US2294862A - Thermionic valve circuits - Google Patents

Description

Patented Sept. '1 1942 UNITED STATES PATENT OFFICE 'rimnmomc VALVE CIRCUITS Bertram Morton Hadfleld, Harrow Weald, England, assignor to Associated Electric Laboratories Inc., Chicago, 111., a corporation of Delaware Application July 10, 1940, Serial No. 344,642 In Great Britain August 5, 1939 scams. (curs-171) The present invention relates to thermionic valve circuits and is more particularly concerned with thermionic valve circuitsin which a substantially constant alternating output may be which produces substantially zero anode cur rent. At any given value of control grid potential, the slope of the tangent to the characteristic will represent the mean change of anode current per unit change of control grid potential, so that if an additional alternating input be applied to the control grid. the output of alternating anode current will be proportional to the product of the input alternating control grid voltage and the slope of the tangent, the latter being expressed as a ratio of the change in anode current per unit change in control grid potential.

It can be shown that the slope of the tangent,

as defined above is proportional to the controlgrid potential (reckoned from the zero anode current potential) and as the latter is proportional to the square root of the anode current, it follows that if theisquare roots of the anode currents at various control grid potentials are plotted, a straight line will be obtained which represents the variation of the slope of the tangent at these control grid potentials.

between the alternating input and the outputwill be a parabola, whose maximum value occurs at a control grid potential of one half the difference between the initial bias (with no input) and the bias necessary to produce zero anode current. The output will be zero-when the alternating input is zero and also when the bias due to the alternating input is such as to produce zero slope (i. e. zero anode current).

Figures 1 and 2 show characteristics explaining the operation of the invention.

According to one feature of the invention a direct current bias, whose value is proportional to the alternating input, is applied to a control grid of a thermionic valve in series with the alternating input, the initial bias on the control grid being very small so that a large anode ourrent flows in the absence of an alternatingim put.

put with inputcan be determined by taking the product of the control grid directv current potential (due to the alte'matinginput) and the slope of the control grid/anode current char- As describedabove, the output of alter nating anode current will be proportional to the acteristic at each potential. Since the relation between these two variables is linear, the relation Figure 3.and 4 showspeciiic embodiments of V the invention.

Thus referring to Fig. 1 of the accompanying drawing, the mutual conductance or slope of the grid-volts/anode current characteristic has been plotted on a verticalaxis as a, whilst the corresponding grid voltage Va is plotted on a horizontal axis. As pointed out above the relation between 9 and Va is a straight line such as AB, if the grid volts/anode current characteristic is a parabola. -'I'he parabola APO represents the relation between the output and input voltages.

Let the difierence in biasses on the control grid occurring at an input bias 01 W2. The change,

of output above this value is small depending on the working limits assigned to V, For instance, assuming that V varies from W3 to 2W3, then vG will varysfrom 2G/3 to G/3 respectively, .and

therefore the output at these values is '2VG/9. The change in output therefore, over this range of input levels (6 db. on voltage ratio basis) is from VG/4 to 2VG/9, or a change 'of- 11.2%.

As' a further feature of the invention it is proposed to reduce the rate of change of direct current bias on the control grid due to the alterhating input, at a predetermined value of the bias, leavingthe alternating .input to the grid unaltered. For thepurpose ofiliustrating the effect of this alteration, the initial working range taken above will be considered, i. e. a working range of 6 db. over a change of- V from V/3 to At a bias of 2V/3 on the control grid due to the alternating input 1. e. at the point x, let

the rate of change of such bias with increasing input be reduced to one quarter. It calf be (1+p), where n is the number or times the rate shown that this amounts to altering the slope of changeol grid biasis altered.

of the linear relationshp between V and at this Urn) point to one quarter of that formerly obtaining m i. e. changing AB to CD. The intercepts of this 6 working range= -(l p), from (3) new linear relationship onthe V and G axes (V I can be shown to be 2V and 'G/2 respectively. =n(1+p) (7) Hence with increasing input theoutput will describe a fr sh parabola whose maximumwm It can be shown that values of p greater than 1 occur et-en in corresponding to v and at an 10 are not profitable, besides adding to the comanode current/ control slope of the 0f the bias circuit Likewise Values value f the maximum being th refore 4. of 11 greater than 3 do not add materially to the This ts the same maximum value as was tt range and affect the change of output to ,a greater previous t e redu ti n-of the t of increase degree. If n is taken at 3 the calculated range is of bias, whilst at an input corresponding to 15 16 I 4V/3 the output will against 2VG/9 Hence t The process of reduction of rate of change of resultant t t curve 1 he CQPO and bias with increasing input at such predetermined working range has been extended by a further 4, points can; of course, be extended almost inde- 6 db. with the same change in putput of 11.2% fini l each r uction iving an extension or as was, formerly obtained. 20 working range of amount equal to the initial h erder estimate the change of output voltrange (the range being expressed in terms of ratio age f r a given range f mputhlevelsi with one i. e. deci-bels for instance). The determination change of bias ratio, the general case will now he of the points at which reduction of rate of change taken I e is desired depends on the amount of change toler- InFig. 2 the same basic characteristic is shown, able in the 013911?- ovel the Wmking but the rate of change of grid bias is altered, at. Furtherymra by using two W m push-pun a given input by so that the intercepts f. and applym the direct current b as derived from the new slope/grid bias line are Glu and 12V re.- the inputs to the control grids m Parallel" the speet1ve1y Let t f rm of t equation for t changes in the direct current component of the slope/grid bias line be g=A+C'Vg, then for t anode currents are prevented from aflecting the original line, waveform of the alternating output. In addition,

- a as is well known,the output impedance,such as g=G+- -Vg (1) a transformer, need not be designed with a view to carrying large direct currents.

and for the expanded line. In order that the application of the invention G G Va may be more easily understood, reference should g=; (2) be made to Figs. 3 and 4 of which Fig. 3 shows a specific method for obtaining the desired reduc- Equatins ese wi eiv t e p i o intersection tion in rate,of change of control grid bias at G g 40 a specific bias while Fig. 4 shows acomplete cir- +T" 9 ;+,;T 9 cult. The input transformer, which may also supply the alternating input to the control grid ,,Vg= or grids, is arranged with a winding, which by "(l-n) 4 means of any well known rectifyingsystem supe 1v (3) plies a direct current bias between the cathode 1+1 and control grids. For a reduction in rate 01' Th change of say, one quarter, the bias is developed 8 van}? of at the pant 9f intersection across the following network shown in Fig. 3.

Between the positive and negative output termi- V 1 5o nals of'the rectifier system is connected in series G 4 aresistance R, a battery E (or other source of TT-Fr' direct current potential) in such a manner as to tend to oppose currents from the rectifier vsysgg g output at the point of tem, a rectifier MR in such a manner as to tend e v I to allow currentsto pass from therectifler sys- Q ,L L (5) tem, and a resistance r. The cathodes of the -i- 0 f" I valves are connected to the junction of R and E. But th maximum t t is 4 V whilst the grids are connected (via the push-pull T erefore the ratio ofminimum to maximum out- 2:?51 vintage) the negative put is 4n 11 the backwardresistance 01' the rectifier be .very large compared with R, then substantially +n) f 1 1 no potential exists between the cathodes and grids It theworking range is expressed as the ratio in the absence of an alternating input. When of the grill bias at the point of intersection to an alternating input is applied, until the voltage that which'initially produces the same output, delivered by e r fi sy m exceeds no then from inspection it is plain that if the former c r en will pass throu h the rectifier MR an by V1 then the latter 'will be V-Vl. Also the the whole of the bias is applied to the control total range will be the "initial" range (from V1 grid of the valve When the rectifier voltage to V-Vl) multiplied by the number of times the exceeds E a current passes through the rectifier t i change grid bi is lt d pl s n MR. in its forward dllECtiOll, 50 that it becomes Therefore the working rangeis of small resistance compared to 1'. Therefore (V1) for increased input the rate of change of bias voltageiis reduced to r/R+r, after a voltage E is attained-and if a reduction ratio or one cuarter.

' sources, such as the 50 cycle mains supply. This anacea is desired 1' will be made equal to one quarter of thesumofRandr. I i

For a range of levels of. some 20 db. it is generally not necessary to apply more than one "reduction in ratio as most valve characteristics 5 'tend to deviate from the true parabola at low anode currents, the effect being to prolong the characteristic over a further small range of control grid potential. This factor gives .an extra working range over and above that formerly l0 calculated. Likewise the points of minimum out- 'put indicated by the calculations (1. e. where the rate of change of bias is altered) are not so pronounced in. practice, because of the gradual change of; the resistance .of the rectifier MR from a high backward value to a low forward value. 'This factor may be made use of witlr advantage to produce a more nearly constant 7 output over the desired range.

Referring now to Fig. 4,-in which an input transformer Tl supplies both the grid bias and the alternating voltages to push-pull pentodevalves Vi and V2'from' its secondary winding. Apush-pull rectifier system P and Q is shown supplying unidirectional voltage to the grid bias 5 circuit. The output impedance-R0 may be made Owing to the balanced nature of the output circuit, it is apparent that the device is immune from surges on the supply. i 4

As the portion of the bias network applied-between the cathodes and grids is of relatively high impedance in the absence of an alternating input it is found in practice that voltages may be 4 picked up on this impedance from surrounding results in a corresponding modulation, of the output and can be eliminated by placing a suitable .condenser Cl across the impedance. Such 5 a device however,.introduces1a relatively long charging time constant for input voltages producing a bias :gvhich is sumcient to cause .the network to assume its reduced ratiocondition 7 (since the condenser CBI is now' effectively 5; charged via R and discharges via r). .In orden ,to obviate this effect it is proposed to connect a further condenser C2 across R: such that the time constants 02R and "Cl: are equal. The

condenser for the rectifier system which produces the bias.

In order that full advantage may be taken of the parabolic nature of. the anode current/grid voltage characteristic of a thermionic valve (since a more rapid attainment of constancy of output with low levels is thereby achieved) it is desirable that the static characteristic normally obtained by neglecting-the eflects of an anode load impedance, shallbe used. For this reason it is preferred to use pentode valves, .in which the anode loa has relatively little effect on the static characte 'tic.

It will .be realised that if the output is to be kept reasonably undistorted at input le els ex-, ceedingvthe upper limit of the workin range, so as to obtainthe full benefit of the reduction in output at this point, the alternating input to the grids must be kept low enough to avoid anode bend rectification, It has been found that if Q I A the ratio of the bias voltage produced by the rectifier systemto .the alternating peak input per 'grid is some ten to one thebutput remains substantially pure. I

4 Despite the reduction in output which this entailsas compaied with the output of the valve as a linear amplifier for the same anode impedan'ce, it is possible to attain a substantial percentage .of the maximum output by the use of pentode valves in conjunction with high anode load impedances. As the effective alternating output current over the working range will be constant and smaller than that giving themaxincrease the working anode impedance (over and above that normally used), to a value approximating to the dynamic impedance of the pentode valve, without introducing distortion due to curvature of the anode or grid characteristics, and more especially when a, push-pull system is used.

.In the foregoing description of the principles and embodiments of the invention, where reference is made to the control grid, this electrode need not necessarily be that nearest the cathode or filament, although this is generally advantageous since the maximum sensitivity is here obtained. The invention can be applied to control on the suppressor grid of a pentode valve or valves in push-pull, whose anode current/suppressor grid characteristic is closely parabolic.

In using the suppressor grid, advantage can be taken 'of the fact that the gain of the.valve as a whole tends to remain constant with variations in'suppressor potential (since the screen current and conductance are increased as those of the anode are diminished), by deriving the bias proportional to the alternating input from the common electron path (for instance from a cathode'impedance). Likewise other types of valves having oneor more control grids with a suitable parabolic characteristic can be us such as the heptode and octode.

The biasproportional to the input need not be derived directly therefrom and more especially when the resulting load onthe input is not permissible; it can be derived from a thermionic amplifier or from a' self rectifying valve of the cathode follower type.

One'of the applications of the invention occurs imum output from the valve, it is possible to I in the-reception of signals of alternating current 2 over telephonic lines, where the invention forms the first stage of a receiver connected to the line.

- Such signals may be received. at widely differing condensers may then form the normal storage levels and may be of. definite frequency or frequencies, whilst it is desired to prevent the response of the receiver to currents which may rents becomes higher.

I claim; i I 1. An amplifier having a space dischargedevice and an input transformer, a rectifier circuit ineluding a portion of the secondary of said trans-:

former for initially applying-a grld 2. An amplifier as claimed in clain'i 1 in which.

said network is nonconductive until the grid potential reaches a certain point and then becomes conductive to act as a shunt for a part of said rectifier circuit.

3. In a thermionic amplifier, an input transi'ormer, a space discharge device, a connection from said transformer to the grid of said device for supplying alternating current thereto for emplification, a rectifier circuit also connected to said transformer and to said grid to supply biasing potential thereto derived from the input, a second rectifier circuit connected to said first rectifier circuit, a source of potential and a rectiher in said second circuit normallyeflective to prevent any flow of current therein, said second circuit eilective when the current in said first circuit reaches a certain value to cause a current to fiow therein from the first circuit to thereby effect a change in the ratio of thebiasing potential to the input volume.

4. In a thermionic amplifier, a pair of space discharge devices connected in push' pull rela-- tion, an input transformer therefor, the outer ends of the secondaryofsaid transformer connected through a pair of rectifiers to the filamentsof said devicesia connection from each control grid of said devices to a point on said secondary, a second connection from the filaments of said devices through a source of current and a rectifier to the center point of said secondary, said pairof rectifiers supplying a grid potential to said control grids whenever al- 4 ternating current is supplied to the primary of said transformer, said last connection being effective, after the input -volume reaches a,cer-

tain point, to vary the rate of change in the grid

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