US2226995A - Television amplifier - Google Patents

Description

Dem 1., 40' K. SCHLElNGER 2,226,995

TELEVISION AMPLIFIER Filed May 4, 1935 7/21 50? for m l M frequently described in the literature.

Patented Dec. 31, 1940 UNITED STATES PATENT FFICE TELEVISION AMPLIFIER,

Kurt Schlesinger, Berlin, Germany, assignor, by

mesne assignments, to Loewe Radio, Inc., a corporation of New York Application May 4, 1935, Serial No. 19,816 In Germany May 9, 1934 1 Claim.

The invention relates to the construction of an amplifier with condenser resistance coupling which does not possess the disadvantages of the previously known photo-cell amplifiers of this kind for television transmitters, viz., the so-called fog formation. To explain the physical occurrences it is necessary to deal briefly with the formation of the fog.

When employing a normal resistance amplifier behind the photo-cell of a transmitter, which is furnished for example with a Nipkow disc as the scanning means, disturbing shadows of opposite intensity value occur behind White or black portions of the image, as known for a long time and For example, there occurs behind a horizontal, brilliant white portion a dark shadow, which is all the darker and endures all the longer the more brilliant and the greater the duration of the original White picture. If not merely one line, but a plurality of, for example 20, lines are transmitted with uniform white or black color inthe original, the disturbing echo will also always endure a comparatively long time, and since the same in turn also affects a plurality of lines, a wide strip of the received image may under certain circumstances become fogged. This is referred to as fog formation.

In explanation of this appreciable interference, which has been known for a long time, it has repeatedly been set forth that in the case of an image current conspicuously slow frequencies occur which, as well known, the resistance amplifier is unable to transmit, and that the separation of these low frequencies, which are below the image-change frequency, cause the interferences referred to. As a remedy there has been recommended in generaltheuse of the carrier frequency method, in which as a matter of fact stripes of this character no longer occur. The general contention has been that the television transmitter problem is not to be solved with resistance capacity coupled amplifiers, as in the case of these amplifiers the fog formation is not a to be avoided.

The subject matter of the invention is an amplifier which, whilst avoiding the carrier current method permits of the production and reproduc- 1 tion of images completely free of reverberatory effects.

The invention is described more concisely in conjunction with the drawing:

In the drawing:

Fig. 1 is the diagram of an ordinary resistance capacity couple-d connection.

Fig. 2 is an equivalent circuit diagram of the coupling stage according to Fig. 1.

Fig. 3 shows a potential curve to be transinitted, whilst in Fig. 4 shows an exemplary embodiment of the amplifier circuit according to the invention.

Figure 5 shows a modification of the present invention.

In Fig. 1 there is shown a normal tube member 5 5 of a resistance capacity coupled amplifier with the usual battery blocking and the resistance coupling. In the same:

I is the amplifier tube, in practice usually a screen grid tube, 2 the anode resistance 1%,.3 the repeating condenser C of the next stage, 4' the grid leak resistance Rg of the latter, '5 the anode battery decoupling condenser Ca, and 6 the decoupling resistance Rb.

It has found that the interferences referred to 315 in the above (which will be designated in the following reverberatory effect) are not due to the elimination of low frequencies upon the transmission, but are caused by disturbing charg n effects (residual charges) of the condensers in the grid circuit 2, 3, i and in the blocking circuit 2, 5, 6 of the single stages of the amplifier cascade.

It has also been found that the reverberatory effect is due principally to the interfering effect of the last-mentioned circuit.

The manner in which a reverberatory effect fundamentally occurs is explained in conjunction with the substitutional diagram according to Fig. 2.

In this the amplifier tube I is replaced in the -30 known fashion by a generator i and a resistance 8 in series with the generator. There is shown the bridging condenser 3 and the grid leak resistance 4, and there is to be observed the potential at the terminal 9 when the potential curve 35 shown in Fig. 3 reaches the input terminal Hi.

It will be seen that the potential varies from zero only in the positive direction, i. e. the direction corresponding to white image points. Such a curve is obtained in scanning a dark image hav- 4 inga perpendicular white strip by means of successive horizontal lines. It will be assumed that 25 images each consisting of horizontal lines are being transmitted per second and it will thus u dark line occurs after several very bright hori-- zontal lines have been successively transmitted;

or after the transmission of a single very bright or very dark signal. acquired the charge Q mentioned above, cannot immediately assume the potential corresponding to the gray value but lags with a negative residual potential 62- which decreases with time due to discharge of the condenser at a rate in accordance with the time delay constant corresponding to Rg, Cg. The potential e2 is the echo or fog potential and the magnitude of the disturbance can be indicated by the relation Substituting the value e2 and e1 given previously one obtains the following important equation:

According to the invention, the grid and anode circuitsof the individual stages are so construct.- ed and dimensioned that the magnitude of N remains below a definite limiting value (e. g. be-

low 0.1, .05, .01 or the like) so that noticeable echo disturbances no longer appear in the image.

It is evident from Equation 1 that even very short signals of the duration T will cause an echo effect if they are repeated frequently.

It will also be clear that it is necessary to make the resistance 4 (of value R and the grid/cathode resistance of the succeeding valve as large as possible. The eifect of ,both these measures is to reduce as far as possible the current in the grid circuit thereby reducing residual charges on the condenser 3. As it is not usually practicable to increase resistance 4 (of value Rg) beyond 0.5 megohm, the value of condenser 3 (of value Cg) should be increased correspondingly. If n is 'made equal to 50, i. e. about one third of the image and T is the line period and N is equal to .01 (which practicallyamounts to complete disappearance of the fo-gdisturbanoe) thenthe grid time constant Tg=RgXCg=1 second. Suitable values for Hg and C are therefore 0.5 megohm and 2 microfarads or 1 megohm and 1 microfarad or 0.1 megohm and microfarads. It has been found however that in practice it is not necessary to have such a large time constant and that a time constant of 0.1 second is suflicient in most cases. Suitable values for R; and C in this case are l megohm and 0.1 microfarad.

Practical investigations have shown that the operation of the grid circuit 2,3, 4 in the manner described above is not the main cause of the fog formation but that the phenomena is due to a much greater extent to similar operations taking place in the anode circuit 2, 5, 6. In the same way as in the case of the grid circuit one obtains acorresponding equation to Equation 1, viz.

It will be seen from Equations 1 and 2 in the foregoing that in the case of both the grid circuit and the anode circuit, the time constant The condenser, since it has where n is the number of peaks per second, T is the time period of each peak and N is the desired ratio between the echo potential and the fog potential. Applicant has further found that in both cases this value should be made less than or sub-- stantiallyequal to ll 4N where Tb is the duration of a line period.

A selection of suitable dimensions for the elements in the grid and anode circuits should be made in the manner already described in all the stages of the amplifier since if only one stage is treated in this manner the production of the fog formation may still occur in other stages. For avoiding fog disturbance the circuit elements of the gridand anode circuits of the several stages are in practice dimensioned in such manner that the value of N amounts to 0.1 to 0.01. It is advantageous that for the last stages of the arrangement the value of N amounts to nearly 0.1 whilst for the preceding stages there are used practically smaller values of N amounting to 0.02 to 0.01.

All considerations given here in respect of the transmitter should naturally also be employed in television receiver construction in all cases in which resistance capacity coupled connection is used.

It is further possible to avoid the disturbances above disclosed in the anode circuit without using the special dimensioning of the single circuit elements but with using the circuit elements commonly employed.

This may be accomplished, for example, by the provision of means for reducing to zero the alternating potentials which are linked up with the anode block during operations and cause the residual charges resulting in the interference.

For this purpose there may be provided a special push-pull tube, which supplies a potential equal in amplitude to the anode potential but of reversed phase.

An arrangement of this kind is illustrated by way of example in Fig. 4.

,As shown by the drawing, there is provided a phase reversal tube I, which operates on an anode resistance 2' preferably equal to the resistance 2, and the anode circuit potential of which is raised to the same amplitude as the anode circuit potential of the main tube by coupling the grid circuit with a corresponding tapping of the anode resistance 2 of the main tube, so that the same supplies a reflectory-reverse potential of the same amplitude.

The fluctuations in potential of the main tube flowing to the condenser 5 over the resistance 2 are completely compensated by the potential fluctuations of equal amplitude but reverse phase supplied by the phase reversal tube, so that the condenser 5 is wholly relieved.

The elements 4 and 6 are biasing resistances. Further the anode of the phase-reversing tube is connected via condenser 3 to a separate output terminal. This separate terminal may be used as well as-that terminal adjacent to condenser 3 of the main amplifier l as output ter minal. Either the one or the othermay be used as output terminal. This enables to tap either positive or negative amplitudes of the image-contents signals which is most advantageous for television.

Similar arrangements using such an additional push-pull tube may be employed in. each stage of the amplifier arrangement.

The charging fault of the anode block may also be eliminated without the use of special push-pull reversing tubes by making the alternating potentials zero during operations at the anode block by means of a neutralizing branch.

Fig. 5 shows a corresponding circuit diagram. There are shown three tubes of a resistance amplifier. The last thereof, which possesses the most powerful anode potentials, is guarded against fog formation by a glow lamp l I.

By means of a neutralising connection system according to the invention, comprising the resistances 2| or 23 in conjunction with a potentiometer tapping at the working resistance 2 or 2", there is conducted to the anode blocking condenser 22 or 5 of the preceding stage a potential of equal amount and reversed phase such as obtained by the same from the anode circuit of the appertaining amplifier stage. In this manner it is accomplished that the blocking condenser becomes free of alternating potential. The tapping at the anode resistance 2 must correspond with the amplification ratio of the appertaining stage, and the series resistance 2| or 23 must be just as large as the anode resistance of the previous stage. In practice the former is, for example, 0.1 megohm, and the latter approximately 5,000 ohms. It is possible by means of a galvanic neutralisation of this nature to affect the fog formation quite considerably, and with correct adjustment of the condensers even to overcome the same entirely.

Further, according to the invention, the transmission element 3, 4 (according to Equation 1) is, with the insertion of a suitable value of 1; dimensioned for example,

In the arrangement according to the invention care should also be taken to avoid grid current in the following amplifying tubes, and also a quality of insulation of this tube which is large as compared with the external grid resistance fitted.

I claim:

An amplifying device for television picture signals wherein the picture signals constitute voltage variations occurring within a frequency band from at least one megacycle to substantiallyzero comprising a first electron tube having a cathode, a control electrode and an anode, means for impressing the picture signals upon said control electrode, means including a series connected load impedance and decoupling resistance for maintaining said anode positive with respect to said cathode, a condenser connected between a point of fixed potential and the junction of the load impedance and decoupling resistance, a second electron tube having a cathode, a control electrode and an anode, means for coupling the control electrode of said second tube to an intermediate point on the load impedance of said first tube, a load impedance for said second tube connected between the anode of said second tube and the junction of said first-named {load impedance and decoupling resistance whereby amplified picture signals of the same intensity and opposite phase relationship are present at the anode of each tube and whereby the total anodic current present in said decoupling resistance will remain substantially constant, and a utilization circuit coupled between the point of fixed potential and the anode of one only of the tubes whereby the potential at the junction of the load impedances and the decoupling resistance remains sufficient- 1y fixed to permit substantially linear amplification of all frequencies occurring within the frequency band occupied by the picture signals.

KURT SCI-ILESINGER.

Images