US2424893A - Amplifier circuits - Google Patents

Description

. July 29,1947. H. l.. MANSFORD 2,424,893

AMPLIFIER CIRCUITS l Flled Aug 22 1945 *x -vWs/L Q Patented July 29, 1947 u ,n

UNITED STATES 4- PATENT OFFICE AMPLIFIER crncm'rs v Hugh Lyon Mansford, ckenh'am, Imglanel,as-A

signor to Electric &Musical1ndustries Limited, Hayes, England,l Aajcompany-oi -Great k"Britain Application August 22, 1945,;se14a1No. 612,033

In Great Britain April 2 4, 1944 ,section 1, P'ubiicLaw cemnugust s, 194s.

`Patentexpres April 24, V1964 The present invention relates to amplifiers and more particularly to ampliers operating down to lovv frequencies including direct current(D.'G.').

In known ampliers the tubes of the ampliiier are usually coupled in cascade so that the anode circuit of each tube is in parallel with the v"Source of anode voltage.v When low frequencies, including D. C., are to be amplified, the couplingsbetween successive tubes are usually such as will pass D. C. and generally include means for providing grid bias so as to avoid impressing the positive voltage of the anode of one tube upon the grid of the next tube. v j

Known arrangements of this kind suffer `from the disadvantage that instability can ariseas 1va result of back-coupling via the impedance of the source of anode voltage.

It has also been proposed to connect the -individual tubes of a multi-tube amplifier with their anode-cathode paths in series across the source of anode voltage. This reduces the instability due to coupling via the impedance of the :sourcev of anode voltage but has the great disadvantage that because substantially the same current 'mus-t flow through all the series-connected tubes,-fthe change of current caused by the signal applied to the control electrode of the rst tube of vthe series is the same as the change of current in the rest ofthe tubes. The arrangement is thus equivalent to a single tubeamplier with an increased anode impedance andvvith the output taken from only part of this anode impedance and therefore gives substantially no more amplication than can be obtained from a single tube-amplifier.

The object of the present inventicn'istoprovide an amplifier in which thedisadvantages above referred to are substantially avoided.

According to the present invention thereislprovided an amplifier comprising a source of anode voltage, a vplurality of pairs vof thermionictubes' the tubes of each of said pairs having their cathodes connected to a common point-and theiranodes connected to a common point, means associated with each of said pairs for Vvarying lthe voltage of the control electrodes of each tube of each pair diiierentially'so as to causeequal and opposite changes in the anode currents 'of' both tubes of each pair, whereby in operation vthe total anode curr-ent of each of said pairs remains7 substantially unchanged, and connections ybe tween the common points to whichthe-cathodes comms. (01179-171) f Preferably, said means for varying the voltages of the control electrodes of each of said pairs other than the lowermost pair comprises impedances included between the anodes of the tubes of the preceding pair and the common point to which said anodes are connected and connections between said anodes and said control electrodes respectively. and if desired a resistance may be connected in shunt with at least one of said preceding pairs so that the total anode current of at least one pair is greater than that of a preceding pair.

In order that the said invention may be clearly understood and readily carried-into effect it will now be more fully described with reference tothe accompanying drawings, in which:

IFig.` 1 illustrates a simplied circuit according tothe invention, and

' `Fig. 2 is a circuit diagram illustrating a practical embodiment of the invention.

l Referring now to Fig. 1, reference numerals l, la indicate triode rtubes of the lowerrnost pair having cathodes, control grids and anodes respectively numbered 2, 2a, 3, 3a, 4, ldat. Cathodes `2, 2a aredirectly connected to a common point, namely', to thepositive -pole of -a source of grid bias 6, the negative pole of which is connected to the negative pole of the source of anode voltage (not shown). The anodes 4, 4a are also connected to a common point via impedances 5, 5a respectively,

Signals to be `amplified are applied to grids 3, 3a, in push-pull by Way ci input terminals I and I. The changes of the anode currents of tubes l, la will herefore be equal and opposite and the total anode current passed by these tubes in operation will beconstant. A further pair of tubes, indicated in'Fig. 1 by numerals 1, 1a, have cathodes 8,8m control grids 9, 9a anodes I, Ilia, respectively, their cathodes 8, 8a, being directly connected to the common point to which anodes 4v, 4a of the preceding pair of valves are connected and their anodes I0, Illa being connected to a further common point via impedances Il, lla. Control .grids` 9, 9a are directly connected to anodes 4, Aa., respectively and due to the voltage drop across impedances 1.5, 5a, tubes 1, 1a are therefore driveninpush-pull when tubes Vl la are so driven. AS n :the case of tubes I, la., the-total current ovving into the commonpoint to which anodes t0, 40a are connected ,is constant due to the fact that the current changes `in the anode circuits ofjtubes 1, 'la are equal and opposite and the presence of tubes 11,' la in series with the anode cur- ,polarising voltage.

are similar in principle but differ in detail in one or two respects. Thus, the tubes in Fig. 2 are shown as pentode valves having their suppressor grids connected to their cathodes and having the screening 4grids `of each pair connected together and'viaafeed resistance to a source of suitable As shown, the screening grids of tubes I, Ia are fed via resistance I1 conimpedances I6, IEa is taken to the positive pole of the source of anode voltage so that the same constant current flows through the anode circuits of each pair of tubes I2, I2a, 1, 1a, and I,V Ia in series. It will be appreciated that further pairs of tubes driven in push-pull can if desired be connected in series with the three pairs shown in Fig. 1.

In the circuits shown in Fig. 1, each of the tubes operates as an amplifier for push-pull signals, and, due to the absence of coupling capacities, the ampliiication is maintained down to the lowest frequencies including, D. C. Further, under conditions of proper balance there will be no variations in the current supplied by the source of anode current due to signals and as there is no feedback path whereby voltages developed across the impedance of the source of anode voltage by small out-of-balance signal currents which may arise in practice can be fed back to the early tubes of the amplier, there is vno likelihood of instability from this cause as is the case with the normal cascade amplifier operating down to the lowermost frequencies. A further feature of the-circuit arrangement illustrated in Fig. 1 is that the amplier is relatively insensitive to signals which are applied to the tubes of each pair in the same sense. In so far as signals of this kind are concerned, each pair of tubes may be regarded as being connected in parallel and behaving in a similar manner to previously proposed series connected amplifier arrangements hereinbefore referred to which are equivalent to single tube amplifiers with tapped down anode impedances. This feature is` advantageous in that out-of-balance currents tend to be suppressed and slight ripple on the source of anode voltage does not give rise to any troublesome effects.

It will be appreciated that, provided each lpair of tubes I, Ia, 1, 1a, I2, I2a, etc., are driven in push-pull, the relative phase in I which, for example, tubes I and 1, are driven is immaterial.4 Thus, control grid S can, if desired, be connected to anode 4a and control grid 9a connected to anode 4 instead of as shown in the gure. Further, if it is not desired to amplify the lowest frequencies, direct connections between the control grids and the anodes of the tubes of the preceding stage need not be used and may be replaced, for example, by resistance couplings or transformer couplings. n

As the later stages of the amplifier have to handle 'larger signal current variations than the preceding stages of the amplifier Vit is inconvenient to have all the ypairs of tubes of the amplifier Vpassing the same total anode current. Reference will now be made to Fig. 2 which illustrates a, circuit arrangementV in which this lmitation is avoided and some other modications are introduced-for convenience. In Fig. 2 elements corresponding to elements of Fig. l. are given the same reference numerals and it will be seen that the arrangements: of Figs. 1 and 2 nected to cathodes 8, 8a, the screening grids of ytubes 1, 1a are fed via resistance I8 connected to cathodes I3, I3a and the screening grids of tubes I2, I2u. are fed via resistance I9 connected to the positive pole of the source of anode voltage.- Battery 6 of Fig. 1 has been replaced by the resistance 20 common to the cathode circuits of SLVtubes I, Ia,`and a potentiometer comprising series connected resistances 2| and 22 across the source of anode voltage is provided for adjusting the-bias on grids 3, 3a, grid 3a being connected through leak resistance 23 to the junction of resistances 2I and 22, the input circuit, notshown, providing a circuit feeding the bias Y to control grid 3.

The use of tubes of different current carrying capacities for the different stages of the amplier is made possible by means of shunting resistances 24 Vand 25. Resistance 25 is connected between cathode I3,V I3a and the negative pole of the source of anode voltage and has a value such that it-,passes the difference between the total anode and screen current passed by valves I2, I2a. and the total anode and screen current passed by tubes 1, 1a. Similarly, resistance 24 connected between cathodes 8, 8a. and the negative pole of the source of anode voltage passes the difference between the total anode and screen current drawn by tubes 1, 1a. and the total anode and screen current drawn by tubes I, Ia. It will be appreciated that although these shunting resistances are illustrated as resistances they can if desired take the vform of further apparatus including tubes such as, for example, further amplifiers according to the present invention, providing that these latter. arearranged to operate with a constant total anode current of appropriate value.

L* `lit may sometimes happen that the negative bias applied, for example, to control grid 9 by the voltage drop across resistance 5 is unsuitable and this may be adjusted by the introduction of a suitably chosen resistance between control grid 9U and ranode 4 and an additional resistance of suitable value between control grid 9 and a point of constant positive voltage such as cathodes I3, I3a`or the positive pole of the source of anode voltage. Alternatively, or in addition, resistances may .beintroduced between the common point of, for example, resistances 5, 5a and cathodes 8, 8a.

Provided that resistance 20 is suitably chosen, one ,of the -grids 3, 3a, may be earthed and the signal appliedV unbalanced to the other control grid, the 4signalrvoltage developed across resistancelnrserving to vary the cathode potential of the otherV tube and thereby driving it in antiphase. In vvsome cases, the resistance 20 may taire theform of the anode cathode path of a high impedance tube such as a pentode and the total current'v taken by tubes I, Ia can then be adjusted by` achange of the bias on the control grid and thefscreening grid of this high impedance tube and, if desired, the output of the amplier can be controlled or lreduced to zero by application of asuitable negative bias to this tube. By adding further'tubesy 3. and 3a' arranged similarly to tubes 3, V3a,l and a further tube serving as a common resistance, said tubes 3' and 3a' sharing anode resistances 5 and 5a with tubes 3 and 3a respectively, a convenient switching circuit is provided which enables the signals applied to tubes 3, 3a or 3', 3a' to be alternately fed to the output of the ampliiier as the tubes in the common cathode circuits are alternately rendered conducting and non-conducting by switching signals.

Although the invention has been described with particular reference to amplification operating down to very low frequencies including D. C. it will be understood that it may be used With advantage even when it is not desired to amplify down to such low frequencies.

What I claim is:

1. An amplifier comprising a source of anode voltage, a plurality of pairs of thermionic tubes, the tubes of each of said pairs having their cathodes connected to a common point and their .anodes connected to a common point, means associated with each of said pairs for varying the voltage of the control electrodes of each tube of each pair differentially so as to cause equal and opposite changes in the anode currents of both tubes of each pair whereby in operation the total anode current of each of said pairs remains substantially unchanged, connections between the common points to which the cathodes and anodes of tubes of diierent pairs are connected such that the anode circuits of said pairs are effectively 'connected in series across said source, a pair of input terminals connected to the control electrodes of the first pair of tubes, and a pair of output terminals connected to the anodes of the last pair of tubes.

2. An amplier according to claim 1 in which said means for varying the voltages of the control electrodes of the tubes of each of said pairs other than the first pair comprises impedances included between the anodes of the tubes of the preceding pair and the common point to which said anodes are connected and connection between said anodes and said control electrodes respectively.

3. An amplier according to claim 2 in which a resistance is connected in shunt with at least one of said preceding pairs so that the total anode current of at least one pair is greater than that of a preceding pair.

4. An amplifying system for direct or alternating potentials, comprising two sets of thermonic tubes each of which is provided at least with a cathode, a grid and an anode, the tubes of each set being arranged in series ywith the anode of one tube connected to the cathode of a succeeding tube, a connection between the cathodes of corresponding tubes of said sets, a source of anode potential supply having its positive terminal connected to the anode of the last tube of each set and its negative terminal to the cathode of the rst tulbe of each set, means for supplying the potential to be amplied to the grid of the rst tube of each set, and means for deriving amplified output potential from the anodes of the last tubes of each set.

5. A system for amplifying direct or alternating potentials, comprising two sets of thermionic tubes each having at least a cathode, a grid and an anode, the tubes of each set being arranged in series with the anode of one tube connected to the cathode of a succeeding tube through a resistance, a source of anode potential supply having its positive terminal connected through a resistance to the anode of the last tube of each set and its negative terminal connected through a resistance to the cathode of the first tube oi each set, a connection between the cathodes of corresponding tubes of said sets, a connection from the grid of each tube except the first to the anode of the preceding tube, means for supplying the potential to be amplified to the grid of 'the first tube of each set, and means for deriving amplified output potential from the anodes of the last tubes of each set.

6. An amplifying system as dened in claim 5 wherein a resistance is connected between the common cathode connection lof the rst tubes of each set and the common cathode connection of another pair of corresponding tubes.

HUGH LYON MANSFORD.

REFERENCES CITED The following references are of record in the Name Date Mitchell et al Sept. '7, 1943 Number

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