US2227157A - Grid bias modulated amplifier - Google Patents

Description

Dec. 31, 1940. H. RODER GRID BIAS MODULATED AMPLIFIER Filed Nov. 12; 1938,

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TUNED ANODE C [R C U! T Inventor Hans Rod r, by 5.49 Hi Attovney.

ACTUAL 4 amp you-Ass Patented Dec. 31, 1940 UNITED STATES GRID BIAS MODULATED AMPLIFIER Hans Roder, Schenectady. N. Y., assignor to General Electric Company, a corporation of New York Application November 12, 1938, Serial No. 240,141 '1 Claims. (01. 179-1715) This invention relates to circuits for amplify.- ing and modulating high frequency electric currents.

In television transmission it has been a serious I 5 problem to modulate and amplify electric waves over an enormous channel width. The normal is an object .of my invention to provide improved apparatus which by means of degenerative feedback in a grid modulated stage of amplification provides faithful reproduction over a band width 15 suitable for television transmission or the like.

It is the further object of my invention to provide an improved type of amplifier stage for amplification of television signals which will have no regenerative or degenerative effect upon preceding stages, while it is itself subject to a degenerative feedbackvoltage. It is also an object of my invention to provide an improved type of amplification by means of electron discharge amplifier devices wherein the devices are biased be- 25 yond anode current cut oil and wherein degenerative feedback voltage is used with such apparatus.

The novel features which I believe to be characteristic. of my invention are set forth with par- 30 ticularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof may bestbe understood by reference to the following descrip- 35 tion taken in connection with the accompanying drawing, in which Fig.1, of the drawing illustrates schematically a circuit embodying my invention. Fig. 2 is a somewhat more schematic diagram of the circuit illustrated in Fig. 1; Fig. 3 40 is a vector diagram supplementing Fig. 2, and Fig. 4 is a modification of the embodiment of Fig. l, and Fig. 5 represents certain characteristics pertaining to my invention.

In Fig. 1 the primary winding ID of a coupling 45 transformer II is supplied with ultra-high frequency carrier voltage from any suitable source, not shown. A secondary I! of the coupling transformer II is tuned to the operating frequency by a capacitor I3 connected in parallel with the sec- 50 ondary I2. Two electron discharge amplifier devices l4 and ii are arranged in balanced, or pushpull, relation having their cathodes connected together and to ground. The grids of the devices It and ii .are connectedby means of capacitors 55 i6 and i'l'to the respective ends of the secondary I! of the coupling transformer II. The capacitors l6 and II are bypassed by suitable grid leaks l8 and I9 respectively, arranged in the conventional manner. A'common point in the cathode circuit of the devices l4 and I5 is connected through a source of grldbiasing potential 20, and a biasing resistor 2| to the center point 22 of the transformer secondary l2. The grid biasing potential 20 is sufliciently negative to provide class C operation of the amplifier, that is to say that 10 current flows in the anode circuit of the discharge device only during peaks of the grid voltage.

The tuned anode circuit of the apparatus is formed by the inherent capacities of the devices l4 and I5 and by the primary 23 of a coupling transformer 24, the ends of which are connected to the anodes of devices I 4 and I5. The center point of the transformer primary 23 is connected through a suitable source of anode potential 25 to a common point on the cathode circuits of devices l4 and I5. The secondary of the transformer 24 may supply modulated high frequency current from the apparatus shown to any desired utilization circuit. A capacitor 2'I'is connected between the anode of device I4 and that end of 25 the transformer secondary l2, which is connected to device IS. A similar condenser 28 is connected between the anode of device l5 and that end of the transformer secondary l2 which is connected to the grid of device l4. These condensers pro- 3 vide degenerative feedback voltage on the grids of devices I4 and I5, as will be explained below.

An electron discharge amplifier device 29 acts as a modulator and has its anode connected through a coupling capacitor 30 to the center tap 22 of the transformer secondary l2. The cathode may be grounded in the conventional manner and the grid of device 29 supplied with a modulat ing signal. A suitable source of anode potential for the device 29 is indicated as being supplied ,to the anode through a resistor 3|. Modulating voltage supplied by device 29 will vary the grid bais of devices l4 and i5 and thereby modulatethe carrier signals amplified through those devices.

To consider the operation of .the electron disw charge amplifier circuit shown, let Ep represent the plate voltage and let Ed represent a degenerative feedback voltage which is a definite portion of Ep introduced into the control grid circuit in 'shown to be equal to 2R be represented by a. I have determined that in so far as the control grid circuit is involved, an electron discharge device working into a tuned anode circuit in class C operation appears to be working into a tuned circuit which has twice the value of the anode resistance connected across its terminals. It is known also that if a real resistance be added in parallel to the tuned circuit, the band pass characteristic of the amplifier becomes broader; in other words, by the addition of a resistance in parallel .to .the tuned anode circuit an electron discharge amplifier apparatus may be made to have a broader channel width of reproduction. If a sumciently small resistance be placed in parallel with the tuned anode circuit a sufficiently broad frequency band may be obtained to satisfy fidelity requirements for television transmission. The very serious objection to this procedure is that a large .portion of the am-, plified power is consumed in the resistance. It is possible to avoid placing any resistance in'parallel with the tuned anode circuit and at the same time secure the desired wide band characteristic necessary in television applications in accordance with my invention by decreasing the apparent anode resistance of the electron discharge device.

This may be conveniently accomplished by introducing a certain amount of degenerative feedback voltage from the anode circuit to the grid circuit of the amplifier.

For example, if a given discharge device having the anode resistance Rp, as determined by the change in anode voltage divided by the change in anode current, be operated class C, as mentioned above, or class B, its anode resistance can be It can also be shown that, if a degenerative voltage be applied to such a discharge device when operated class B or class C, the anode resistance may be represented by the expression; 2R /(1+ a); in which 1!. is the amplification factor of the discharge device and w=EdlEp as mentioned above. a The expression is an exact analysis of class '3 operation, and a close approximation of class C.- The amplification factor it may be of the order of 30 to 50 or greater, and the value of a may be of the order of /5 to %0. Thus the internal resistance of the discharge device, operated class B or C, as it appears to the tuned anode circuit may be reduced to 0.5, 0.1, or even less, of the .value which it would have were no degenerative voltage employed. This operates very effectively to increase the band width of the class B or C amplifier, while at the same time it does not increase losses in the circuit or reduce the power capabilities thereof.

Fig. 5 illustrates the improvement in band pass and power handling capabilities of the circuit produced by degeneration. This figure includes a two curves A and B showing the relation between the peak power output per tube plotted as ordinants and band width of the tuned amplifier plotted as abscissas. Curve A represents this relation for a class C amplifier operating without degeneration and curve B represents the same'relation for the same amplifier operating class C with degeneration in accordance with my invention. It will beseen that for a given band width the power output is greatly increasedby the degeneration, or for a given power output the band width is greately increased.

It is true that by this arrangement somewhat more exciting voltage is required for the grid of the discharge device, but since this can be conveniently obtained and suitable channel-width" is attainable only with difiiculty, this is no disadvantage. y In the embodiment shown in the drawing, degenerative feedback voltage is applied to the grids of a circuit which may be called a neutralizing bridge. The bridge neutralizes the amplifier in so far as it prevents the effects of any change in the anode circuits of the devices it and I5 from appearing at the tuned grid circuit comprising transformer secondary l2 and capacitor l3. That is any transfer of energy between the input and output tuned circuits except through the electronic action of the discharge devices is prevented through neutralization. This bridge circuit, however, does not neutralize insofar as the grids of devices It and I5 are concerned, but it actually applies a degenerative feedback voltage thereto. It may be said to compensate for the voltage induced at the ends of the transformer secondary l2 by the inherent capacity between the anodes and grids of devices H and I5. In order that this bridge may be effective, it is necessary that it balance. The conditions of balance are fulfilled when the reactance of capacitorfl; the reactance of capacitor 28; the sum of the reactances of capacitor l1 and the capacitance between the anode and grid of device i5; and the sum of the reactances of capacitor I6 and the capacitance between the anode and,

grid of device It are all equal.

In operation, since the bridge is balanced no voltage is fed from the anode circuits of devices I4 and ii to the tuned grid circuit including transformer secondary l2 and capacitor 13. However, a degenerative feedback voltage is applied to the grids of devices 14 and I5 because their plate-grid capacitance and the capacitances i8 and I1 respectively act as voltage dividers. This action may be best understood by reference to Fig. 2.

In'Fig. 2, the elements shown are identical with those in Fig. 1 and have the same numerals. Ep represents one-half the voltage across the tuned anode circuit. C14 represents the capacity between anode and gride of device l4 and C15 that for device l5. If we consider this bridge representation of the circuit of Fig. 1, when no voltage exists across the tuned grid circuit II-H, En will be distributed across each arm 'of the bridge. Since the capacitors l6 and-l1 are made quite large with respect to the capacity between the grid and the anode of device I or device l5, most of the voltage Ep will be distributed between grid and anode across C14 and C15 and only a small fraction will be distributed across capacitors I6 and II. This is represented in Fig. 3, by the vectors Ep, E14 and E16 which are allin phase.

We may now consider the bridge of Fig. 2 in the condition when it has no voltage on its tuned anode circuit, and a voltage E; across each half of the tuned grid circuit. This voltage E distributes itself across each arm of the bridge in much the same manner as Ep. As before, the major portion of the voltage exists across C14, but it exists in phase opposition to the voltage induced thereon by Ep. If now we consider that, .due to the voltage E a grid voltage exists represented by vector Em with respect to the fiiaphase opp sition. Hence the voltage Era introduced in the grid circuit by means of capacitor l6 from the plate voltage Ep is a degenerative voltage whose value may be controlled by the ratio between the capacity of capacitor i6 and that between the grid and anode of the device 14.

In order that it may be more clearly understood a concrete numerical example of voltage distribution will be set forth wherein the actual grid voltage is calculated and the actual amount 10 of degenerative feedback voltage is determined.

Assuming E, 200 volts E,= 100 volts, and

capacity of 16 capacity of C it will be seen that Ep will appear across C14 and 20 IS in the ratio of 160 volts to 40 volts. s being only 100 volts will appear across '16 and C14 in the ratio of 20 to 80 volts. Since the point between C14 and I6, which is at the grid potential is 80 volts away from filament potential due to E3 and is 40 volts away from filament potential in the opposite direction due to Ep, it will be seen that when both E1) and Eg are applied the voltage at this point will become 40 volts, which is the difference between these two vectorial quantities. The voltages given here are purely for purpose of illustration, since in the more usual case the ratio of amplification of the system is far greater than the 2 to 1 ratio above assumed. Similarly, the ratio of the capacitances, or in other words, the amount of degenerative voltage,

may be greater than above assumed, as mentioned before.

It is-to be understood that the invention described herein may be used in different sorts of apparatus. A slightly different modification is shown in Fig. 4, in which the amplifying circuit uses a single electron discharge device I4. It will be noted that all numerals in Fig. 4 are identical with the numerals in Fig. l, and are applied to corresponding parts which perform corresponding functions. The connection to capacitor 30 marked with an arrow is intended as before to be connected to a modulating stage. The capacitor 21 is as before connected between the anode of device l4 and that end of the transformer secondary l2 which is opposite to the end connected to the grid of device l4. By this connection a phase reversal of voltage is obtained to effect neutralization and degeneration. It must be borne in mind that although neutralization; is ffected at the terminals of the tuned grid circuit [2- and I3, there is an actual degenerative voltage applied to the grid of device [4 due to the voltage drop across the capacitor I6. The operation of the device illustrated in Fig. 4 is identical with that of Fig. l, as may be seen 'from the schematic representation in Fig. 2. If the lower two arms of the bridge and the lower half of the transformer primary 23 be omitted, the schematic diagram of Fig. 2 will represent the embodiment of Fig. 4. It will be seen, therefore, that the operation is identical as the voltage Ep upon the transformer primary 23 will be distributed across capacitor 21 in one arm and 7 across capacitor 16 and the anode and grid of device M in the other arm as was explained be fore. Likewise, the grid voltage Eg will be'similarly distributed. In other words, the operation of capacitor IS in connection with the anode and 15 grid of device It will continue to operate as a voltage divider or potentiometer in exactly the same way as in the embodiment of Fig. 1. The vector diagrams of Fig. 3 are valid for Fig. 4.

It is quite obvious that other methods of obtaining neutralizing and degenerative voltage 5 may be employed. While in the drawing I have illustrated my invention only in a modulated stage which is necessarily operated class C, it may be employed in a subsequent stage to amplify the output from the modulated stage. Such 10 an amplifier is necessarily operated class B, that is, withthe grid bias volta e adjusted approximately to anode current cut off on the absence of that I do not wish to be limited thereto, since different modifications .may be made both in the 25 circuit arrangement and instrumentalities employed, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by 30 Letters Patent of the United States is: a

1. The combination, in a circuit having input and output terminals, and including an electron discharge amplifier device having a. cathode, grid, and anode, of compensating means for supplying 85 a voltage to said input terminals derived from said output terminals and of such phase and amplitude as to compensate for the voltage induced at said input terminals through the inherent capacity between said anode and grid, and means 40 for supplying a degenerative feed-back voltage from said output terminals to said grid without disturbing said compensating means whereby a wide band amplification characteristic is produced. d5

2. In combination, an electron discharge amplifier device having cathode, grid and anode electrodes, an input device having one side connected to' said cathode and its other side connected through a condenser to said grid, an out- 60 put circuit connected between said anode and cathode, whereby the voltage between said anode and cathode is distributed on the elements comprising the capacity between said anode and grid, said condenser, and said input device in series, 55 means to supply from said output circuit to the point between said input device and said condenser a voltage of equal value and opposite phase to the voltage supplied to said point from said output circuit through said discharge device 50 and condenser, the reactance of said condenser being sufiicient to maintain on said grin sub-. stantial degenerative alternating voltage, whereby the transfer of energy between said output 65 circuit and input device, except through electronic action in said discharge device, is prevented and the voltage of said condenser produces degeneration in said amplifier.

3. In combination, an input device, an output device, an electron discharge amplifier connected between said input and output devices, said amplifier having an anode,-catliode and grid, said input device being connected between said grid and cathode through an impedance and said out- 1 fective on said grid to produce degeneration in' said amplifier.

4. The combination, with an electron discharge device having a cathode, a grid and an anode, an

input circuit connected between said grid andcathode, and an output circuit connected between said cathode and anode, of means for supp ying a degenerative feedback voltage f1 om said output circuit to said grid, and means for preventing transfer of electrical energy from said out put circuit to said input circuit, said last means comprising means for supplying to said input circuit a voltage of equal intensity and opposite phase to the voltage supplied thereto from said output circuit.

5. The combination, in an amplifier having an input circuit and a tuned output circuit, of an electron discharge device having a grid, an anode and a cathode, and a condenser connected in series with said grid, cathode and input circuit, the voltage between said anode and cathode being distributed across the capacity between said anode and grid, said input circuit and said condenser all in series, the voltage on said condenser being degenerative and said condenser having such reactance that said voltage on said grid substantially'reduces the resistance betwen the anode and cathode of said device and thereamplifier. by broadens the band pass characteristic of said 6. The combination, with an electron discharge device having a cathode, a grid and an anode, an input circuit connected between said grid and cathode and a tuned output circuit connected between said cathode and anode, of means for supplying a degenerative feedback voltage from said output circuit to said grid to reduce the resistance between said cathode and anode and produce a wide band pass characteristic through said device, and means for preventing transfer of electrical energy from said circuit to said input circuit, said last means comprising means for supplying to said input circuit a voltage of equal intensity and opposite phase to the voltage supplied thereto from said output circuit through such other paths as exist between said output circult and said input circuit.

7. The combination, in an amplifier comprising an electron discharge device having an anode, a cathode and a grid, a circuit between said grid and cathode to be'supplied with a carrier wave to be modulated, a tuned circuit between said anode and cathode, and means to supply a signal voltage to said amplifier to modulate said carrier wave whereby said carrier wave and modulation side bands appear in said tuned circuit, of means for supplying a degenerative feedback voltage from said tuned circuit to said grid to reduce the resistance between said cathode and anode, thereby to provide more uniform amplification for said side bands,-and means for preventing the transfer of electrical energy from said'tuned circuit to said first circuit, said last means comprising means for neutralizin in said first circuit the voltage supplied thereto from said tuned circuit, whereby said amplifier has a wide band pass characteristic and voltage in said tuned circuit cannot induce voltage in said first circuit.

HANS RODER.

CERTIFICATE OF CORRECTION. Patent no. 2,227,157. December 51, 191;,0.

mus RODER.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1, second column, line 14.5, for Y'bais" read --bias--; page 2, first column, line '71, for "greately" read -greatly-; and second column, line )5, for

"grids" read -.-grid--; page 1+, first column, lines 57, 58 and 59, for the syllable and words "thereamplifier. by broadens the band pass characteristic of said" read thereby broadens the band pass characteristic of said amplifier.-; and that the said Letters Patent should be read with this cor- I'ection therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this llth day of February, A. D. 19141.

Henry Van Arsdale,

(Seal) Acting Commissioner of Patents.

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