US2197029A - Line amplifier for telephony purposes - Google Patents

Description

April 16, 1940. G. H. BAST ET AL 2,197,029

LINE AMPLIFIER FOR TELEPHONY PURPOSES Filed Nov 22, 1954 3' k gZ SZZM Patented Apr. 16, 1940 UNITED TATES PATENT, OFFICE LINE AMPLIFIER FOR TELEPHONY PURPOSES Gerardus Henricus Bast and Frederik Hendrik Stieltjes,- The Hague, Netherlands, assignors, by mesne assignments, to International Stand-' ard Electric Corporation, New-York, N. Y., a,

corporation of' Delaware I Application November 22, 1934." Serial No. 754,332 7 In The Netherlands November29, 1933 -1 clai (01. 179-170) The invention, relates to communication line such a load as a communication line, which shall have a higher amplificationthan a corresponding output stage of known "type, which uses a triode, and shall at the same time have an available power output at least asgreat as the corresponding stage using a triode.

Anotherspecific object is to provide an output,

stage, such as above,'in which the power required for heating the cathode and supplying potential to the anode and screen, is equalto or less than in the corresponding known type of output stage using a triode.

According to this invention, the above and:

- other objects are attained by the use of a pentode Y whose degree of screening is above a certain value, connected so as to work into a load whose impedance bears at least a four. to one ratio to certain characteristicsv of the tube.

-'..In the accompanying drawing, several em- .39. bodiments of the invention are illustrated- Fig. 1 represents schematically" an amplifier according to our'invention applied to a system in which it is especially advantageous. In this system the amplifier is connected in the junction between a two-wire line and a four-wire line.-

Figs. 2 and 3 represent further special applications wherein single stage amplifiers constructed according to the .principles of our invention are provided with negative back coupling.

s In order to more precisely define the above special relationships which are an essential feature of the invention, it isnecessary to refer to a number of partial derivatives which define the characteristics of a pentode. Also,ifor;analyz- 4. 6 ing thev performance of an amplifier. output stage in accordance with our invention, andfor comparing it with the performance of a corresponding known outputstage using a triode, certain other partial derivatives are useful. To avoid 9,- complex expressions, all these partial derivatives will be denoted, hereinaftenhy convenient designations as follows: v

' s=partial derivative of anode current with respect to control grid -potential-all other electrode potentials being constant. With respect to the usual an-'- ode current-grid voltage curve, s is, the slope of the curve. This, derivative s is well knownas the mutual conducte ance of the'tube. s=second partial derivative of anode current with respect to control grid potential'all other electrode potentials being constant. With respect to the usual anode current-grid voltage curve, s"-

is the curvature. of the curve. l g=partial derivative of anode potential with respect to control grid potentialiall other electrode potentials as well as the anode current being constant. This de-' rivative g iswell. known as the amplification factor of the tube. 1 R1=partial derivative of anode potential wit respect to anode currenteall other electrode potentials being constant. This derivative R1 isgenerally known as the A. C. resistance or' internal resist ance of the tube.

K =partial derivative. of control .grid potential with respect to total emission current-all "other. electrode potentials be. ing constant. Y

K partial derivative of anode potential with respect to total emission currentall other electrode potentials being constant.

Ks =partial derivative of screen grid potential with respect to total emission currentall other electrode potentials being constant. I

The amplifienaccording'to the invention is characterized in 'this, that for the power-valve" a pentode is used, wherein 'in the operating region the ratio ofthe small variation of the total emission current which results fromagiven.

small variationin the screen-grid voltage (other voltages being constant) to the small variation ofthe total emission currentwhich results from the small variation in the anode voltage (other ratio may be called the screeningratiofl voltages again being constant) is of the order of or higher.

and whereby, (at least at those frequencies at which the greatest sound energies occur) the load impedance is at least equal to four times the resistance represented by Ksg.

(2) Ru lKsg The ratio KG K measures the extent to which. the. screen. grid shields the anode. For convenience therefore this The detailed structural designs whichmay be used to produce a screening ratio of vmere'than 50 are well known. In fact pentodes whose screening ratio is far above 50 have already been known;

although these tubes were not used as power pentodes but rather for radio frequency ampli fication where freedom from feedback is the primary requirement.

It is assumed that the .maximum available, alternating current power is determined, by the maximum allowable percentage of harmonics. Moreover with line amplifiers this percentage is lower than with other applications, since in general in an amplified line more than one amplifier occurs and all said amplifiers deliver the same power.

The maximum alternating current power of the triodes ordinarily employed in line amplifiers is not determined by a sudden limit (e. g., by the I attainment of a zero value of v anode voltage or anode'current) but ratherv the limit is reached before this point. Thisis due to the fact that the characteristics are curved. The most important harmonic possesses-a frequency which is twice the fundamentalfrequency and is herein called the second harmonic.

' To-a close degree of approximationthe percentage of the second harmonics may be represented by /W R, (3) h-eIOOp Ru-Ri:[ Ru

In this equationh is the percentage, W the delivered alternating current power, R1 the internal tube resistance as previously defined, Ru the exterior resistance seen from the anode, and p is given by the equation: I 3

-The amplification A, defined as the ratio of the voltage across the normal telephoneimpedance (obtained on the secondary of an ideal output transformer transforming the said secondary into Ru at theprimary) to the grid voltage is given by the equation: i

in which g is the amplification factor and s the mutual conductance of the triode as previouslydefined. I

As. far as the percentage of harmonics is concerned the following is observed:

The factor indicates the influence of the smaller'current variation which in case of a higher resistance is triode used for line amplifiers; the screen grid is arranged in the place of the anode of the triode I (that is to say: gs pentode g triode) The ideal- I necessary for obtaining the same alternating current power, the factor indicates a kind of anode reaction. V As far as the output power is concerned, however, it is seen that the amplification becomes maximum at Ru=R1i Therefore this is the normal relation for line amplifier triodes.

It can be seen from Equation 3 that by-increasing Ru beyond the above mentioned value (R1) the harmonics are reduced in a two-fold way. Since, however, the amplification decreases when Ru increases beyond R1 the above is not applied.

Now let us consider a somewhat idealized pentode havingthe following properties: cathode and control grid have the same structure as in a ization of the pentode consistsin this, that it is,

- assumedthatthe entire emission current goes to the anode and thatthere is .no anode-reaction whatever. Then with the same voltages (and heating'current power) thewsame currents and consequently the same (curved) Ia.V;, teristics are*obtained.

The percentage of harmonics roo /2% The amplification:

If both-of these tubes are operated into loads" characwhose-impedance equalsaRr of the .triode,.then. the pentode produces twice-the-percentageoi:

harmonics with-the same alternatingcurrent power and. also gives twicethe amplification-of.

the triode.

In order toobtain anequally small percentage;

of. harmonics the, pentod'e; shouldbe. loaded .by:

fourtimes-theRrof theg triodeh Then. the am; plification. becomes: four times, (viz.- twice two;,

own interiorresistance;- I

An. actualpentode, however, is. not like above assumed.- idealized pentodeafori 1. The-anode.voltageadoes infiuencethe total emiz-zsioncurrent.v

times). as high aswithsthetriode loaded by itsrf 2. The emission current isdividedbetween the.

anode and thescreen grid. v 3, An" anode voltage variation more strongly influences the division between anode and. screen grid than the total emission current.

' Consideringonly the first of the above tioned practical limitations, if itis desired to realize the possible higherarnplification of'the pentode when the pentode is operating into as load whichis at least four times R1 of the triode, it is necessary that p 1 1 1 a it This has been done by making:

50 Considering now, the second of the above mentioned practical limitations, this reduces the eifective .mutual conductance s. The necessity of maintaining a small anode reaction is, therefore, even greater, in order that a high amplification may be obtained in spite of this reduction in mutual conductance.

Considering now the third of the above mentioned practical deviations from ideal, in general one can express a small change in anode current by the following equation:

' 2b(AV,,AV,,)+'c(AV,,) etc. in which Va and V represent the anode voltage and the grid voltage, respectively.

With a triode very approximately With a pentode such is not the case because the total emission current is divided; between the anode and the screen grid, and because the said division depends on the value of the voltage a SR IE and also 'ga i and 2SR5 c A consequence of all these relationships in a pentode is that by a right. choice of the proportion V (which amounts to a correct choice of the loading impedance) it may be arranged that of the linear factors the SAW; predominates by far, whereas the square factors balance one another, at least if Theresult of this isthat if the load impedance is equal to the interior resistance the alternating current amplitude is a larger fraction of the normal anode-current than the alternating voltage amplitude isof thenorrnal anode voltage. In other words; if the alternating signal input were increased the limit of zero anode current would be encountered-prior to the limit of zero anode voltage.

Consequently there is a suitable margin for increasing the loading resistance'without reducing the amount-of power which can be handled before encountering a discontinuity; the same is true of the corresponding pentode. This margin is still further increased in proportion as the maximum allowable alternating current power with the triode is reduced relative to the total supplied direct current power; I The above applies only to pentodes. With tetrocles the additional discontinuity at Va =Vsg imposesan additional limit on power and for this reason only pentodes are considered.

From the above analysis it appears that if the conditions according to the invention are met, a line amplifier is obtained having a higher amplification as well as a smaller percentage of harmonies for'an equal delivered alternating current power, in comparison with the corresponding triode. put energy is concerned, pentodes connected in accordance with our invention are definitely superior to well matchedtriodes, provided that the load impedance is practically constant, and furthermore it can be seen that a higher amplification may be obtained, which represents a considerable advantage for line amplifiers. I

The higher amplification which is obtainable according to our invention provides an opportunity of decreasing'the transformation proportion of the input transformer so that a larger width of the band and a better quality are obtained, because it is not necessary to make such high de- It is thus evident that-so far as the outmands upon the said'itransformer since the am-- plification obtained with the pentode is very high.

If Ri Ru a simple arrangement may be made for the change from four-wire into two-wireconnections, whereby the acting losses are compensated by a higher amplification in the direc-' tion 4-wire to 2-wire. It is true that then the maximum delivered power will decrease but in general the maximum output energy is. not required in points in which one does not pass again into a four-wire connection.

In Fig. l of the drawing suchan arrangement is illustrated.

In such an arrangement the incoming four-wire circuit is coupled over the input transformer I to the grid circuit of the pentode 2, whereas the outary winding 4 of an output transformer 5, the primary winding 6 of which lies in the anode circuit of the pentode 2.

v The winding of a second transformer I, the

put two-wire circuit 3 is connected with the secondsecondary 8 of which is connected to the balance impedance 9, is in serieswith the said winding. The two-wire conductor 3 and the balance 9 are connected in series in such a manner that their free terminals I0, I l produce'a difference in voltage equal to zero if the line-impedance and the balance-impedance are equal. The incoming four-wire branch I2 is connected to these points.

Both transformers are of the type usual for ordinary one-way amplifiers. With a very large R1 in comparison with Ru the amplification between the grid circuit and the two wire circuit will be the same, independent of whether the second transformer is or is not present. In this connection it should be noted that the condition above specified ior'the load Ru (namely that it must be at least four times Ksg) does not in any way prevent the load from; being small in comparison with Ri. The effect of the currents set up by the two-wire line through the valve impedance is so very small that it may. beneglected; so that for speech transmitted from the twowire line to the four-wire line the efiect, is practically the same as if terminals 3 were simply connected to terminals it in series With balancing resistance 9. In this case Ru is equal to twice the optimum output value (since the transformers 5 and 'l' are the same type used for simple line amplifiers) so that the energy to be delivered does not reach fully the optimum value. Further in case of Ri Ru the load impedance maybe artificially increased in any frequency ranges which do not require the full maximum power output, thus permitting the attainment at certain frequencies of an amplification somewhat higher than-the normal amplification. By this arrangement the unequal attenuations of the line may be compensated.

A further advantage is that the invention by providing an unusually high amplification offers an opportunity of reducing the high amplification by the use of negative back-coupling, while still having as much amplification as Would be given by an ordinary triode.

Consequently the variations in the amplification grade in question being due to voltage-variations, ageing of the valves and the like become much I smaller which is important for the upkeep of lines having a plurality of amplifiers, whereas the percentage of harmonics decreases considerably. This method with an auxiliary amplifier has been proposed already, but the use of pentodes makes it possible to yield a similar efiect without extra valves by very simple solutions of which Figure 2 illustrates an embodiment.

In the grid circuit of the pentode 2 a resistance l3 is inserted between cathode and earth, whereas the transformer is likewise connected to earth over a second resistance 26. The latter has been designed for coupling back the alternating current, whereas the resistance l3:substantially sup:

plies the negative grid voltage.

Many variations of this example are possible.

For example the resistance [3 could be replaced by a choke-coil with shuntresistance', whereby the resistance I4 may be abandoned.

Figure 3 shows a somewhat modified arrangement. Here the resistance M is arranged between the resistance l3 and earth. The high resistance l6 arranged parallel-to l4 and the condenser l5 provide theright grid bias.

It is also possible to make the back-coupling plification becomes of the order of 125 to 3'75 and consequently four to twelve times that in amplifiers having a corresponding triode.

We claim:

In a communication system a communication,

path which includes a two-Wire line and a four- The degree of amplification of the amplifier is in that case equal Y to S'Ru and for s=2-5 mA/V the degree of amwire line, having outgoing and incoming circuits, a combined amplifier and. repeater connected so as to couple the two-wire line with the four-wire line, comprising in combination a pentode includ-- ing a cathode, a control grid, a screen grid, a suppressor grid and an anode, the screen grid being designed to substantially completely shield the anode, an input transformer for supplying the control grid and cathode oi the pentode with voltages derived from the four-wire circuit, two output transformers having their primaries con-.1-

nected in series with said anode, their secondaries being connected in series and in opposition in the outgoing circuit of the four-wire system, the cir-" cult of the two-wire system being connectedto one of the secondaries, and a balance network being connected to the other secondary.

GERARDUS HENRICUS BAST. FREDERIK HENDRIK, STIELTJES.

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