US1687245A

US1687245A - Amplifying

Info

Publication number: US1687245A
Application number: US609932A
Authority: US
Inventors: Raymond A Heising
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1922-12-30
Filing date: 1922-12-30
Publication date: 1928-10-09
Anticipated expiration: 1945-10-09

Description

Patented Oct. 9, 192$ TATES ATENT OFF-ICE.

' RAYMOND A. nnisme, or miLLBUnN, NEW .nmsnv, ASSIGNOB 'ro ,WESTERN ELEC- rnrc- COMPANY, rNcoarom'rnn or NEW YORK, N. Y., A. CORPORATION or NEW YORK.

AMPLIFYING.

Application filed December 80, 1822. Serial No. 609,932.

This invention relates to the amplifica-- tionof modulated waves and more particularly to the amplification of modulated high frequency Waves for radio transmission.

It is well knownhow high frequenc waves may be modulated in accordance wit speech-or other signals to roduce a modulated wave from which t e unmodulated carrier component is suppressed or how the unmodulated carrier component may be suppressed after modulation. Such a modulated wavewith the unmodulated component of carrier frequency suppressed is often referred to as a pure modulated wave. Oftentimes, for one reason or another, one of the side bands or side frequencies resulting from modulation is also suppressed, and the remaining wave is then referred to as a single side frequency without unmodulatedcarrier. In the present s ecification, any modulated wave from whic the unmodulated carrier frequency component has been entirely or largely suppressed will be referred to as a' zero carrier modulated wave or zero this difficulty is surmounted b carrier wave. This definition is intended to include a wave wherein both side frequencies are present or wherein only one side frequenc is present.

A di culty present in amplifier systems in general and especially important in -amplifier systems for amplifying considerable power, as in radio transmission, is the inefficiency of vacuum tubes when used asam-- plifiers. For efficiency a high negative voltage must be impressed upon the grid and considerable distortion occurs. In a system heretofore devised and used by applicant, operating the tube at such a point on its c aracteristlc as to amplify the modulated portion only of an incompletely modulated wave.

In order to use vacuum tube amplifiers of high efficiency for am 'lifying zero car-' rier waves, other di culties which arise must be overcome. Llhese difficulties and the method of surmounting them will be discussed hereinafter.

- One system described herein for exempliffying the inventiori consists of an arrangement in which zero carrier modulated waves are supplied to a space discharge amplifier of the usual type havin an anode-cathode outputcircuit and a gri -cathode input'ei'rcuit. The grid is adjusted to a negativepos tential such that space current just begins to flow when no waves to be amplified are applied to. the input circuit. A rectifier is connected; across the input circuit in such a manner that the application of waves to be am lified to the input circuit causes the grid to ecome more no ative as the amplitude of the applied waves ecomes lar er. The system operates without appreciable loss of the signaling frequency components by which the carrier wave is modulated and is highly efficient.

In another system, current of the modulating wave form is rectified and ap lied to the the grid potential thereof to become more negative as the amplitude of the applied Waves becomes larger and vice versa, to 7 cause the grid potential to become less negative as the amplitude of the applied waves becomes smaller.

It is an object of this invention to amplify zero carrier waves by vacuum tube amphfier systems working at highef'ficiency. In this specification, the ex ression high efliciency is used to descri e the condition of o 'eration in which the ratio of the energy of t e desired wave form produced in the output circuit to the total energy supplied to the amplifier system is hi h.

The novel features believe to be characteristic of the invention are set forth with particularity in the appended claims' The lnvention itself, hoWever,, both as to organization and method of operation, together with further objects and advantages thereof will best be understood from the following detailed description taken in connection wit the accompanying drawings wherein Fig. 1 is a circuit diagram of a system for amplifying'zero carrier waves with high eflicienc Fig. 2 is a diagram illustrating high e ciency amplification of ordinar modulated waves by a vacuum tube amplifier system Fig. 3 is a diagram illustrating the inapplicability of the method illustrated in Fig.

2 to zero carrier modulated waves; Fig. 4: 1s a dlagram indlcating the mode of operaamplif ing zero carrier modulated waves.

In ig. 2, the curve 0 represents the grid- ,voltage-plate-current characteristic of a vacuum tube amplifier in which grid voltages are plotted along the axis OX and late currents are Eplotted along the axis "Y. The valueof represents the normal negative id voltage to be used when the amplifier notions to amplify an incompletely modulated high frequency wave represented by the curve w, this wave being modulated in accordance with a. speech frequenc wave s. When the wave is applied to the input circuit of the amplifier the id voltage will at times durin each high equency cycle become of suc value that space current will flow in the output circuit. This will result in the production in the output circuit of a substantially completely modulated high frequency wave of slightly distorted wave form. For radio transmissionthe distortion introduced is a disadvantage, but not such a disadvantage as to overcome the advantage inherent in this method of amplification.

A consideration of the wave form of the 1 high frequency modulated wave applied to the grid circuit in accordance with Fig. 2

- as compared with the wave form of a zero carrier modulated wave indicates at once that the method of Fig. 2 is inapplicable for the amplification of zero carrier modulated waves, inasmuch as the wave form of the latter is such that the-envelope of the high frequency wave which represents the wave form of modulating wave is constantly descending to and crossing and recrossing the zero axis. Thus in Fig. 3 the curve a represents a zero carrier modulated wave consisting of both side bands and 8 represents the modulating wave. It appears at once that any system of amplifying in which only the higher peaks of the wave 2' are amplified will result in a loss of important signaling components. Thus, for example, it would not be desirable to amplify only those portions of J the wave indicated as lying to the right of the dotted line mn. Furthermore, if all portions of the wave lying to the right of the central axis MN are amplified, the system will be inefiicient from. the standpoint of power consumption.

Consider now the mode of operation indicated in Fi 4. In this case the normal negative gri voltage E is adjusted to that which space current begins to flow as in the case of Fig. 2. This results in good amplification of those portions of the zero carrier modulated wave of small amplitude. If now the negative rid voltage is caused to become considerab y more negative during those portions of the speech frequency cycle when the zero carrier wave is of large amplitude, efficiency of operation and absence of signal destroying distortion will both result. In order to accomplish this, the negative grid voltage E is caused to vary in accordance with the heavy curve MN as indicated in Fig.

4. The wave form'of the curve-MN sugests at once that it is periodic and that its undamental frequencyis twice that of the signaling wave a. It is therefore contemplated that any method of operation or means for performing. such method whereby the grid voltage an amplifier for amplifying a zero carrier wave is caused to pass through variations at double the modulation frequency of the modulating wave is an embodiment of the present invention. It also appears that the principles of the invention are applicable in amplifying any modulated wave of which the amplitude at times approaches zero, i. e. complete modulation.

Arno g the possible arrangements functioning in accordance with the diagram of Fi 4, is the circuit arrangement indicated.

inFi 1, which will now be described.

11 circuit arrangements comprise an oscillation enerator which may generate any desired frequency of carrier wave. By way of illustration, it may be assumed that the frequency produced by the oscillation generator 1.is 100,000 cycles per second. The microphone circuit 2 typifies any desired source of modulating Waves, oscillations, or impulses in accordance with which the oscillations produced by the generator 1 are to be modulated. The vacuum tubes 3 and their associated circuit arrangements constitute a modulator 3 of the well-known carrier suppression type. The function of the modulator is to modulate the waves from the genig. 1, the tube 1 and its associated erator 1 in accordance with waves from the source 2 and largely suppress the unmoduallel, the zero carr'er waves produced by the modulating system may be separated from any interfering waves y means of a selective circuit arrangement 6, and increased in energy in one or more stages of amplification by means of vacuum tube amplifiers 7 and 8. The amplifiers 'Z and 8 are of a conventional type well known in the art and need not detailed description. The selective circuit 6 preferably consists of a band pass filter of the t pe described in U. S. patent to Campbell 0. 1,227,113, granted Ma 22, 1917. The filter 6 may pass a range 0 freqliliencies consisting of the lower side band, t at is, for example, from 98,000 to 99,800 cycles per second; or the upper side band, that is, from 100,200 to 102,000 cycles per second; or it may pass both side bands, that is, from 98,000 to 102,000 cycles per second.

In the present instance, we will assume that both side bands are transmitted by the filter 6 and that the unmodulated carrier com onentis wholly suppressed by the modu ator 4 3 and 4. The curve 2 in Fig. 4 represents same modulated wave as the curve 2 in Fig.

' 3 but in the case of Fi 4, the-zero axis'of I rectifier.

the wave 2 is curve instead bf being straight as in Fig. 3.

It is in the power amplifying system 4 that large amounts. of power are dissipated, this power being supplied from the space current source 9. The ower consumption in the amplifiers 7 and 8 is too small to necessitate the application of the invention to their input circuits. In accordance with the present invention, the battery 10 should be adjusted to or at about the point where-space current flows in the amplifier tubes 5. A rectifier 11, preferably in the form of an evacuated-tube having a cathode .and an anode, is connected across between the rids and the filaments of the tubes 5 so that incoming waves will be partially rectifiedin a manner tending to make the grid more negative. The greater the amplitude of the waves, the more negative the grids will become. Hence, when waves of large amplitude are being amplified, the efficiency is correspondingly high and when waves of small amplitude are being amplified the tubes work only on the lower part of their characteristicand but little energy is dissi ated. See paper Modulation in radio telep ony, lroc. I. R. E. August 1921, Appendix. It is preferred to include ter 10 in series with a high resistance 12, an to shunt the rectifier 11 around the resistance 12. Another resistance 13 is preferably included in circuit with the rectifier to determine the potential applied to the The resistances 12 and 13 should be varied both with respect to each other and the other constants of the system, until a desired operating condition is obtained.

The operation of the system is indicatedin Fig. 4 and has been described above. The grid potential of i value at which space current just begins to flow. Incoming high frequency waves then have some of their current rectified in the rectifier 11 and cause the grld potential of the tubes 5 to fall to a still more negative .value. During those parts of the modulating wave cycle when the high frequency waves are of small amplitude the negative charge --on the grid caused by the rectified current will leak off through the resistance 12. The curve MN (Fig. 4) represents the variation of grid potential. The negative potential of the grid thus rises and falls in such a manner that when large waves are amplified, the grid is at such a high negative potential the batv the tubes 5 is set at the as to create an eflici'ent operatin condition. When small waves are ampli ed, only a small amount of power is needed, hence the average efiiciency over .a complete low frequenc cycle remains fairl high. Whatever istortion is introduce does not result in loss of essential elements of the voice wave. The high ower waves in the output circuit are trans erred to the antenna circuit 14 by means of the coupling between thecoils l5'and 16..

trated'is one of the arrangements, but by no means the only arrangement which may be devised for carrying out this method.

Thus, in the circuit of Fig. 5, a portion of the modulating wave energy is utilized to control the grid potential of the power amplifier. Modulating waves are applied from the microphone .circuit 2 to the modulator 3 which is supplied from source 1 with a carrier wave to be modulated. The resultant zero carrier modulated waves are applied to the input circuit'of a power ampli} fier system 4. Some of the speech frequency energy from the circuit 2 1s amplified by the amplifier 17 and applied to a double rectifier system 18 consistingof two hot filament evacuated tubes 19 and 20. The tubes 19 and 20 rectify the alternate half waves of the alternating current of speech frequencyv the amplifier 17. and thus cause supplied by the power tubes of the system the grids of ,4 to vary in potential in accordance with the curve MN. of Fig. 4. The resistance 21 serves asan impedance element across which the \e'lectiomotive force resulting from the rectifigg current is applied to the grid circuit of e tubes 4.

The novel features which are believed to be inherent in the invention are defined in the appended claims.

What is claimed is:

1. The method of repeating at high efiiciency, modulated wave by means of a space dischar e repeater havin a control element whic comprises supp ying to the control element a negative voltage of such a value as to just allow the repetition of waves of small amplitude and automatically increasing this negative voltage in a ratio proportional to the magnitude of the waves when waves of large amplitude are to be repeated. I

2. The method of repeating a zero carrier modulated wave comprising a carrier wave as regards power consumption, a

control element which comprlses var inggrrid periodithe polarizing potential of the equency of cally at a frequency twice the the lower frequency wave. 7 a

3. The method of repeating a wave of a given frequency having amplitude variations in accordance with a modulating wave of a given frequency by means of a space discharge device having a grid or control element which comprises varying the potential of the grid periodically at a frequency twice the frequency of the modulating wave. 4. The method of repeating a high frequency wave of-variable amplitude by means of a space discharge device which comprises rectifying a portion of the high frequency wave energ by means of a device other than the space ischarge device and applying to the space discharge device a potential difference resulting from the rectification to control the repetition of thehigh frequency waves by the space discharge device.

5. The method of high frequency wave amplification by a discharge device amplifier containing a control element which comprises applying a portion of the high frequency energy which is to be repeated to a rectifier and applying to the control element of the amplifier a resultant negative voltage which "varies. periodically at a frequency twice that of the high frequency wave.

6. A vacuum tube repeaterhaving input electrodes, a rectifier connected between said electrodes, a resistance in series with said mew.

rectifier, and ahi h resistance connected between said electro es in parallel to said rectifier which supplies rectifiedcurrent theretofor applying a varying potential between said electrodes.

7 In combination a vacuuin tube repeater for repeating only impulses of a zero carrier wave of one polarity, a rectfiier in shunt thereto for varying-the grid potential of said repeater at a frequency double the frequency of the modulating wave, and a ance in series with said rectifier. 1 I 8. The method of amplifying a wave having components of fixed and variable freresistquencies by means of a space discharge device having an anode-cathode and control electrode or grid which comprises varying the grid polarizing potential as a function of the amplitude of one of the wave coinponents. f

9. A vacuum tube repeater having grid, filament and plate electrodes, an input circuit including said grid and filament electrodes, means for impressing-waves to be repeated on said grid and filament, a high I RAYMOND A. HEISING.