US2458849A - Direct-coupled amplifier with direct-current feedback - Google Patents

Description

Jan. 11, 1949. R. J. GRAMBSCH- 2,458,849

DIRECTv COUPLED AMPLIFIER WITH I DIRECT CURRENT FEEDBACK Filed Aug. 20, 1945 FIG.I

' responsive to Patented Jan. 11, 1949 grasses DIRECT-COUPLED AMPLIFIER WITH DIRECT-CURRENT FEEDBACK Russell J. Grambsch, Marshfield, Wis. Application August 20, 1945, Serial No. 611,659 1 Claim. (01. 179-171) The present invention relates to electronic amplifiers, and it relates more particularly to a multi-stage direct coupled electronic amplifier having an inverse feedback characteristic.

It is often necessary to have high gain amplifiers which are adapted to pass pulse-type signals of electrical energy. Due to the character of the pulses, it is necessary, in order to maintain signal fidelity, that such an amplifier have a wide band pass characteristic. characteristic are often referred to as video amplifiers, and if a high gain is desired, they usually have a plurality of amplifying stages.

It is generally desirable that video amplifiers be both large amplitude and small signals- In'present multidifliculty is encountered in amplitude pulse-type stage video amplifiers,

' amplifying both large amplitude and small amplitude positive pulses,'and to still provide an output having a, waveform similar to the original input signal.

Most of the difficulty encountered in present video amplifiers is due to large amplitude pulses overloading the amplifiers, thereby causing grid current to charge the coupling condensers used between stages of the amplifier.

It is to be understood that a positive pulse as mentioned above and as hereinafter referred to is a pulse having a positive going leading edge and a negative going lagging edge. Similarly, a negative pulse is one having a negativegoing leading edge and a positive going lagging edge.

It is an object of the present invention to provide a multi-stage video amplifier whose output waveform is of substantially the same shape as the waveform of the input signal.

It is another object of the present invention to provide a Video amplifier which cannot be overloaded by large amplitude signals.

It is yet a further object of the present invention to provide a reliable multi-stage, direct coupled video amplifier which is substantially non-critical with respect to circuit components.

The invention, however, will be more fully understood from the following description, when taken in connection with the accompanying drawing wherein:

' Fig. 1 shows a simplified schematic diagram of V a conventional condenser coupled amplifier; and

Fig. 2 shows voltage waveforms which enable a 7 better understanding of the present invention; and g Fig. 3 shows a'schematic circuit diagram of one embodiment of the present invention.

An understanding of difficulties encountered in 1 conventional condenser coupled video amplifier- Amplifiers having such a so as to prevent this large negative charge from apparent that if a number of tion after the duration of the pulse. This surge in the opposite direction may for simplicity stages may be obtained by reference to Fig. 1. Assume that a negative pulse is applied to input terminal 2 and that a resultant amplified, but

as that developed at anode 4. A large amplitude signal drives vacuumtube I4 into grid current, and a low impedance charging path is provided for coupling condenser 8, the result being the side of the coupling condenser nearest control grid I2 attains a large negative charge. This charge may be sufficient to cut off vacuum tube I4. The discharge path for coupling condenser B is then pro- 'vided through resistance I0.

As the discharge time constant is long (coupling condenser 8 is large), condenser 8 maintains its charge for an appreciable length of time and vacuum tube I4 remains cut off. It is therefore apparent that if the large amplitude signal is closely followed by a signal of low amplitude, said second signal cannot cause vacuum tube It to conduct and hence said second signal is not amplified and does not appear at anode I6 of vacuum tube I 4.

On the other hand; if condenser 8 is made small being held by the condenser and hence the loss of small signals, it is diflicult to maintain the waveform of a pulse-type signal. The effect of smaller coupling condensers is shown in Fig. 2. In the drawing, the applied input signal is shown as Fig. 2a. The signal at control grid [2 of vacuum tube I4 is shown as Fig. 2b. It should be noted that the coupling condenser charges during the pulse causing a sag in the top of the pulse and causing a surge of voltage in the other direcvoltage be referred to as an overshoot and such overshoots are highly objectionable.

The waveforms shown in Fig. 2 have been considered only for one coupling circuit and, in fact,

in most video amplifiers, a number of amplifying stages are present and hence a number of coupling circuits. Thus, if the coupling condenser is made small, a number of overshoots will be produced, the number being proportional to the number of coupling condensers. It is therefore such stages of amplification are present, more pulses than were originally applied appear at the output, and the signal may be distorted so much as to be of little use.

Another example of difficulties caused by these undesirable overshoots is when a series of unequal amplitude pulses are amplified and employed to actuate an associated electronic circuit. Even in such aninstance where fidelity .of waveform is not of paramount importancea negative overshoot is troublesome. This condition is illustrated in Fig. 2. In the drawing, Fig. 2c shows a large amplitude positive pulse closely followed by a small amplitude positive :pulse. Fig. "2d shows how this small amplitude :pulse maybe superimposed on a large negative .overshoot lin the positive pulse. It is therefore evident that this results in the loss of the small amplitude pulse.

In view of the foregoing description, it is evident that a number of difiiculties are encountered in-obtaining a video amplifier of the desired characteristic. Briefly, if the coupling condensers are too large, the waveform is maintained, but small signals are'bloc'ked or'lost. If the coupling condensers are smaILthe-WaVefQrm is-distorted,

'overshoots are introduced, so as to introduce extraneous signals and small signals superimposed on =overs'hoots may be lost. 'Thus video amplifiers must be designed for specific purposes andaregenereilly responsive only for signals of a distinctive :character. The present invention -overcomes thesedifiiculties by means of a'novel yet simple and ireli-able electronic :amplifier, said amplifier'consisting basically of two direct coupled stages having 'an'inverse "feedback charac- Lteristi'c. One particular advantage of the direct coupling used in the present invention is that the circuit is substantially non-criticalwithrespect to circuit components "such as vacuum tubes. The functioning ichara'cteristics 'of this circuit -Wl1l nowice described.

Referring now .to Fig. 3, which shows one emb'odiment of the *presentinvention, consider a "negative pulse-"type signal applied to input terminal :21). :input coup'lin'g capacitor '22 and is developed across .grid ieak resistance 124,:said .gri'd leak resistance wbein'g :connected between control grid 26, and cathode 28 of vacuum tube 31]. The

negative pulse applied to control grid 26' causes "a positiveipulse to the "developed at anode 62 of vacuum tube 'r3il,:'sai'd anode .-32"being *connectedthrough resistance 34 andresistance flfi to 'asuitalzile source :of positive potential "38. The

function of "resistance :34 is wto provide :inverse feedback for vacuum tube ill as will be "more fully explained "hereinafter. The positive pulse developed at anode "32 is then applied directly by means o'f connectionfi to control grid 44 of vacuum tube"40 to cause anegative pulseto =be develop'edzat anode 46 of vacuum tube 40.

Assume'that the negative'pulse appliedto input terminal '20 is o'f sufficient magnitude to drive vacuum tube '30 to cut-off thereby developing ,a L: largeamplitude positive ,pulseatanodejZ. .Al-

though .the ipositive ;.pulse .causes .grid scurrent (from vacuum tube-451) to .flow through resistance 34 and resistance 35, there is no coupling condenser to 'be' charged. Therefore, ,lpractically .as soon as the -lar-ge -negative pulseis removed from input'terminalrzii the circuit returns to normali'an'd is then'able'to respond to a smallamplitude 1 pulse. Relatively large, :undistorted negative pulses'may thus "be -dew-doped at :=an0de 46 The negative signal :passes through of vacuum tube 40, and they may be taken through condenser 48 to output terminal 50.

The direct coupling from anode 32 of vacuum tube 30 to control grid 44 of vacuum tube 40 causes a positive voltage (with respect to ground) on said control grid. Therefore, cathode 52 of said vacuum tube must be raised above ground to obtain prqper bias for vacuum tube 40. This may be accomplished .E-by connecting cathode 52 of vacuum tube '49 through cathode biasing resistance 54 and by-pass capacitance 56 to ground.

In order to maintain a steady bias on cathode 5? :and .to it'hus gprevent degenerative feedback, Which'reduces the gain of the amplifier, capacitance 56 .should be reasonably large. This requirement normally causes difliculty, in that large i'amplitudegpositive ;pulses applied to control grid 44 causes by-pass condenser 56 to charge up, the result being that a large bias is maintained on cathode 52. This mayibe suificient to make vac- 'uum tube 40 non-wresponsive to small amplitude pulses. This :undesira'ble eifect is counteracted in the present invention by the :provision {of an inverse -"feedback voltage applied from anode :45 of vacuum tubez4fl, :through inverse rfeedback .resistancelii land connection 412 to control grid 44 of said :vacuum tube. The action :of this inverse feedback lm-ay be readily understood by again considering :a -.-rel-atively .large amplitude :nega- 'tive :pulse. applied to sinpult terminal :20.

Such a pulse :is amplified .and inverted in phasebyvacuumztube 3.9 :andcis applied as a positive signalrtoicontrolggrid:44 of vacuum tube All. If lthiS rpulse acauses increased anode current in :vacuum tube Mil, ithe :charge on by-pass condenser :56 --is likewise :increased, thereby :causing a corresponding increased bias voltage 'tobe developed sacross .zrcathode :resistance .54. This bias voltage ismaintained iby- -condenser I 56 -.even after .the :negative :pulse has been :removed frominput 'terminal'ifl. lI-he'ieffeQt of this :bias is to-cause the :anode current through vacuum tube 40 to decrease, "thereby causing :anode -46 to rise in potential. This rise :is communicated through inversetfeedback resistance -34-zand connection .42 wto :control grid- 4 4-.

Thus, it pan :be seen that the increased potential 0f tcathode 52 has caused the potential -of controlgrid 44 to :also :rise. ,In other words, :thezpotential ionzthe controlagrid ffollows the cathode potential as capacitance 56 ,is charged. there is :then very .little difference between thergridttocathodepotential, vacuum tube 40 is :able to respond (to :a small ,-signal applied ,to input terminal :20 land .it is apparent that the zentireamplifier is responsive-to both large amplitude and small amplitude :pulses.

By-pass condenser 56 may be eliminated in certain .instances without too great ,a loss in amplification. iI-This is qespecially. true if .cathode TBSlSt32I1Gr54 is madeassmallas practicable. such :as byrnperating vacuum tube 3.0 ata lowanode voltage.

iAnother radvantageiof :the .pres.ent invention is that due to, the inverse ifeedback characteristi the circuit is substantially non-criticalwithregards to variationin .ci-rcui-t components. By way -;of example, -,consider the effect ;of changes iinsthe characteristics'of the-vacuum tubes. It is zwellrknown that-the emission of vacuum tubes may change with age and it is also knownthat .vacuumtubes of thewsame typez often vary slightly in electrical(characteristics.

:Assume fthatazthezemission :from :cathode :28 of avacuum tube .='3llsshould decree se thereby-causin a smaller current to flow through inverse feedback resistance 34 and resistance 36. The effect of this decreased emissivity causes the potential of anode 32 to rise, this rise in voltage being transmitted to control grid 44 of vacuum tube 4!}. This rise in potential in control grid 44 causes an increased anode current to flow through anode load resistance 36 and thereby decreases the voltage at anode 46. This decrease is communicated through inverse feedback resistance 34 and connection 42 to control grid 44 thus maintaining the voltage at control grid 44 substantially constant.

In a similar manner, an increase in emissivity causes anode 32 of vacuum tube 30 to drop in potential. This drop in potential is then communicated through connection 42 to control grid 44 thereby causing the potential of anode 46 to rise. The rise in potential is transmitted through inverse feedback resistance to control grid 44 thereby maintaining the voltage of said grid substantially constant.

It is to be understood that although the above invention has been described in connection with pulse-type input signals, the amplifier is adapted to be responsive to other types of input signals. Similarly, the invention is not to be construed as being limited to a two stage amplifier but may consist of a series of cascaded stages.

While there has been here described one embodiment of the present invention, it will be manifest to those skilled in the art that various changes and modifications may be made therein without departing from the invention. It is therefore aimed in the appended claim to cover all such changes and modifications as fall within the spirit and scope of the invention.

I claim:

A multi-stage video amplifier having at least a first vacuum tube and a second vacuum tube, each of said vacuum tubes having at least a cathode, an anode, and a control grid, a resistance connected between the anodes of said Vacuum tubes as a load resistance for said first tube and feedback resistance for said second tube, a further load resistance for both tubes from the anode of said second tube to a source of anode current, a direct electrical connection between the anode of said first vacuum tube and the control grid of said second vacuum tube, means for raising the potential of the cathode of said second vacuum tube above the potential of the cathode of said first vacuum tube, means for applying a signal to be amplified to the control grid of said first vacuum tube to provide a signal at said anode of said first vacuum tube whereby said signal is applied to the control grid of said second vacuum tube by said direct-current connection and whereby an amplified output is obtained at said anode of said second vacuum tube.

RUSSELL J. GRAMBSCH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,173,426 Scott Sept. 19, 1939 2,261,356 Foster Nov. 4, 1941

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