US2541198A - Amplifier - Google Patents

Description

l. R. BRENHOLDT 2 Sheets-Sheet 1 Feb. 13, 1951 AMPLIFIER Filed Aprill 14, 1948 J/ EIE. Z

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JNVENTOR. ffm/m9' @Pemba/df 'BY /Pfw////LMWQ A? 7 'VORNE V Filed April 14, 1948 2 Sheets-Sheet 2 atentecl Feb. 13, 1951 2,541,133 AMPLHFIER rving R. Brenholdt, Chicagct Ill., assignor, by mesne assignments, to the`U1'iited States of America s represented by theUnitedStates Atomic Energy Commission Application April 14, 1948, Serial No. 21,022

My invention relates" to amplifying circuits and more particularly to a sensitive amplifier for handling minute currents and which utilizes tubes whose filaments are supplied from a single high voltage power supply with protective control circuits for preventing injury to the tubes due to vVoltage surges, changes, or abnormal conditions.

In the prior art it has been the general practice to supply each of the laments of the tubes of a circuit from a low voltage heater current power `source or supply. This has been particularly true of sensitive direct current voltage ampliers employed to measure very small currents where regulator tubes are required to maintain supply voltages as nearly constant as possible. See Vance 2,190,743. In these cases performance suffers from the lack of adequate voltage regulating systems and protective devices.

Applicant with a knowledge of these problems in the prior art has for an object of his invention the provision of a voltage regulator system for the power source of an amplier circuit which is instantaneously responsive to changes in supply voltage for maintaining the voltage output at substantially a fixed value.

Applicant has as another object of his invention the provision o f Van amplifier system yr-:nnoloying tubes whose llarnents are fed from the high voltage power supply source with a protective arrangement for safeguarding the filaments from injury or being burned out by variations in voltage from the power source.

Applicant has asa further object vof his invention Athe provision of a safety device for a voltage control circuit vwhich prevents injury to components of a system fed through the voltage control circuit upon failure of any of `the elements of such circuit to properly function.

Applicant has as a still further object of his invention the provision of a negative feedback amplifier circuit employed in a `system for the continuous measurement of minute currents with an A. C. operated instrument Vhaving negligible zero drift.

Applicant has as a still further object of his invention the provision of a direct current degenerative feedback voltage amplifier for feeding a sensitive mlliarnmeter and for measuring weak currents with safeguards in the form of voltage regulating and overload control circuits to prevent injury to the elements of the system resulting from failure of components of the voltage regulating system to 4properly function.

Other objects and advantages of rny invention `will sssssffrsm the iallewins spesiseticn `and s claims. (ci. ira-1&1.)

scsompsnying drswinesyand the novel features thereof 'vvllbe particularly pointed put in the anln'e'iged claims."

" In the "drawings,

Figli i'sascheinatic of a direct current negativ-@feedback amuser:

Figure `2'is ascneniatic of pre-amplifier porion of the circuit Vof current measuring sys- Figure 3 4is a schematic of another portion of the 'circuit of my current measuring system incorporating the"iinproved voltage regulating and `cverloacl protective circuits..

The prsent enbrats'whch represents the Drsierrd embed-intent Of. tisi'riverito, 'was esracially designed' t@ detect' 'and measure the extremely smallcurrentswhich flowto the collecting platein'a mass Aspect inciter.4 These currents are 'of' the or er `o`f5 1l0f1% to 5 v10y`19 amperes. Accordingly, these currents could not operate a normal 'voltmeteij or' other "measuring device dire'ctlybecauseV the amperagewould be far too low.

Referring particularly to Eig. 1 wherein Af des.. ignates schematicallya direct current amplifier, such amplifier is fed' from" the "collector plate I3 of a mass spectrometer ,and associated with in- Put sind olifeutrsssters Rr and Rc. rspctively. The output is fed back in serieswith the input in such a manner as to provide degeneration.

In this arrangement the voltage change is represented as follows:`

`In the above equation Vc is the voltage change; Ys is the input voltage or potential at the collector `of the spectrometer; and-G is the normal gain of the amplifier A without considering the feedback connection. Any voltage at the input is thus matched by the output to an accuracy of f G equals 1000, then the output matches the input by one part lin 1001. By making G very large, the output yoltage may be made negligibly different from the input voltage. `It will beseen further that if the gain G` is large between the input and output, a voltage gain of substantially unity may be obtained.

.The current gain, however, is only unity when 151:30, i. e., when the resistance or impedance of the input .circuit equals the resistance or imredancs of the output Cirsuit. If R1 is made many times the value 0f Ro. a lars?. currar-.1t amplifie@- from the cathode. of 2060 ohms these grids function in such a way tion may be obtained. The current gain is equal to RI/Ro. It can be seen, therefore, that the calibration of this system depends upon the input and output impedances R1 and R0, and that tube characteristic changes only slightly affect the accuracy in calibration. The amplifier, being `a direct current amplifier, will permit a definite calibration and equilibrium condition which will resist any changes by a factor of 'l-l-G. This facilitates holding zero points in measuring direct currents. Y

A measure of the input current or voltage may be obtained by measuring the output voltage or current in connection with the output impedance Ro and its relation to the input impedance R1, as will be seen by referring now to Fig. 2.

In Fig. 2 the collector plate of a conventional mass spectrometer is generally indicated at I3 rhis feeds through line SI into the suppressor grid I5 of the first stage I5 of a preamplifier. Line 5I is alsoconnected at 32 through input resistor 3i, line I4, direct current source 49, andY output resistor 3o to ground. The small shunt condenser 52 is employed for the passage of 'transient currents, since this low impedance to high frequencies gives a good transient response.

'The output circuit of tube I6 is directly coupled to the control grid 2l of the amplier 22 at 20. The output circuit ofthe second'stage 22 of the preamplifier in turn feeds directly into the control 'grid circuit of amplifier 25 at 24,'and the output of amplifier is directly coupled at point 2l to the control grid 28 of tube 29, which tube functions as a Vcathode follower to couple the direct current amplifier system, just, described, back to "the input circuit through the outputresistor 3l) and the cathode lcircuit of tube 29 in order to profduce a negative feedback. The cathode follower vis used in this arrangement to provide a low impedance current source. A miliammeter I2 and variable resistor 160 provide a circuit bridging the output resistor 3S and Ef, M. VE. source d@ to measure the current flow.

In an instrument such as a mass spectrometer, the minimum collector current which can be detected and/or measured with any degree of accuracy will depend primarily upon the residual rent, generally known as grid current, is the most ltroublesome and it is for this reason that the tube I6, preferably type 954, is connected as it is.

The voltages on the lements of the tube IB are Vvery low, that is, considerably lower than the ionization potential of any gas which may be present in the tube. Grids 53 and 6a, which are normally the control and screen grids respectively, draw the greater part of the total current Due to the series resistor 65 as to stabilize electron emission which arrives at the plate from the cathode. For example, if emission tends to increase due to a change in cathode temperature, the grids, since they are drawing current, will go less positive due to the additional voltage drop across the 2000 ohm resistor. Since their potential influences the cur--l rent flow to the plate, a balance or regulating condition is attained. 1

The third grid I5, which is normally the sup-l pressor grid, is very little affected by the space charge electrons surrounding the cathode. However, some small quantity of electrons will strike the grid and some will leave. The tube is operated in such a Way that the sum of these positive and negative grid currents is zero or nearly so. Inherent grid current uctuations in this circuit are of the order of 5x1015 amperes.

Considering the operation of the apparatus more in detail, it is necessary first to regulate the apparatus so that the milliammeter I2 will readV zero when no current is flowing from the collector plate I3. Assuming that the milliammeter reads upscale when the platev I3 is disconnected from the system, there must be a voltage drop between the point I I and the ground. Due to the connecting line Ill, this places a certain charge on the grid I5 of the rst stage tube I5. Whatever the nature of this charge onthe grid I5, some current will flow across the tube I5 between lament and plate and consequently will ow through the resistance Il.

As will become clearer hereinafter, it is necessary to decrease this current flow in order to bring the milliammeter I 2 to a zero reading. The method selected for decreasing this flow of current was to increase the resistance in the cathode circuit of tube I5 by adjustment of the variable resistances shown at I3, I8. rThis increase will result in a larger IR drop across the resistances I8, I8. As a result, the cathode of tube I6 be'- comes more positive. It follows that less current flows in the anode-cathode circuit of tube IS so that current flowing across the resistance Il to the point 2e is decreased. There are other ways in which this current could be decreased. However, it was not found to be feasible to control the value of the resistance Il, since in the arrangement employed the resistance was located so far from the control panel that high impedance at point d@ would cause noise pick up. This is especially true where the pre-amplier is sepatakes place because the voltage drop across the resistance I1 is decreased, while the high side 4D is held at a fixed voltage by the line 4I, since the point 42 is maintained at a constant regulated voltage.` amplifying tube 22 becomes more positive. This increases the flow of current across the tube 22 and therefore increases the flow of current through the resistance 23. Since the high side of this resistance is also tied to the line 4I and is therefore maintained at a fixed potential, the point 24 becomes less positive or more negative. As a result, the grid 25 of the tube 25, biased by E. M. F. source 66, becomes more negative, thus decreasing the current flowing across the tube 25. 'This makes the point 2l more positive for reasons 'similar to those given in explanation of the increase in positive charge at point 20. As a result, :an increase in positive potential appears on the grid 28 of the tube 29. It may be remarked that the tubes 22 and 26 greatly amplify the value of the voltage introduced at'their grids 2i and 25 respectively, and tube I6 has some amplifying effect. At any rate, the net result of the operation of these tubes is to produce a voltage change As a result, the grid 2l of theV `drop across the resistor 3| autres at the grid 28 which is approicimately` 20,000 times the voltage change at the grid I5. Thus by decreasing the now of Vcurrent across the tube IE through adjustment of resistances I8, IB a relatively large increase may be effected in the value of the voltage at the 'grid 28. The increase in voltage value at the grid 28 immediately causes 'an increase in the flow of current across the tube 29. This takes place because the plate of the tube 23 is tied to the fixed voltage line 4I. The flow oi' current across the tube 23 opposes the ow of current in the circuit including resistor 3). By `properly adjusting the variable resistances I8, I8 the flow of current may be reduced to zero or at least balanced to such an extent that the m'illiammeter I2 will not indicate any reading. At this 'adjustment the apparatus is ready to be operated to indicate very small current iiow from the spectrometer collector plate I3.

Assuming that the spectrometer is in` operation Vso that the aforementioned small current ows 'from collector plate I3, this current will flow along the line I4 through the input resistance 3i and from lthe plate I3 which is positively charged by the positive ions collected upon it. Ordinarily ,d

this now of current is very small so that despite the large size of the resistor 3i, the potential drop across it is also relatively small, the range being from 1 millivolt to 20 volts. This means that the current nowing is of the order of -1.`25 1014 to 2.5 10*1 amperes. However, the voltage from the point 32 to ground, is from substantially zero (ground voltage) with no ion current fiowing, to --.001 maximum voltage. This occurs because of the operation of the negative feedback system. As pointed out hereinbefore the voltage value at the grid I5 and therefore at the point 32 with respect to ground is multiplied 4by about 20,000 times at the grid 28. This means that if the voltage value at the point 32 attempts 5to rise, this Voltage value is multiplied by 20,000 times at the grid 22. The construction of the tube 22 is such that the voltage value `at its grid is reflected in its cathode. Thus a voltage rise fat the point 32 would establish an amplified voltage drop across the resistor 3B to the ground. This means that there is also an amplified voltage drop from the point I I to the ground. Since the `.point I l is directly tied by the line I4 to resistor 3l, it follows that any attempt at an increased voltage at the point 32 is amplified and immediately fed back to the other side of the resistor through line I4. The feedback is opposite in phase, that is, a change in positive charge at "32 or I5 is fed back through line I4 as anegative charge and 20,000 times larger in value. The current flowing across the resistor 3l is collected by the mass spectrometer plate I3. Msc, the voltage value at the line I4 must be A20,000 times greater than the value at the 4point 32. This means, for present purposes, that the point 32 possesses practically zero lor ground voltage since the voltage drop across resistor 3l does not exceed volts with maximum signal current. Naturally if the value at the point 32 were actu- -ally zero, the amplifying system would not work..

It is approximately goml of the signal voltage developed across resistor 3I.

Because of the aforementioned Acharacteristic of the apparatus to maintain the point 32 substantially at ground voltage, changes in voltage are reflected as changes in the Voltage value at the line i4. IThis vmeans that the voltage drop at the point 32 from `line I4 is substantially the same as the voltage drop from theV point II to the ground. Such being the case, while there is any current flowing in the line I4, there must be a voltage drop from the point II to the ground. Since the miliiammeter I2 is connected in this circuit, it will indicate the voltage drop from the point ii to the ground and therefore will vary linearly with voltage drop across the resistor 3 I. Voltage drop across the resistor 3I is caused by the current flowing in the line I4 from spectrometer collectorplate I3. Accordingly the readings on the milliammeter I2 vary linearly with the current flowing from the spectrometer plate i3 and serve as a measure of such current.

As indicated hereinbefore it is necessary to provide a fixed voltage at the line fil from power source 43, referred to in more detail hereinafter, in order to provide for proper operation of the apparatus. This voltage regulation is obtained by operation of the tubes 43, 43, and d4. Regulation is effected at the point '42 and the voltage delivered has been Xed at volts. If this voltage tries to increase, more current will ow and the grid 135 of the tube I4 biased by E. M. F. source 6'! will become more positively or less negatively charged. This means that more current will flow across tube fie between cathode and plate. As a result, the grids 40, 43 of the tubes 43, 43 will become more negatively Vcharged and cut down the current flowing across the tubes 133, e3. It will be noted that the current reaching point 42 flows across these tubes since it flows from point i3 which is B+, along the line lil, and through the tubes 43, 5.3 to point 42. Accordingly the tube 44 operates as a sensitive ampliiier so that any slight change in potential at the point 42 is instantaneously reflected in an inverse manner to increase or decrease the flow of current through the tubes 43, e3 and maintain the current flow constant at the point 42, and therefore to provide constant voltage at that point.

The power supply to the line 4i is provided by a conventional rectifying circuit indicated generally at 48 and employing a typical power transformer and a double wave rectier tube in a center tap arrangement. The purpose of this rectifying device is simply to supply the system from an A. C. source and to change the alternating current into direct current supplied at i6 which acts as the high voltage point. As indicated current fiows through tubes i3 to point d2 and from this point through the varioustubes I3, .22, 23, and 23 to ground as at et* and 69, returning through ground 'I0 to the center tap of the secondary of the power transformer of the power supply.

The portion of the circuit, including tubes 53, 55, comprises a safety device for preventing the voltage at point 42 from becoming dangerously excessive for tubes I 6, 22, 2G, lili and E5 during the warmup period or due to breaks in the iiiament circuit.

It will be noted that the tubes d3, i3 which supply the current are connected in parallel and are heated directly from the power transformer winding 50 whereas the other tubes are heated from a rectiiied current provided at the point 42. Accordingly if the tubes 43, 63 are energized first, it is possible that an excessive amount of current would be fed through the tubes to the point c2 before the control tube 44 heated up. This might injure or burn out the filaments of all the tubes except the tubes 43, 43. Furthermore, it will be seen that the filaments of the tubes I'B, 22, 26, 29, 44 and 55 are all connected in series, and if the ilament in one of these tubes burns out or the current to the tubes is otherwise cut off accidentally as by pulling a cable plug, it is possible that a voltage surge to the iilaments of the remaining tubes might take place. ,This could ruin the other tubes by caus- Y ing filament to cathode breakdown.

The tubes are protected from these exigencies by means of a safety device. This safety device, generally designated I, operates as follows: Before the control tube @lil warms up, the neon tube 53 acts to connect point 54 to the grids d5, 5. This applies a predetermined xed voltage to the grids t6, 56 and prevents an excessive ow of current through the tubes d3, 43. As the control tube 4A warms up so does the tube 55, and as the tube 55 gets warmer and warmer the point 55 becomes more and more positive due to the iiow of current across the tube 55 between anode and cathode. As the point 55 becomes more and more positive the potential drop between the points 5l and 58 decreases. Finally this potential decreases to such an extent that the neon tube 53 will cut out, since this tube responds to some specific threshold potential across it for operation. However, this tube will not cut out until the tube @il is warmed up, because the filaments of the tubes 55 and 411 are connected in series. Accordingly no excessive amount of current can flow to the filaments in the amplifying system until the control tube 44 is in operation. Should, through accident or inadvertence, an excessive voltage appear at points lt and 42 and consequently at grids 65, 4S, there willV be a large potential drop between the points 5l and 58 which will place the safety device in operation again. This may save many tubes from being'burned out acci` dentally through improper operation of the ampliier or through accident. In order to maintain a uniform potential on the anode of tube 55 voltage regulator tubes ll, 'il are provided.

Concerning the operation of the resistance El] connected in series With the meter l2, the numbers on Fig. 3 are multiplying factors; that is, the reading on the meter is multiplied by the number indicated to give the actual voltage difference. This is necessary since the currents flowing to the plate IS may vary considerably, that is, by a factor of 1000 or more. Accordingly, the use of the resistances with the meter permits a single instrument to cover the necessary wide range.

Further it will be understood that lilter l2, test jack i3, shunt resistors such as 7M, l5 associated with the heater laments, load resistors and many other items of conventional equipment are employed in a conventional manner in this system, l vand have not, therefore, been described in great detail.

Having thus described my invention, I claim:

1. An amplifying system ci the lcharacter described comprising ampliiier including a series of direct coupled electric discharge devices having their heater elements connected in series, a power source for supplying a, common potential to the elements of the discharge devices oi said amplifier, means for maintaining the potential of the source at the ampliiier at substantially a uniform level, said means including an electric discharge device interposed between thve power source and the amplifier for the passage of current, means responsive to changes in supply potential for controlling the current flow to the amplifier, and means responsive to overload conditions for limiting the passage of current through said last named dis-V charge device.

2. An amplifying system of the character de-l scribed comprising an ampliiier including a of the amplifier, means for maintaining the po-V tential of the amplifier at substantially a uniform magnitude, said means including at least one electric discharge device interposedbetween the power source and ampliiier for the passage of current, an additional electric discharge device responsive to changes in potential of the power source for altering the conductivity of said first electric discharge device, and means for limiting the conductance of the electric discharge device o1" said current passage means during periods oi overload.

3. An amplifying system of the character described comprising an amplifier having a series of electric discharge devices connected in cascade and having their heater elements connected in series, a source of electric power forsupplying a common potential to the elements of the devices of the amplifiers, and means for coupling said power source to said ampliiier, said means including an electric discharge device having its anode-cathode circuit interposed between the source and the amplifier, and an additional electric discharge device responsive to changes in potential delivered by said source to the amplifier for altering the conductivity of said rst named electric discharge device for maintaining the potential at said amplifier substantially constant,

' said additional discharge device having its heater element in series with the heater elements of said series of electric discharge devices, and means for limiting the flow oi current throughsaid first named discharge device during the warm-up period of said series of discharge devices and said additional discharge device.

4. An amplifying system of the character Vdescribed comprising an amplier including a plurality of electric discharge devices terminating in a cathode follower andv having their heater elements connected in series, a degenerative feedback circuit for the amplifier and current responsive means in the output thereof, a source of electric power for providing a common potential for energizing the elements of the devices of the ampliiier, means for coupling the source to the amplifier, said means being re-v sponsive to changes in the potential delivered to the amplifiers for altering the impedance once and maintaining the voltage level substantially constant, and means responsive to overload conditions for limiting the changes in impedance of said coupling means.

5. An amplifying system of the character described comprising an amplifier including a plurality of electric discharge devices terminating in a cathode follower and having their heater elements connected in series, a degenerative feedback circuit for the amplier and current measuring means in the output thereof, a source of electric power for providing a common potential to energize the elements of the discharge devices of the amplifier, and means for coupling the source to the amplifier, said means including an electric discharge device having its anodecathode circuit interposed between the source and the amplifier, means responsive to changes in potential at the amplifier for altering the conductance of said electric discharge device to maintain the potential level substantially constant, and means responsive to a predetermined overload potential for limiting the conductance of said coupling means.

6. An amplifying system of the character described comprising an amplifier including a plurality of electric discharge devices terminating in a cathode follower and having their heater elements connected in series, a degenerative feedback circuit for the amplifier and current measuring means in the output thereof, a source of electric power for providing a common potential to energize the elements of the discharge devices of the amplifier, and means for coupling the source to the amplifier, said means including an electric discharge device having its anodecathode circuit interposed between the source and the amplifier, an additional discharge device responsive to changes in the common potential at the amplifier for altering the conductance of said first electric discharge device to maintain the potential substantially constant, and means bridged across said additional discharge device and responsive to overload potentials thereacross for limiting the conductance of said conducting means.

7. An amplifying system of the character described comprising an amplifier including a plurality of electric discharge devices having their filaments connected in series, a source of electrical energy for supplying a common potential to energize the elements of said devices, means for coupling said power source to said amplifier, said means including an electric discharge device having its anode-cathode circuit interposed 10 tential, and means coupled to said input circuit and responsive to potentials above a predetermined value for limiting the operation of said discharge device and prevent imposition of excessive potentials on said amplifier.

8. An amplifying system of the character described comprising an amplifier including a plurality of electric discharge devices having their filaments connected in series, a source of electrical energy for supplying a common potential to energize the elements of said devices, means for coupling said power source to said amplifier, said means including an electric discharge device having its anode-cathode circuit interf" posed between the source and the amplifier, an

additional electric discharge device responsive to changes in potential at the amplifier for regulating the conductance of said first named electric discharge device to maintain the potential at the amplifier at a substantially constant level, and space discharge means bridged across the output of said second discharge device and responsive to the operation thereof for limiting the operation of the coupling means.

IRVING R. BRENHOLDT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,445,278 Helsing Feb. 13, 1923 1,814,158 Holden July 14, 1931 2,171,614 Wendt Sept. 5, 1939 2,206,123 Rinia et al. July 2, 1940 2,252,057 Blessing Aug. 12, 1941 2,356,296 Zinn Aug. 22, 1944 FOREIGN PATENTS Number Country Date 515,158 Great Britain Nov. 28, 1939

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