US2475547A - Thermionic valve amplifier - Google Patents

Description

Patented July 5, 1949 UNITED STATES OFFICE 2,475,547 THERMIONIC VALVE AMPLIFIER Bertram Morton Hadield, Harrow Weald, England, assignor to Automatic tories Inc., ware Application Gctober 2, 1944,

Electric Labora- Chicago, Ill., a corporation of `Dela- Serial No. 556,879

` In Great Britain December 14, 1943 9 Claims. (Cl. Uli- 171) frequencies may simple manner be substantially improved in a without affecting the nominal gain and with substantial stabilisation of the nominal gain. It is a further object that the foregoing improvements should be applicable to different specimens of valve and to the same valve during its life. A further object is to ensure that the conditions of stability against selfoscillation remain unaffected when overall negative feedback is used.

According to the invention in order to `obtain a phase displacement of a valve which is substantially zero between the input and output for a large range of frequencies by means of negative feedback, impedances lare provided in the cathode and anode circuits respectively having susceptances and conductances of such values that the ratio of the susceptance to conductance of ductance at low frequencies.

In order to obtain conductance of the Valve circuit at low frequencies.

The invention is applicable to triode valves as well as to screen grid valves such as the pentode.

`Serti-on of a suitable inductance in series with le conventional anode resistance. In amplifiers for very large `band width operation (such as those used for television), even this remedy is insuiiicient, `and the basic time constant of the at lower frequencies.

In addition the presence of any series'resistance negative feedback effect on each stage is detrimental, for a given stage gain, since the anode resistance must be increased, `and with it the time constant.

By means of the invention the stage gain may be retained even with series resistance negative feedback for such stage Wthout introducing an aditional phase angle due to the increased anode in fact with a much better phase angle characteristic. Improvement is also obtained as regards constancy of amplitude response with frequency.

The invention vwill be better understood by referring to the accompanying drawings in which:

Fig. 1 illustrates in diagrammatic form the general principles of the invention.

Figs. 2, 3 and 4 illustr-ate examples the invention to a pentode.

Fig. 5 illustrates the application of the invention to a multi-stage amplifier.

Fig. 6 illustrates a graph showing the relation of phase angle and amplitude response with frequency under different conditions.

Referring to Fig. l, let Vl lbe a pentode or tetrode valve, with anode and cathode admittances Aa and Ac. Let the supply busbars lbe as shown marked plus and minus, let the screen be taken for convenience to the positive busbar, and let the anode output voltage V be obtained from an input e. Using the normal differential valve parameter g for the mutual conductance between grid and cathode, or, in other Words, the rate of change of current in the cathode circuit with respect to a change in grid voltage which should be distinguished from the more commonly spoken of mutual conductance between grid yand anode, and assuming the anode impedance of the valve te be infinite and the anode current to be 7c times the cathode current (where lc is a constant for the given valve) then i of applyingr If Aa and Ac be expressed conventionally as GnH-ylSa Iand Gc-l-ylSc respectively, where Ga, Gc are the conductances and Sa, Sc, are the susceptances of the `anode and cathode circuits, then Gain: lag (Gc-H150) In this expression, let

I ;.g.Gc P Ga(g-l-Gc) (3) then P is the nominal gain (since it consists of a real number) and the remainder gives the lphase angle c as follows:

Now in a conventional stage, the cathode admittance is either innite (i. e. Gc infinite, for instance, or is finite and real (i. e. Sc is zero). In the first case becomes or, substituting from (3) Sal lag Therefore the phase angle is a constant for given values of Sa, P, 7c and g, and cannot be reduced for a given value and gain except by reducing Sa. In the second case, qb becomes made sensibly zero at lower frequencies, by making Se Sg Sc c g Q c Gc-'Ga-Fg--Gc or Gc-Ga 1 y) (5) Thus, by applying Equation 5, and despite the increase in anode resistance necessitated by the finite value of Gc in order to maintain the former nominal gain, the resultant phase angle may be made Zero over a substantial frequency range, while the ultimate phase angle at infinite frequency remains unaltered. The latter statement can be seen from Equation 4, since the expressions for all three component angles tend to infinity at innite frequency, and the net result must be minus infinity, which is the same result as for a conventional stage.

The frequency band over which the phase angle remains sensibly Zero by application of Equation 5, for a given gain, has an optimum value determined by the conditions which make the individual angles of Equation 4 a minimum. This is best seen by substituting in (4), and also substituting for Ga in term of P from (3),

For given, finite, values of the variables, it is obvous that the rst term is the largest. Hence the conditions giving a minimum for this term, must also give the optimum Zero phase angle frequency bandwidth. For given values of Sa, P, 1c and g, it follows, by differentiation, that this term is a minimum when Gor-g.

Hence by introducing a cathode conductance (Gc) equal to the grid to cathode mutual conductance (y) of the valve, an optimum frequency bandwith is obtained over which the phase angle is sensibly zero, for a given gain, while the ultimate phase angle at infinite frequency remains unaltered. Also since the nominal gain expression (Equation 3) is only multiplied by the constant its frequency characteristic will remain as before, while the resultant amplitude response taking into account the phase angle term of Equation 2 must be borne constant since the phase angle itself has been reduced to zero. Again, and for the reason that the phase angle degenerates ultimately to its normal value, it follows that the ultimate amplitude response must also degenerate to its normal value, i. e. zero at infinite frequency. Therefore the problems of self-oscillation in a multi-stage amplifier using Overall negative feedback with stages arranged according to the invention, are not increased, and may in fact be improved if advantage be taken of the individual feedback per stage to reduce the normal value of overall feedback.

With the above optimum condition, the phase angle is Sa.P Sal3 -1 -1 t tan 4. hg 2.tan 2. hg (7) and as regards the bandwidth for which the phase angle is sensibly Zero, it can be seen that for a value of Sa.P

lag qs is -0.1 compared with tional stage phase angle 35 for the convenwhere Gc is infinity. For an equal of -0.1 in the latter case a moderate angle such as 1, the respective values of SoulJ lag are 0.106 and 0.017, i. e. a bandwidth gain of 6:1`

By re-writing Equation 6 as follows:

g will give identical answers. can be shown that the value of Sa.P 10.9

varies only from 0.106 to 0.09 (i. e. 15%) f( changes in the ratio of Gc to g from 3 to 0.3 and for a 1 degree phase angle for instanc Thus there is in fact a wide tolerance permissib in practice in the application of the inventio provided the tolerance is taken about the o;

ratios of Gc am In this manner i awww? is made greater than Sa G'z:` 1 +2? and the onlyeiect i on 5 will be to glve a ypositive the specic improvements in vthe it is clear that the `most likely form of cathode circuit 'to employ is one ofrsimilar form to the anode circuit, otherfulness lies in the application of Equation y5 and a value for Gc at around the g of the Valve.

Fig. 2 shows load resistance, Rc the cathode resistance and 'Cc `:he cathode capacitance. Hence l Gat-rt 'where w is 'the frequency in radians per secondol he applied input e). The common factor hich may be put as wo where @v0-Cmp) 1d 'since wo is a constant for a given nominal iin, Ca, 7c 'and g, i-t will be `used as 'the frequency ference value "in this -andpther specific embodiants. Fig. '6 `shows the relations fbetween `the phase gle and :amplitude response with frequency, rves l and .3 respectively lbeing for the conventional circuit (1. e. Gc=innity) and curves. 2 and 4 :respectively for Gc= g andy valve and nominal gain `(1. e. Sa, P, k and g constant) in each case.

angle v:and The dotted curves 5 .and 6 phase angle of making quen'cy and `then simultaneously. The amplitude in curves 3 and 4 degenerates to zero at inni'te frequency while the phase angle degenerates to -90 in curves I, 2, 5 and 6 at infinite frequency.

"g, 3 shows a further of the invention,

around fifho'se predicted above,

J7g; fsettled, they-can :be applied to "the practical casse where Ca is comprised only by that of the anode and its associated components to earth. For instance, it has been found that the best value for Cc in the case of Fig. 3, is about 60% of its theoretical value, as calculated by the above method.

, With these arrangements the buildup time for attainment of 95% steady state, were respectively as l:0.5'l:0.3 for the conventional stage compared to Figs. 2 and 3 designed according to the invention, and where all three stages had the same valve and the same nominal gain. These `figures demonstrate the advantage given by the invention to any amplifier.

In the practical application of the invention to any single stage, the following two points facilitate its use. Use of the optimum condition, where Gc=g, or any convenient ratio about this value ysuch as from 3 to 0.33, means that Rc is of the same order as the mutual resistance and so the grid bias conditions normally used are automatically satisfied, since such values of Rc will give biases around one half of the usable grid Ivoltage between anode current cutoff yand the inception of grid current. Generally speaking, therefore, the only changes needed to the conventional cathode circuit are the addition of a suitable value for Cc and any further components in series with Rc (such as Lc and Kc). As regards the addition of series components such as La and Lc, it is obvious that La will be small for the usual order of Ca (i. e. of the millihenry order) and is therefore convenient, whilst it is equally obvious that Lc will be approximately .equal to La divided by the stage gain and so will be even more convenient to apply without affecting the bias conditions.

In order to maintain the nominal gain, by comparison with a conventional stage, it is necessary to increase Ra for the Rc used, according to Equation 3. Although this effect has been fully accounted for in the previous discussion of the frequency characteristics, it may cause a partial redesign in order anode voltage requirements. If, for instance, a conventional stage using the maximum permissible Ra is to be modified, it follows that increase of Ra alone will cause anode overloading by virtue of the decreased anode to negative busbar steady voltage. However, this can be avoided by redesign of the steady voltage conditions so that a for instance by Ausing a lower value of screen voltage. Alternatively, and where suitable` frequencies are involved, Ra may be shunted by a choke..

In many cases however, Rc is not at its maximum value permitted by the steady voltage conditions, to keep the anode time constant small, such as in television amplifiers. In these cases it is highly probable that `Rai may .be increased at least to twice its value, Athus permitting the immediateapplication of the invention in its optimum stage of Gc=g.

As regards the application of the invention to a multi-stage amplifier, Fig. 5 demonstrates a typical setup. Valves VI, V2 to Vn have anode Aand cathode admittances Aal, AaZ, to Aan, and Acl, A02, to Aon, in accordance with the invention, and are coupled in cascade by any wellyknown means such as grid condensers and resistances C2, R2, C3, R3, to Cn, Rn, as shown. Pro- '.vision has also been shown 5 mittance Af and the coupling -condenser 2, and the 0 related in any specic manner (such as t9 mionic valves 7p impedances feedback of conventional type into the cathode circuit of VI via Ric, or the grid circuit of VI via Rig, according to the phase of the output voltage to the input, and via the feedback ad- C. In the case of use of Rfc, decoupling means Rd and Cd have been shown so that no additional local feedback on Vi can be given by the voltage on Ric. The input is applied between terminals I, output is taken from the anode circuit of Vn, via terminal 0 and one of the supply busbars marked plus and minus. As mentioned before, the degree of overall feedback may be reduced at least by the local amount given by Acl,

15 A02, etc., on each stage, and also on account of the improved phase angle and amplitude characteristics of each stage, so that the care necessary to prevent self-oscillation as high frequencies need not be so great as normally.

While the case of the pentode or tetrode valve has been taken as being the simplest to demonstrate the invention and as being the type of valve moet likely to be used in practice, the methods shown and described can be applied to other valve types. Generally speaking, however, the application is less eiiicient if the anode potential variations affect the cathode current or grid potential. Again it is not essential to the invention that the steady electrode voltages shall be has been shown for the screen and anode voltage supplies) since the functions and derivation of the invention are concerned with the differential operation of the valve and its associated circuits.

I claim:

i. in a Wide band amplier, a thermionic valve having at least a cathode, a grid, and an anode, an input circuit coupled to said grid, an output circuit coupled to said anode, and impedances `4c in the anode and cathode circuits of values such that the ratio of susceptance to conductance of the impedance in the cathode circuit is substantially equal to the ratio of the susceptance to conductance of the impedance in the anode circuit times the sum of the grid to cathode conductance and the conductance of the impedance in the cathode circuit, divided by the grid t0 cathode conductance, whereby substantially zero phase displacement between signal waves in the v input and output circuits is obtained over a wide frequency range.

2. A wide band amplifier as claimed in claim l wherein the conductance of the impedance ir the cathode circuit is made substantially equai to the grid to cathode conductance of the valvf bandwidth over is substantiall;

in order to obtain the optimum 'which the phase displacement zero.

3. In a wide band amplifier, a plurality of ther each having at least a cathode, grid, and an anode, said valves connected in cas cade, an input circuit coupled to the grid of th first valve and an output circuit coupled to tl* anode of the last valve, and impedances in tt anode and cathode circuits of said valves havir values such that the ratios of susceptance to co! ductance of the impedances in the cathode ci cuits are substantially equal to the ratios of tl susceptance to conductance of the correspondii l in the anode circuits times the su of the grid to cathode conductance and the ,co ductance of the impedance in the cathodecirci divided by the grid to cathode conductance the corresponding valves, whereby substantie for overall negative yzero phase shift between the input and outp signal waves is obtained over the major portion 9. In a wide band amplier, a thermionic valve of the operating range. having at least a cathode, a grid, and an anode,

4. A wide band amplifier as claimed in claim an input circuit coupled to said grid, an output 1 wherein the conductance of the impedance in circuit coupled to said anode, similar complex the cathode circuit lies between one-third and 5 iinpedances connected in the anode and cathode three times the grid to cathode conductance of circuits of said valve, the ratio of the susceptance the valve. to conductance of the cathode impedance being 5, A wide band amplifier as claimed in claim approximately twice the ratio of the susceptance l wherein the impedances in the anode and cathto conductance of the anode impedance, whereby ode circuits each comprise a resistor shunted by sensibly ZelO DhaSe displacement beWeeIl Siga condenser. nal waves is obtained in the input and output 6. A wide band ampliiier as claimed in claim Circuits @Ver a Wide frequency range. 1 wherein the impedances in the anode and cath- BERTRAM MORTON HADFIELD. ode circuits each comprise a resistor and inductor in series shunted by a condenser` REFERENCES CITED 7- A Wide band amplifier as Claimed in Claim The following references are of record in the 1 wherein the impedances in the anode and cathme of this patent; ode circuits each comprise a resistor and inductor in series shunted by a condenser of values such UNITED STATES PATENTS that the inductance equals the product of the Number Name Date capacitance and the square of the resistance. 2,231,374 Stillweu Feb 11, 1941 8. A Wide band amplier as claimed in claim 2,269,593 Schade t1511113, 1942 1 wherein the impedances in the anode and cathode circuits each comprise an inductor in series FOREIGN PATENTS with a resistor, a rst condenser in shunt of the Number Country Date inductor, and a second condenser in shunt of the 465,118 Great Britain Apr. 30, 1937 series combination. 484,102 Great Britain May 2', 1938

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