US2198323A - Amplifier - Google Patents

Description

April 23, 1940. WAGNER 2,198,323

AMPLIFIER Filed D80. 30, 1936 nvpz/r 52 /5 1 25/ 28 AAAA AAAAAA v V INPUT INVENTOR HERBERT M. WAGNER Patented Apr. 23, 1940 joFFics Herbert M. Wagner, Newark, N. 1., assignor, by

meme alignments, to Radio Corporation or America, New York, N. Y., a corporation of Delaware Application December 30, im, Serial No. 118,220

My invention relates to electron discharge devices and circuits therefor particularly suitable for the amplification of audio frequency voltages andcurrents. i i In one form of electron discharge device. referred to as a beam tube, there is provided a straight tubular cathode having emitting sections spaced along its surface so as to provide electron beams directed toward the anode suri rounding the cathode and an intermediate control grid, the wires of the grid being positioned opposite the non-emitting sections 01' the cathode. In tubes of this type the control electrode or control grid draws relatively small currents even though the control grid becomes positive with respect to the cathode,'substantially all of the current going to the anode in beams between the grid wires. Such a tube is particularly usein] for amplification purposes and is capable of relatively large outputswith small distortion oi the kind usually due to grid current, which in the conventional type of tube causes considerable distortion. Notwithstanding the practically negligible grid current in this form of tube, when used in conventional circuits there is still a possibility of some distortion caused when very small currents flow to the grid and cause a. variable voltage drop in the grid circuit due to its impedance.

It is the principal object of my invention to provide an electron discharge device and circuit therefor especially suitable for audio frequency amplification and capable of delivering at high eiiiciency a practically distortionless signal of high power to an output load, and particularly to provide an amplifier making use of an electron discharge device which draws a comparatively negligible control grid current when the control grid swings positive.

In the preferred embodiment of my invention I make use of a low impedance input circuit and electrically couple this circuit to the input of a beam type of tube. This provides a voltage source .or input circuit of low internal impedance for a beam tube having a low control grid current characteristic, the resulting combination providing an amplifier capable of delivering a high power output at very high efilciencies with practically no distortion.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing in which 3 mm; (c1. na -111i Figure 1 is a vertical section through one ior n of beam tube forming a part of my invention and Figures 2 and 3 are circuit arrangements incorporating the beam tube shown in Figure 1.

.In Figure 1 is shown one form of beam tube which can be used in my invention. The envelope III is provided with the usual press II which supports the electrodes. The cathode I2 is provided with a plurality of spaced emitting sections l3 along its length. These emitting portions may be formed as recesses to assist in the formation of beams, so that the electrons will pass between the grid wires of the control electrode IE to the anode IS, the grid wires being opposite the nonemitting sections of the cathode. Rods I! are preferably secured to the cathode parallel to the grid side rods and mask the portion of the cathode opposite the grid side rods by splitting the beams and thus prevent the grid side rods from drawing current. While therewill be some deflection of the electrons of the beam by the grid wires particularly at the lower and upper portions oi. the beam when the grid goes positive, because of the position of the grid wires and their proportion the grid, even when it goes positive, will draw only a relatively small grid current. A tube of this type is described by Harry 0. Thompson in the Proceedings of the Instituteof Radio Engineers, October, 1936, pages 1290'to 1292 inclusive. Under the same operaiting'donditions two tubes identical in all respects except for the oathode showed the following results which indicate the wide difierences in grid current between a tube with a conventional fully coated cathode and a tube having a beam forming cathode such as described. With a beam forming cathode and with the anode and grid at 60 volts positive potential the grid current was 2.7 ma. and the plate current 93 ma. With a conventional fully coated cathode with the anode and grid again at 60 volts positive potential, the grid current was 56 ma. and the plate current 152 ma. In the first case the grid current was about 3% of the anode current and in the second case about 37% 01' the anode current. Comparison of the beam tube with other conventional tubes shows the same striking differences in grid current. Thus while a tube of the beam type is particularly suitable for amplification purposes and far superior to the conventional type of tube, it is still subject to some distortion when used in conventional circuits.

The reason for the distortion when the grid of an amplifying tube draws current is known to be due to the drop in voltage across the input caused by the unavoidable impedance oi the input circuit. This condition makes it necessary to bias the grid negatively to prevent 'it drawing current and limits the permissible swing on the grid to 5 the region or negative bias with a resulting loss or efliciency. If the input circuit has relatively negligible resistance or impedance and is capable of supplying all of the grid current. necessary there will be very little loss in the input circuit and the amplification in the output oi the ampliiying tube will be a true and accurate reproduction oi the input. That is, if the grid driver impedance is low the variable voltage drop is low and hence the distortion will be low. In con- 1 ventional circuits using conventional tubes, this condition cannot be obtained.

In Figure 21 show one preferred embodiment of my invention. In the figure the envelope l0 of the beam tube contains the beam type cathode IS, the control grid l4 and anode it, which form the output section of the amplifying unit. Any form of tube which has the characteristic of drawing a negligible control grid current even though positively biased can be used however.

The input circuit for the tube includes the driver tube 20 provided with cathode 2|, grid 22 and anode 23, the grid being connected to the input terminal 24 and the cathode through the cathode resistor 25 to the input terminal 26, the

input circuit including the resistor 21 electrically connected across the input terminals. This cathode resistor provides the input circuit for the beam tube and has a comparatively small im- The cathode 2| of the driver tube 20 tube by means of the condenser 28, a biasing and leak resistor 29 being connected between the grid I I and the cathode l3. The anode and cathode of the beam tube are connected to the output terminals 30 and 3|, the anode being connected through the output transformer 92 to the positive side of the voltage supply source 93 to which the anode 29 of the driver tube is directly connected' In one example of my inet'vention which is capable of delivering a substantially undistorted output of about 5 watts the cathode resistor 25 is of the order of 3000 ohms, the condenser 28 has a capacity of .5 microfarad, and the resistor 29 a resistance of 9000 ohms.

In operation with no signal applied to the grid 22, which is normally negatively biased, a certain amount of current 'will fiow through the tube causing a drop across resistor 25. The grid H 56 of the beam tube however is at zero or ground 60 nal received, this voltage being applied through condenser 28 to the grid ll of the beam tube. With the conventional tube when the grid I4 is swung positive, a grid current will flow causing a flow of current through resistor 29 so that the grid will become considerably negatively biased, this bias building up so that the tube will go practically to the cut-off condition. that is the grid will become so negatively biased that very little, if any, response will' be caused in 79 the output circuit. With the beam tube due to the fact that only very small grid currents are drawn, there will be very little bias developed across the resistor 29 even though the grid should swing positive. Furthermore, because 01 the relatively small grid current and the ctfective low impedance of the cathode driver cir cult, the loss of input voltage appearing acros resistor 28 is relatively small. The current iiow ing in the anode circuit it through the primar oi' the output transformer 82 is therefore a rela tively true reproduction oi. the voltage applied t the grid ll from the cathode circuit of the driv er tube 28. In this way an input circuit com prising the cathode circuit of the driver tub and having a comparatively low impedance 1 combined with a tube drawing a relatively smal grid current, the combined result being an am plii'ying unit or circuit arrangement in which th output may be of comparatively high power am yet at the same time provide a distortionless sig nal. That is, the input voltage swings will b truly and accurately reproduced in the outpu of the beam tube.

In the modification of my invention shown i1 Figure 3 I eliminate the coupling condenser 2 and the resistor 29. In this circuit arrangemen the grid is always operated in the positive re gion, the minimum swing being limited to zer swing in the negative direction. Here agaii because of the low grid current and low cathod circuit driver impedance what small current ma: be drawn by the beam tube III will cause a rela tively negligible loss in the input voltage so the the reproduction of the impressed signal will b true and accurate.

While I have indicated the preferred embodi ments of my invention of which I am now awar and have also indicated only one specific appli cation for which my invention may be employed it will be apparent that my invention is by m means limited to the exact forms illustrated o the use indicated, but that many variation may be made in the particular structure use and the purpose for which it is employed withou departing from the scope of my invention as se forth in the appended claims.

What I claim as new is:

1. An amplifier including an electron discharg device having a cathode, a control grid and a1 anode, and means for forming a beam oi 'elec trons, said control grid lying outside the pat] of said beam, a second electron discharge devic including a cathode, grid and anode, a resisto in series with the cathode of said second elec tron discharge device, input terminals connect ed to the grid of the second electron discharg device and the end of said resistor remote from the cathode of the second electron discharge de vice, the grid of said first electron dlscharg device being capacitatively coupled to the oath ode of said second electron discharge device, am the cathode of said first electron discharge de vice being connected to the end of the resisto remote from the cathode of said second electroi discharge device, and output terminals connecteq to the anode and cathode oi said first electron discharge device.

2. An amplifier including an electron dischargi device having a cathode provided with means to forming beams of electrons, a grid, the wires 0 which lie outside the paths of the beams o electrons and an anode, a second electron dis charge device including a cathode, grid and an ode, a resistor in series with the cathode oi sail second electron discharge device, input termi' nals connected to the grid 01' the second electroi discharge device and the end of said resistor re mote from the cathode of the second electron discharge device, the grid oi said first electron dis charge device being capacitatively coupled to thi cathode of said second electron discharge device, and the cathode of said first electron discharge device being connected to the end of the resistor remote from the cathode of said second electron discharge device, and a resistor connected between the grid and cathode of said first electron discharge device, andoutput terminals connected to the anode and cathode of saidfirst electron discharge device.

3. An amplifier including an electron discharge device having a cathode, a grid and an anode, and the characteristicpf drawing a comparatively low grid current'when the grid is positively biased, a second electron discharge device including a cathode, grid and anode, a resistor in series with the cathode of said second electron discharge device, input terminals connected to the grid of the second electron discharge device and'the end of said resistor remote from the cathode [of the second electron discharge device,

the grid of said first electron discharge device being capacitively coupled to the cathode of said second electron discharge device, and the cathode of said first electron discharge device being connected to the end of the resistor remote from the cathode of said second electron discharge device, and output terminals connected to the anode and cathode of said first electron,

HERBERT M. WAGNER. 20

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