US2211010A - Thermionic amplifier - Google Patents

Description

Aug. 13, 1940- c. E. HALLMARK THERMIONIC AMPLIFIER Filed Nov. 30, 1958 N @N ooo o nvent xr Clyde E. Hallmark Gttorneg www Patented Aug. 13, 1940 UNITED STATES ampio 'Y v THERMIoNIo 'AMPLIFIER s@ oiyde'njnaihnark, ,oamien,'N."J., assigner to j j Radio Corporation of America, da corporation 1 of Delaware Application November.30, .1938,fseria1 No. ziaaov v 'I'his invention relates to thermioni'c amplifiers and more particularly to a device/adapted to amplify currents having a wide range of frel quency down to and including zero cycles per '5 second. v

I am aware of the existence of thermionic amplifiers which are adapted to amplify low frequency and direct current impulses. Such dew vices usually include a plurality of direct-coupled l1I) tubes. Since the grid of one tube necessarily has the direct current potential of the anode of ythe preceding tube, separate batteries' for each tube are usually used. VIf the batteries are to be serially connected, or if a rectifier and filter system is to l5 be used, the cathode of each successive tube must be connected to a more positive potential in order to maintain the proper grid bias. As a result the overall voltage required becomes very large.

This difficulty is overcome in accordance with 120 the present invention by converting the signal which is to be amplified into a modulated radio frequency signal, amplifying the radio frequency signal as much as is desired, and demodulating to obtain the original signal frequency greatly L amplified. In accordance with this invention a system is provided for obtaining from the rectifier an output voltage suitable for driving the grids of a pair of push-pull, direct-coupled output tubes which may be voltage or current am- I pliiiers.

Among the objects of this invention are the provision of an improved method for amplifying currents having a Wide range of frequency, down to and including zero cycles per second, the provision of an improved method of driving a pair lof direct-coupled push-pull ampliers.; and the provision of means for carrying out the above obv s tentiometer is connected to the control grid v1 of a multi-electrodev modulator tube 9.v y`

A local oscillator I l is provided for the generation of radio frequency carrier currents. This oscillator consists of a' therrnionic triodeA I3 which y is connected to a tank circuit I5"-in the usual 7 claims.` (ocllfiicLljvi) o f i" 'v H,

manner. The cathode I1 of the modulator tube 9 is connected toa suitable point between the vterminals of thetank circuit inductor I9. Input terminal 3 is connected to a point on inductor I9, andgrounded. The terminal of inductor I9 which f5 is near the ground connection, above referred to, is coupled by means of a capacitor 2I to the control grid 1. It is to be noted that low frequency currents which are impressed on the input terminals are "*1'0 veffectively connected between the grid 1 and the ,capacity of the grid to the cathode I1, since `the rg0 cathode I1 islnot atground potential with respect to the radio frequency currents. This neutralizing voltageis equal in amplitude and opposite in phase ,to thefvoltage induced on the grid from the cathode. Modulationof the radio frequency 25 currents which are impressed on the modulator 9 iseffected by varying the grid-cathode potential by means-.of the input voltage.

The modulated carrier current' present in' the plate of the modulator tube 9 has a greatlyinrg0 -creased amplitude due to the amplication'of this tube. yThe output is coupled bymeans of ares- =onant transformer 23' 'to a pair of diode'rectifiers 25 and 21. Transformer 23 may be an intermediate frequency transformer `of theftype usedin '35 superheterodyne receivers, or the like, depending upon the frequency range'desired in the amplifier. .Iv-f

The rectifying action of diode 25'produces a unipotential voltage across the diode load resistor um 29 whose amplitude issubstantially equal to the peak amplitude of the applied voltage. 'I'he plateyside of the resistor 29 is grounded so `that la potential which is positive with respect to ground appears across the resistor and is impressedA on the grid of an output amplified tube 3 I.

The diode 21 is energized by an identical radio frequency voltage but is isolated with respect `to direct `current voltages by the coupling capacitor 33.

Similarly, `a unipotential voltageis den50 veloped across diode load 4resistor 35. In this case, however, the cathode side of the resistor is connected to a source of fixed' positive potential.

vlin the instant case, the fixed potential is obtained -from' a direct current source 31 and, for con- 55 venience, has a value of 90 volts. This value is conveniently maintained by a gaseous discharge regulator tube 39.

The plate end of the cathode resistor 35 is connected to the control grid of a second output amplifier tube 4|. Consequently, the direct current potential, with respect to ground, present on the control grid of amplier tube 4|, is determined by the diierence between the fixed positive potential of 90 volts and the voltage developed across resistor 35 by the rectifying action of diode 21. Returning to a consideration of the local oscillator it is seen that the intensity of oscillation is controlled by a potentiometer 43 which is connected between a source of fixed potential and ground. The value of the voltage applied to the local oscillator is selected at that value which will cause a carrier voltage to be impressed on the rectiers 25 and 21 whose peak value is in the order of volts in the absence of a modulating input signal.

In the absence of an input modulating voltage, and after the adjustment mentioned above has `been made, a positive direct current potential of approximately 45 volts will be impressed on the grid of amplifier tube 3|. The grid potential of amplifier tube 4| will be equal to 90 volts minus 45 volts, or a positive direct current potential of 45 volts. In such a case, a source of biasing voltage must be provided. This is accomplished by means of a resistor 45 which is connected between the cathodes of amplifier tubes 3| and 4|,

.and ground. The value of this resistor is chosen so as to develop volts. Consequently, in the absence of an input modulating voltage, the steady state grid bias on the amplifier' tubes 3| and 4| is substantially minus l5 Volts, which is a suitable bias for the particular tubes used.

It is to be noted that there is a direct current path between the input terminal and the grid 1 of modulator tube 9. As a result, the voltage of grid 1 will be varied by the application of an input voltage having any frequency down to and including zero cycles per second. The input voltage thus changes the amplitude of the carrier frequency voltage which is applied to the rectifying tubes 25 and 21, and consequently changes the amplitude of the direct current potential which is developed across the respective diode load resistors 29 and 35. If the change causes the voltage across resistor 29 to become more positive, the voltage on the grid of amplifier 3| likewise becomes more positive, but the voltage on the grid of amplier 4| becomes more negative since the difference potential increases negatively. Out of phase potentials are therefore applied to the two amplier tubes thereby obtaining pushpull operation.

In addition to obtaining a push-pull voltage, an effective tWo-to-one voltage gain is realized by the system illustrated. A change in the amplitude of the radio frequency carrier which increases the potential of the grid of tube 3|, with respect to ground, from 45 to 46 volts, at the same time reduces the voltage of the grid of tube 4| from 45 to 44 volts. Thus an effective change of l volt at the rectifier produces an effective change of 2 volts between the grids of the amplifiers which is the voltage doubling effect realized in addition to the amplification produced by the modulator tube 9.

Amplifier tubes 3| and 4| are coupled, respectively, to rectiers 25 and 21 by means of resistive-capacitive networks 41 and 49 which serve to lter radio frequency currents from the input to the amplifier tubes. In the circuit which I have illustrated it is to be noted that no low frequency bypass capacitors have been used. The present amplifier will amplify direct current impulses or audio frequency impulses up to a sub- 5 stantial frequency limited primarily by the band Width of coupling transformer 23.

In the example illustrated, output tubes 3| and 4| are preferably beam power tetrodes, although `other tubes are suitable. The particular application in mind was in connection with a cathode ray oscillograph, and the amplier was utilized to provide horizontal or vertical deflecting voltages for the cathode ray beam. 'Ihe particular cathode ray tube utilized was of the type having magnetic deilecting coils and consequently required high current ampliers to operate the device. Other applications would require high voltage output and in such cases other output tubes Would be more satisfactory.

`While I have shown a preferred embodiment of `this invention, it is to `be understood that modiunipotential voltages whose amplitudes vary in ,35

accordance with the amplitude of said local oscillations, a fixed biasing potential eiiectively connected in series with one of said unipotentlal voltages, said Xed potential having such a value that difference Voltage is developed which, for ,.40

a given oscillation amplitude, is equal to the amplitude of the other of said unipotential voltages, and c-i like polarity with respect to ground, and means for varying the amplitude of said oscillations from said given value in acccrdance-45 with voltage which is to be measured, whereby an amplified output potential is developed between the other of said unipotential voltages and said difference voltage.

2. The method of amplifying an electrical im,- pulse which includes the steps of generating local oscillations, modulating said oscillations in accordance with said impulse, amplifying said modulated oscillations, and demodulating said amplified oscillations, to obtain two unipotential voltages, combining one of said unipotential voltages with a fixed voltage to obtain a difference voltage Whose amplitude is normally equal to that of the other of said unipotential voltages,

and of like polarity, and obtaining an output volt- 50 1 age whose amplitude is proportional to changes from said normal condition due to changes in said voltage to be amplified.

3. In an amplifier responsive to a range of -frequencies down to and including zero cycles perl5 second, the combination which includes a local i oscillator, a modulator, means for modulating the amplitude of said local oscillations in accordance with a signal, means for amplifying said modulated oscillations, a pair of rectifiers, means forum impressing said amplified oscillations on said recfixed biasing potential effectively connected in series with one of said uni-potential voltages, said fixed potential having such a valuev and polarity that a di'erence Voltage is developed which, for a given oscillation amplitude, is equal to the amplitude of the other of said oscillations cause said other uni-potential voltage and said difference potential to vary in like degree and opposite sense, a pair of thermionic output tubes having grid, cathode, and anode electrodes, said grid electrodes being connected respectively to said load resistors, a biasing source connected between said cathode electrodes and ground, and output terminals connected tosaid anode electrodes.

4. In an amplier, the combination which includes a local oscillator, means for modulating said oscillator in accordance with an input voltage which is to be amplified to obtain a modulated `carrier, means for amplifying said carrier, a pair of rectiers coupled' to said amplifying means, and means connected to said rectiers for deriving two rectified voltages having normally equal amplitudes and polarities, with respect to ground, said rectified voltages changing in opposite directions in accordance with changes in the amplitude of said input voltage.

5. In an amplifier, the combination which includes a local oscillator, means for modulating said oscillator in accordance with an input voltage which is to be amplified to obtain a modulated carrier, means for amplifying said carrier,

and means connected to said amplifying means for deriving two rectied voltages having normally equal amplitudes and polarities, With respect to ground, said rectified voltages changing in opposite directions in accordance with changes in the amplitude 4of said input voltage.

6.'.In a device of the character described, the combination which includes a source of carrier frequency currents ofk normally constant amplitude, means connectedto said source for Ideriving two rectified voltages of normally equal amplitudes and 'of the same polarity, their amplii tudes changing in opposite directions upon the variation of said carrier frequency currents from' said normal value, and'means for varying the amplitude of said carrier frequency currents.

7. In a device of the character described, the combination which includes a source of carrier frequency currents of normally constant-amplitude,- a pair of rectiers having input and output circuits, said input circuits being connected to said source, and said output circuits including means for developing output voltages of normally equal amplitude and polarity, but varying in opposite directions in accordance with variations of said carrier frequency currents from said normal amplitude, and a pair'of output devices having their input circuits energized, respectively, by said output voltages.

CLYDE E.

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