US2092496A - Amplifier - Google Patents

Description

H. BRANSON Sept. 1, 1937.

AMPLIFIER Filed Nov. 26,

H/.S T MEA/E "Patented Sept. 7, 1937 UNITED STATES PATENT OFFICE Harry Branson, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application November 26, 1932, Serial No. 644,41*!V 'l Claims.

My invention relates to ampliiiers and, more particularly, to amplifiers intended for intermittent periodic operation. i

In connection with recent developments in television, the need has arisen for an amplifier that may be rendered substantially instantaneously operative or inoperative without causing the appearance of disturbing impulses in the output circuit thereof.' Such an amplifier is especially adapted for use in transmitting television pictures and accompanying sounds over a single radio channel and is also useful for the purpose of introducing synchronizing signals in the same channel as that which carries the picture impulses. y

Furthermore, an` amplifier of the type described may be used to reduce the signal from any source to zero at definite periods, such as for use in connection with cathode ray television ltransmitting tubes, and it may also be utilized for introducing more than one signal into a system where the signals must exist at separated times insofar as the entire system is concerned.

Ampliers heretofore constructed for the,

aforementioned purposes have been open to the serious objection that, wheneve' their condition is changed from inoperativeness to operativeness, and vice versa, at rapidly recurring intervals, very objectionable harmonic impulses and impulses occasioned by abrupt variations in the A plate supply to the amplifier tubes appear in the output.l Obviously, these disturbing impulses mar the effect of a received picture whether they appear at the transmitting or at the receiving end of the system, and militate against the usefulness of the amplifier when employed for any of the aforementioned purposes.

It is, accordingly, an object of my invention to provide an amplifier the condition of which may be substantially instantaneously -altered from inoperativeness to operativeness, and vice versa, without causing the appearance of objectionable impulses in the output circuit thereof.

A further object of my invention is to provide an amplifier the gain in which may be instantaneously altered from zero to maximum, and vice versa, without the introduction of a hangover effect, which would interfere with the proper amplification of incomingor outgoing sig- A still further object of my invention is to provide an amplier such that it may be utilized for introducing more than one signal into a transmitting or receiving system, whether radio or carrier current.

(ci. 17a-70)" The foregoing objects and others appurtenant` thereto I prefer to accomplish in brief as follows: I provide in my improved amplifier a plurality of main amplifying tubes connected in cascade and having a final output circuit across which appear the amplified signals. In addition. I provide what will hereinafter be termed a controltube which is utilized to substantially instantaneously control the gain in the Vfirst of the main amplifier tubes in response to a controlsignal. I also provide an auxiliary thermionie tube, the output circuit of which is differentially included in or coupled to the final output circuit of the system and so interconnect the control tube with the intermediate tube that the latter serves to introduce into the nal output circuit impulses that neutralize whatever transient potentials may appear therein when the gain in the first amplifier tube is abruptly changed. i

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and advantages thereof, will best be understood from the following description of a specific 4embodiment when read in connection with the accompanying drawing, the single figure of which is a diagrammatic View of an amplifier including a preferred embodiment of my invention.

Referring specifically to the drawing, my iinproved amplifier includes an amplifying thermionic tube I preferablyy of the screen grid typev having an input or control grid 3, a cathode 5, a screen grid 1, 'and an anode 9. The anode output circuit includes a resistor Il which is connected to the cathode 5 through a source I3 of high potential and a resistor I5.

'Ihe tube I is provided with a grid resistor I1 and it is normally biased to the cut-off potential by a source I9, the positive. terminal of which is connected to the junction between the negative terminal of the high potential source and the resistor I5. The output resistor II is coupled to a second amplifier tube 2i, which may be of any desired type, through a stopping condenser 23 and a connection 25 common to the cathodes of the several tubes.

The output circuit of the second amplifier tube 2l includes a resistor 21 through which plate potential is supplied to the tube from the high potential source.

' The first amplifier tube I, as above mentioned,

is normally maintained in the inoperative con- 50 the said tube.

plied to the grid thereof from the source I9. In this condition, the gain, of course, is zero, and any .-signal which is applied to the input terminals thereof is not amplified.

In order that the gain in the tube I may be shifted from zero to an operative value, I provide a control tube 29 having an anode 3| which is connected to the cathode terminal of the resistor I5, and a cathode 33 which is connected to the grounded terminal of the same resistor through a potential source 34. 'I'he control tube is provided with a grid resistor 35 and with a source of grid bias potential 31, the voltage of which is adjusted to either bias the tube to the cut-off point, or to the point at which it operates best, depending upon the shape of the control impulse. For example, if a source 38 of square top waves is available for control purposes, the control tube 29 is merely biased to the point where it functions as a linear amplifier. If, on the other hand, only a source of sine-wave potential is available for control purposes, the control tube is biased to the cut-01T point, which permits only the peaks of the positive half-cycles to be effective. u

Irrespective of the manner in which the control tube 29 operates, however, a positive pulse applied to the grid thereof permits space current to flow therein, which current, in flowing through the resistor I5, develops a potential that opposes the potential applied to the grid of the tube I from the bias potential source I9. Accordingly, as soon as the tube 29 is made conductive by a control impulse, the amplifier tube r I abruptly becomes operative and the amplified incoming signal is impressed upon the grid of the amplifier tube 2| and appears across the output resistor 21 thereof.

By reason of the abrupt change in plate current flow in the first amplifier tube I, undesirable impulses appear across the output resistor I I thereof. For the purpose of minimizing the effect of such impulses, I provide a neutralizing tube 39, also preferably of the screen grid type having a cathode 4|, a control grid 43, a screen grid 45, and an anode 41. The cathode is directly connected to tlie cathode of the first amplier tube and the control grid is connected to the low potential` end of the grid resistor I1 of The anode of th'e neutralizing tube is connecte/d to the anode of the amplifier tube 2|, the connection including a resistor 49 thgugh which plate potential is supplied to the tu The screen grid 45 may be connected to the screen grid of the amplifier tube and be supplied with potential over a common conductor 5| leading to the highfpotential source I3.

To understand the operation of the entire system including the neutralizing tube, let it be assumed that a signal, representative of picture frequencies or the like, is being impressed across the input terminals of the first amplifier tube I, which normally has zero gain.

At the same time, let it be assumed that a control impulse is being applied periodically to the input terminals of the control tube, such signal being either square top from any desired source, or sine wave as hereinbefore described. Between impulses applied to they control tube, the amplifier tube I is inoperative and it is rendered operative to amplify the signals impressed thereon only during intervals of operativeness of the control tube. 4

The action of the control tube, however, is

abrupt and the surge of plate current in the tube I which flows at the instant the tube 29 unlocks", so to speak, the said amplifier tube causes transient, undesirable potentials to appear across the output resistor II and, ultimately, across the resistor 21. At the same time, however, the auxiliary tube 39 is rendered operative and its plate current abruptly rises to cause the same transient potentials to be developed across the output resistor 49 connected to the plate thereof. It will be noted, however, that the potentials developed across the resistor 49 by reason of abrupt changes in the conductivity of the tube 39 are 180 out of phase with the analogous potentials developed across the -resistor 21. Insofar as the output circuit of the system is concerned, which serially includes the resistors 21 and 49, the transient impulses are can'celled, and do not appear.

From a consideration of the` foregoing, it might be inferred that, if transient impulses are neutralized, the input signals ,would also be neutralized during periods of activity of the amplier tube I. This is not the case, however, since only a fractional part of the amplified signal potential appears across the resistor I5 and such potential, by reason of the circuit connections, when applied to the neutralizing tube, is in proper direction to offset the degeneration caused in tube I by the presence of the said re" sistor.

In describing the operation of my improved system, it has been assumed that the several resistors |I, 21, and 49 have been properly correlated to the gain in the tubes I, 2|, and 39. If the gain in the tube 2| is unity, the resistors 21 and 49 may have approximately the same magnitude. It is, however. difficult to provide a tube wherein the gain is unity and, for best operation, IA have determined that the output resistor should be equal in value to the sum of the resistance of the resistor 49 and the combination` of the resistance of resistor 21 with the plate impedance of the tube 2|.

In an actual amplifier which I have constructed according to my invention, the tubes and 39 are of the screen grid type known as Radiotron 57, and the tube 2| is of the type commercially known as Radiotron 1'7l-A. The resistor II has a value of 2,000 ohms, the resistor 49 1,500 ohms, and the resistor 21 1,000 ohms. Furthermore, the resistor |5 in the cathode circuit of the tube has a magnitude of 7,50 ohms, while the plate supply for the system provides a potential of 250 volts. The tube 29 is, preferably, of the type known as Radiotron 56, which requires a cut-off potential of 30 volts.

An amplifier including a preferred embodiment of my invention is normally inactive and is rendered operative only when the control tube is energized. It will, however, be obvious that such condition may be reversed, the amplifier being normally operative and being rendered inoperative in accordance with impulses applied to the input circuit of the control tube. One Way of obtaining the last-mentioned condition is to adjust the bias battery I9 to deliver a less negative potential, whereupon theblas on the tube I is the algebraic sum of the potential supplied therefrom, the drop across the resistor I5, by reason of space current in the said tube, and the drop across the same resistor in the opposite direction occasioned by the flow of space current in the tube 29, which is so biased as to permit a small amount of current to flow therein. 'I'he control signal must,

under these conditions, be negative in order that the space current in the control tube may be reduced orstopped, whereupon the grid of the tube I is immediately biased negatively to the cut-off point. Another'way of obtainingthe same condition is to omit the bias-battery I9 and to reverse the connections of the anode 3| of the control tube and the source 34 to the resistor I5. 'I'he control tube then functions as firstdescribed, namely, becomes conductive inresponse to the control impulse and develops-a potential across the resistor in the direction to bias` the amplifier I to the cutoff potential. Although I have chosen a specific embodiment of my invention for purposes of illustration, many modifications thereof will be apparent to those skilled in the art to which it pertains. My invention, therefore, is not to be limited except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention: w

1. In combination, means for producing control impulses, a plurality of thermionic tubes connected invcascade, means for altering abruptly the gain in at least one of said tubes in response to the occurrence of said impulses, and means for compensating disturbing impulses occasioned by said impulses, an electric discharge tubev having an in- ,put circuit and an output circuit, means for amplifyingwanted signals appearing in said output circuit, means for abruptly altering the gain of said'tube in response to the occurrence of said control impulses, and means for balancing out impulses appearing in said output circuit resulting from said abrupt alterations in the gain in said tube.

4. In an amplifier, a thermionic tube having an input circuit, across which potential changes representative of signals may be impressed, and

an output circuit, means for abruptly altering the gain in said tube, an amplifier coupled to said output circuit, said amplifier havingian output circuit in which may appear impulses occasioned by abrupt changes in the gain in the first referredto tube, and means, energized solely in response to abrupt changes in gain in said thermionic tube, for neutralizing impulses in the output circuit of the amplifier.

5. In combination, a plurality of thermionic tubes each having an input circuit and an output circuit, connections between the input circuit of one of said tubes and the output circuit of another of said tubes, connections between the inl second mentioned tube give rise to mutually opposing effects in the said common output circuit.

6. In combination, a pair of electric discharge tubes each having an input circuit and an output circuit, a source of control voltage, said'input circuits being connected in parallel with respect to said source, a source of signal voltage connected across only one of said input circuits, a signal output circuit, means for coupling one of said tube output circuits to said signal output circit, and

means for coupling the other of said 'tube output circuits to said signal output circuit in reversed phase relation rwith respect to the coupling between said one output circuit and said signal circuit.

7. In combination, a pair of electric discharge tubes each having an input circuit and an output circuit, an impedance unit common to said two input circuits, means for impressing a control voltage across said impedance unit, a second impedance unit included in one of said input circuits only, means for impressing a signal voltage across said second impedance unit, a balancing circuit,

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