the power bandwidth and the output voltage re-

sponse time.

AN007385-27

FIGURE 26. Typical Output Saturation Characteristics

AN007385-28

FIGURE 27. Time Delay Generator

www.national.com13

The LM139, like other comparators, is not internally fre-

quency compensated and does not have internal provisions

for compensation by external components. Therefore, com-

pensation must be applied at either the inputs or output of

the device.

Figure 28

shows an output compensation

scheme which utilizes the output collector pull-up resistor

working with a single compensation capacitor to form a

dominant pole. The feedback network, R

and R

sets the

closed loop gain at1+R

or 101 (40 dB).

Figure 29

shows the output swing limitations versus frequency. The

output current capability of this amplifier is limited by the

relatively large pull-up resistor (15 kΩ) so the output is

shown boosted with an external NPN transistor in

Figure 30

The frequency response is greatly extended by the use of

the new compensation scheme also shown in

Figure 30

. The

DC level shift due to the V

of Q

allows the output voltage

to swing from ground to approximately one volt less than

. A voltage offset adjustment can be added as shown in

Figure 31

AN007385-29

FIGURE 28. Non-Inverting Amplifier

AN007385-30

FIGURE 29. Large Signal Frequency Response

AN007385-31

FIGURE 30. Improved Operational Amplifier

www.national.com 14

DUAL SUPPLY OPERATION

The applications presented here have been shown biased

typically between +V

and ground for simplicity. The

LM139, however, works equally well from dual (plus and mi-

nus) supplies commonly used with most industry standard

op amps and comparators, with some applications actually

requiring fewer parts than the single supply equivalent.

The zero crossing detector shown in

Figure 10

can be imple-

mented with fewer parts as shown in

Figure 32

. Hysteresis

has been added to insure fast transitions if used with slowly

moving input signals. It may be omitted if not needed, bring-

ing the total parts count down to one pull-up resistor.

The MOS clock driver shown in

Figure 16

uses dual supplies

to properly drive the MM0025 clock driver.

The square wave generator shown in

Figure 13

can be used

with dual supplies giving an output that swings symmetrically

above and below ground (see

Figure 33

). Operation is iden-

tical to the single supply oscillator with only change being in

the lower trip point.

Figure 34

shows an LM139 connected as an op amp using

dual supplies. Biasing is actually simpler if full output swing

at low gain settings is required by biasing the inverting input

from ground rather than from a resistive divider to some volt-

age between +V

and ground.

All the applications shown will work equally well biased with

dual supplies. If the total voltage across the device is in-

creased from that shown, the output pull-up resistor should

be increased to prevent the output transistor from being

pulled out of saturation by drawing excessive current,

thereby preventing the output low state from going all the

way to −V

AN007385-32

Av≅100

FIGURE 31. Input Offset Null Adjustment

AN007385-33

FIGURE 32. Zero Crossing Detector Using Dual

Supplies

AN007385-34

FIGURE 33. Squarewave Generator Using Dual

Supplies

AN007385-35

FIGURE 34. Non-Inverting Amplifier Using Dual

Supplies

www.national.com15

MISCELLANEOUS APPLICATIONS

The following is a collection of various applications intended

primarily to further show the wide versatility that the LM139

quad comparator has to offer. No new modes of operation

are presented here so all of the previous formulas and circuit

descriptions will hold true. It is hoped that all of the circuits

presented in this application note will suggest to the user a

few of the many areas in which the LM139 can be utilized.

REMOTE TEMPERATURE SENSOR/ALARM

The circuit shown in

Figure 35

shows a temperature

over-range limit sensor. The 2N930 is a National process 07

silicon NPN transistor connected to produce a voltage refer-

ence equal to a multiple of its base emitter voltage along with

temperature coefficient equal to a multiple of 2.2 mV/˚C.

That multiple is determined by the ratio of R

to R

. The

theory of operation is as follows: with transistor Q

biased

up, its base to emitter voltage will appear across resistor R

Assuming a reasonably high beta (β≥100) the base current

can be neglected so that the current that flows through resis-

tor R

must also be flowing through R

. The voltage drop

across resistor R

will be given by:

and

(31)

As stated previously this base-emitter voltage is strongly

temperature dependent, minus 2.2 mV/˚C for a silicon tran-

sistor. This temperature coefficient is also multiplied by the

resistor ratio R

This provides a highly linear, variable temperature coefficient

reference which is ideal for use as a temperature sensor

over a temperature range of approximately −65˚C to +150˚C.

When this temperature sensor is connected as shown in

Fig-

ure 35

it can be used to indicate an alarm condition of either

too high or too low a temperature excursion. Resistors R

and R

set the trip point reference voltage, V

, with switching

occuring when V

. Resistor R

is used to bias up Q

some low value of current simply to keep quiescent power

dissipation to a minimum. An l

near 10 µA is acceptable.

Using one LM139, four separate sense points are available.

The outputs of the four comparators can be used to indicate

four separate alarm conditions or the outputs can be OR’ed

together to indicate an alarm condition at any one of the sen-

sors. For the circuit shown the output will go HIGH when the

temperature of the sensor goes above the preset level. This

could easily be inverted by simply reversing the input leads.

For operation over a narrow temperature range, the resistor

ratio R

should be large to make the alarm more sensitive

to temperature variations. To vary the trip points a potentiom-

eter can be substituted for R

and R

. By the addition of a

single feedback resistor to the non-inverting input to provide

a slight amount of hysteresis, the sensor could function as a

thermostat. For driving loads greater than 15 mA, an output

current booster transistor could be used.

FOUR INDEPENDENTLY VARIABLE, TEMPERATURE

COMPENSATED, REFERENCE SUPPLIES

The circuit shown in

Figure 36

provides four independently

variable voltages that could be used for low current supplies

for powering additional equipment or for generating the ref-

erence voltages needed in some of the previous comparator

applications. If the proper Zener diode is chosen, these four

voltages will have a near zero temperature coefficient. For

industry standard Zeners, this will be somewhere between

5.0 and 5.4V at a Zener current of approximately 10 mA. An

alternative solution is offered to reduce this 50 mW quies-

cent power drain. Experimental data has shown that any of

National’s process 21 transistors which have been selected

for low reverse beta (β

<.25) can be used quite satisfacto-

rily as a zero T.C. Zener. When connected as shown in

Fig-

ure 37

, the T.C. of the base-emitter Zener voltage is exactly

cancelled by the T.C. of the forward biased base-collector

junction if biased at 1.5 mA. The diode can be properly bi-

ased from any supply by adjusting R

to set l

equal to 1.5

mA. The outputs of any of the reference supplies can be cur-

rent boosted by using the circuit shown in

Figure 30

www.national.com 16

AN007385-36

FIGURE 35. Temperature Alarm

AN007385-37

FIGURE 36. Four Variable Reference Supplies

www.national.com17

DIGITAL TAPE READER

Two circuits are presented here–a tape reader for both mag-

netic tape and punched paper tape. The circuit shown in

Fig-

ure 38

, the magnetic tape reader, is the same as

Figure 12

with a few resistor values changed. With a 5V supply, to

make the output TTL compatible, anda1MΩfeedback re-

sistor, ±5 mV of hysteresis is provided to insure fast switch-

ing and higher noise immunity. Using one LM139, four tape

channels can be read simultaneously.

The paper tape reader shown in

Figure 39

is essentially the

same circuit as

Figure 38

with the only change being in the

type of transducer used. A photo-diode is now used to sense

the presence or absence of light passing through holes in the

tape. Againa1MΩfeedback resistor gives ±5 mV of hyster-

esis to insure rapid switching and noise immunity.

PULSE WIDTH MODULATOR

Figure 40

shows the circuit for a simple pulse width modula-

tor circuit. It is essentially the same as that shown in

Figure

with the addition of an input control voltage. With the input

control voltage equal to +V

/2, operation is basically the

same as that described previously. If the input control volt-

age is moved above or below +V

/2, however, the duty

cycle of the output square wave will be altered. This is be-

cause the addition of the control voltage at the input has now

altered the trip points. These trip points can be found if the

circuit is simplified as in

Figure 41

. Equations 13 through 20

are still applicable if the effect of R

is added, with equations

17 through 20 being altered for condition where V

≠

/2.

Pulse width sensitivity to input voltage variations will be in-

creased by reducing the value of R

from 10 kΩand alter-

nately, sensitivity will be reduced by increasing the value of

. The values of R

and C

can be varied to produce any

desired center frequency from less than one hertz to the

maximum frequency of the LM139 which will be limited by

and the output slew rate.

AN007385-38

Q1 =National Process 21 Selected for Low Reverse β

FIGURE 37. Zero T.C. Zener

AN007385-39

FIGURE 38. Magnetic Tape Reader with TTL Output

AN007385-40

FIGURE 39. Paper Tape Reader With TTL Output

www.national.com 18

POSITIVE AND NEGATIVE PEAK DETECTORS

Figures 42, 43

show the schematics for simple positive or

negative peak detectors. Basically the LM139 is operated

closed loop as a unity gain follower with a large holding ca-

pacitor from the output to ground. For the positive peak de-

tector a low impedance current source is needed so an addi-

tional transistor is added to the output. When the output of

the comparator goes high, current is passed through Q

charge up C

. The only discharge path will be the 1 MΩre-

sistor shunting C

and any load that is connected to V

OUT

The decay time can be altered simply by changing the 1 MΩ

resistor higher or lower as desired. The output should be

used through a high impedance follower to avoid loading the

output of the peak detector.

For the negative peak detector, a low impedance current

sink is required and the output transistor of the LM139 works

quite well for this. Again the only discharge path will be the 1

MΩresistor and any load impedance used. Decay time is

changed by varying the 1 MΩresistor.

CONCLUSION

The LM139 is an extremely versatile comparator package of-

fering reasonably high speed while operating at power levels

in the low mW region. By offering four independent compara-

tors in one package, many logic and other functions can now

be performed at substantial savings in circuit complexity,

parts count, overall physical dimensions, and power con-

sumption.

For limited temperature range application, the LM239 or

LM339 may be used in place of the LM139.

It is hoped that this application note will provide the user with

a guide for using the LM139 and also offer some new appli-

cation ideas.

AN007385-41

FIGURE 40. Pulse Width Modulator

AN007385-81

VA=UPPER TRIP POINT

AN007385-82

VB=LOWER TRIP POINT

FIGURE 41. Simplified Circuit For

Calculating Trip Points of

Figure 40

AN007385-43

FIGURE 42. Positive Peak Detector

AN007385-44

FIGURE 43. Negative Peak Detector

www.national.com19

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NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DE-

VICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMI-

CONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or sys-

tems which, (a) are intended for surgical implant into

the body, or (b) support or sustain life, and whose fail-

ure to perform when properly used in accordance

with instructions for use provided in the labeling, can

be reasonably expected to result in a significant injury

to the user.

2. A critical component is any component of a life support

device or system whose failure to perform can be rea-

sonably expected to cause the failure of the life support

device or system, or to affect its safety or effectiveness.

National Semiconductor

Corporation

Americas

Tel: 1-800-272-9959

Fax: 1-800-737-7018

Email: support@nsc.com

www.national.com

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Fax: 81-3-5639-7507

AN-74 LM139/LM239/LM339—A Quad of Independently Functioning Comparators

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.

Application Note 74 LM139/LM239/LM339 A Quad Of Independently Functioning Comparators AN 0074

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