US1890503A - Tunable dynatron amplifier - Google Patents

Description

Dec. 13, 1932. P. o. FARNHAM TUNABLE DYNATRON AMPLIFIER Filed Oct. 17. 1931 Zn R H 0N m T M 0 N F T. Dru W T m n M n m u N wm m m u xx n l m m m 0 W l IJ". =N. u m w u N m R Ll m k m x E v Q v m RN\K\N$- I A 662$ J r v G k i EEE QDCMGQ R Patented Dec. 13, 1932 warren srares r ,OFFIQE PAUL O. FABNHAM, F BOONTON, NEW JERSEY, ASSIGNOR TO RADIO CORPORATION OF AMERICA, A CORPORATION, OF,DELAW'ARE TUNABLE DYNATRON AMELIFIEB,

Application filed October 17, 1931.

The present invention relates to amplifier circuits, and more particularly to methods of, and means for, rendering the dynatron suitable for use 'in tuned high frequency ampli- The conditions which give rise to oscillations, and which cause variations in gain and/or selectivity with changing adjustments of the operating controls in'thecase 1c of the dynatron are quite different from those which produce disturbing eiiects in the case of the usual or positive resistance tubes, the known methods of securing stability, satisfactory sensitivity and selectivity character istics are not applicable to the dynatron. Hence, in my copending application Serial No. 523,859 filed March 19, 1931there have been disclosed methods of, and'circuit arrangements for, employing the dynatron in amplifier stages of the type having a resonant output circuit. It has been pointed out in the disclosure of the latterthat one of the main objects of the arrangements therein was to provide, in a dynatron amplie5 fier having a tuned output circuit, methods of and circuit elements for automatically maintaining an approximately constant ratio between the magnitude of the plate resistance and the plate load for'preventing that ratio from reaching or exceeding unity.

My aforesaid application shows, from theoretical and practical considerations, that a dynatron amplifier having a tuned plate load impedance will oscillate at any frequency for which the plate load is a pureresistance that is greater than the plate resistance. This oscillating condition can be. reached either by adjusting the control grid bias towards less negative values, or by increasing the plate load. An amplifier system adapted to be selective over a band of frequencies should retain certain features of performance through its entire-range of adjustment, it being required that the amplifier should not 4.5 oscillate at any adjustment of the amplification control, nor should it oscillate at any frequency to which the system may be tuned.

lVithout going into details concerning the various arrangements employed in the said copending application, it is sufficient to point Serial No. 569,421.

out herein that the tendency to oscillate as an electrode voltage is adjusted to increase the amplification, is overcome by a simultaneous and automatic adjustment of another electrode voltage. Reverting to the question of freedom from oscillation as the output circuit of the dynatron amplifier is tuned over a band of frequencies, it has long been recognized. that the impedance, at resonance, of a tuned circuit varies with the frequency to which the circuit is tuned. If the plate load impedance should vary widely with the tuning of the output circuit, these variations will be greatly exaggerated in the gain-frequency characteristic of the dynatron stage. Thus, unless due precautions. are taken, the tuning of the output circuit may so alter the magnitude of the load impedance that oscillatory conditions are established.

Optimum conditions so far as concern stability of oscillation with tuning may be obtained by maintaining a constant ratio between the magnitudes of the external plate load and the plate-cathode resistance. This condition maybe realized, either completely or to an extent sufiicient to prevent oscillation, by maintaining the plate load substantially invariant with'tuning for a fixed plate resistance, or by varying the plate resistance as a substantially linear function of a varying plate load. In my said copending application, and in accordance with the first of the aforementioned methods, the plate circuit includes a compensating or corrective network, inaddition to the tuned output circuit, which maintains the load impedance, as viewed from the plate and cathode terminals, substantially independent of the tuning of the output circuit.

One of the main objects of my present in vention is to provide a dynatron amplifier including a tunable output load, which includes simplified devices to maintain a substantially constant load impedance in said output in order to secure stability of the dynatron amplifier. 1

Another important object of thepresent invention is to provide a method of, and circuit arrangement for, preventing self-oscillation throughout the entirev range of tuning of a dynatron amplifier having a tuned output circuit, which method includes introducing a frequency variant resistance in the tuned plate circuit of the dynatron amplifier to compensate for frequency variation of the tuned circuit impedance.

Still other objects of the present invention are to improve generally the simplicity and efliciency of devices for maintaining a substantially constant load impedance in a tuned dynatron amplifier circuit, and particularly to provide an amplifier of the dynatron type Which is not only stable and reliable in operation, but includes simplified circuit elements.

The novel features which I believe to be characteristic of my invention are set forth in particularity'in the appended claims, the invention itself, however, as to both its organization and method of operaton will best be understood by reference to the following description taken in connection with the drawing in which I'have indicated diagrammatically one circuit arrangement whereby my invention may be car ied into effect.

In-the drawing,

Fig. l diagrammatically shows a conventional type of radio receiver employing a tuned dynatron radio frequency amplifier,

Fig. 2 diagrammatically shows a simplified circuit illustrating the operation of the present invention.

Referring to the accompanying drawing in which like characters of reference indicate the same parts in the two figures, there is shown in Fig. 1 a radio receiver of conventional construction, wherein the reference numeral 1 designates a source of signal energy. The latter can consist, for example, of an antenna circuit, but it is to be understood that it may comprise any other source of modulated carrier energy. The source 1 is coupled, as at M, to the tunable input circuit of a dynatron radio frequency amplifier, the output circuit of the amplifier being couple d, as at'M to the tunableinput circuit of a second stage of dynatron amplification. The output of the second dynatron amplifier is detected, as at 2, by any well known form of detection circuit. The detected energy can then be utilized in any desired manner, or be amplified by one or more stages of audio frequency amplification before utilization. These various steps of reception of the collected energy are very well known, and need not be gone into in any further detail in this application.

Considering now the constructional details of the first stage of dynatron amplification, it is pointed out that one satisfactory embodiment of'the invention for securing operation which conforms to the requirements pointed out heretofore is illustrated diagrammatically in the said first dynatron stage. The electron discharge tube 3 is of the tetrode, or screen grid, type having an input circuit which comprises the resonant circuit L, C connected between the control grid 4; and equi-potential cathode 5 which is energized for electron emission by a filament 6. The source of current supply for impressing voltages on the tube electrodes is indicated diagrammatically as a battery B, but it is obvious that any suitable source, such as the customary power supply devices, may be our ,ployed. As indicated, a negative bias E is applied to the control grid, and the screen grid 7 is connected through a lead 8, to a point on the source 13 at which the voltage is greater than the supply source voltage which determines the voltage of the anode, or plate 9.

The tuned output circuitcomprises a twin winding, consisting of the primary inductance coil L and the secondary inductance coil L the latter being shunted by the variable tuning condenser C It will be noted that the coil L is arranged in series between the negative terminal of the source B and the anode 9 of the first dynatron tube, a resistor R being connected in series between the coil and the anode. A capacity C is connected in shunt with the resistor R, and the function of this aperiodic reactive network will be described in further details at a later point.

Whereas in my aforementioned copending applicationthe tunable output circuit has been shown connected directly in the plate circuit of the dynatron tube, it will be noted that in the present application the actual resonant circuit consisting of the coil L and the condenser 0 comprises the tunable input circuit of the second dynatron amplifier stage 3. Electrically, of course, the twin winding coupling M and the condenser C may be considered as connected directly in the output circuit of the first dynatron tube 9. The twin winding coupling is desirable for various reasons, which reasons need not be mentioned in this application, and is therefore disclosed as employed in this receiver. It is to be clearly understood that the constructional details of the second dynatron stage 3" is exactly the same as those shown in connection with the first tube 3.

Fixed condensers 10 are provided, in connection with the first dynatron stage, to bypass the screen grid low potential side of L and cathode leads for carrier frequencies. In my aforementioned copending application various arrangementshave been disclosed for securing stability for variation of gain control. These various arrangements can all be summarized by stating that the mode of operation upon which they are based comprises overcoming the tendency to oscillate as an electrode voltage is adjusted to increase the amplification, by a simultaneous and automatic adjustment of another electrode voltage.

For1the purposes of the: present discussion, it may be assumed that a device is employed in the first dynatron amplifier stage which permits control of the tendencyto oscillate as the gain control element 11 is adjusted to vary the amplification of the dynatron' amplifier. In other words, in order to simplify the description of the present invention which primarily deals with the maintenance of a constant load impedance during tuning of theidynatron amplifier, there-has not been shownany circuit elements for gainvariation stability, it being clearly understood that any of the devices shown in my aforesaid copending application can be employed for this purpose.

Considering, then, the problem sought to be solved in the present application, the disposition of the network R, C between the anode 9 and the coupling M results in the introduction of a frequency variant resistance in the tuned plate circuit of tube 3 to compensate for frequency variation of the tuned circuit impedance. In Fig. 2' there is diagrammatically shown an analytical circuit to explain the operation of the present invention. Only those portions of the circuit of the first stage of the dynatron amplifier shown in Fig. 1 are disclosed in Fig. 2 in order to preserve simplicity of description.

Thus, it is to be understood that the variable condenser C shown in dotted lines, shunted across the coil L represents the tuning capacity C reflected from the secondary L The capacity G also shown in dotted lines, represents the plate-cathode capacity of the tube 3. The network RC may be considered, as in effect in the tuned circuit of the output of tube 3 since the inherent platecathode capacity C is shunted across the coil L At low frequencies C is large, and the current flowing through C is small, thus reducing the effective value of the impedance of the network RC. At high frequencies, CT is small, C carries relatively large current, and the impedance of the network RC is important.

As the resistor R is shunted by the capaclty C, the effective resistance of RC decreases with increasing frequency. As illustrative of one practical arrangement, the following values are cited as suitable for the network RC. For a frequency range extending between 600 to 1500 KC inclusive, the resistor It may have a value of 5000 ohms, while the capacity C may have a value ranging between 40 to M. M. F. The effective resistance of RC for R=5000 ohms and for a value of C equal to 50 M. M. F., at 1500 KC will be about 690 ohms, while at 600 KG the value would be about 2500 ohms. This increase of resistance of the network BC with decreasing frequency keeps the total impedance of the plate load approximately constant, and hence, the gain scribed one arrangement for carrying my invention into elfect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications' may be made without departing from the scope of my invention as set forth in the appended claims.

' What I claim is: Y Y

'1; Ina dynatron amplifier, the combination with. an electron tube and means energizing said tube for dynatron operation of a tuned output circuit, including a tunable resonant network, whose impedance at resonance varies with the frequency to which said network is tuned and means in series between the tube anode and the network in said output circuit for maintaining the total impedance of said output circuit substantially constant during tuning.

"2. In a dynatron amplifier, the combination with an electrontube and means energiz ingisaid tube for dynatron operation of a tuned output circuit, including a tunable resonant network, whose impedance at resonance varies with the frequency to which said network is tuned, and means in series between the tube anode and the network in said output circuit for maintaining the total impedance of said output circuit substantially constant during tuning, said means comprising an aperiodic reactive network.

3. In a dynatron amplifier, the combination with an electron tube and means energizing said tube for dynatron operation of a tuned output circuit, including a tunable resonant network, whose impedance at resonance varies with the frequency to which said circuit is'tuned, and means in said output circuit for maintaining the total impedance of said output circuit substantially constant duringtuning, said means comprising an aperiodic reactive network in series with said tunable network.

at. In a dynatron amplifier, the combination with an electron tube and means energizing said tube for dynatron operation of a tuned output circuit including a tuning ca pacitor whose impedance at resonance varies with the frequency to which said circuit is tuned, and means in said output circuit for maintaining the total impedance of said output circuit substantially constant during tuning, said means comprising a network consisting of a resistor shunted by a capacitor other of said output circuit substantially constant during tuning, said means comprising an aperiodic network in series between the tube anode and the said resonant network whose effective resistance increases with decreasing frequency.

6. In a dynatron amplifier,the combination with an electron tube and means energizing said tube for dynatron operation of a tuned output circuit whose impedance at resonance varies with the frequency to which said circuit is tuned, and meansin said out put circuit for maintaining the total impedance of said output circuit substantially constant during tuning, said means comprising a network consisting of a resistor shunted by a fixed capacity whose effective resistance increases with decreasing frequency whereby the total impedance of said output circuit is maintained substantially constant over the tuning range of said circuit.

. 7. In a 'dynatron amplifier, the combination with an electron discharge tube and means energizing said tube for dynatron operation, of a tuned output circuit including a tunable resonant network, whose impedance at resonance varies with the frequency to which said circuit is tuned, and an aperiodic reactive network, including a resistor and a capacity in shunt therewith, in series between the anode of said tube and the said resonant network for maintaining the effective impedance of said circuit substantially constant as the output circuit is tuned over its frequency range.

v 8. An amplifier circuit comprising a tube and means. applying to the electrodes thereof, direct current potentials efi'ective to give the plate impedance a negative characteristic,.and an'output network for said tube, said output network including a tunable resonant circuit which acts in series between the oathode and anode of the tube, and an aperiodic reactive circuit between said anode and said resonant circuit for maintaining the effective resonant impedance of the entire circuit between said anode and cathode substantially constant over the tuning range of said resonant circuit. I I

9. In combination with a screen grid tube having a'resonant input circuit coupled between the input electrodes of the tube, a resonant output circuit. a coupling element in the anode circuit of the tube for coupling said output circuit to said anode circuit. a

ne ates PAUL O; FARNHAM.

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