US2363659A - Electric circuit - Google Patents

Description

Nom 28, 19. s. DOBA, JR

ELECTRIC CIRCUIT Filed Jan. 3l, 1942 2 Sheets-Sheet w Dn N C m TA, l m A V WD 5 Vl Nov. 28, 1944. s, DQBA, 1r-g` 2,363,659

ELECTRIC CIRCUIT Filed Jan. 3l 1942 2 Sheets-Sheet 2 VOLTAGE ACROSS CONDENSER 22 VOLTAGE APPL/E0 T0 6R10 27 0F 7118i' V2 vanua: 4r wH/cH wat v2 atc/Ns im coA/oucr VOL TAGE ACRQJ'S GONDENS E l? Jl ANU RSISTANGE J0 CURRENT THRU T'UDE V4 CURRE N T THPU TUBE V5 CURRENT THRU 70H5 V6 /NVE/VTOR s. 005,4, JR.

A TTORNEV Patented Nov. 28, 1944 ELECTRIC CIRCUIT Stephen Doha, Jr., Long Island City, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Y Application January 31, 1942, Serial No. 429,067

17 Claims.

Another object is to provide a novel -form of' generator of recurrent waves oi saw-tooth wave form.

In a sweep circuit suitable for use with high quality cathode ray tubes, particularly of the type used as television receivers, it is desired that the sweep voltage be linear with time and have small distortion, that the sweep have a small return time and that a relatively high output voltage which is balanced with respect to ground be obtained. Itis also desirable that only high vacuum tubes be used. I'he sweep circuit of this invention, the novel features of which will be pointed out below, meets these specifications.

In many known sweep circuits, so-called constant current devices are used to enable the charging or discharging of the sweep condenser to be substantially linear with respect to time. Even with such devices the characteristic curve of current versus time is not exactly linear and itis frequently dimcult to adjust the devices properly. In accordance with the present invention in an important aspect this disadvantage is overcome by doing away with a constant current device and employing instead a non-linear sawtoothed wave generated by a self-oscillating .multivibrator arrangement in which correctness in wave form is purposely made subordinate to simplicity and cost of the apparatus and compensation for the non-linearity of the wave is subsequently made. preferably in an amplier stage through which the wave is passed, to produce a saw-toothed wave the slowly changing portion of which varies linearly with time. found that compensation may be effected by properly choosing the time constant of a capacity-resistance combination in the feedback of a feedback amplifier.

A feature of this aspect of the invention is the amplifier, which has a special form of feedback circuit. band of frequencies in proper amplitude relations to avoid the rounding ed of the corners of the saw-toothed wave which occurs when the very high frequency components oi the wave are reduced or suppressed. but it does this in a manner to introduce the desired compensation for lt has been This amplifier not only passes a wide the rather wide departure from linearity in the wave generated by the multivibrator.

In order to produce a balanced output to be applied to the deilecting plates of -the cathode ray tube (so that distortions of the television image and of the shape of the field caused by the use of unbalanced sweep voltages are prevented) an inverting stage comprising a single high-vacuum tube has been used in the prior art, theinput circuit of the inverting tube being connected to one plate of a pair of sweep plates and the output circuit of this inverting tube being connected to the other of the pair of plates.

With a gain equal to unity and a mil-degree phase shift in the inverting tube, the saw-toothed voltage wave applied to one plate of the pair is equal to and degrees out of phase with the voltage wave applied to the other plate of the pair. This arrangement has been found to have the disadvantage, however, that power dissipation is high. Although the plate current in the last tube of the amplier is low except during the iiyback time, when it is high, just the reverse of this is true in the inverter tube because oi the phase reversal: that is, the plate current is high during most oi the time and low during the yback interval (which is of the order of a few per cent of the total sweep interval). This plate current in the inverter tube, being high most of the timefcauses a high power dissipation. An additional feature oi' the sweep generating means of this invention is an arrangement, hereinafter described in greater detail, in which this disadvantage is overcome by adding a second tube in the phase inverting stage. the cathode of this second tube being connected to one ci the pair of deecting plates and the output circuit of the amplifying stage being connected to the other of the pair. This added tube has a low current during the sweep time, thus not dissipatlng much power. While the iirst tube in the inverting stage draws e. relatively lhigh current during the sweep time, this current is much lower than that which would be present if the first tube in the phase inverting stage were connected directly to one or the deilector plates (the second tube being, of courset omitted) due to the fact that the parasitic capacities existing across the anode circuit of the first tube of the inverting stage are much lower than those eidstins across the deflector plate circuit. Thu: the total power consumption in the output tubes is reduced. ,Y

In accordance with s preferred eminent oi eeeeeco low impedance' compared to the impedance of the charging petti including anode resistor it, the portion of the sweep weve produced by this discharge is steep (see line i--t of Fig. 2c) This low impedance of the discharge path through Vn results from e. positive pulse from V1. This pulse existo long enough for condenser 3l to recharge to e. substantiel velue. Without this pulse the condenser Sii would charge only to e. small velue before the potentiel drop across, re-

sistencc would come the potentiel of the grid V: to reach. it blocking velue. That is, when the dischcrgng action of the sweep condenser starte, the discharge current begins to recharge condenser 3i, thus cousine the potential of 'the grid 21 to fell relatively to that of cathode it, This action is eeen to be in c direction to tend to decrease the rete of discharge oi condenser 22. The discharging action of condenser 22 ceases abruptly when the pulse from V1 ceases. The sweep condenser 22 will then immediately begin to be charged again to initiate another cycle. As the sweep condenser 22 is connected in the circuit between the control grid it and the eethode l2 of the tube Vi, the potential variations due to the charging and discharging ci this con. denser constitute the input weve for the tute Vi. which tube would amplify seid weve ec auch, v were it not for elements new to be `ciecuseed. The'perellei connected resisterlce le end condenser il, which ere included in ootl'x the cetha ode-control grid end the cathode-colado circuits of the time V1, acquire (when current flows) e feedback voltage ecioes their terminele which le in opposition to that across the sweep condenser 22 and of such amplitude that the grid i3 oi the tube Vi does not et any time become positive with respect to the cathode it. Dieter-tions of such nature es to produce enero pulses in the weve shown in Fie. 2o wave applied between the control grid @i and the cathode 2B or the tube V2) cre caused by this feedbackvoltcge and by the attenuation of the low enci cem-ed by the coupling cir-f cuit between the tubes V1 and V2 comprising resistore i8 end S2 sind the condenser 2i. 'me

voltage Fic. 2o) applied to the input circuit4 of the tube ifs le essentielle' the time derivative of the voltage (Fie. 2c) applied to the coritzol grid It of the tue-e Vi. The pulece in the weve shown in. Fig. 2t drive the control grid 2 oi the tube Vi sharply im c. positive direction, time permitiera@ c discheme ce the condenser., emi theo ebruptry ceese. eo tliet time Vc ioloeie eeein.

The period cf free oeeilieticoe may changed by vng the reeistcmce ii the resistance 8@ is mede large the chemo f the condenser deye elcwiy, thus imcreecice the teorico. of caw-tooth weve. This normeii cet clieiitly longer thee met t... 1w peices.; when these e. letter here control. l icei w with eclenf Mum etileec :ieee wie, hundred eet' cestiA plitudc may be tlcmcter et. which comtroi," ci the ewpe cendeoeeif tit. varied. the, che.; the 'e The output coi e condenser .ce once of' the tette it le zweeft wie lice e. eletti? en exieiitirei not e. line The outmit volteee ie emilie e t e .e not r .1c-.e @nohref-L ey Ete increased effects. Y The ce of the ce e of chere/e esisto? moet plete if tml motion ci time. directly to the (that is, the input' control grid ci' the rst tube in the ieedbeei: m plier E which preferably comprime two tubes Je and Vl. It can be shown by rccthemeticei analysis and veriied experimentally that the dii ference betweentheeectee emctoiis ic simile!" to that duc to c series cepecittmoe in e coneteot resistance iine. This conception leeds to the introduction ci means in the amplifier feedback. poeh which gives good ccmoeneetioii. Given the sewtooth wave shown in Eig.

,e(t =A(i-/f)-Bf/f' 0 i V erwan-effe J t where T is the time constant end is eoucl to time product CR, to is the lperiod for one complete eycle, end tx le the period for the slow@ .n f Y g portion of the cycle. Milet 1 fo ffl-5' wo2=rfm woi, mui-T" and so il 2c Then Equation i mcy ce written in terme ci em) where e veries between 0 and 2e.

With no lose oi generality the memorie time of ece) may be teleco eeunity end by die cerding c. direct current term, the um met be teiten ce cero. `iettii'ig.v eie =c1 xl -t-eeie) there t; ced

i... -z me e c e 2fe ci wwe-e) 1 v alla :c: rd. 1 The voice of any onic component, o, et c. time t will then be given by:

Usine the estetico in accordance with circuit theory that coe ncct=si sin mi, we lieve Lemert-lng the' velu ci' one? emi eeiei, eti-ei integre. cocu the collection ci terme, there is obtained leem Also, Equations 3 and d will reduce to:

, ideal wave. Equation may then be seen to represent this wave multiplied by a pure amplitude factor affecting all harmonics equally and multiplied by a distortion factor which aifects the various harmonics differently. Since en when 'r is a time connector no product this ,distortion factor may be represented as a oontinuous function of frequency. 0r Y where T=C'R, m=e.

To correct for this factor a correcting network,

.the transmission characteristic of which varies directly as y is supplied. This connecting network which com prises a two-stage amplier having a feedback circuit between the output circuit of the second stage and the input circuit of the first stage will now be described'.

Referring now to the amplifier B, tube Vs oomprises a cathode Il, an anode 5|. a control grid 52, a screen grid il, and a suppressor grid 54 while the tube V4 comprises a cathode 00, an anode 0|, a control grid 62, a screen 'grid 68 and a suppressor grid iid. The cathode 50 is connected through the parallel-connected resistance 00 and condenser $6 to ground. The screen grid 53 is connected through condenser i1 to the cathode 50 and through resistance 58 to a point 59 having a direct potential of about 300 volts positive. 'I'he'suppressor grid 56 is connected directly to the cathode 50 while the anode 5| is connected through an anode resistor 1| to the point 58 of positive direct potential. A coupling condenser i2 is 'provided between the output circuit of the tube Vs and the input circuit of the tube V4. The resistance 65 and the. condenser 58 are used to apply gridvoltage between the cathode t8 and the control grid 62 thereof and resistor 'I3 is used as a grid leak. The suppressor grid 8d is ccnnected directly to the cathode 60 while the screen grid 63 is connected directly to the point 59 of positive direct potential. The anode 8| of the tube V4 is connected to a point H having a di rect potential of about 1,000 volts positive through resistances 1B and i8, a variable con denser il being shunted across the resistance F5. A condenser 78 is connected from the common terminal 19 of the resistances le and i6 back to the cathode 50 of the tube Vr.

The low frequency response of the amplifier, with which we are here concerned. can be de termined from the following well-known equation for the eilective amplification u' of a feedback amplifier:

rml-

assenso where a represents the amplification without feed back and is the feed back ratio. Since the reactance effects of the condensers 50 and 0| (leading to one of the plates of a pair of decting plates) will be small at the frequencies involved, p is approximately:

21 Hjucl where o is the feedback ratio existing if the capacity C of condenser 'I8 were infinite. and R' is the eifective resistance facing condenser 18 (R'the resistor 65 plus the resistor 1l). The eifective ampliiication is then:

l l d@ +566" the approximation holding when .CR' 1 u which is effectively fulfilled in practice. Equation 13 is of the form required (see Equation 10) in order to compensate for the non-linearity of the input sweep voltage. For substantially exact compensation, the product of the capacity of condenser 'I8 and the resistance R (that is, Ru+Rss) facing this condenser should be equal to the product of the capacity of the sweep condenser 22 and the resistance of the resistor element I3. The effect of condenser $0 may here be neglected because the capacity of condenser 58 is small compared with that of the con denser l0.

The amplifier comprising the tubes Va and V4 thus compensates for the non-linear voltage wave produced by .the `mu1tivibrator arrangement A comprising tubes Vi and V2. The feedback is less at low frequencies and greater at high frequencies. is connected to the balancing and inverter stage C comprising the tubes Vs and Ve, these two tubes being used to obtain a balanced output. Bhunt type feedback is incorporated in the inverter to obtain unity amplification.

The tube V5 preferably comprises a cathode 00,

an anode 0|, a control grid 02, a screen grid 03 and a suppressor grid 0|, while the tube Vs preferably comprises a triode having a cathode 05, an anode 90, and a control grid 91.

the tube Vs, one terminal of the resistance |00 being connected to ground and also, through resistor |00, to the common terminal oi' condansers |01 and |00. Connected-between the common terminal of the condensers |02 and |01 and the cathode 96 ofthe tube Vs is a resistor |09, this resistor shunting the condensers |01.

The output of the tube Vc The anode 6| of the tube Vi is connected to the control A resistor |05 serving as g, grid leak is also connected in circuit between the grid 92 and the cathode 90 of condenser, means associated with said condenser for varying the voltage thereacross to set up the other side of each of said sawtcoth waves, means upon which the voltage across said condenser is at all times impressed, and means cooperating with said last-mentioned means for compensating for the curvature in said curved side of each wave comprising a circuit consisting of capacity means and linear resistance means together having a time constant substantially equal to that of said rst-mentioned condenser and resistance means. n

3. A generator of saw-tooth waves of the form having one side at least somewhat curved comprising a condenser, means for varying the charge on said condenser to generate said curved part of each wave comprising a resistance means connected in circuit with said condenser, the rate of varying said charge being predominantly dependent upon the resistance of said resistance means and the capacity of said condenser, means associated with said condenser for varying the voltage thereacross to set up the other side of each of said saw-tooth waves, means upon which the voltage across said condenser is at all times impressed, and means cooperating with said lastmentioned means for compensating for the curvature insaid curved side of each wave comprising a multistage amplier having a negative feedback circuit between a later stage and an earlier stage consisting of capacity means and resistance means together having a time constant substantially equal to that of said first-mentioned condenser and resistance means.

4. A generator of saw-tooth waves of the form having one side at least somewhat curved comprising a condenser, means for varying the charge on said condenser to generate said curved part of each wave comprising a resistance means of constant value connected in circuit with said condenser. the rate of varying said charge being predominantly dependent upon the resistance of said resistance means and the capacity of said condenser, means associated with said condenser for varying the voltage thereacross to set up the other side of each oi said saw-tooth waves, means upon which the voltage across said condenser is at all times impressed, and means cooperating with said last-mentioned means for compensating for the curvature in said curved side oi. each Wave comprising a two-stage amplifier having a negative feedback circuit between the output circuit of the second stage and the input circuit of the first stage consisting of capacity means and resistance means of constant value together having a time constant substantially equal to that of said :mst-mentioned condenser and resistance' means.

5. In combination, an impedance means one terminal of which is maintained at a fixed. potential, means for generating a, periodically Varying potential at the other terminal of said impedance means, a pair of like impedance elements having a common terminal maintained at a fixed potential, one of said pair having its outside terminal effectively connected to the terminal of said first-mentioned impedance means having a oi' the first of said pair oi' impedance elements with respect to said iixed potential of said common terminal and equally different from said fixed potential of said common terminal, said last-mentioned means comprising phase inverting means connected to said first impedance means, single stage means for repeating the output of isaid phase inverting means without phaseI inversion, and means for impressing the output of said repeating means upon the second of said pair of impedance elements.

6. In combination, an impedance vmeans one terminal of which is maintained at a fixed poten-Y tion, means for generating a periodically varying potential at the other terminal of said impedance means, a pair of like impedance elements having a common terminal maintained at a xed potential, one of said pair having its outside terminal effectively connected to the terminal of said first-mentioned impedance means having a varying potential so that the potentials of these two connected terminals vary in fixed phase relationship with each other, and means controlled by the potential difference across said iirst impedance means for giving the outside terminal of the second of said pair of like impedance elements a. varying potential such that it is always opposite in sign to that of the outside terminal of the first of said pair of impedance elements with respect to said fixed potential of said common terminal and equally different from said xed potential of said common terminal, said last-mentioned means comprising phase inverting means connected to said first impedance means, means for repeating the output of said phase inverting means without phase inversion, and means for impressing the output oi.' said repeating means upon the second of saidv 'pair of impedance elements, said repeating means comprising a cathode, an anodan'd' a control element, said cathode being connectedto the second of said pair of impedances,"

7. In combination, an impdance means one terminal of which is maintained at a xed potential, means for generating a periodically varying potential at the other terminal of said impedance means, a pair of like impedance elements having a common terminal maintained at a xed potential, one of said pair having itsoutside terminal eiectively connected to the terminal of said first-mentioned impedance means having a varying potential so that the potentials of these two connected terminals vary in xed phase relationship with each other, and means controlled by the potential difference across said iirst impedance means for giving the outside terminal of the second of said pair o1' like impedance elementsa varying potential such that it is always opposite in sign to that of the outside terminal of the first of said pair of impedance elements with respect to said fixed potential of said common terminal and equally different from said iixed potential of said common terminal, said last-mentioned means comprising phase inverting means connected to said first impedance, means for repeating the output of said phase inverting means without phase inversion, and means for impressing the output of said repeating means upon the second of said pair of impedanceelements, said phase inverting means ha'ving a high internalimpedance compared with that of said repeating means.

8. The combination of elements as in claim 5 in further combination with a pair of deflecting plates in a cathode ray tube, and means for connesting the output terminals of said pair of like aseaoss impedance means respectively to the plates oi the pair.

9. A sweep circuit comprising means for gen@ erating a saw-tooth potential wave in which for the maior part of each cycle there is a relatively slow change in one direction and for the rest of each cycle a relatively rapid change in the opn posite direction, said wave being non-linear in `Vthe slowly changing portion, means for amplifying said potential wave, said means comprising means for compensating for said non-linearity of said potential wave without changing the shape of the wave produced by said generating means, whereby the output potential wave of said ampli iler has a slowly changing portion which is substantially linear, a pair of deecting plates in a cathode ray tube, means ior connecting one terminal of said amplifying means to one of said pair oi defiecting plates, a phase inverting circuit, and means for connecting the output wave o said amplifying means through said phase inverting circuit to the other oi said pair of deiiecting plates.

l0. A sweep circuit comprising means for generating a saw-tooth potential wave in which ior the major part of each cycle there is a relatively slow change in one direction and for the rest of each cycle a relatively rapid change in the opposite direction, said wave being non-linear in the slowly changing `portion, means for amplifying said potential wave, said means comprising means for compensating for the non-linearity of said potential wave whereby the output potential wave of said ampliiler has a slowly changing portion which is substantially linear, a pair ci' deilecting plates in a cathode ray tube, means for connecting one terminal of said amplifying means to one of said pair of deiiecting plates, a phase inverting circuit, means for connecting the output wave of said amplifying means through said phase inverting circuit to the other of said pair of defiecting plates, said phase inverting circuit comprising a high impedance tube followed by a low impedance tube, the connection to the second of said'pair oi deilecting plates being made from the cathode of said low impedance tube.

11. An oscillator comprising a rst tube having an anode, a cathode and a control element, a second tube having an anode, a cathode and a control element, means for coupling the anode oi the iirst tube to the control element of the second tube. said coupling means being such as to pass high frequencies and greatly attenuate or suppress low rreouencies, means for coupling the anode of the second tube to the control element ci the rst tube, a iiist condenser, means for connesting said ilrst condenser between the anode ci said second tube and the cathode thereof through a circuit which includes a second condenser with a resistor shunted thereacross, means including a source oi potential for relatively slowly varying the potential across said first condenser, and means including the anode-cathode path of said second tube for relatively quickly varying the potenial across said rst condenser by allowing the condenser to discharge therethrough, said disccharge causing the second condenser to be charged.

12. The combination of elements as in claim 11 in which said resistor is variable to change the period of oscillations.

13. The combination of elements as in claim ll in further combination with means to apply synchronizing signals to the control means of one of said tubes.

14. An oscillator comprising an electron discharge device having an anode, a cathode and a control element, a sweep condenser, a second condenser, s. rst resistor shunted across said second condenser to discharge said condenser relatively slowly during the charging of said sweep condenser, means for connecting the sweep condenser and the second condenser in a series circuit including the anode-cathode path of said discharge device, means for connecting the second condenser and said first resistor shunted thereacross in a circuit between the cathode and the control element of said discharge device to vary the potential of saidV control element with respect to said cathode, a second resistor, va source of potential for relatively slowly charging said sweep condenser through said second resistor. said discharge device being blocked during at least the major portion of the charging time by the voltage between the terminals oi said first resistor which voltage decreases with respect to time during the charging interval to swing the potential oi said control element with respect to said cathode in a positive direction thus tending to unblock said discharge device. and means including the anode-cathode path of said discharge device for relatively quickly discharging said sweep condenser when the device becomes unblocked and thereby charge said second condenser whereby the voltage across the terminals of said rst resistor is raised by an amount sumcient to block said tube.

15. The combination of elements as in claim'ld in further combination with means including a second electron discharge devide for amplifying and* distorting the wave produced across the sweep` condenser, and means for applying the resultant wave between the control element and the cathode of said iirst electron discharge device.

16. A saw-tooth wave generator comprising an electron discharge device having an anode, a ,cathode and a discharge control element, a source of direct potential in series with said anode and cathode, a resistance, a second resistance shunted by a condenser. both said resistances being in series with said source and said anode and cathode, said second resistance being also in the input circuit of said discharge device, a second condenser having one terminal connected to said anode and the other to that terminal of said second resistance remote from said cathode, means for applying a steep sided pulse of voltage to said input circuit to cause said discharge device to pass current and charge said drst condenser to a potential difference such that when said voltage pulse ceases said discharge device is blocked, whereby said iirst condenser then discharges through said second resistance.

i7. In combination, means for generating a potential wave in which for the major part of each cycle there is a relatively slow change in one direction and for the rest of the cycle, a relatively rapid change in the opposite direction. said wave being non-linear in the slowly changing portion, electric circuit means, including a negative feedback amplifier, independent or said wave generating mea-ns, for producing from said wave an amplified and distorted version thereof in which the slowly changing portion is substantially linear, and means for utilizing said output potential wave.

STEPHEN DOB, Jn.

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