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Патент USA US2122851

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My 5, 1938.
B. J. THOMPSON
2,122,851
AMPLIFYING CIRCUITS
Filed Dec.‘ 29, 1957
Inventor‘:
BrowderJ-Tho p'son,
by 1/0/1476
‘
His Attorney.’ I
Patented July 5, 1938
2,122,851
UNITED STATES PATENT oFFicE
2,122,851
AMPLIFYING' CIRCUITS
Browder J. Thompson, Basking Ridge, N. J., as
signor to General Electric Company,
ration of New York
Application December 29, 1937, Serial No. 182,267
4 Claims. (Cl. 179—171)
The present invention relates to an improved meral 4. This ampli?er includes a cathode 5, an
amplifying circuit especially useful in connec
anode 6, a control grid 1, a screen grid 8 and a
tion with control apparatus such as photosensitive space charge grid 9 whose nature and function
devices which are adapted to produce very small will be more fully explained hereinafter. The
5 potentials varying in accordance with a physical
output voltage of the ampli?er, which is de-‘ 5
condition.
veloped across a resistor It], may be impressed
It is one object of the invention to provide a on the control grid of a suitable triggering or con
circuit of the type in question which can be oper
trol device, exempli?ed in the present case by a
ated satisfactorily from an alternating current thermionic discharge tube l2. For impressing a Y
10 power source.
difference of potential between the various elec
Known types of photosensitive devices are trodes of the ampli?er and associated devices
characteristically of very high impedance and re
there is provided a source of alternating current
quire to be used. with an amplifying circuit which potential, illustrated as including a transformer
has an input impedance of commensurately high having a secondary l3 and a primary M.
value. However, it has been observed that as the
The photosensitive device is connected in the
input impedance of an amplifying circuit is in
cathode-control-grid circuit of the ampli?er 4
creased, the relative importance of interelectrode
capacitance also increases. With a circuit adapt~
ed to be energized by alternating current and hav
ing a su?iciently high input impedance to assure
proper cooperation with a conventional ‘photo
sensitive device, the effect of interelectrode capac
itance may be such as to prevent the normal func
tioning of the ampli?er.
It is a more particular object of this invention
as applied in connection with an amplifying cir
cuit adapted for use with an alternating current
source to provide means for effectively neutralizing
the capacitance between the control grid and the
remaining electrodes of the ampli?er.
In accordance with a preferred embodiment of
the invention this object is accomplished by suit
ably connecting a condenser between the control
grid of the ampli?er and a point of the alternat
ing current source of supply whose voltage phase
relation with respect to the connection point of
. the input impedance element cathode is opposite
to that of the remaining electrodes. In this way
the tendency of the control grid to accumulate
charges as a result of its capacitive relation to
the other electrodes is offset by the equal and
opposite e?ect of the neutralizing condenser.
The features of my invention which I desire to
protect herein are pointed out with particularity
in the appended claims. The invention itself, to
gether with further objects and advantages there
of will best be understood by reference to the fol
lowing description taken in connection with the
accompanying drawing in which the single ?gure
' shows schematically a control circuit suitably em
bodying the invention.
Referring to the drawing, there is shown a
photo-sensitive device (comprising a cathode I
and an anode 2) connected to a vacuum tube am
pli?er which is designated as a whole by the nu
and is associated with an input resistance I 6. As
previously noted, for purposes of cooperation with
the high impedance photosensitive device this
resistance should have an extremely high value,
for example, on the order of 1,000 megohms.
As a result of the high impedance of the in
put circuit thus formed, the capacitance between
the control grid and the anode and various other
grids of the ampli?er 4 may have a relatively im
portant effect on the operation of the ampli?er.
These capacitances (indicated in dotted outline
as condensers ll, l8 and I9) permit a substantial
flow of alternating current through the resistor
it.
With a low value of the resistor Hi the re
sultant drop across it and the attendant swing
of the control grid voltage may be negligible.
However, with a resistance of the value required
in the present connection the potential developed
across the resistor from this cause may be suffi
cient to cause the control grid to swing positive
at the same time as the grids 8 and 9 and the
anode B and thus substantially to nullify the
effect of variations in the current ?owing through
the photo tube and to prevent the normal func
tioning of the ampli?er.
In order to overcome this difficulty, I provide
a neutralizing condenser 22 connected between
the control grid I and a point 23 of the alternat
ing current source whose voltage phase relation
with respect to the connection point 24 of the
resistance !6 is opposite to that of the points
to which the remaining electrodes are connected.
By proper adjustment of this condenser it is pos
sible to eliminate the blocking effect referred to
and obtain. good performance of the ampli?er.
The requirement for neutralization is that the
voltage between the points 23 and 24 multiplied
by the capacitance of condenser 22 must equal
the sum of ( 1) the voltage between the point 24
2,122,851
2
and the screen grid connection point multiplied
by the capacitance between the control grid and
screen grid plus (2) the voltage between the
point 24 and the space charge grid connection
point multiplied by the capacitance between the
if used will increase the effective ampli?cation of
the device and shield the control grid 1 from the
effects of the anode ?eld.
Although I have described my invention espe
cially in connection with a photosensitive device it
space-charge grid and control grid plus (3) simi
lar products of voltage and capacitance for such
other electrodes as may be involved. The simplest
method of arriving at the proper value of the
10 condenser 22 is to put in a small ?xed condenser
and to slide the tap along or to put in a small
variable condenser with a ?xed tap and to ad.
just the condenser until in either case satisfac
tory operation is achieved.
A further dii?culty resulting from the use of
15
the high resistance element 16 is the fact that
any current however minute, which may flow from
the cathode 5 to the control grid 1 during the
half-cycle in which the latter is positive will pro
is by no means limited to such use and may be
alternatively employed in connection with any
electrical device, the electrical characteristics of
which are variable automatically in. accordance
with variations in a physical condition to which 10
the device is subjected. Furthermore, while I
have shown a particular embodiment of my in
vention, it will be understood that many modi?
cations may be made by those skilled in the art
without departure from the invention. I aim by
the appended claims to cover all such modi?ca
,20 duce a considerable voltage drop across the re
sistance I6. This drop will tend to charge the
condenser 22, the interelectrode capacities I1, I8,
and I9, and such stray capacities as may exist
going disclosure.
I claim:
'
ampli?er having a 20
plurality of electrodes including at least a
1. In combination,
an
cathode, an anode and a control grid, an alter
electrode is negative with respect to the cathode
such charged capacities will oppose the action
nating current source having various points
thereof connected to said electrodes for supplying
desired potentials thereto, an input circuit in
cluding a high impedance element connected be
tween the control grid and a point on said alter
nating current source, and a condenser connected
of the photosensitive device and render the con
between the control grid and a point of said source
between the control grid and the associated ele
25 ments of the circuit. During the portion of the
alternatingr current cycle in which the control
.30 trol circuit unable to respond in the intended
V44)
tions as fall within the true scope of the fore
whose voltage phase relation with respect to the 30
manner to the stimulus of a light beam impinging connection point of the impedance element is op
posite to that of the points to which the elec
on the photosensitive element.
This difficulty is overcome by providing electro- ‘ trodes of the ampli?er other than the control
static means associated with the control grid 1 grid are connected, the condenser being effective 35
to neutralize the capacitance existing between the
and the cathode 5 for preventing current flow be
control grid and the said other electrodes.
tween them. This. means may suitably comprise
2. In combination, an ampli?er having a
a space charge grid 9 which is adapted to be
plurality of electrodes including a cathode, a
negatively charged whenever the control grid be
comes positive with respect to the cathode, so control grid and an anode, an alternating cur 40
rent source connected to said electrodes ior im~
that its e?ect is to inhibit the movement of elec
trons from the cathode. As illustrated, the de
sired potential relationship is obtained by con
necting the grid 9 to a point of the alternating
current source whose voltage phase relation is
45 opposite to that of the control grid when referred
to the cathode as a reference point.
In order further to block the random electron
current which tends to flow from the cathode
during the period in which all the electrodes are
50 at substantially zero potential, additional means
are provided in the connection between the grid 9
and the transformer secondary 13 for de?nitely
biasing the grid negatively with respect to the
cathode 5 whenever the control element ceases
55 to be negative with respect thereto. This means
as illustrated comprises a condenser 20 in, par
allel with a resistance 2!, these elements being
so related in magnitude that any charge accumu~
lated on the condenser will not be lost in less than
a full cycle of the alternating current. Under
these conditions current drawn by grid 9 during
the period of normal conduction of the ampli?er
charges the condenser 20 and thus maintains a
negative bias on the grid when a reversal of the’
.65 alternating potential occurs.
The use of a space charge grid in this connec
tion and apart from the use of a neutralizing
condenser as claimed herein is fully described
and claimed in my copending application S. N.
-70 101,612, ?led September‘ 19, 1936 and assigned
to the same assignee as the present invention. '
The electrode 8 comprises a screen grid which
may be interposed between the anode and the
control grid if desired. This element does not
.78 constitute an essential part of my invention but
pressing desired potentials thereon, an electrical
device connected in, the cathode-grid circuit of
the ampli?er and having its electrical character
istics variable automatically in accordance with
variations in a physical condition to which the
device is subjected, means including a high ini
pedance element connected between the control
grid of the ampli?er and a point on said alternat
ing current source for causing variations in the
electrical characteristics of said device to affect 50
the potential of the control grid in a desired man
ner, and a condenser connected between said grid
and a point of said source whose voltage phase
relation with respect to the connection point of
the impedance element is opposite to that of the 55
points to which the electrodes of the ampli?er,
other than the control grid, are connected, the
condenser being effective to neutralize the capaci
tance existing between the control grid and the
60
remaining electrodes of the ampli?er.
3. In combination, an ampli?er having a
plurality of electrodes including at least a
cathode, an anode, a control grid, and a screen
grid, an alternating current source for impressing
desired potentials on said electrodes, a photo 65
sensitive device connected in the cathode-con
trol-grid circuit of the ampli?er, a resistance ele
ment of at least about 1,000 megohms connected
between the control grid and a point of said
alternating current source, and a condenser con~ 70
nected between the control grid and a point of
the alternating current source whose voltage
phase relation with respect to the connection
point of the resistance element is opposite to that
of the points to which the remaining electrodes,
2,122,851
other than the control grid are connected, the
condenser being of su?icient capacity to neutral
ize the capacitance existing between the control
grid and the said remaining electrodes.
4. In combination, an ampli?er including at
least an anode, a cathode, a control grid between
the anode and cathode, and. a space charge grid
between the cathode and control grid, an alter
nating current source for impressing desired
10 potentials on said electrodes, a photo-sensitive
device connected in the cathode-control-grid cir
cuit of the ampli?er, a high impedance element
connected between the control grid and a point
on said alternating current source associated with
3
the cathode-control-grid circuit, means connect
ing the space charge grid to a point of said source
whose voltage phase relation with respect to the
cathode is opposite to that of the connection point
of said impedance element, and a condenser con
nected between the control grid and a point of
said source whose voltage phase relation with
respect to the connection point of the impedance
element is opposite to that of the space charge
grid, the condenser being effective to neutralize 10
the capacity existing between the control grid and
the space charge grid.
BROWDER J. THOMPSON.
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