US1986554A - Direct coupled amplifier - Google Patents

Description

Jim. 1, 1935. B. A. J. TEN BRINK 1,986,554

DIRECT COUPLED AMPLIFIER Filed July 3, 1931 JMIO Patented Jan. 1935 l electrical amplifier consistinglof two 'vnves; there.

voltage is added, for the firstjvalve, to that of the DWWIQ P Barend Albert Jan'tenBrink, The Hague, Netherlandsg assignornto HeinrichWillem Brcy, The-Hague,Netherlands r Karel De Application .iul a 1931 Serial No. 548,664 "I11,the Netherlands July 11, 1930 The invention relates toelectrical amplifiers and more, particularlyto direct-coupled electrical amplifiers or so-called direct-current amplifiers,

inwhich aresistance is employed for the coue pling of two amplifying valveswhich resistance is not only connected with the anode of the first valve but also on the sam'e side with the 0f the following valve. I l c a it By. the e gpressionffeed device as hereafter used is meantasourcegof current fromwhich tappings can be taken only by means ota resistance. connected inparallel with it.- Snch a-resistance, however, will'invariably introduce thefdrift effect, referred to hereinafter on page 2,; paragraph, 2, of the specification. The great advantage of the invention isthat although feed} devicesare used for, the current supply the circuit arrangement of these feed devices is such that resistances for tapping of suitable working voltage ion the individual tubes are not necessary. l i

According to the invention in a directecoupled is available a considerably higher' lpreferably twice or more times) voltage forfthe 'feedingof the first valve than for the feedingof the second valve. In addition to the usual feed devicdlthere maybe provided an additional feed device which supplies no energy to thelsecond valve but whose usual feed device.

In order that the invention may beclearly un-- derstood and readilycarried into effect, it will now be described more'iully with reference to the accompanying drawing, inwhich-z- Figure 1 illustrates diagrammatically a known circuit arrangement of a direct-coupled electrical amplifier, and

Figures 2 and 3 illustrate aiagrammaucau twc examples of an arrangement of direct-coupled amplifier in accordance with theinvention.

In electrical amplifiers-of the. kind above referred'to, the grid of the second .valve obviously, has the, same voltage as the anode of the first valve. This grid, thereforacompared with the corresponding cathode, should be negative, this being necessary for f distortionless amplification,

so that the cathode of the second valve compared with the cathode of the first valve must have a stillhigher'positivevolt'age than the grid of the second Ivalve. This has led to the choosing in known circuits of feed voltagesfor the anode circuits of both valvesin the manner diagrammatically illustrated in Figure 1. In that figure two valves I and @II are "represented which, are ied from a feed device connected withthe terminals A, A and generating a total voltage V which, by means of a resistance R, can be tapped off at different points. "1 is the coupling resistancebetween the two valves, and the terminals of the load circuitof the second valve are represented by 2. By virtue of the points of connectionof the separate anode andcathode conductors, to the resistance R of the anode feeddevice, itis, seen that essentially the same feedvoltages. are sup plied to the two valves, which, however, have a different level. The feedvoltage of the valve II lies as a whole higher than the reed voltageof the valve I. The filament of the-valve II has, compared with the lower-terminal A,jsuch a posi tive voltage that thegridof the valve II receives the desired negative grid bias compared to the, corresponding cathode.

' In the above-described known circuit, neither of the two valves utilizes the whole of the voltage generated between the terminals A, A, a

circumstance which is disadvantageous for various reasons. There is ,alimitation .ot the grid amplitude which is capable of amplification with out distortion, and moreover, the second valve is not in a position to. give itsmaximurn output. Further, the action of the first valve which is already encroached uponas a result of the provision of the coupling resistance 1. (dropof potential) is still f urtherreducedso that the advantage of the direct'coupling for distortionless amplification is not made full use of.

By means of the invention, the removal of these disadvantages is made possible. The basis of the invention is formed by the recognition that there are various advantages in-supplying to the first valve a considerably higher (e. g. twice or several times'highem voltage than to the second valve. As regards the first valve, ;.there is clearly apart of the feed voltage lost in the coupling resistance. and this part is greater theigreater is 40 chosenthe coupling resistance with the object of obtaining .a higher, amplification;

As now the feed voltage of ,the first valve is considerably raised, the significance off this loss is reduced not only for the reason that the valve itselihas at its disposal a sufiicient voltage in order to ,be able to work under favourable condi tions, but there moreover arises the possibility of employing a coupling element whose resistance is largefcompared with the internal resistance of the valve and which is so suited to this valve. that the, distortionless amplification attained reaches a very much higher value than in known circuits. The arrangement according to thein vention isof far reaching significance, especially if the first amplification stage is equipped with a screened grid valve, which in general has a considerably higher internal resistance than a threeelectrode valve.

The raising of the feed voltage of the first valve is attained, according to the invention, in that, besides the usual feed device, there is employed afeedhdevioewhose voltage; for the first valve, is added to that of the existing feed device and which, however, supplies no energy to the second valve. There is thereby the ,fur ther possibility of employing the whole voltage cf the usual feed device for supplying the secondva1ye and thereby to amplify without distbrtionagreate energy. I

Another and not less importantadvantag'e of the above described method of feeding is present in that the drift effect which makes itself so uhpleasantly noticeable with direct-coupled amplifiers-and which has up to the present stood the way of thefpractlical application of jsuch amp-lifier's, is completely removed. In the circuit arrangement according to Figure 1, there is the dini'culty'thatthecircuits of the two valves mutu= ally reactionaccount of'their connection with the: sainefeed device so thatthe stabilityof the I amplifier as a Whole is endangered and in conse quen e' distortionlessamplification is quite 131- possible.. With-nothing to apparently disturb the state of equilibrium vof the system, the plate current of the 'last tube begins to drift until it iqm e e 10 i. ofliii aI d; t e ri isj'entirelyfoif the cor'rect portion of its operating on racteristici difficulty is either altdg'ethr er unavoidable or avoidable only with the aid of special compensating. circuits. On the ether hand, with the arrangement according to the invention, in which use is made of additional anode feed device which only supplies energyfto the first but not to the secondvalv'e, this diffi:

culty no longer arises. The arrangement is stable withoutmore ado-. II

The second additionalfeed d only the firstvalve, need onl put. In a direct-current inpli w I serves mainly only for voltage 'a' season, and in consequence of this' it tali esfa much smaller current than thesecond valve which' ha's, to supply the consumption circuit (for example, a loud speaker) I with energy. I I I I f t I I consequently, even d sregarding the remaining advantages; the efiiciency ofi he amplifying ar rangement according to the inventionis consider ce, as it feeds ablybetter'than that oflinown circuits which it is endeavoured to obtain a better'amplification by'raising the feed voltage. In he latter case it is, .of course,fnecessary todinie'nsion the whole feed device according tothe current taken by the second valve. There is thereby occasioned'a high continuous loss 'of energy. According to the invention, a considerable adi /"ance is "therefore ob tained'not only 'as regardsamplificationbut also inthe economy of the whole arrangement with small costs. I

Referring now'to Figures? and 3 ofthe'acc'oI r'ipanying drawing, Figure Zrepresents adii ec'te' current amplifier consisting of 'two'valves with an indirectly-heated screen-grid valve I, which is connected by means of va coupling resistance 1 withIa threeaelectrode valve II, 'i Sarje' theinput terminals, and 17, the output terminals 'of the amplifier which "are connected to the, arrangement through a transformer'lfi. t I II I Forf'eeding the two valves there are provided rv a small out first va e according to the invention, two feed devices A and B (which are surrounded with dotted lines) and each assumed to consist of a transformer fed from the mains, with an appropriate rectifier. The feed device A consists of the transformer 4 and a rectifier 6, the feed device B of the transformer 3 and the rectifier. 5. Thetwofeed devices are connectedwith one another-at the point 2 and are connected to the two valvesin such a zmanner that there is fed to the anode circuit of the valve, l the sum of the two voltages, but to the anodecir'cuit of the valve 2, onl; the voltage of the feeel device;B, Ete ckoned from the point 2 outwards-,athe. twofeed devices therefore generate er grid amplitude and to obtain a greater ohtputf opposed voltages.

I A smoothing circuit is connected with the feed device A which is formed by a resistance 11 and the choked and two condensers 8 and '8 and the other of two resistances "9 and 9' and two condensers 10 and 10''. The direct voltage of the point E is therefore again positive compared with that of the point D. II t FroniFigure 2 it isseen that the one end of the coupling resistance 1 inthe anode circuit of the valve I, is connected with the point E so that between this point E and'thecathode side of the valveI there" is moviued'a jfeed'voltage corresponding to the of the voltages of the two feed de'vices A and B; The resistance 13 and the condenser '14 connected in parallel with it in the cathode connecting conductor of the valve I serve in known manner to conv y automatically to the grid of this valve the desiredv negative grid bias.

Regarding the valve II, it isseen that this receives between thecathodeiand the output transformer 16, a feed voltage afforded only and deter mined by the feed device The anode'conduc tor 18 is connected with the output circuit of the feed device B behind thefirstsmoothi'ng circuit 7, 8, 8'; the smoothing jc irc'uit 9, 9', 10, 10' infiuences, therefore, only the anode current of the valve 1. The reason of this connection will be more fully explained below.

On consideration of the voltage distribution over the who'le amplifier arrangement, it is seen that the cathode of the valve II has a positive voltage compared withthat or the valveI which is approximately equalto the voltageof the fee deviceA. I I I The negative grid bias of the valve II is obtained by virtue of the fact that in the coupling resistance 1 there is'produced such a drop of potential that the potential ofjthe'anode'of the valve I is somewhat lower than'that of the cathode of the valveII or thatofthepoint of connection 2 of the two feed devices 'A and B. The screen grid of the valve I is'only'fedfrorn the feed device A and assumes through the drop of potential in the resistance 19 a potential which is again somewhat lower than the anode potential of this valve. In the above-mentionedcircuit the valve I as well as the valve II receives a feed voltagewhich ensures an optimum for the amplification under the special'circumstancesdetermined for these valves. I I I Asthe first valve'only takes a very small curr nt rrom the series connection of the two feed devices, in consequence of the coupling resistance 1,'the feed device A is onlyvery lightly'loaded; it,

therefore; needs to be designed "'onlyfdr .a' small output' which is advantageousto the emciency of.v 'the whole arrangement. Further, the direct voltage afforded'by the feed device A, will be approximately equal to the peak value of thevoltw age generated in the secondary winding of the transformer 3. By means of this high voltage;

one isiin" a position to adapt the icouplingu'esistance 1 in an appropriate manner to the valve I.

As already mentioned, there thus occurs a'high amplification, while the valve, however, receives a sufficient voltage to enable it to work on the kinds of valves may be employed with equal:

success.

The feed device B is provided with two smoothing circuits for the reason that practice has shown that thefeed current of the valve I should be according to Figure 2.

usually smoothed more than that of the valve II;

"This is attained in that the usual smoothing circuit '7, 8, 8' of the feed arrangement for the end valve II is arranged with a second smoothing circuit 9, 9, 10, 10'. As, however, is to beseen. from Figure 3, the connecting in series of the two smoothing circuits is not absolutely necessary.

V Figure 3 represents a somewhatldifierentcircuit arrangement of the direct-coupled amplifier according to the invention in which a simplification has been provided in the formation of the two feed devices, whereby the cost of the arrangement is reduced. The operation otherwise agrees completely with that of the arrangement In the'arrangement of Figure 3, the two recti fiers 5 and 6 are connected with the secondary;

winding of the same transformer 4. Thus, therefore, the two feed devices A and B are, as it were, connected with one another, whereby one transformer is saved. The low loading of the rectifier 5 for the feeding of the valve I makes this change possible without further ado. In order to be able to regulate within certain limits the voltage afforded by this rectifier, a tapping-off switch is provided between the rectifier and the secondary winding of the transformer 4.

The circuit arrangement according to Figure 3 deviates further from that accordingto Figure 2 advantage that the drop of potential arising from the anode current of the valve II in the choke 7 is not subtracted from the feed voltage intended for the valve I.

In the circuits according to the invention, it is advantageous to connect the point of junction of the two feed current devices to earth. The advantage is thus obtained that the maximum voltage with respect to earth is reduced. This choice of earth connection is, however, not ab- On the other hand, the whole solutely necessary. The earth connection could be made in the usualmanner on the cathode ofzthe valve I.

'Ifqthe point of connection of the two feed devices is earthed, it is obviously necessary to insulatethe input circuit of the amplifier from the amplifier stages which may precede it, or from other devices, and this may be brought about, for example, by the use of an input transformer. v

The supply to the cathodes of the amplifieror rectifier valves is not represented either in Figure 2 or in Figure 3 as this forms no part of the invention.

It is to be understood that the valve I may act not only as amplifier but also as detector.

Also it is obvious that the above-described circuit arrangement, insofar as it concerns the rectifier, may be made double-acting (full wave rectification). Usually such an arrangement will only come into consideration with very large outputs. As a result of the very high voltages with which the arrangement according to the invention works compared with known circuits, the

smoothing condensers employed in most cases completely suffice to bring about a sufiicient smoothing with simple rectifiers also.

. What I claim is:--

1. A direct coupled electric amplifier comprising a first valve and a secondvalve, a high voltage feeddevice comprising a rectifier and having its terminals connectedwith the cathode of the first valve'and the anodes of both valves respectively, an additional feed device comprising a rectifier and having its terminals connected with the cathode of the first valve and the cathode of the second valve respectively, said additional feed device having a connection for the anode of the first valve through the high voltage feed device.

2. An amplifier according to claim 1, in which the point of connection of the two feed devices is connected to the cathode of the second valve, the other terminals of the two feed devices are respectively connected with the anode conductors of, the two valves and with the cathode conductor of the first valve, and smoothing circuits interposed in each of the connections from said other terminals.

3. A direct-coupled electrical amplifier comprising two valves, a high voltage feed device, an additional feed device, a connection between the two. devices, the common point of connection between the two devices being connected to the cathode of one of the valves, the opposite terminal of the high voltage feed device being connected to the anodes of the two valves, the opposite terminal of the additional feed device heingconnected to the cathode of the other valve, and smoothing circuits interposed in the connections from said opposite terminals.

4. Amplifier according to claim 1, in which the point of connection of the two feed devices is connected to the cathode of the second valve, the opposite terminal of the one feed device is connected to the anode conductors of the two valves, the opposite terminal of the other fed device is connected with the cathode conductor of the first valve, andthe feed device providing the anode conductors of the two valves with current is provided with two smoothing circuits, of

which one is interposed in the connection between said feed device and the anode conductor of the first; valve, and the other is interposed in alricde contiuetor'of the secia'fid valve'or the" Ode endiictors or :bothvalves. 1

5. A direct-coupled electrical amplifier accbrd iii-g: tdlaini 3 in which thefirst' valve 'h'aea screen grid connected through a-fGSiStflllG with the" point (if 'comiec'tibn of the two? feet! devices.-

6. A direct coupled electrical amplifier accord ing to claim '3 in which'the high voltage feed device emdthe' additional feefi device comprise 9L" c'bmmen secondary winding ore; fee'd transformer and two separate rectifiers,one end of salidiwind-T ing' beingconnectedfio the cathode of onebf the valves-and other points df said Winding beingconncted'to the anode 'of one rectifier and to the cathode of the other rectifier respectively;

" BAREND ALBERT JA-N TEN BRINK;

Images