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Патент USA US2410077

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Oct- 29, 1946-
Filed March 5, 1943
- Patented Oct. 29, 1946
Kenneth S. Johnson, South Orange, N. 1., aa
signor to Bell Telephone Laboratories, Incorpo
rated. New York, N. 'Y., a corporation 0! New
Application March 5, 1943, Serial No. 478,113.
8 Claims. (Cl. 178-44)
The present invention relates to circuits using
negative impedance and more especially to the
provision of auxiliary impedance, which may
comprise a network'of impedances, tor modify
ing the characteristic of the negative impedance.
One known‘ type of negative impedance device
Fig. 5 shows one method of coupling a ther
mistor to a line according to the invention;
Fig. 6 shows a circuit for utilizing a thermistor
in accordance with the invention; and '
Fig. 7 shows curves to be referred to in the de
Fig. 1 is the approximate circuit equivalent of
consists or a substance having a negative tem
a thermistor. The thermistor when suitably
perature coeilicient of resistance connected in
biased to its negative resistance region behaves
circuit with a source or biasing current which
like a series positive resistance R shunted by a
raises the temperature of the substance to such 10 negative resistance —r and inductance L in series
a point that its resistance changes in response to
with each other. As stated, it is the temperature
small impressed current ?uctuations in such
time lag which imparts to the thermistor the re
manner that the terminal voltage across the sub
actance e?ect represented in the equivalent cir
stance decreases as the current through the de
cuit by the inductance L. As the frequency in- I
vice increases. In other words, the device op
creases, the negative resistance effect eventually
erates on a falling ‘portion of its volt-ampere
disappears and the thermistor behaves as an
characteristic. This type of negative impedance
ohmic resistance R. In terms of the thermistor
device has been termed thermistor because its re
this occurs when the frequency exceeds the
sistance changes as a function of its temperature.
highest frequency which the temperature varia
This and other types of devices that exhibit a
tions can follow. In terms of the equivalent
characteristic with a region in which the slope is
circuit, this occurs at the frequency at which the
downward are referred to as negative resistance
impedance of the inductance L becomes so great
devices since the derivative of voltage with re
that substantially no current can traverse the
spect to current variation is negative in sign.
path through -r and all of the current must take
It has been shown that the time lag of heating
the path through R.
inherent in a thermistor is equivalent to an in
In accordancewith the invention, this time lag
ductance in the equivalent electrical network.
of the thermistor is reduced or substantially neu
For some purposes this time lag is undesirable
tralized by connecting in shunt across the ther
and what is desired is a negative resistance with
30 mistor terminals a circuit comprising a positive
substantially no reactance.
resistance of numerical value nearly equal to r in
It is an object of the present invention to com
series with a capacity C of proper size, as indi
pensate or neutralize to a material degree the
cated in Figs. 2 and 3. It can be shown that the
reactive component oi’ impedance of a negative
impedance Z of the circuit of Fig. 2 (which is
resistance device such as a thermistor.
A further object of the invention is to provide
a negative resistance whose magnitude varies
with frequency.
35 identical to the circuit of Fig. 3 but uses a differ
ent notation) is
_ Thermistors that are suitable for carrying out
the objects of this invention may comprise any
of a number of known substances including par
ticularly semiconductors, such as silver sulphide,
boron, oxides of uranium or other substances
or, using the notation of Fig. 3,
having negative temperature coe?icients of re
sistance. Such thermistors have been con 45
Z_ .(r+ Ar):c+ry
structed having a frequency response range cov
ering the range of commercial speech and even
extending to many kilocycles per second. One
type of thermistor construction capable of re,
sponding to frequencies of this order 01.’ band 50
width is disclosed in United States patent to G. L.
Pearson 2,276,864, granted March 17, 1942,
“7 Ar+i (av-21)
any = r(r + Ar)
As Ar approaches 0, Z approaches the value
The nature and objects of the present inven
which, since it contains no reactance term, rep
tion will appear more fully from the following de
tailed description together with the accompany 55 resents a pure resistance whose magnitude varies
with frequency. Moreover, the resistance can be
ing drawing in which:
positive or negative depending upon the fre
Figs. 1 to 4, inclusive, are impedance network
diagrams to illustrate how the equivalent circuit ‘ quency. This effect is illustrated by the graphs
in F18‘. 7.
of a thermistor may be combined with a‘ com
Curve '1‘ is the measured curve of a boron ther
pensating network according to the invention;
mistor without the compensating shunt circuit
frequency variations into amplitude variations.
of the invention. Its resistance is negative at all
of the frequencies plotted but the value is de
creasing with increasing frequency so that at
some frequency in the neighborhood of '7 kilocy
cles its resistance will turn positive. Solid line
curve A is in two parts (A1, A1‘) and represents
A source of variable frequency is shown at II
which may in practice comprise a vacuum tube
oscillator with a movable condenser plate for
varying the frequency. The output of source II
is sent over a circuit or line 2| to the point at
which the variations in frequency are to be indi
the ideal case in which A1‘ is zero and, therefore,
cated. Resistance 22 (which could be replaced
by transformer I l and condenser ll of Fig. 0,
if desired) is for coupling the line H to the
1' is numerically exactly equal to —r. This curve
passes from minus in?nity to plus in?nity in go-'
ing through a frequency of about 5 kilocycles.
indicator circuit comprising thermistor units II
This curve represents an unstable case and can
to I8 (Fig. 4) and a suitable current responsive
not be actually realized in practice but can be
element shown for illustration as a moving coil
Dotted curve B is a curve calcu~
lated for the case where Ar has a small positive 15 instrument 23 having a pen 24 movable across
a paper 2! fed along by rollers 2| to make a
value large enough to give stable operation. This
curve is interesting in exhibiting increasing neg
ative resistance eifect with rising frequency over
a considerable range below 4 kilocycles and a
steeply rising characteristic between 4 and 5 kilo~
cycles, this steep portion being entirely in the
negative resistance range for several hundred cy
cles. This offers the possibility of a circuit with
net gain and with its gain varying rapidly as
a function of frequency. The computations for 25
these curves assumed values as follows:
record of the frequency variations.
The frequency of source 20 may be varied by
hand to transmit signs or signals or may be
varied by a meter, ?oat, steering wheel or any
other movable member whose position or move
ment is to be indicated at a distance. The re
sulting changes in frequency are translated by
the thermistor circuit into amplitude variations
which are recti?ed by any suitable type recti?er
21 shown as a solid element rectifier, such as
copper oxide. The recti?er current flows through
R=600 ohms
the colic! the instrument 23 and the alternating
current, component flows through shunting con
denser 28. If operation is desired in accordance
L=0.00577 henry
C=0.096 microfarad
—r=-240 ohms
30 with curve B of Fig. 7, the source 20 may have
a mean frequency slightly above 4 kilocycles (e. g.
The curve A was computed for r=240 ohms and
4,200 cycles) and the maximum range of varia
curve B was computed for r=250 ohms.
tion may be between 4,000 cycles and 4,400 cycles.
In the case of curve B since the neutralization
Over this entire
of the inductive reactance L is incomplete, the
impedance Z is not a pure resistance but has 35 and varying at
mistor acts as
a phase angle varying with frequency. The real
time as a very
or resistance component is plotted in curve B.
changing frequency modulated waves to ampli
tude modulated waves. By using two channels
like the one in Fla. 6 and suitably coordinating
them, a telautograph system may be provided.
Other applications of the invention will occur to
persons skilled in the art, from the illustrative
examples that have been given showing the
The invention provides what is believed to be
a highly novel circuit combination or unit capable
of general application. This unit is illustrated in
Fig. 4 as comprising a thermistor l0 having a
battery II and variable resistance I! for applying
an adjustable bias to the thermistor to bias it
to the desired point along its negative resistance
The current. may be passed
through the thermistor -by a conductive path or
. characteristic.
range the resistance is negative
a steep rate so that the ther
an ampli?er and at the same
eil'ective conversion circuit for
45 principle of operation.
The invention is not to be construed as
limited to the speci?c forms that have been dis
closed but the scope is de?ned in the claims,
applied to a heating winding surrounding the
thermistor. Choke coil i3 is for preventing cur
rent variations representing signals or other
which follow.
neutralizing circuit already described comprises
current supply circuit for causing said thermistor
waves to be ampli?ed from being shunted through 50
the bias current circuit. The compensating or
What is claimed is:
1. A circuit comprising a thermistor and a bias
to develop an impedance effect having a negative
resistance component and an inductive compo
ance and positive inductance e?’ects inherent in 55 nent, and a circuit connected in shunt relation to
said thermistor and comprising positive resistance
the thermistor operation as already set forth.
and capacitive reactance proportioned to sub
Fig. 5 shows a circuit for introducing into a
stantially neutralize the inductive reactance of
line or circuit represented by the fragment of
said thermistor impedance.
line II a resistance or a gain varying with fre
a resistance l4 and capacity it proportioned in
magnitude with relation to the negative resist
quency over a definite range.
The thermistor is . - 50
coupled to the line I8 through transformer I‘!
inserted between the thermistor and a bridge
across line l6, including condenser II which may
2. A circuit comprising a thermistor and a bias
current supply circuit for causing said thermistor
be a stopping condenser or a tuning condenser
to develop an impedance effect having a negative
resistance component and a reactive component,
and a circuit connected in shunt relation to said
i‘! may serve to step the impedance up or down
and a reactive impedance proportioned to sub
for tuning the shunt branch. The transformer 65 thermistor and comprising positive resistance
stantially neutralize the reactive component of
said thermistor impedance.
resistance having a highly positive temperature
3. In combination a thermistor, means to bits
coe?lcient of resistance to insure stability if 70 said thermistor to the negative resistance region
of its volt-ampere characteristic whereby said
high local circulatory currents should tend to
as between the line and the thermistor circuit.
The resistance I‘ may be partly or wholly a
thermistor exhibits negative resistance over a
given frequency range with an inductive re
actance, and a passive network connected in
sated themistor of the invention for converting
75 shunt across said thermistor of such character
6 shows a circuit for using the compen
istic that the combination thermistor and net
work has a resultant impedance closely approach
ing a pure negative resistance whose magnitude
comprising a compensating network in shunt
relation to said thermistor comprising positive
resistance and capacitance.
7. In combination with a circuit carrying cur
rents of di?erent frequency, a circuit unequally
responsive to currents of di?erent frequencies
comprising a thermistor shunted by resistance
and capacitance of such value as to give a result
ant impedance approaching a pure resistance
shunt across said device, said positive resistance
being nearly equal in magnitude to the negative 10 whose magnitude varies markedly at said dif
ferent frequencies.
resistance of said device and said capacitance
8. A circuit comprising a pair of terminals and
being of such value that said inductive compo
two parallel branches connected between said
nent is substantially neutralized.
terminals, the ?rst branch comprising a negative
5. A frequency-amplitude conversion circuit '
comprising a negative resistance device having 15 resistance device which is electrically equivalent
in impedance to a pure resistance connected be
inherent reactance and a compensating network
tween said terminals, shunted by a negative re
having resistance and reactance shunted across
sistance of value --1' and an inductance L in
said negative resistance device for giving a result
series with each other, and the second branch
ant impedance approaching a pure resistance
consisting of a resistance r+Ar in series with a
whose magnitude varies with frequency.
‘ 20
capacitance C of magnitude given by
6. A circuit for converting frequency variations
to amplitude variations comprising a thermistor
varies with frequency over said frequency range.
4. In combination, a negative resistance device
whose impedance includes an inductive compo
nent and means comprising a positive resistance
and a capacitance in series with each other in
of sumciently high response speed to permit its
temperature to follow said frequency variations,
said thermistor exhibiting an inductive reactive 25
effect, and means to reduce said inductive e?ect
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