US2258598A - Amplifier - Google Patents

Description

Ot.14,l941. J. w. coNKLlN` l 2,258,598 A AMPLIFIER.v

Fi 1ed July s1, 1940 2 sheets-sheet 1 fr. F'.

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01.14,1941. J w, CQNKLIN' 2,258,598 AMPLIFIER r Filed July 3l, 1940 HG. Z.

Y nventor JameGl/V Con/ Z in (ttorneg 2 Sheets-Sheet 2 1 Patented Oct. 14, 1941 AMPLIFIER James W. Conklin, Audubon, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 31, 1940, Serial No. 348,742

7 Claims.

My invention relates to television transmitters and particularly to direct current-coupled ampliers or modulators employed in such transmitters. l

In ltelevision systems, it is generally preferred degeneration e ect. l

that the transmitter be so designed as to transmit the direct current component of the picture signals. This necessitates direct current coupling between the modulator and power amplifier tubes.

The conventional D. C. coupled amplier requires a biasing source such as a battery above ground potential `to keep the amplier tube grids at,d the desired negative bias. The use of such a battery is undesirable in transmitters where high voltages and large power outputs are encountered. The capacity to ground of such a battery may cause diiculties also where the transmitter is being operated at a high frequency.

An object of my invention is to provide an improved direct current coupling for electron discharge tubes which does not require a biasing source at' a high `potential above ground and which does not introduce excessive degeneration at the low frequency signal components.

A further object of my invention is to provide an improved television transmitter of the type which will transmit the direct current component of the picture signal. l

A further object of my invention is to provide an improved resistance-coupled amplifier or modulator.

In vpracticing my invention, accordingto one preferred embodiment, the plate of a modulator or amplifier tube is coupled through a direct current connection to the control grid or grids of the succeeding amplifier (there being no voltage sourcein this connection), while the cathode of the modulator tube is operated below ground potential by grounding the positive end of the modulator tube plate supply. Any voltage drop across the plate supply impedance will change the cathode potential with respect to the control grid potential and cause degeneration unless proper steps are taken to prevent it. At the lower frequencies, in the region of 60 cycles, for example, such degeneration may reduce the gain of the modulator tube by one-half.

I avoid such undesired degeneration by making f the control grid of the modulator tube change in potential substantially the same amount as and substantially in phase with the cathode potential changes. Thus, the degenerating voltage causes no change inthe potential of the control The grid potential is madeV to follow the cath ode potential by employing a high plate impedance tube such as a tetrode or pentode in the amplifier stage immediately preceding the modulator tube and by connecting the cathode of the modulator tube back to the plate ofthe high impedance tube, through a condenser and through part of the resistance in the plate circuit of the high impedance tube. As a result, the modulator tube grid is made to follow any variations in cathode potential since the grid is coupled to the said plate. However, since the plate current of a high impedance tube is substantially independent of changes in its plate potential, these voltage variations do not affect the signal output of the high impedance tube and, therefore, do not affect the input signal supplied to the modulator tube.

The invention will be better understood from the following description taken in connection with the accompanying drawings, in which Figure 1 is a circuit diagram of a television transmitter embodying my invention, l

Figure 2 is a circuit diagram of an amplifier or modulator designed in accordance with another embodiment of my invention, and

Figure 3 is a circuit diagram of an amplier or the like designed in accordance with another embodiment of my invention.

Referring to Fig. 1, my invention is shown applied to a television transmitter of the type wherein the carrier wave is negatively modulated by a composite signal consisting of picture signal and synchronizing pulses.

By negative modulation is meant modulation in such direction that a decrease in light in the picture results in an increase in the carrier wave amplitude. Thus, in the case assumed in this application where synchronizing pulses are in the direction representing black in the'signal, these pulses are represented by maximum carrier amplitude.

The transmitter comprises the necessary terminal equipment and amplifiers indicated by the block I0 for supplying the above-mentioned composite signal which is to modulate the transmitted carrier wave. Them-odulating signal includes regularly recurring line synchronizing pulses, these pulses preferably being set on pedestals, the tops of which reach black level. The modulating signal also includes regularly recurring framing pulses.

The terminal equipment I0 may be the same as that described in Patent 2,192,121, issued February 27, 1940, to A. V. Bedford, and assigned to the Radio Corporation of America. As indicated above, the pedestals appearing in the output of such terminal equipment are caused to g-o to a fixed level such as black in the picture, whereby the direct current component of the picture signal may be reinserted after the Signal has passed through alternating current amplifiers.

The terminal equipment I0, if preferred, may be substantially the same as that shown in Vance Patent 2,137,039, issued November 15, 1938, and assigned to the Radio Corporation of America. In the Vance patent, peaks of the synchronizing pulses go to black or to a fixed level beyond black.

The modulating signal is amplified by a suitable amplii'ier I I and supplied to the control grid of a modulator tube I2 with the synchronizing pulses of negative polarity. The plate circuit of the modulator tube I2 is direct current-coupled to a power amplifier I3 forV modulating the radio frequency carrier supplied from va suitablel carri'er wave source I4.

The power amplifier I`3 comprises the ampliier tubes -IS andl- I"I which Vhave a certain output rating determined by the plate dissipation.

It-Will be noted thatV the direct current connection between the plate of the modulator tube I2 and the control grids 'of 'the power amplifier tubes VIii and II'I does notV contain' any source of f at the desired bias potential. Such a battery 2 necessarily is above ground potential and obviously'lis undesirable in a high power, high frequency transmitter.

In accordance with one feature of my invention, the plate of the modulator tube I2 is operated near ground potential, the cathode of this tube'being at a high voltage below ground potential. Theplate supply for the modulator tube is indicated at I9, this plate supply preferably being of the usual rectier-lter type supplied from the fiO-oycleV power line and having the usual voltagedivider 2l connected thereacross.

The positiveside of the `voltage supply I9 is connected* to ground and is also connected through a comparatively lowl impedanceresistor 22 to the plate of the modulator tube I2. The resistance Vvalue ofresistor 22 is so selected as to applylthe VdesiredV negative bias kpotential to the control'grids of the power amplier tubes i and II. across resistor 22 also supplies the desired bias or reference Voltage for a diode 23 comprising part of a direct current reinserting circuit which preferably 'is 'included in the transmitter. This D. C. reinserting circuit is described and claimed in my copending application' Serial No. 348,743, filed July 3l, 1940, entitledDirect current restoring apparatus, and assigned to the Radio Corporation of America.

The negative terminal of the voltage supply IS is connected through a conductor 24 to the cathode of the modulator tube I2. Where the modulator tube is of the screengrid type, the screen grid is connected to a suitable positive point on the'voltagedivider 2| as illustrated.

Referring now to the input circuit of the modulator tube I2, and to the preceding amplifier tube I I which supplies picture signal and synchronizing pulses to this input circuit, the amplifier tub-e I I is of the high impedance type such as a tetrode As will be explained later, the Voltage drop i or a pentode whereby changes in its plate voltage will not noticeably affect its current output. A suitable operating voltage is applied to the plate of the tube II through a comparatively high impedance resistor 2G, a comparatively low impedance resistor 21, and, if desired, an inductance or peaking coil 28 for holding up the high frequency response of the amplifier.

The plate circuit of the tube II is connected through a coupling condenser 29 to the input circuit of the modulator tube I2.

In the specific application here described in which the D. C. reinserting circuit covered by my above-mentioned copending application is included in the transmitter circuit, the desired negative bias for the control grid of the modulator tube I2 is applied through a grid leak resistor 3i from a section X of a voltage divider 32 and from an integrating circuit 33 in series therewith. The grid circuit of the modulator tube may be traced from the cathode of the modulator tube I2 through the conductor 24 to the integrating circuit 33 andthrough the integrating circuit and the secti-on X of the voltage divider 32 to the grid leak resistor 3|' and to the control grid of the modulator tube.

It will be apparent that the voltage divider section X applies a negative bias to the modulator tube grid while the voltage drop across the integrating circuit 33 opposes this negative .bias and, as will be explained hereinafter, reduces it by an amount depending upon the magnitude of the synchronizing pulse peaks which are passed by the reference level setting diode 23 described hereinafter. It is, of course, this variation in the bias applied to the modulator tube that varies the D. C. output of the modulator tube to give the desired D. C. insertion. l l

As previously indicated, one of the main objects of the present invention is to avoid any substantial amount of degeneration which might be introduced because of the impedance of the plate supply I9 (and voltage divider 2l) of the modulator tube. It will be understood that, at the lower frequencies, this impedance may be of substantial magnitude even' though large bypass condensers have been used, such bypass condensers being included in the circuit represented by the block I9. As previously stated, such impedance will cause a degenerative action, in the absence oi means to prevent it, as a result of the voltage drop thereacross varying the potential of the modulator tube cathode with respect to ground and thus varying the potential difference between the control grid and the cathode of the modulator tube.

I prevent such a variation in potential diiierence by connecting the cathode of the modulator tube I2 through a large capacity condenser 36 to the junction point of the resistors 26 and 21. Thus the cathode of tube I2 is coupled through the plate resistor 21 to the plate of amplifier II and through the coupling condenser 29 to the grid of the modulator tube I2. Now, so far as voltage drop across the impedance of the plate supply I9-2I is concerned, the control grid and the cathode of tube I2 are tied together whereby the grid will follow any changes in cathode potential. The result is that the variations in cathode potential do not cause degenerative feedback.

Since the amplifier tube II is of the high impedance type, the voltage variations impressed upon its plate through the condenser 36V will not cause any variations in the plate current through the plate resistors 26 and 21. It is important current as a result of the potential variations of the modulator tube cathode, as otherwise there would be introduced a voltage drop across the resistor 21 which would be applied to the control grid of the modulator tube.

It may be noted that the low frequency components which might introduce a large amount of degenerative feedback may be of the order of 60 cycles and may represent the fundamental frequency of the vertical synchronizing pulses, for example, or they may be ripple voltages due to imperfect filtering in the plate supply.

The present invention does not necessarily include the D. C. restoring circuit, but, in the particular example being described, the direct current connection between the modulator tube and the power ampliiier is used because it is desired to transmit the D. C. component of the picture L signal, and this preferably is accomplished by utilizing the D. C. restoring circuit described and claimed in my above-mentioned copending application. As previously stated, it includes the diode 23, which is biased to the desired reference voltage to permit only the peaks of the synchronizing pulses to pass therethrough and produce a voltage drop across a resistor 31 in series with the diode 23 when the peaks exceed the said reference voltage. to the proper value by'adjusting a tap 38 on the voltage divider 2|, the voltage between the tap 38 and the positive end of the voltage divider 2| being less than and in opposition to the voltage drop across resistor 22.

The synchronizing pulse peaks appearing across the resistor 31 are -fed through a coupling condenser 39 to the grid of an amplifier or control tube 4|, which preferably is biased substantially to cut-off, whereby pulses of current flow through the plate circuit of tube 4| to build up the previously mentioned bias control voltage across the integrating circuit 33. The plate circuit of the control tube 4| may be traced from its plate through the conductor 24 to the integrating circuit 33 and through the integrating circuit and a portion of the voltage divider 32 to the .cathode of the control tube.

The integrating -circuit 33 comprises a resistor 42 and a condenser 43 having such resistance and capacitive value that the charge applied to the condenser 43 (as a result of the peaks of the synchronizing pulses reaching the grid of the control tube 4|) will hold over for several successive line synchronizing pulses. This time constant, in other words, is similar to the time constant employed in other D. C. inserting circuits.

The voltage divider 32 is connected across a suitable voltage supply 46, such as a battery or the usual rectifier-filter supply unit. It will be noted that the voltage drop across the section X of the voltage divider 32 applies a negative bias to the grid of the modulator tube I2. The voltage divider 32 also supplies the plate voltage and the grid bias voltage for the control tube 4I, the grid bias prefer-ably being sufcient to bias tube 4| substantially to cut-orf.

An adjustable tap 41 on the voltage divider 32 is adjusted to a point where the value of the modulator tube bias insures that the signal peaks (in this case, the synchronizing pulse peaks), at least, will pass through the modulator tube I2 under all signal conditions so long as synchronizing pulses are being supplied from the generator I0 whereby the signal peaks in the mod- The reference voltage level is set ulator tube output will just exceed the reference voltage. At all times, therefore, a voltage will be fed back to establish the desired equilibrium condition. This bias adjustment, for example, may be such that, in the absence of a modulating signal, the modulator tube |2 is biased to hold the carrier wave -at an amplitude representing black in the pictlure.

In Fig. 2, there is shown an embodiment of my invention in Whichundesired degeneration resulting from the use of self-biasing is avoided. Referring to Fig. 2, there is shown an amplier comprising an amplier tube 5| of the high impedance type which is resistance-coupled to a second amplier tube 52 by means of plate resistors 53 and 54 in the plate circuit of the first tube and by means of a coupling condenser 56.

The plate ofthe amplifier tube 52 is supplied through a .plate resistor 51 with an` operating potential from-a suitable voltage supply 58 having a voltagedivider 59k connected thereacross.

The desired negative bias for the control grid of the second tube is provided by means of a cathode resistor 6| shunted by a condenser 62, the lower end of the cathode resistor 6| being connected to ground.. In this embodiment, the negative terminal of the plate voltage supply 58 is connected to the grounded end of the `cathode resistor 6|. The voltage drop appearing across the cathode resistor 6| is applied to the control grid of the second tube through a grid leak resistor 63.

As is Well known `in the art, the above-described self-biasing circuit 6|62 introduces a certain amount of degeneration at the lower frequencies. I avoid substantially .all of this degeneration tby` coupling the cathode of the second tube to its control grid through a condenser 64, through the plate resistor section 54 of the high impedance tube 5| and through the coupling condenser 56. Thus, as previously described, the control grid of tube 52 is made to follow any potential variations of its cathode due to the impedance presented to the lower frequencies by the cathode self-biasing circuit 6 |--62.

In Fig. 3, there is illustrated another embodiment of my invention as applied to an amplier feeding into a coaxial cable or the like. The amplier comprises a high impedance ampliiier tube 66 which is resistance-coupled to a second amplifier tube 61 by means of plate resistors 68 and 69 and a coupling condenser 1|.

In this embodiment, the plate of the second tube is operated at substantially ground potential, the `plate supply, such as the battery 12, having its positive side connected to ground Iand to the plate of the second tube through a comparatively low impedance output resistor 13.

The coaxial cable 14, comprising an inner conductor 16 and an outer grounded conductor 11, preferably is properly terminated as by means of a resistor 18 of suitable value. It will be evident that, with this arrangement, there is no high voltage difference between the central conductor 16 and the outer conductor 11 of the cable. Therefore, there is no danger of voltage breakdown in the cable.

Since in this embodiment of the invention, as in the rst-described embodiment, the cathode of the amplifier tube 61 is being operated below ground potential, there might be serious degeneration at the lower frequencies because of the unavoidable plate supply impedance. Such degeneration is avoided in the manner previously 'described byeou'pnngi aeondense'r 19 fbetweenfthis cathode -.and the juncti'onpoint Vof "the -plate `re- "sistors'SB andSSof the high'impedance tubeBS.

` Fromf the' foregoing," it 'will be l"apparent vthat I have 'provided a simple f and effective circuit' for opera-'ting an amplifier ormodulator tube with its output electrode at or vnear 'ground'potential without introducing undesirable degeneration.

vIt'iivillalso be apparent that Ihave provided an lnprov'edftelevision' transmitter'so" de'signed as to transmit -the direct "current"'component of apicv4ture'signal'without 'theV use of Abiasing batteries lor' the like' above ground potential.

Iclaim'asrnlnvention: 1."-An" amplifier comprising a vfirst amplifier tube and a second amplifier l tube which are impedance-coupled, eachv of Asaid ltubesY having a plate electrode, a control grid-and a cathode,

-Qmeans ifor operating -s'aid secondtube with its cathode substantiall-7 below ground potential, and means for couplingf the cathodeof said'second tube toits vcontrol grid through a portion of the coupling impedance inv the plate circuit-of said rst tube. y

2,. The invention accordingto claim l wherein the nrst emph/fier tube is of th high plate impedancetype. n A

3. An amplifier Ycomprising av first amplifier j tube and a second amplifier tube which are rehsistancefcoupled, each 'of 'said tubes having a plate electrode, a control grid and a cathode, means for operating the plate electrode'of said 1 second tube at a potential nearer ground Athan the potential of its cathode,y and means for coupling thecathode of said second tube to its con- "trol grid through a portion `of the coupling impedance in the .plate circuitl of said first tube.

4. The inventionl according to claim `3 wherein the 'rst amplifier tube is of the high'plate impedance type. y .y

l 5. In combination', a high plate impedance ampliiier tube, a second amplifier tube, each of said tubes having a plate', a controlgrid anda cathode, means for coupling said tubes, said means comprising a plate resistor for said rst tube and f a coupling condenser' connected between the plate of the- Ifirst tube andthe control grid of the second tube, means 'foroperating the second tube with its cathode belowground; and afcondenser "connected'be'tween 'the cathode of said second tube and aLpoint onv said plate resistor whereby the cathode and control 'grid'of the second tube -are connected together through at least a portion of'said plate resistor.

y 6. In'combination, a high plate impedance amplier tube, asecondamplifier-tube, each of said tubes having a plate, a 'control grid and al cathode, means/for couplingr said tubes, said means comprising a low impedance'resistor and a comparatively `high impedance resistor connected in series'in" the platev circuit of the rst tube with the -low' impedance resistor adjacent to vits plate and a coupling condenser connected betweenthe plate of the Arst tube and the control grid of the f "second tube, means for operating the' second tube with its cathode below "ground, and a condenser connected-betweeny the' 'cathode of said'second 'tube and the junctionpoint ofsaid two resistors 'whereby the cathode and control grid of the'secnond tube are connectedtogether through the'low `impedance resistor.

' 7-. 'In combination, a high plate impedance amv'plier tube, a modulator tube and a radiofrequenoyamplifier-tube. each; of saidv tubes having a plate, a control'grid and a cathode; a direct current connection'between said modulator tube and said radio'frequency amplifier, a voltagesupply'for said modulatorv tube connected between 'its plate v'and its cathode with the positiveside'of connectedtogether'through at least a portionof said plateresistor. I Y a JAMES W. 'CONKLIN

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