436,734

PATENT SPECIFICATION

Application Date: April 17, 1934. No. 11602/34.

Complete Specification Left: April 15, 1935.

Complete Specification Accepted: Oct. 17, 1935.

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PROVISIONAL SPECIFICATION

Improvements in and relating to Electrical Amplifiers, such as may be used for example in Television

We, ALAN DOWER BLUMLEIN, a British Subject, of 7, Courtfield Gardens, Ealing, London, W.13, and CECIL OSWALD BROWNE, a British Subject, of 29, Monkís Drive, West Action, London, W.3, do hereby declare that nature of this invention to be as follows:-

The present invention relates to electrical amplifiers such as may be used for example in television or for other purposes where the oscillations to be amplified may contain components of very low frequency including substantially zero frequency.

In television transmission systems the elemental areas of the image to be transmitted are usually caused, by a suitable scanning device, successively to influence a photo-electric cell, thereby giving rise in the cell to picture currents representative of the brightness of these elemental areas. It is known that in practice for these currents to be truly representative of the brightness they must contain components of very low frequency extending substantially to zero frequency. The zero frequency of D.C. component is the component which takes count of the absolute brightness of the elemental areas as distinct from differences in brightness.

The picture currents are generated in the photo-electric cell are usually caused to flow through an impedance, such as a pure resistance, and the potential differences set up across the impedance are amplified. It is of course necessary that the amplification of the system as a whole should be substantially the same at all frequencies within the picture signal range.

The difficulty however arises in providing an amplifier capable of amplifying equally all frequencies within the range down to substantially zero frequency. This is mainly because the currents generated in the picture cell are of very small magnitude and very high magnification of the picture signals is therefore necessary. On the other hand slight and almost inevitable changes in the potentials of the batteries, or other voltage sources, associated with the amplifier, particularly the early stages thereof, may have amplitudes comparable with or greater than the amplitudes of picture signals of similar low frequencies. When greatly amplified in an amplifier capable of amplifying very low frequencies these potential variations of the batteries often produce such bias in the later stages of the amplifier that these stages are rendered inoperative or at least are caused to operate very unsatisfactorily, and in any case the ratio of wanted signal to "noise" or unwanted signal is so low at the low frequencies as to be impracticable.

As an example a practical case may be considered. A photo-cell and an associated thermionic valve input circuit may have a capacity to earth of say 50 micro-microfarads. The highest frequency to be transmitted from the photo-cell may be 500 kilocycles per second and the photo-cell may be fed with polarising potential through a resistance of 5000 ohms. With this value of resistance the capacity of the photo-cell and associated circuits will cause the response at 500 kilocycles per second to be 2 decibels below that at the lower frequencies. If the resistance is made any larger than 5000 ohms, therefore, the response at 500 kilocycles per second will be more than 2 decibels below that at low frequencies and this is usually not permissible.

If now the brightest picture spot produces a current of 0.2 microamperes in the photo-cell, the peak signal representing this brightness will be 1 milli-volt on the grid of the first valve. If the amplifier is capable of amplifying frequencies extending substantially to zero, any change in battery voltage, however slow, equivalent to 1 milli-volt in the grid circuit of the first valve will produce a "noise" signal equal in amplitude to the maximum picture signal. Clearly the use of a D.C. amplifier under these circumstances is practically impossible.

It is an object of the present invention to provide means whereby the above difficulty can be removed or reduced.

According to the present invention, in apparatus comprising a source of oscillations to be amplified capable of delivering oscillations having frequencies extending substantially to zero frequency and an amplifier capable of amplifying these oscillations, there are provided means whereby the amplitudes of oscillations of the lower frequencies applied from the source to the input of the amplifier are increased relatively to the oscillations of higher frequencies and means whereby the distortion thus introduced can be subsequently corrected.

The amplitudes of the lower frequencies may be increased for example thirty or more times at the expense of the higher frequencies fed to the input of the amplifier by suitably mis-matching the impedances of the source and the input circuit. The amplitude level of the lower frequencies in the input circuit may in this way be made less than that due to potential variations of batteries or the like associated with the early stages of the amplifier and the amplification provided by the amplifier may be made correspondingly smaller for the lower frequencies than for the higher frequencies.

In one arrangement according to this invention a photo-electric cell has its anode connected through a resistance to the positive terminal of a source of potential, the negative terminal of this source being earthed and connected to the cathode of the cell. The control grid of any amplifier valve is conductivity connected to the anode of the cell and the cathode of the valve is connected to earth through a suitable source of grid bias. The value of the resistance is made so high that the voltages develop across it, for any given change in illumination of the cell, at low frequencies are much greater than at high frequencies. The high resistance enables the lower frequencies to be passed on efficiently but the stray capacities associated with the photo-cell cause several attenuation of the higher frequencies.

For example taking the values given above, the resistance may be made 5 megohms (instead of 5000 ohms) and the voltage on the grid of the first valve for maximum brightness at low frequencies will be 1 volt. The self-capacity of the input circuit will however now reduce the response for all frequencies above 500 cycles per second (where it is 2 decibels below the D.C. level) and the loss of response at 500 kilocycles will be about 58 decibels compared with the response for D.C.

The amplified associated with the photo-cell is D.C. coupled and, for this purpose the valve above mentioned may be conductively coupled to a second stage of amplification and so on.

Suitable means are provided in the amplifier whereby its amplification at high frequencies is made much greater than itís amplification at low frequencies, in the particular case above mentioned the amplifier would be arranged to provide at high frequencies an amplification which was about one thousand times that at low frequencies. This correction in the amplifier is preferably provided in stages so that a part of the necessary correction is effected in each of a number of stages of the amplifier. The correction may be for example be made by providing, in parallel with the anode-cathode paths of the valves, parallel resonant circuits adjusted to resonate at a frequency above the highest picture signal frequency to be amplified. The correction may however be effected in any other known or suitable manner. Care must of course be taken that any phase distortion introduced by the correcting means is not such as to produce appreciably different time delays in oscillations of the different frequencies.

It is necessary to arrange that the leakance between the electrodes of the photo-electric cell and between the input electrodes of the first amplifying valve are negligible compared with the conductance of the 5 megohm feed resistance or else the leakance may be included as an effective part of the 5 megohm resistance. The leakage between electrodes may be reduced in any known or suitable manner, for example by using guard rings.

Provided that the insulation of the components is sufficiently good, a feed resistance having a value higher than 5 megohms may be used and the amplitude ratio "signal" to "noise" at low frequencies will then be still further increased.

It must be understood that the self-capacity and resistance values given above are only given by way of example. The invention may also be applied to sources other than photo-electric cells. Similarly the invention is not limited to television but is applicable together systems involving the amplification of a band of frequencies including substantially zero frequency.

The invention may also be applied to cases where the higher frequencies are much more attenuated than the low frequencies owing to inductive reactance.

Dated this 17th day of April, 1934.

REDDIE & GROSE,

Agents for the Applicants,

6, Breamís Buildings, London, E.C.4.

COMPLETE SPECIFICATION

Improvements in and relating to Electrical Amplifiers, such as may be used for example in Television

We, ALAN DOWER BLUMLEIN, a British Subject, of 7, Courtfield Gardens, Ealing, London, W.13, and CECIL OSWALD BROWNE, a British Subject, of 29 Monkís Drive, West Acton, London, W.3, do hereby declare the nature of this invention, and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:-

The present invention relates to electrical amplifiers such as may be used for example in television or for other purposes where the oscillations to be amplified may contain components of very low frequency including substantially zero frequency.

In television transmission systems the elemental areas of the image to be transmitted are usually caused, by a suitable scanning device, successively to influence a photo-electric cell, thereby giving rise in the cell to picture currents representative of the brightness of these elemental areas. It is known that in practice for these currents to be truly representative of the brightness they must contain components of very low frequency extending substantially to zero frequency. The zero frequency or D.C. component is the component which takes count of the absolute brightness of the elemental areas as distinct from differences in brightness.

The picture currents generated in the photo-electric cell are usually caused to flow through an impedance, such as a pure resistance, and the potential differences set up across the impedance are amplified in a suitable amplifier. In the past, the aim has usually been to make the amplification provided by this amplifier substantially the same at all frequencies within the picture signal range.

The difficulty however arises in providing a satisfactory amplifier capable of amplifying equally all frequencies within the range down to substantially zero frequency. This is mainly because the currents generated in the picture cell are of very small magnitude and very high magnification of the picture signals is therefore necessary. On the other hand slight and almost inevitable changes in the potentials of the batteries, or other voltage sources, associated with the amplifier, particularly the early stages thereof, may have amplitudes comparable with or greater than the amplitudes of picture signals of similar low frequencies. When greatly amplified in an amplifier capable of amplifying very low frequencies these potential variations of the batteries often produce such bias in the late stages of the amplifier that these stages are rendered inoperative or at least are caused to operate very unsatisfactorily, and in any case the ratio of wanted signal to "noise" (or unwanted signal) is so low at the low frequencies as to make such working impracticable.

As an example a practical case may be considered. The stray capacity to earth of a photo-cell and an associated thermionic valve input circuit may be, say, 50 micro-microfarads. The highest frequency to be transmitted from the photo-cell may be 500 kilocycles per second and the photo-cell may be fed with polarising potential through a resistance of 5000 ohms, the arrangement being such that the stray capacity referred to is in shunt across this resistance. With this value of resistance, that is, a value of the same order as the impedance of the stray capacity of 500 kilocycles, the effect of the stray capacity of the photo-cell and associated circuits is that the response at 500 kilocycles per second will be more than 2 decibels below that at the low frequencies and this is usually not permissible.

If now the brightest picture spot produces a current of 0.2 microampere in the photo-cell, the peak signal representing this brightness will be 1 milli-volt on the grid of the associated valve. If this valve forms part of an amplifier which is capable of amplifying frequencies extending substantially to zero, any change in battery voltage, however slow, equivalent to 1 milli-volt in the grid circuit of the first valve will produce a "noise" signal equal in amplitude to the maximum picture signal. Clearly the use of a D.C. amplifier in these circumstances is practically impossible.

It is an object of the present invention to provide means whereby the difficulty discussed above can be removed or reduced.

The invention consists in providing apparatus comprising a source of oscillations including a light-sensitive device such as a photo-electric cell, and an amplifier for amplifying oscillations delivered by said device, said amplifier being capable of amplifying oscillations within a range of frequency extending substantially to zero frequency, in which said device is coupled to said amplifier by means of a circuit such that, at the input of the amplifier the amplitude of an oscillation corresponding to a given change in intensity of the light falling on said device which takes place at a frequency near to the lower end of said range is much greater than the amplitude of an oscillation corresponding to an equal change in light intensity taking place at a frequency near to the upper end of said range and in which said amplifier is constructed and arranged to amplify oscillations at frequencies near to the upper end of said range to a greater extent than oscillations near to the lower end of said range, wherein said amplifier comprises a plurality of amplifying stages, each of said stages being arranged to amplify most efficiently a different band of frequencies within said range.

The invention will be described, by way of example, with reference to the accompanying drawing, in which

Fig. 1 shows one arrangement according to the invention, and

Fig. 2 illustrates a modification of Fig. 1

Referring to Fig. 1, a photo-electric cell 1 has its cathode 2 connected through a resistance 3 to earth, and has its anode 4 connected to the positive terminal of source 5 of potential, the negative terminal of this source being earthed. The control grid of screened grid amplifier valve is conductively connected to the cathode of the cell and the cathode of the valve is connected to earth through a suitable source of grid bias (not shown). The value of the resistance 3 is made so high that the voltages developed across it, for any given change in illumination of the cell, are much greater at lower frequencies than at high frequencies. The high resistance enables the lower frequencies to be passed on efficiently but the stray capacities associated with the photo-cell cause severe attenuation of the higher frequencies.

For example, reverting to the example given above, the resistance 3 may be made 5 megohms (instead of 5000 ohms) and the voltage on the control grid of the valve 6 for maximum brightness at the extreme low frequencies will be 1 volt. The self-capacity associated with the input circuit of the valve 6 will however, cause the response for all frequencies above 500 cycles per second (at which frequency the response is 2 decibels below the D.C. level) to be reduced more and more as the frequency increases. The attenuation at 500 kilocycles will be about 58 decibels compared with the response for D.C., this value corresponding to a voltage ratio of rather less than 1 to 800.

The anode of the valve 6 is connected through resistance 7 to a point at a suitable positive potential in a suitable anode current source (not shown); the anode is also connected though an inductance coil 8 and a circuit comprising a resistance 9 in parallel with a condenser 10 to the control grid of a second screened grid amplifying valve 11, and the control grid of this valve is connected through a resistance 12 to a suitable source of biasing potential.

The resistances 7, 9 and 12 in series constitute a potential divider, and their values are so chosen that for a D.C. picture signal, the amplitude of the signal voltage applied to the control grid of the valve 11 is substantially the same as that applied to the control grid of valve 6. The valve 6 thus effectively provides no amplification of the D.C. picture signal component.

The valve 11 is coupled to a third valve 13 which in turn feeds a fourth valve 14, and the interstage couplings between valves 11 and 13 and between valves 6 and 11 comprise the same elements, although in some cases these elements have different values. The elements of the coupling between valves 11 and 13 are given the same references, with dash suffixes, as the elements of the coupling between valves 6 and 11. The coupling between valves 13 and 14 comprises three resistance 711, 911 and 1211 arranged as a potential divider, the resistance 911 being shunted by a condenser 1011. Inductance coils 15 and 16 respectively are connected in series with the resistances 711, and 1211. The values of resistances 71, 91 and 121 and 711, 911 and 1211 are so chosen that the valves 11 and 13 provide substantially no amplification of the D.C. picture signal component; the amplification of this component is substantially confined to the amplifier stage including valve 14.

The condensers 10, 101, 1011 serve effectively to short circuit resistances 9, 91, 911 at the higher frequencies, and thus to increase the amplification at these frequencies; the condenser may have successively smaller values, so that they are effective over different parts of the frequency range to be amplified. For example, condenser 10 may effect the desired correction for a band of frequencies immediately above 500 cycles per second, for example, while condenser 101 is of a smaller capacity than condenser 10 and provides correction over a second part of the frequency range. Over most of the frequency range dealt with by condenser 101, the condenser 10 has an impedance so low compared with resistances 7 and 12 that it can be provide no further increase in the amplification of the higher frequencies. Similarly, condenser 1011 is arranged to increase the amplification at the extreme high frequencies.

An additional increase in the amplification at the extreme high frequencies is effected by inductances 15 and 16. The inductances 8 and 81 serve to prevent falling off of the amplification at the extreme high frequencies due to the effect of stray capacities associated with the anode and grid circuits between which they are connected.

The amplifier described may be arranged to correct almost exactly for the attenuation of the higher frequencies taking place in the input circuit, while at the same time introducing negligible phase distortion. It will be appreciated that since there is substantially no D.C. gain the valves 6, 11 and 13, the need for large and critically adjusted negative grid bias for these valves is eliminated.

Reference is now directed to Fig. 2, which shows an alternative form of input circuit for an amplifier for use, for example, with a photo-electric cell; like parts in this figure are given the same references as in Fig. 1.

It will be seen that in Fig. 2 the resistance 3 of Fig. 1 is replaced by two resistances 17 and 18 in series, the latter resistance being shunted by a condenser 19. The resistance being shunted by a condenser 19. The resistance 17 may be 5000 ohms, the resistance 18, 5 megohms and the capacity of the condenser 19 may be 0.05 microfarad. Then at frequencies of about 1000 cycles per second, the effective impedance in series with the photo-cell is substantially only the 5000 ohms due to resistance 17, but for D.C. and low frequency signals, both resistances 17 and 18 are effective.

The valve 6 of Fig. 2 may form part of a D.C. amplifier similar to that shown in Fig. 1, except that it will be necessary to make the condensers 10, 101 and 1011 of rather larger capacities so that they are effective at lower frequencies. Similarly, inductances 15 and 16 may be dispensed with, the resistances 711 and 1211 being made such that higher gain is obtained at higher frequencies in the final stage of the amplifier.

The arrangement shown in Fig. 2 is convenient for two reasons. Firstly, as the resistance 18 is shunted by a condenser of capacity 0.05 microfarad, its residual capacity and inductance can easily be made small in effect compared to this condenser, or these residuals may be allowed for in the effective value of this condenser. Secondly, the amount of correction required is not so dependent on the particular values of the unavoidable capacity in the photo-cell and grid circuit of the first valve, so that the correcting circuits can more easily be designed and the values of the correcting condensers become larger and more accurately adjustable.

The amplifier arrangements described are given by way of example, and many modifications and elaborations thereof, within the scope of the appended claims, will occur to those versed in the art. Correction for the attenuation of high frequencies may, for example, be made by providing, in parallel with the anode-cathode paths of the valves of the amplifier, parallel resonant circuits adjusted to resonate at a frequency above the highest picture signal frequency to be amplified. The correction may however be effected in any other known or suitable manner. Care must of course be taken that any phase distortion introduced by the correcting means is not such as to produce appreciably different time delays in oscillations of the different frequencies.

It is necessary to arrange that the leakance between the electrodes of the photo-electric cell and between the input electrodes of the first amplifying valve are negligible compared with the conductance of the high feed resistance employed. For this purpose, there may be employed valves in which not only is the leakance made very low, but in which guard rings are provided to prevent leakage between high potential electrodes and the grid; photo-electric cells having guard rings are also preferably employed.

Provided that the insulation of the components is sufficiently good, a feed resistance having a value higher than 5 megohms may be used and the amplitude ratio of "signal" to "noise" at low frequencies will then be still further increased.

It must be understood that the values allotted to the various circuit elements in the arrangements described above are only given by way of example. The invention may also be applied to sources other than photo-electric cells; for example, the invention is applicable in television and like systems in which a cathode ray tube having a light-sensitive screen in the form of a mosaic of photo-electric elements, for example, is employed as a scanning device. Similarly the invention is not limited to television but is applicable to other systems involving the amplification of a band of frequencies including substantially zero frequency.

The invention may also be applied to cases where the higher frequencies are much more attenuated than the low frequencies owing to inductive reactance rather than to the effects of stray capacity.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:-

  1. Apparatus comprising a source of oscillations including a light-sensitive device such as a photo-electric cell, and an amplifier for amplifying oscillations delivered by said device, said amplifier being capable of amplifying oscillations within a range of frequency extending substantially to zero frequency, in which said device is coupled to said amplifier by means of a circuit such that, at the input of the amplifier, the amplitude of an oscillation corresponding to a given change in intensity of the light falling on said device which takes place at a frequency near to the lower end of said range is much greater than the amplitude of an oscillation corresponding to an equal change in light intensity taking place at a frequency near to the upper end of said range, and in which said amplifier is constructed and arranged to amplify oscillations at frequencies near to the upper end of said range to a greater extent than oscillations near to the lower end of said range, wherein said amplifier comprises a plurality of amplifying stages, each of said stages being arranged to amplify most efficiently a different band of frequencies within said range.
  2. Apparatus according to claim 1, wherein amplification of oscillations of frequencies at the lower end of said range is substantially confined to one or more final amplifying stages of said amplifier.
  3. Apparatus substantially as described or as shown in the accompanying drawing.

Dated this 15th day of April, 1935.

REDDIE & GROSE,

Agents for the Applicants,

6, Breamís Buildings, London, E.C.4.

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Leamington Spa: Printed for His Majestyís Stationery Office, by the Courier Press. - 1935.