US2444854A - Television amplifier - Google Patents

Description

K. SCHLESINGER 2,444,854

TELEVIS IQN AMPLIFIER 2 Sheets-Sheet 1 Wer A TTOQNFY v July 6, 1948.

Filed June 12, 1946 y 5, 1948- K. SCHLESINGER 2,444854 TELEVISION AMFLIFIER Filed June 12, 1946 2 Sheets-Sheet 2 007 07' WITH PLATE INDUCTAIVCE7 EXPANDD .em/AL g ORIGINAL WITH IVO PLATE r SIG/VAL INDUCTANCE g O l 5 o 'f, I '2 3 4 2 MC. INVENTOR KURT SCH ESI NGER ATI'ORN EY Patented July 6, 1948 UNITED STATES ATENT OFFICE TELEVISION AIVHLIFIER Delaware Application June 12, 1946, Serial No. 676,190

Claims. l

This invention is a continuation-in-part of my copending application for Letters Patent of the United States, Serial No. 488,609, filed May 26, 1943, now abandoned, and it relates to television amplifiers. In 'general, it is particularly concerned with that portion of the television amplifier system which is to derive its input energy from a suitable television scanning or camera tube whereby the signal energy resulting from 'scanning in the camera or scanning tube is converted into an amplified train of video signalling energy wherein the amplification level is held at least uniform throughout the wide frequency band of the impressed signal energy.

In television apparatus the video signals which are to be transmitted may be developed from any sort of a signal source, which might be very broadly characterized as being of the so-called "constant-current variety. Signal sources of these general types are disclosed in the 'book entitled "Principles of Television Engineering" by D. G. Fink, published by McGraW-Hill Book Co., Inc., New York, in 1940, where, particularly in the portion of that book between pages 81 and 117, various forms of scanning systems and television video signal sources are set forth. These forms include, among others, photocells (phototubes) operating in combination with scanning discs or the equivalent, image dissector tubes or without electron multipliers, and storage types of scanning tubes, such as those conventionally known in the art as "Iconoscopesf' "Image Iconoscopes," "Orthiconsi' and Image Orthicons," these last two types of storage tubes being used With or without electron multiplication. Still further, the signal source may be of the types commonly known in the art as the monoscopes, the phase-majectors, the monotron, or the like.

In any case, in an arrangement of the type herein to be described, the output from the instrumentality which produces the video signal is loaded by a resistor which is normally of a rel-. atively high value such that there results, in combination with the natural and stray capacitances, an input resistance-condenser combination supplying signals to the signal amplifier where the resistance-capacity combination has a time constant which is greater than that of the duration of the shortest signal which occurs. This means that the higher frequency video signals (that is, those signals which produce the ,sharpest picture delineation or detail). are attenuated. The apparatus and circuit herein to be described is concerned with overcoming the high frequency drop or 'attenuation -o i the input signal which 'would otherwise occur with increasing frequency.

While, as above explained, many and various.

forms of signal sources may be used to supply the input video or picture signals, this applica-v tion and disclosure will refer to one general form of such types of signal sources and, accordingl'y, from purely an illustrative viewpoint, it will be assumed that the television camera or scanning tube or signal source is of the electronic variety and also the "storage" type for the purpos'e of providing some intensiflcation at least .oithe:

output signal resulting from scanning. r

In a television system of such an electronic type, an optical image of any character is cast and focused upon a light sensitive electrode member or surface within the scanning or camera tube. The electrode member used to receive such an optical image is commonly referred to as the "mosaic" element of the tube, although in some instances, it may be termed the image receiving target or plane. This mosaic element usually comprises a conductive area, known generally as the signal plate, which supports an insulatin element or dielectric upon which minute, isolated droplets or particles of light sensitive or photoelectrically active material are coated. Thus, the photosensitive particles each form one element of a condenser with a common dielectric for all condensers and a common-second element, which is the conducting plate. vIn this way, an optical image cast upon'thelight sensitive elements produces charges across the mosaic, With the charges at different areas being proportional to the brilliance of the imping-- ing optical image. The charges are sequentially neutralized and released as video signalling energy to an external circuit by means of a scanning beam or cathode ray beam which is suitably defiected and caused appropriately'to traverse the mosaic element by element and line by line a predetermined number of times per second.

Purely for more specific illustrative purposes, reference herein will be made to one of the forms of storage type canning tubes Which may be used for such purposes, which tube has become known in the art as the "Orthicon, and which'has been described in some detail on pages 111 et seq. of the above mentioned book entitled Principles of Television Engineering. For the purpose of this invention and its description, refering a scanning tube of this variety,xwhich papers above named also discusses generally the pre.--

amplifier of a television system in that portion which begins on page 392 thereof and which continues on page 396 to describe the transfer, of the pre-amplifier output to `a coaxialrcable which is terminated in an extremely low resistancc or impedance to energize the remaining amplifiers of this system.

It, accordingly, becomes one of the objects of the present invention to provide an amplifier circuit for use with television apparatus wherein the signaloutput from an appropriate scanning systenfor camera tube is amplified to be delivered to;'an' Output' circuit with substantially uniform response over a; very wide band of frequencies generat'ed,"which,,in the gpresent apparatus, may be frequencies covering 'a frequency band' of the' orderotfourto six megacycles and higher; for example.

A further object of the invention is that of providing'anamplifier which is particularly usable in'connectionwith a signal source where there is a high impedance signal input with falling cr droopingfrequency response.

`Still 'a' further object of the invention is that of providing a television amplii'ier wherein losses inherent in the signal source shall be compensated by'providing anamplifier response wherein the amplifie'r gain shall rise with increase in frequencyofinput in a characteristic manner.

Othr objects of the invention are those of providing' a television amplifier whose Output can readllybe -made to match the low 'impedance of concentric cable customarily used to connect the outputof the' pre-amplifier to the v control, mixing or-lln'ea-mplifier and, at the same time, the invention hasas an object that of-providing an am-` plifir which will develop a reasonably high signal voltage across the very low characteristic impedance of such a connecting cable.

' Further than' this, the present invention has as another of its' main objects, that 'of providing a pre=-aniplifier system wherein the "hum level shall beso -low as to be of but a. negligible por tio'n' of the total Output signal.

Other objects and advantages of the invention are those of overcoming one or more known' defects-:ofprior artsystems and to improve upon the' prior art systems by providing an amplifier of improved efficiencyrand ease of adjustmentduring operation whilegat 'the same time, reducing the-'number ofcomponentparts and the complexities of the circuit to a minimum.

Otheriobjects and advantages ofthe invention wilrbecome apparent and at once suggest themselves to *those skilled in the art to which the 'invention is *directedwhen 'the' following specification'is considered together with the accompanying drawings; wherein; v

Figd is 'a schematiczillustration of 'one' form- Figa is a series -of curves to-indicate the re- 4 sponse of the system to be described herein for difierent conditions of operation.

If reference is now made to the drawings for a further understanding of this invention, it will be seen, by a consideration of Fig. 1, that the scanning or camera tube il is arranged to supply its output signal into a group of tubes which can readilybedivided into .four separate classifications of which the first tube '5 of the .sequence may be considered as the camera matching input tube. This tube is arranged to receive its input signal from the camera or scanning tube l and to feed-its -output energy to a voltage amplifier tube 25rWhCh,.in turn, energizes an equalizer and hum -compensaton-tube35 which supplies its Output energy to the finalcable matching tube 45. The tube supplies its energy to a suitable load circuit which is represented by the coaxial cable section schematically shown at 55.

For the purpose of coupling the voltage amplifier tube 25 to .the .equalizer and hum cornpensator tube 35,. there vis included a suitable coupling tube' .which derives its input energyv directly from'the.output of the voltage amplifier 25'and delivers 'its output as a cathode coupled element to the.input circuitof the equalizer and hum compensator tube 35.

Referring nowmore particularly to .the circuit herein disclosedJt will be assumed, for reference purposes, that the ,scanning or Camera tube H is in the nature .of the "Orthicon" tube, hereinabove mentioned and-described in the Fink publication, as wellasin the mentioned articles by Iams and Rose; 'To this end; it is usually customaryto have the. signal plateor mosaic electrode '3 of the scanning or. camera tube H connected to ground 2fl" (or at least held at substantially cathode potential of the camera tube), whichmay beprovidedthrough thecoaxial load .cable I" and the.

load resistor I 9.'

For' the formation. and development .of signals from theiforthicon or other form of Camera tube (represented'in 'purely schematic form in Fig. l by the'tube' l i) an optical image of a subject 2! is directed upon the mosaic element ,13 by Way` of a conventionally' represented optical system '22. The *mosaic or signal plat '3 of the camera tube II 'is then scanned by a scanning beam, conventionally shown: at 23, which is subjected to the inuence of'suitable defiecting fields .(not represented), so that trains of ,video signals are supplied'to'the coaxial cable Il and, when so connected; produce a voltage drop across resistor '9. In the design of the apparatus and circuit herein described, the resistor element is is usually chosen between ten and thirty times as large as would be compatible with faithfully high frequency transmission. n This selection is made because ofa resulting improvenent in the signal-to-noise ratio, as comparedto the low impedance unit. However, when this is done, the higher frequency components which are developed across theoutput resistor '9 under such Circumstances, form but a small' fraction of the low frequency components. Accordingly, equalization of the dropping response* 'characteristic at the high. frequencies formsanessentialpart of the function of the preramplifier circuit herein to be described. The outer shell .of the cable` l" is grounded at Zil.

The energy Output resulting xfrom scanning in the'. camera tube. is then supplied by a coupling condensor 2" to ,theogrid or control electrode 29 of the- Camera matching input tube i5. This tube i hasits cathode elementil connected to ground zllthrough the cathode resistors 33 and.

33 and its grid or control electrode 29 is connected at an intermediate point on the cathode resistors (such as that point intermediate the cathode resistors 32 and 33) through the grid leak resistor 34. In this connection, resistor 32 is relatively small as compared to resistor 33. suitable plate voltage for the tube '5, as well as for the other tubes of this series, is provided from a source of energy (not shown) connected at the input terminal 36 and supplying its voltage by Way of conductor 31 to the plate or anode tube '5 through the plate resistor 38.

In some 'cases, substantially perfect insulation may exist between the backing plate is of the nosaic and the remainder of the camera tube II and the rest of the discharge space in it, or, under some circumstances, the signal electrode '3 may have a certain positive bias applied to it without impairing the operation (while the structure of the signal source would be different to some extent, under such Circumstances, a case of this nature would be where the photoelectric tube served as the original signal source). Under these conditions, the capacitive coupling provided by the condenser 2" and the resistor 34 might be replaced by a direct conductive coupling from the upper end of the load resistor '9 to the grid or control electrode 29 of the tube '5 and, in this case, the resistor 34, instead of connecting at its upper end to the grid or control electrode 29, would connect to thelower end of the load resistor '9 and to the junction point of the cathode resistors 32 and 33. Then, between the lower end of the load resistor '9 and ground 20, a condenser element, substantially like the condenser 27, might be included. However, it should be pointed out that even under these Circumstances the condenser suggested between the lower end of the resistor '9 and the ground 29 might be omitted, as well as the resistor suggested to be connected between the lower end of the resistor '9 and the junction of the cathode' resistors 32 and 33, since such condenser and resistor combination serve to smooth. If the load resistor '9 is relatively large (or the order of one hundred thousand ohms or more), the resistor and condenser may be omitted.

The type of camera tube hereinabove suggested is generally so designed as to make desirable a grounded signal plate '3. Acordingly, a conductive coupling between the camera tube and the control grid 29 of the camera matching input tube I 5 is not desirable so that the capacity coupling 2" supplies the Output energy from the camera tube Il to its matching input tube l5. The particular form of triode tube chosen as the tube is should preferably be of the type such that low losses occur. The capacity across the signal input resistor, while of itself rather small, is, nevertheless, large enough to cause a loss in the transmission of the high frequency components relative to the low frequency components in the range of approximately 1 to 30. The invention provides for keeping this capacity as small as possible and for compensating for the efiects of the high frequency losses as much as possible. The cable length "l should be made short and with an extremely small size wire (such, for example, as No. 32) for its inner conductor, which means that the total capacity across the signal plate to ground is primarily due to the signal plate itself. It is desirable to make th coupling condenser 21' reasonably large, so that the time constant is such as to make the design particularly suitable for low frequencies.

In order that suitable plate current may flow through the tube '5, the connection of the grid leak resistor 34 to the point between the cathode resistors 3.2 and 33 furnishes a means whereby automatic grid blas is obtained from the 'cathode of the tube ratherthan from the plate. obtaining grid bias in this manner has an additional advantage in that efficient 'hun" filtering (about twenty times that obtained in the plate circuit of the tube by means of the circuit elements including the :plate resistor 38 and condenser 46, later to be described) is brought about by the shielding eiect of the grid 29 with respect to the cathode element. It was above noted that the capacities across the load 19 for the camera tube H should be as small as possible and, to this end, the camera matching input tube '5 has its Output signal connected to the voltage amplifier tube 25 in such a way that the voltage amplifier tube appears as a cathode load on the camera matching tube '5, rather than on the camera output resistor '9. The connection of the input grid or control electrode il of the tube 25 is by way of the series peaking inductance element 42 and the coupling capacity 43. A connection of this general type, including the series peaking element 62, has been set forth and described more particularly in my copending application, Serial No. 485,981, filed May 7, 1943, and now issued as United States Letters Patent No. 2,384263, granted September 4, 1945, for an invention entitled "Video amplifier." The application above named describes in detail the functionin-g of a self-biased cathode follower and reference may be made thereto for further details of this particular type of tube connection.

A connection of the self-biased type of cathode follower hereinabove described is particularly useful in supplying the Output energy of the camera matching tube I 5 to the input circuit of the voltage amplifier 25 because it serves to reduce both the influence of any ripple in the plate supply Voltage and at the same time any noise due to the ,so-called shot effect."

In the arrangement shown, the plate resistor 33 of the tube l functions together With the capacity 45 to form a plate filter circuit whereby the effect of the power supply voltage appearing in conductor 3? and a possible cycle ripple (assurning the rectified voltages are derived from a 60 cycle power supply) may be reduced to a substantial extent, but n ot enough to be negligible as compared to the signal input level. However, With the connection of the tube '5 as a cathode follower stage, the circuit is such that any hum voltage appearing at the plate of the tube l5, assuming the tube to be a pentode, for instance one `of the 6J7 type, would have to carry through all of the suppressor, screen, and control grids of the tube 15 in order to aiect the cathode al. In this way, the effects of the hum. are reduced a still further num ber of times which correspond to the amplification factor of the tube in triade-connection, with a final result that the hum voltage, which is impressed upon the signal as it is fed from the cath-- ode circuit of tube IE to the control electrode 4! of the voltage amplifier tube 125, may be made to be but a small percentage of the average video signal voltage which is supplied from the camera tube l l to the input of the camera matching tube l5.

It will'be apparent that further filtering may be obtained by feeding the screen electrode of this tube through a resistance-capacity filter connected to the late electrode. In such a case, any

residual -plate "hum7' -wouldbe r able 4 to affect: the cathode nly .after the shielding effect of both ,the screen-grid: and: controlv grid. is overcome; In usual practice, hWeVe1',:`the triade-connection; as shown; is usually sufficient hum" filtering.

The voltage amplifierttube 25,- which'by way of illustration may be a-BAC'? or-l852 type, is the only tube stage which 'provides voltage amplificatio-n,- andthis: tube functions, together with the coupling tube 85, to :supply energy to the equalizingandccmpensating tube, which also may be a16AC7 type; for instance' The general connections of the coupling tube 85* (preferably of the GJ type, for example) have also been described in the above mentioned copyending application entitled Video amplifier and, accordingly, detailed description thereof need not be'made herein, although reference may be had to the companion case. Withthe signalsupplied to the control electrade. or grid n of the voltage a mplifier tube 25 hy wayof the peakinginductance 42 and the coupling capacity 1l3, it will be appreciated that any degeneration shouldbe avcided in the voltage amplifier tube 25. Accordiriglmthe cathode element id-thereof connects preferably directly to ground. suitable bias for 'the voltage amplifier tube 25 is preierablyprovided by the blas source 59, which is conventionally represented by a' battery so in order that hum may he minimized. At this point it may be mentioned that rectification of the A. C. heater voltage for the tube 25 may provide the blas voltage. Such .rectification 'may' be provided by a copper-oxide rectifie', a vacuum tube (such as the 61-16, for'instance), or by any other suitable means. However, at this point it is usually not desirable to use a cathode resistor bias as the degencrative-eifectthereof results in a loss of amplification. The load resistor l for the tube is connected in series* with' a filter resistor' which, in turn, connects to the power supply conductor 37. The capacity 'element 53 forms, with the resistor 53, a time constant circuit-later to be described.

The input capacity and coupling is such that the lowest frequency energy (such as that 'representing the usual 30 cycle frame frequency) is transmitted undistorted, and the Output coupling for the tube noticeably reduces the effect of any 60 cycle power supply line interference. The time constant of the filter network comprising the resistor 53 and condenser 5 1 is such as to give adequate low frequency emphasis in the signals passed through the tube and the voltage dropacross the feeder resistor 53 provides, at the same time, necessary plate voltage for the coupling tube 65. The condensor 53, in this instance, is preferably of the paper variety, in that it is a reasonably large :condensor and this type is usually to -be preferred in the particular use over the electrolytic variety. However, the filtering eiect of .the condenser-resistor combination 54, 53 is generally designed so as to have only moderate effeet and if, for example, the power supply line 3" happened to have a ripple of even 3% at 120 cycles and 200 volts, it will be evident that the ripple is of the order of six Volts and the filter 53, 5 would normally transmit approximately oneeighth of this 'voltage so that more efficient humelimination is provided by the compensation of such ripple effects in the next succeeding stage.

The Output energy of the coupling tube 65 is fed across its cathode resistor BT by Way of the peaking'inductance 68 and the coupling condcnser 69 into the input circuit of the equalizertubea. This equalizer' tube circuit 'is preferably of the variety which provides a gain which increas es with frequency 'and, 4 as: shown, this, is generally'. accomplishedzby the degenerativeamplifier. connections as shown more'particularly byFlg. 2, which cause the system to act generally as a "frequency-expander, which increases the bandwidth to any desired amount, although this occursat the expense of over-all gain.

Reference 'will be'made to Fig. 2 for afurther complete understanding of the invention, but first it Willbe seen, from the showing of Fig. 1, *that the signals sent through the peaking inductance 68 and the coupling condenser 59 are suppliedto the 'control 'electrode l' of theiequalizer and hum compensating tube 35. This tube has itsgridor control electrode ?l suitably biased by the grid resistor ?3. The cathode element-,wi l` is appropriately biased to provide both adequate and critical degeneration by the series connected cathode resistors 'FS and 'l which are shunted torground :ill by the Variable capacity element 17. In the arrangement shown-the grid resistor 13 corresponds approximately to the resistor 34* associated Withthe tube 35 and' the cathode 14 connectsto ground 25 through the cathode resistor "Fi and the series connected resistor '19, and thence through a very low variable resistor- Si, which last named resistor, when functioning in combination with the capacity element 83, serves :in a-manncr later to be explained (particularly in connection with Fig. 3) to provide forhum compensatio-n. The Output energy from tube 35,is fed from the anode through the coupling condensor 8? to the grid ocontrol electrode sfl of the final cable matching tube 45. The plate load for the tube 'is provided by the plate resistor 9' connected in series with the shunt pealdng inductance 83, as well as the resistor element of a filter combination comprising resistor and condensor 96.

Considering now the circuit schematically reprcsented lay-Fig. 2, the Camera tube herein assumed as the Orthicon device l I is schematically represented so that its plate or mosaic element '3, when scanned by a suitable scanning beam, provides a signal across the load resistor #9 across which some distributed capacity value exists, represented in Fig. 2 by the capacity 22, which is the inherent Stray capacity of the scanning or' camera tube which cannot be avoided. The connections by Way ofthe tubes '5, 'z and 65 intermediate the Camera tube Output and the equalizing* tube 35 are indicated by the dotted outline in Fig. 2 separating the coupling condensor 21 and the control electrode 'H of the tube 35. Also, it will be appreciated 'that, for reasons of simplicity of illustration; some of the connected cathode elements are onitted in the arrangement of Fig. 2, so that the particular showing of the capacitive cathode degeneration provided'by the cathode degenerative resistor and the shunt condenser "'I is made more clearly apparent.

The circuit arrangement shown by Fig. 2is-both schematic and extremely generalized in form, but parts of the systen are shown apart from the showing of Fig. 1 to make still clearer certain of the essential features of the invention. t can be appreciated, in this sense, that the source of video signals which is to be fed to the final Output may be-assumed to be generated by a signal generator tube of the so-called constant-current" type, such as the tube schematically illustrated as 'the "Orthicon" H which is terminated by a relatively large-loadresistor I 9 connected to its signal plate;

As was above explained, 'the inevitable capacity to ground, indicated in the dotted outline of ca-' pacity element II, causes a characteristic decay of the higher frequency signals with respect to the lower frequency video signals across this load resistor W. One of the purposes of this invention, as was above suggested, is to provide cathode degeneration in the compensator tube 35 by means of the cathode resistor 'le which is shunted by the Variable capacitor T'. This resistor combination may be so adjusted and the time constants so established and the relationshipbe tween the cathode, resistance, input resistance and plate inductance so established that the voltage drop produced is just equalized and the plate Voltage drop across the plate resistor 9! assumes a substantially constant value for all frequencies.

It thus becomes apparent that the conditions of the cathode circuit are critical, and that the time constant formed by the product of the cathode resistor 79 and its shunt capacity Ti should be equal to the time constant of the camera tube output circuit including the product of the stray capacity 22 and the load resistor '9. Mathematically expressed, this condition is represented as follows:

where n: represents the cathode resistor "9: Ck represents the cathode capacity ll; rm represents the resistor 19; cm represents the input capacity 22; and fco represents the cutoff frequency of the signal which will be at frequency f for curve A in Fig. 4. Next, the value chosen for the cathode resistor 19 should be such that the degeneration factor equals the voltage drop at the input system. This condition may be mathematically expressed as follows:

where r; again represents the cathode resistor 79 of the tube 35; gm represents the transconductance of the tube 35; b represents the band width which is to be considered, for example, to 12 for curve B in Fig. 2; and, again, fco represents the cuto" frequency, which will be at frequency fz in curve B in Fig. 2. Summarized in slightly different terminology, it might be stated that the time constant of the equalizer cathode circuit equals the time constant of the input and the degeneration'factor equals the signal attenuation. When these conditions are fulfilled, the plate voltage from the equalizer tube 35 appears to be equalized to the first order and the system disclosed acts and functions as a band-stretcher, so that any desired band width of signals may be uniformly transmitted, even though slightly and somewhat at the expense of over-all gain, but with the product of gain and band width being constant.

Under such Circumstances, some relatively minor high frequency losses occur, but adequate compensattion is accomplished by the inductance element 93 connected in series with the plate load resistor 9! to serve as a peaking coil. Also, Fig. 2 makes still clearer the showing of the inductive plate impedance 93, which serves not only to expand considerably any falling frequency response of the input signal but to yield a substantially constant gain and negligible phase shift over the entire expanded frequency band up to the peaking frequency. This third condition may be mathematically expressed as follows:

Where Lp represents the plate inductance 93; is"

. represents the transconductance of the tube 35.

It might be remarked that the inductance element 93 is limited only by the condition that resonance with the plate capacity of the tube 35, which is represented schematically by the condenser Cp on Fig. 2, should preferably occur outside the frequency band to be transmitted. In this way, the maximum size of the plate resistor 9' is also limited to some extent. It also may be remarked in connection with the peaking inductance 93, that if conditions occur such that resonance with it and the plate capacity Cp of tube 35 is omitted in the upper portion of the frequenc spectrum to be transmitted, the plate coil or inductance 93 may be used to some extent for aperture correction, in which case a larger plate impedance of the tube 35 becomes possible. It thus becomes apparent, from what is shown by Fig. 2, that there has been provided a source of video signals which normally occupy a wide frequency band of the order of 4 to 6 megacycles, for instance, according to presently accepted standards, although for higher definition scanning and increased field repetition ratio the frequency band naturally becomes wider. The frequency band mentioned, for illustrative purposes only, is for a 525 line picture scanning at 30 frames per second interlaced.

From the foregoing it will be appreciated that the problem which is solved by this invention is broadly that of providing equalization of a signal over a predetermined and selected frequency band to include all frequencies from the lowest up to and including some selected high frequency, such as represented by 1'2 in Fig. 4. Thus, for instance, at this selected frequency an input voltage represented by e which may be assumed to be equal to 1.00 in signal amplitude, as per Fig. 4, will be transmitted uniformly throughout the complete frequency band between zero frequency (D. C.) and the selected upper frequency limit f2. Under these circumstances, if it be assumed that the signal for transmission is obtained from a suitable constant current source, such as a television camera tube of the type disclosed, across an input circuit represented by the resistance '9 and the distributed capacity 22, it will be appreciated that the input Voltage may be represented- =m where 1'1 represents the load resistance '9 of the source; cs represents the source capacity (capacity 22); e is the input voltage; is the input current and p y'w.

It will be seen, for instance, that without compensation, as shown by curve A of Fig. 4, the response drops to the half power point where L at frequency f (for instance). This represents a response drop of 3 db. so that the signal response reaches a point which is frequently knownas the "cut-off frequency which has been represented by :a frequency value f on the curve of Fig. 4. This frequency 27rr c. (2)

Thepresent invention solves the problem of' providing ways and means by Which the band of frequencies between zero frequency and the fre-.

quency fl, above noted, whereat the response was where represents the plate current, ei representsthe input voltage, ek represents the cathode Qutputvoltage, gm represents the transconduct- .anceof 'the tube 35, p is as above, n; is the cathocle resistance '19 and ck is the cathodecapacity 1", forinstance. If nowEquation 4 above andjEquation' 1 above be combined andthe plate inductance of the tube 35 be considered, then. the overall amplification of the system shown by. Fig. 2 may be considered as follows:

where e is the Output voltage, as in Fig. r is.

the plate resistor 9' of tube 3.5, Lp represents the plate inductance 93 of tube 35 and the other quantities areas above noted.

From the foregoing it will be apparent that the firstpart ofEquation 5, above, namely.

,gme 1 +gm `k shows 'no frequency values andamounts to a mere representation of the fact that D. C. `amplification takes place.-

The second part 'of Equation 5, above, namely,

1 p ie t 1 p i e becomes-unity where the time constant of the signal: input equals the time constant of the cathode circuit o'f the tube 35, as stated hereinabove should be the case. t

The last portion of Equation 5, namely,

1 (1 +gmrn isonewhere the numerator of the equation would be equal to unity if there were no inductance in the plate circuit of tube 35. This third partof the equation represents thecapacity attenuation where a capacity element, such as capacitor "l, is placed in shunt with the cathode resistance, such as resistor 19. In this thirdportion of the equation that part of the denominator of the e'quation which is represented by is :known as the degeneration` factor. Accordingly, where a falling ofi`- in the response characteristic such as shown by curve Hof FigA isapparent i f thereisno inductan'ce in-the'plate circuitiofthej tube 35, which falling off reaches the cutr-ofifievel" or a point where the signal .is equal to /2 of the input or down 3 db.- at a point, purely by way of example in Fig. 4, of about 4.25 megacycles, it becomes evident that this falling ofi of the response from the amplifier can be avoided,

through the inclusion of some factor which is of equal value out of opposite sign to the capacity factor of the denominator ofthe third portion of Equation 5. This inductance value is reached where the following condition holds true, namely,

r, 2'rfz Thus. the inductive time constant 9 TF may be regarded as some constant which might 'be assumed as equal to a constant factor K. Accordingly, as hereinabove indicated, the following relationships are established' from the circuit hereinabove explained.

c a= .+g,,.rt=s=%f (7) and L,, 1 7; m In Equations 6 and 7 above, it will be seen that' each defines a cathotle circuit in which both'the resistanceand the bypass-capa-cities values are unique and, accordingly, the vaiueof'the cathode resistance may readily be established from the equation 1 k and bythe same 'analysis the 'cathode capacity is equal to a.. f( As is evident from the curve of Fig. 4, theresponse will now be down 3 db. at the frequency fz where there is no plate inductanceelement, such" as the inductance 93. However, the inclusion of inductance 93 tends torpeakthe response and' preventsthe falling ofi as'the frequency-fa is:

reached. To prevent any overshoot it is 'important that the plate resistance values; :that is; the value of the res-istor'el; should be limited; to a value where rp grfzc where C represents the. plate capacity as indicatedby -Figl 2 ofthe drawings. It will be seen that -this value -is onehalf that ofthe resistance value for conventional amplifiers with shunt peaking. Underthese circumstances, the over-all gain in the: 'systemis then represented ras,

& iT S` andit will be observedthat the productof gain and .bandwidth is againconstant. r

In connection with thisinvention, asshereinabove explained, and possibly at theexpenseof further repetition and additional ,reference to the mathematical analysis, it must be. again, em-

phasized in Summary that the scanning system may be looked uponas a constant-current source where Output is terminated by a load resistor (such as resistor ['95 of such a magnitude that an attenuation of the higher 'signal frequencies, as compared to the lower signal frequencies, occurs across this output resistor. This signal energy output which appears across the load resistor '9 is then supplied to a signal amplifier tube '5, which is provided with a Critically controlled cathode degenerative resistor "9 which is caused to become effective at the lower frequencies only, due to the fact that a by-pass condenser 'l" to pass the high frequencies is placed in shunt with it. Then, in addition, an inductive element 93, to provide high frequency peaking, is included in series with the plate resistor al of the signal amplifier tube 35, so that the desired frequency band'of signals is efiiciently transmitted.

The equalization effects resulting from such a condenser shunted cathode degenerative resistor "9 is such that the time constant of the cathode circuit is made equal to that of the signal input and the degenerative factor is made equal to the signal attenuation factor. With regard to the plate circuit of such an amplifier, it will be appreciated that the time constant of the circuit formed by the plate resistor al. and the peaking inductance 93 is at least equal to, or greater than, the shortest signal period to be transmitted.

To state the foregoing slightly differently, it will be seen that the circuits herein described furnish equalizing effects which compensate for the decaying or falling frequency response Which results from an input circuit whose time constant is longer than that of the higher frequency signal components of the video signal Outp t of the video signal source. To this end, .cathode degeneration is provided, and the time constant of the cathode circuit of the tube 35 is made equal to that of the signal input circuit and the controlled amount of degeneration is made equal to that of the signal decay, that is, the ratio of the signal input at the highest signal or cutoif frequencies and at the critical frequency of the input circuit.

Now, considering the circuit of Fig. 3, there is also represented thereby certain of the elements shown more completely by Fig. 1, although Fig. 3, being schematic in nature, serves to explain more readily the operation and functioning of parts of the circuit of Fig. 1. In the circuit of Fig. 3, like numerals represent like parts of Figs. 1 and' 2.

sented in' schematic form in Fig. 3 as the battery 36, although it must be appreciated that this is purely schematic and any desired source of energy may be utilized. Naturally, *because of the ripple effects above explained, it is to be assumed that the operating Voltages for the various tubes of the system will be derived from rec-tified alternating current. 'To this end, the ripple above mentioned, which occurs to some extent with fluctuations in the power supply, is represented in schematic mannerby the element 98 'in Fig. 3.'

It was above pointed out in the discussion of Fig. *1 that the plate filters for the tubes 25 and 35, which filters are represented respectively'by the . filter networks 53, 54 and 95, 96 having time con- :stants which are conventionally designated, respectively, by T and Tz, cannot conventionally 'be designed in apparatus of the nature herein disclosed to be extremely efficient filtering elements, with the result that some fluctuations tend to become effective at-the grid or control electrode "l of the equalizing tube 'and the same energy fluc- It might be remarked' further that the connection terminal 36 of Fig. 1 is repre- 14- tuation is found to be present also at the screen electrode of the compensating tube 35.

. To provide an output signal which becomes substantially completely free from any hum effects, a 'compensating voltage, which is derived from the plate supply line for the various tubes, is fed through an appropriate network comprising the capacity element 83 and the resistor 8', so that a voltage equal, but of opposite sign :to the hum .Voltage at the grid or control electrode 'H, is injected into the cathode circuit of the equalizing tube 35 by way of the connection of the cathode resistor 19 :to the junction point of connection of the capacity 83 and the resistor 81. The resistor 8' is usually of extremely low value with a maximum' resistance of the order usually of about 20 ohm's, which is Variable, as indicated, and usually 'adjusted at approximately its centerm-ost value. The result is that the combination of the condensor 83 and 'the resistor 8! may 'be made such that the hum frequency of cycles, which would normally tend to be introduced from the usual A. C. power supply Which is rectified and flltered, is substantially completely compen'sated. In addition, the hum compensation provided by the network formed in the capacity 83 and the resistor Bi and designated as having a time constant Ts is also effective against any 120 cycle modulation which is caused by magnetic fields of heater elements of the tubes of the system where the heater elements are heated by the usual 60 cycle power supply from the A. C. power supply mains.

continuing now with a consideration of the remaining portions of the circuit of Fig. 1, it was above explained that the output of the equalizing tube 3 5 is fed by way of eondenser ar to energize the cable matching tube 45 (for instance, a tube of the GAG'! type to act as a class "A" amplifier) by application to its grid or control electrode 89. The input coupling, provided by condenser al has a relatively small time constant because the input signal to the cable matching tube 45 has, as above explained, 'already received a low frequency emphasis. A cable matching stage, provided by tube 45, has its screen current adjusted by way of the resistor '01 and is preferably operated in a linear manner over a relatively wide range of plate current. In order to avoid any feedback, the anode or plate electrode loz is grounded through the trap circuit comprising the plate resistor '03 and the capacity ill Connecting to ground 20. The screen electrode 35 of the tube also is grounded by way of the connection of the by-pass condenser In" to the tube cathode '08, so that the grid-to-screen capacitance is reduced by the degenera tion fact-or of the tube. The result is that the total grid capacity of the tube 45 is extremely small and, therefore, can properly be' placed in shunt with the plate resistor 9' of the equalizer tube `45 without impairing uniform transmission of the desired wide frequency band which, as above stated, may be at least in the range of four to six megacycles. The output energy from' the cable matching tube 55 is fed by way of its cathode into the concentric output cable represented at 55 which, in accordance with known practice, has its outer shell connected to ground and which is also terminated in the socalled characteristic impedance" '09 which is generally within the range of 75 to 120 ohms. 'The resultant output signal, as it appears at the output terminals I Hl, is then fed to a suitable line or" control or mixing amplifier, not shown.

Whilerpurely for purposes of illustration various pe of tube components used.-

the ahovedescribed circuithave;been,illustrated` it will beunr derstoodthat these are. shown purelyby Way. of example.,

Having now described the: invention. what, is claimed :is

- 1; Ina television circuit, a. constant current source of video signalling energy; covering arel: atively'wide predetermined frequency band and. across Which source 'an inherent capacity exists, a; load, -resistor connected to the. said ,source and producing an attenuation: of the higher range; of the signalling: frequency. energy as compared to the; lower frequency range; ofthe said energy, a video; signal amplifier tube havingat least a catho de,- an anode and a control; electrode, means: to. impress the: signal Output, from the. said load' factor introduced; thereby is: equal to the voltage:

drop; at. the input due to: the load resistorrof the signal source at the cutofi frequency.

2. In a television:v circuit, a constant current source; of video signallingl energy: covering; a: relatively wide predeterm-ined; frequency band and across.` which source an inherent: capacity exists, a load; resistor connected to the, said-source and producing an attenuation ofthe-higherrange o the signalling frequency` energy as; compared; to. the lower; -frequencynta-nge of" the saideenergya video signal amplii'ler tube havin at least a cathode, an anode and: a: control electrodegmeanstoimpress the signaloutput from' the-said load: resistor upon, the control electrode` of said tube, a degenerative resistor element connected' to: the. cathode. of" said tube to provide; controlledrdegeneration at. the lower-frequencies. of the; said video* signal energy. range, a by-pass condenser connected in parallel 'with the said cathode: degenerative resistorto pass readily the:higher; fre quencies of the said -videc signal frequencyrange, the time constant: Ofr" the cathode, resistor' and cathode capacity.'beingmadezequalgto-that of the inputresistor and* the-input capacityof: the-sig;- nal source, andt-he value ofrthecathode resiston being such that the degeneration' factor; introducedtherebyis equal tmthevoltage dropatthe' input due to the load resistor: of the signal source atfthe cutofi'frequency-,'. a peaking: element and-a load resistor element serially: connected 'ini the: anod'e l circuit: of'said' tube to provide-peaking in 'the higher. frequency. range,` 'and *the ratio of: thesplatiednductance totheiplate resistance being? equal'to` the ratioofthetime constant'of the:` cathode resistor` and cathode capacity' combination to 'the-'quantity one plus the productzof the tube transconductance timesthe tube cathode reslstor;

3. A televisionamplifiercomprising a' substantially constant currentasaurce ofvideo signalling; energy for developing 'videosignal energy' Output: having a frequency range covering*afrequency band between `extremely low and; extremelyhigh frequencies and acrosswhich sourcean inherent capacity exists-,,a. load. resistor-` connectedrto .re-

said* signals, resistive means connected inthe,

cathode circuit'of said-tubez-to provide controlledcathode degeneraticn at. the: lower frequency range of the said signalslonly; a. condenser element connected. inparallel with saidcathode resistor for by-passing the higher frequency vrange of signals to: b anplified, the time, constant. of. the said resistor and capacitor of: the cathode. circuit being: madesubstantially equal tothat of the signal inputz'circuit and :the: degeneration of the low frequency signall'ing energy, being made substantially equal to the: signal' decay at. thehigher frequency range, a resistor and inductance element: serially connectedin the output circuit ofsaid'tube and togetherhaving atime constant substantially' of' the same, order as that of the shortest signal period to. be: transmitted andbeing resonant outside any frequency. band to be 'passed, and a load circuit' connected:

to receive the output energy from saictatube.

4. A television amplifiercomprising; alsubstantially. constant current. source' of vi'dleossignallingy energy for developing video signal 'energy n Output' r havinga frequency range. coyeringa, frequency band between extreme'ly'low and 'extremely high frequencies, a load. resistorv connected' to receive. the; Output ener from: said' video signallingztenergy source, saidmesistiveoutputbeingot such magnitud'e that it, together withthe inherent ca,- pacity of 'the signal source; produces. attenuation of the higher signal. frequencies as compared al to the. lower signals'frequenciesga. therrnionicsignal: amplifier tube for. i amplifyin said; signals;

resistivetmeans connected: in .thecathode' circuit of said` tube` to provide: criticali controlled cathode. degeneration.` at' the: lower frequency range of the said signals on1y;. a; ccndenser elementv signals to be amplified; thetime: constantof. the

said* resistor and capacitor:off thexcathocle circuitbeing made-substantially equal to that of; the signal .input circuit and the, degeneration. of the low frequencysignalling energy being made. substantially equal to the signal decayat the higher frequency.- range; a. resistor and inductance elementserially connected, in the. output circuit' ofsaid' tube and together* having a; time. constant substantially ofrthe same. order as that` of: the shoztestsignal; period-: be; transmitted; a; pling t e to receiv the, Output energy f rom said Output load 'impedance, and azload 'circuit connected.- as a cathode follower to said coupling, tube; t

5. In. a television circuit; a constant; current source of video signalling energy covering a predetermined frequency hand g; a: load: resistor connected: to: receivesthe: outputfrom said source-and tending v to` cause. an; attenuation of' ai higher range of; sigr'alling` v frequencies as: compared: to the lower frequencyrange of the said: vsignalling' frequencies, said constant current source pro:

vi'ciing; an .inherent ande unavoidable, distributed capacity; a video; signal amplifiey tube having'a' predeterminedc transconductancej characteristic' and; having; ati least; a cathode a :control` electrod'e, andan anode; 'meansato impressrthe signal 17 Output developed across the said load resistor upon the control electrode of the said tube for amplification, a degenerative resistor element connected to the cathode of the tube, a by-pass condenser connected in parallel with the said cathode resistor to pass readily the higher frequency components of the said video signal frequency range impressed upon the control electrode of the said tube, a series combination comprising an inductive peaking element and a load resistor serially connected in the anode circuit of the said tube, the said capacitors, resistors, tube, and inductive element bearing a relationship one to another such that l na rich-410" and (fc and z +gm `h

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