US2084180A - Radio signaling system - Google Patents

Description

l. E. BARTON 2,084,180

Filed Jan. 15, 1932 V35 Sheets-Shveet l June 15, 1937.

RADIO SIGNALING SYSTEM June 15, 1937. l.. E. BARTON 1 2,084,180 1 RADIO SIGNALING SYSTEM,

Filed Jan. l5, 1932 5 Sheets-Sheet 2 H/S ATTORNEY.

3 Sheets-Sheet 3 Filed Jan. 15, 1932 o'o'o'o'o'o o INVENTOR. Log E.Bo.1^tof1, www /f/s ATTORNEY.

N nNN Patented June 15, 1937 2,084,180 RADIO SIGNALING SYSTEM Loy E. Barton, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 15, 1932, serial No. 586,874

14 Claims.

The present invention relates to radio signaling systems and more particularly to systems of that character embodying electric discharge devices as audio frequency amplifiers and modulators.

Since the early use of electric discharge devices or vacuum tubes in the broadcasting art, the demand for higher output power from audio frequency amplifier systems has steadily increased, in both radio receiving and transmitting apparatus.

In radio receiving apparatus, as is well known, the demand for higher power output occurs chiefly in connection with the output stage of 15 the audio frequency amplifier, while in transmitting apparatus it occurs both in connection with the power amplifier and in connection with the modulator, used in conjunction with the power amplifier, to apply the modulating or audio frequency signal to the carrier wave.

This demand for higher audio frequency powery has heretofore been met chiefly by the develop` ment of tubes having a higher plate or anode dissipation and a lower plate or anode resistance, and the use of these or available standard tubes in balanced or push-pull relation and in multiple,

both in radio receiving and amplifying apparatus and in transmitting apparatus. This ex.

pedient has the disadvantage that it involves a omultiplicity of tubes and associated apparatus, with corresponding complication and cost. This is particularly true in connection with transmitters arranged for substantially 100 percent modulation and having appreciable power output, for the reason that the power requirements for a suitable modulator therefor is of a corresponding magnitude and cost.)

When the antenna or output current of a transmitter is caused to uctuate according to a modulating wave of any shape, the average antenna current may remain the same, but the effective value of the current increases, therefore, increasing the average power input to the antenna. This increase in antenna power must be supplied (a) by the modulators employed, (b) by an equivalent increase in power, (c) by an increase in the plate current to the power arnpliiier tubes, or (d) by an increase in the average efficiency of the power amplifier tubes.

It is generallyx'conceded that the increased power may best be obtained by method (a) above, in supplying the increased power in the form of audio frequency power to the anode or plate circuit of the radio frequency power amplifier.

However, this has heretofore involved the use of tubes of higher rating, adapted tolcarry the Therefore, in

larger modulator power output.

meeting such increased power requirements, modulators heretofore and at present may often include a plurality of high power in balanced and in multiple rel tubes connected ation, with attendant high cost of operation as well as initial cost.

is assumed as 50 percent.

Table A Percent Modulator output modulation required in watts It is well recognized that 100 percent modulation is the most economical degree of modulation. This is for one reason, among others, that the higher percentage modulation permits the use of a transmitter of lower power for the production of the same signal strength at a given dis- Furthermore, the higher modulated signal is subject to much less tance from the transmitter.

interference.

the transmitter.

tter from zero to o modulate the modulators, and the power requirements thereof, involves a careful consideration and understanding 'of three existing types of vacuum tube or electric discharge ampliers. The classification 4is based upon the operating characteristics depending upon values of grid bias, excitation, allowable distortion, and other features.

A type of amplifier known as a class "A amplifler is the type most used in receiving apparatus in the usual audio and radio frequency ampliflers, and in which'the average value of plate `appreciably when a signal is impressed on the tube grids, and the grids are usually not driven positive. The distinguishing characteristic is that the output alternating current voltage bears a linear relation to the input alternating current voltage.

A type of amplifier, known as a class B" amplifier, is one which is usually employed as a radio frequency output amplifier in a transmitter, and its distinguishing characteristic is that the power output is proportional to the square of the excitation grid voltage. The electric discharge devices or tubes employed therein are biased to essentially plate current cut off or slightly above, and the average value of direct plate current varies with the input alternating current voltage. The plate current flows only during the positive swings of the input voltage. I'Ihe grids may be driven positive until the output voltage begins to deviate from a linear relation with respect to the input voltage.

Since plate current flows only during one-half of the cycle, the plate circuit must be tuned to preserve the input wave form or if an audio or an aperiodic amplifier is desired, two tubes must be used in a typical push pull or balanced arrangement in order that plate current to one or the other of the tubes may flow at all times.

The excitation may be strong enough to swing the grid or grids positive and thus produce an appreciable rectified grid current. Rather heavy loads are therefore placed on the preceding amplifier by the large amount of grid excitation required to overcome the grid losses in the tube. The output efficiency is very good because of the fact that the peak plate or anode current'may be comparatively high for peak positive voltage swings on the grid and the instantaneous direct current resistance of the tube is low during the half cycle the plate current flows.

A third type of amplifier, known as a class amplifier, is a, special type `of radio frequency amplifier or oscillator and is one in which the output varies as the square of the plate voltage within certain limits. The tubes are biased to about double the value for plate current cut off and the grid is driven to a degree such that l small changes in input voltage does not appreciably change the output Voltage.

The excitation peak voltages must be sufficient to drive the grid considerably positive in order to secure large amplitudes of plate current. This results in a very high emciency of tube operation due to the fact that the internal direct current resistance of the tube is very low during the time of plate or anode current flow. However, the grid excitation losses are relatively high and the preceding stage must deliver enough energy to supply these losses. In fact, the grid excitation may go so far positive that saturation 2,084,180 A further lconsideration oi' amplifiers and may be 'reached for a relatively large portion of the cycle.

, The class C to do economically.

The class C type of ampliner is considered to be the most efficient type for transmitters, but the plate' circuit must be tuned if sinusoidal voltage output is desired. For constant input voltage, the output voltage or antenna-current is proportional to the plate voltage. Therefore, this type of amplifier is well adapted to plate modulation. If enough energy is taken from the output o1' a class C amplifier to drive its grid, the ampliij'ler becomes tor, l

power required for modulation purposes may be in the order of the power output of the output amplifier.

In the eld of radio receiving apparatus and the like, employing audio frequency power am- This demand is in part, a result ofdesigns directed at greater fidelity of tone and larger sound coveragefrom the output of an amplifier. The present power output requirements from the amplifier of a radio receiver may be in the order of several watts, while for operating a plurality of loud speakers or other devices for wide sound coverage, the audio frequency power requirements may often be in the order of several hundred watts.`

Because of operating characteristics, whereby it is adapted economically to meet demands for high power output, the present. invention relates more particularly to the class type of amplier as the output stage of an audio frequency amplifier unit for signaling systems, and has for a further object to provide an audio frequency electric discharge amplifier or modulator of that type from which a higher undistorted output power may be obtained at audio frequencies than has heretofore been obtainable by known means or arrangement of electric dischargeV devices in an amplifier, while taking advantage of the desirable operating characteristics of a normal class B amplifier. v

It has been found that an amplifier of the above character capable of delivering a high undistorted audio frequency power output may be adapted to use as a modulator of high power transmitters, as an output amplifier in radio receiving apparatus and the like, and that it may permit the use of ordinary electric discharge devices or tubes of the so called battery type to deliver relatively high audio frequency power in battery and portable receiving apparatus, with economical current consumption.

the usual radio frequency oscillaamounts of audio frequency power in an output amplifier for radio receiving apparatus and the Ito such limits that they may supply an like, without requiring vthe use of electric discharge devices of a corresponding higher power rating orina multiple arrangement heretofore required.

In providing an amplifier for high audio frequency power output from electric discharge devices or vacuum tubes simply and economically as is desirable, several principal governing factors in design must carefully be taken into account. Such factors are:

1. The plate or anode dissipation must be kept within rated or normal values.

2. It is desirable to use as low plate or anode potentials as is possible.

3. It is also desirable that the power required for operation be kept to a low value. This is f particularly true in the case of portable and battery operated amplifier apparatus.

4. The power output, relative to the normal power rating of the electric discharge devices employed, must be kept high.

The class A type of audio frequency amplifier or modulator which is extensively used at the present time, does not satisfy all of the above conditions. The maximum plate or anode dissipation occurs while the amplifier is receiving no signal current and the maximum anode efficiency is relatively low, being normally not over 25 percent at maximum output. The result of the high anode dissipation is that the output or power tubes must be large, and in general a relatively high anode voltage is required. Furthermore, a plurality of such tubes, and at least two in balanced or push pull relation, are required in the usual amplifier. In the last case,the tubes must be of a higher power rating to obtain a higher power output. This type of amplifier is therefore usually uneconomical in the power required for the anode supply.

In the class B type of amplifier, the output or power tubes are biased to essentially anode current cut off and when the input electrodes or grids are properly biased and driven by signal currents to substantially zero bias swing, the first three of the above conditions are satisfied to a fair degree. However, like the class A amplier, this type requires relatively low audio frequency power from the signal source to operate it, and the input system thereto may be the usual relatively high resistance type. An amplifier of the class B` type, however, does not in itself satisfy the fourth requirement above mentioned.

It is, therefore, a further object of this invention to provide an improved audio frequency amplifier arranged to operate the electric discharge devices or tubes therein in such a manner and audio frequency output relatively higher than the normal output of the same electric discharge devices or tubes with the same anode voltage, with a lower average anode dissipation, and with no serious effects upon their life.- Thus, in accordance with the invention, the usual requirements for higher plate or anode dissipation and higher anode voltages, both of which are costly in tube construction and in the matter of the supply of operating potentials, are entirely obviated.

In accordance with the invention there may be provided an audio frequency amplifier by which a relatively high audio frequency output, for exampleof f'lve to ten times the usual or normal output, may beobtained Without increasing the size, rating, or number of electric discharge devices employed therein, and without increasing the anode potential or anode dissipation.

An amplifier of this typel may be used as a source of high audio frequency power for plate modulation of a transmitter as hereinbefore described, and may be applied to any signaling system requiring a relatively high'audio frequency output, with a minimum of equipment. n

Because of the class B type of operation, a varying load is placed upon the anode power supply for an improved amplifier and modulator embodying the4 invention; and accordingly it is -a further object of the invention toy provide means in connection with the source of anode or plate current supply, whereby fluctuations in load are prevented from appreciably affecting the voltage regulation of the source.

The invention will be better understood from the following description when taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the drawings, Figure l is a circuit diagram of an audio frequency amplifier embodying the invention;

Figs. 2 and 3 are curve diagrams illustrating operating characteristics of the amplifier shown in Fig. l;

Fig. 4 is a circuit diagram of equivalent circuits representative of the input and output circuits shown in Fig. l;

Fig. 5 is a circuit diagram of a modulator embodying the invention as applied to a broadcastv transmitter having a high power output amplifier of the class C type;

Fig. 6 is a wiring diagram of an audio frequency amplifier embodying the invention and adapted for battery operation;

Fig. 7 is a circuit diagram of equivalent circuits to those shown in Fig. 6; and

Fig. 8 is a circuit diagram of an alternating current amplifier or modulator embodying the invention and applied to a short wave transmitter system.

Referring to Fig. 1, an improved class B audio frequency amplifier embodying the invention is shown. This amplifier includes an output'stage I0 and a driver stage II therefor, which are so coupled and controlled, that by means of an electric discharge amplier or vacuum tube I2 of relatively low power output in the driver stage, which may be of the class A type for example, the output stage may be driven substantially to the limit of the emission or space charge of the electric discharge devices or tubes therein for maximum audio frequency power output. For a given power output, the latter tubes also may be of normal l power rating as will be seen hereinafter.

The amplifier I2 receives audio frequency signal voltage from any suitable source such as input terminals I3 which are connected with it through a suitable input or grid circuit I4, whereby audio frequency signal potentials applied to said terminals or to the input of the amplifier, are amplified by the device I2 and through its output or anode circuit indicated at I5, are applied 'to the output stage I0.

The output amplifier stage consists essentially of two tubes I9 and I'I connected in push pull or balanced relation between push pull input and output coupling devices I9 and I9 respectively, and biased for class B operation, as indicated.

The grid bias supply indicated is preferably of relatively'low direct current resistance. The output stage is provided with a suitable anode voltage or plate supply source as indicated in the drawings. The anode voltage supply source 10 should have good voltage regulation under wide load variations, as it should be understood that the load current increases with an increase in the amplitude ofv applied signal voltages in this type of amplifier.

The output stage is provided with an input circuit or'system, including the coupling device i9 and the output circuit I5 of the driver stage, having a relatively low impedance in itself, whereby the signal voltage is permitted to carry the grids of the tubes I6 and I1 therein, far into the positive range of operation, without a reduction of the available signal voltage and hence distortion, because of the impedance drop in such input cir'- cuit or system, and without excessively loading the driver stage.

The driver or lrst stage of the ampliier is preferably but not necessarily, transformer coupled to the output stage. In the present example, device I8 is a transformer having a step down ratio from a primary winding 20 in the anode circuit I5, to each side of the secondary winding 2|, which is included in an input or grid circuit 22 for each of the output tubes. It should again be noted that two tubes in balanced or push pull relation is preferred for substantially distortionless class B operation in an audio frequency amplifier, for reasons pointed out in the preceding description of the class B type of amplier.

The step down ratio of the coupling device or input transformer I8 for the output stage is arranged to permit a relatively low reflected impedance, over from the anode or output circuit I5 of the driver stage, in each half of the input or grid circuit 22 of the amplifier I 0, in series therewith. Therefore, when the grids of the tubes I 6 and I1 are driven into the positive range, the low effective input resistance permits grid current to flow without seriously affecting the shape of the signal or input voltage wave as applied at the input terminals I3 of the amplifier.

. The impedance ratio of the input coupling means, furthermore, is such that the current requirements of the grid circuit 22 are met without overloading the driver stage II and introducing distortion, and, is further of an order such that the anode circuit impedance of the driver stage reflected over into each half of the grid circuit 22 of the output stage III in series therewith, is substantially less than thelgrid to cathode impedance of the electric discharge device I9 or I1 in the output stage connected with that half of said grid circuit, when drawing maximum grid current through said circuit.

The output or power amplifier devices I6 and l1 are preferably of the type having a higher,

rather than a lower, amplification factor, and a higher, rather than a lower, internal impedance. Like the devicei2 shown in the driver stage, they are illustratedl in the present example as being of the usual three-element type of vacuum tube, although other types of electric discharge devices may be used. In general, these tubes are of the ordinary audio frequencyamplier type.

In any case, the output tubes I6 and I1 are arranged to be operated in accordance with class "B operation. that is, the control electrodes or grids are biased to substantially anode or spacecurrent cut off as indicated by the legend in the drawings, Fig. 1.

The bias source is located inthe grid circuit 5 22, and is common to both devices I9 and I1.

In the present example, the input or grid circuit 22 for devices I6 and I 'I is provided by a winding or impedance 2I, this being the secondary of the coupling device or transformer I9, and is mid- 10 tapped as indicated at 24 for application of the biasing potential, in the usual manner for push pull circuits.

The load connected with the output stage is also arranged to have a relatively low impedance 15 with respect tothe internal impedance of the output stage. .To this end, the output circuit of the power stage is arranged to have a low ini-' pedance with respect to the internal or plate lmpedance of the electric discharge'devices therein. 20

The output circuit of the power or output stage I0 includes a balanced output or anode circuit 25 for devices I6 and Il, a source of load therefor indicated by output terminals 26, and the output coupling device or transformer I9 which is inter- 25 posed between the circuit 25 and the load, as an impedance transforming means.

The anode or operating potentials for the de;- vices I6 and I'I areapplied through a mid-tap connection 28 on the primary winding 21 of the 30 output transformer or coupling device, and the anode circuit 25, in the usual manner for push pull amplifiers. The plate or anode supply source which may be connected between the cathodes and the tap 29 should, as indicated, have good regulation for the reason that in operation the devices or tubes I6 and I I draw a varying load current in response to changes in the impressed signal voltage.

In accordance with the invention, therefore, 40 the grid'or input circuit per se as distinguished from the entire input circuit or system, of the power amplifier or output stage, must be of low resistance and the input circuit as a whole must be of low impedance, so that the power or cur- 45 rent requirements of the tubes through the grid circuit may be met without seriously distorting the wave shape of the signal applied to the driver stage, and the reected load back into the output circuit, or the output circuit impedance, must be 50 kept low with respect to the internal impedance of the output tubes in order that the tubes-may be driven to the limit of their emission or space charge, without distortion in the plate circuit.

The output power is limited by the plate dissi- 55 pation, the maximum direct current to the plate, or the maximum allowable grid voltage swing within the distortion limits. If the grid voltage swing limits the output because of distortion, it does so, by swinging in a positive direction until 60 it approaches the minimum instantaneous plate voltage. When the two voltages approach each other, the direct current of the grid rises very rapidly with any increase in grid voltage swing.

The plate or anode current swing for a given ex- 65 citation depends upon the load resistance in the plate circuit. 'I'he load resistance, then, bears a very important relation to the tube loss, the maximum power output and the power required to drive the grid, as will be seen herein- 70 after.

Referring now to Fig. 2, 29 and 30 are plate or anode-current,l grid-bias curves for tubes I9 and I 1 respectively, of Fig. 1. The curves are plotted with respect to a grid bias scale or zero 75 plate current line 3I, and an ordinate .32 representing plate current and grid current scales. The curve 29 is plotted from data obtained by varying the applied grid potential and reading the corresponding anode current for one tube such as tube I6, inthe circuit 25 of Fig. 1. The plate current curves are taken with load, that is with a load at 26 reected over in series with one tube in the plate circuit 25.

The zero bias line for curve 29 is indicated at 33 and it will be noted that the normal anode current for class B operation is indicated at 34 through which is drawn the normal negative bias line 35. It will also be noted that the negative bias is such that the point 34 on the curve 29 is substantially at zero or cut off for the anode current. In response to applied signal potentials, the anode current increases along the line 29 to a maximum point such as the point 36 for example. The total grid swing for such operation is indicated at 31, which includes the negative range 38 and a considerably larger positive range 39. 'I'his is thegrid or input signal voltage swing, A, for the tube I6.

It will be noted that the operation of each tube is carried far into the positive range and that after the zero bias at line 33 is passed, the grid begins to draw. an increasing amount of grid current indicated by the curve 40, a point of maximum slope on which is indicated at 4I.

The lower curve 30 for the tube I1, is the same as the upper curve 29 except that it is reversed and shifted along the bias line 3I until the curves 29 and 30 coincide as nearly as possible to a line 42 drawn through the straight portion of the upper curve 29. The bias potential indicated .at the point 43 through which the straight line 42 passes in crossing the zero plate current line or the grid bias line 3I, is the proper normal negative bias potential to use for the two tubes I6 and I1 having characteristics substantially as shown.

If the tubes or electric discharge devices are not similar, or if the anode supply voltage is slightly different from the values indicated by the curves shown, the bias may be adjusted until the anode current is obtained as indicated by the curves.

The relation of the plate or anode current curves 29 and 30 of Fig. 2 is an indication of the operation of devices I6 and I1 respectively in the circuit shown in Fig.v 1. When the grid of one tube is driven in a positive direction from its normal negative bias value, the plate or anode current of that tube increases with the swing, and ows through one half of the primary winding of the output device I9. The output voltage for this half cycle bears a linear relation to the input voltage to the amplifier. At the beginning of the next half cycle the above tube becomes idle because its grid becomes more negative and the other tube functions in a manner similar to the first except that the anode .current ows in the other half of the output winding of the device I9, and the output voltage is therefore 180 d`e grees out of phase with the first half wave.

These two output waves will thus unite to form a wave similar to the input signal wave, with no distortion, if the plate current and grid voltage have a linear relation substantially as shown in Fig. 2. This relation is substantially linear as will be seen by the similarity between the curves 29 and 30 and the straight line 42 drawn through them.

It should be noted that the input coupling device or transformer I8 delivers current to the grids of the output tubes from only one side of the secondary at any particular instant, which must be considered in the design of this device.

If the signal swings to the right, as indicated, in Fig. 2, from the zero center line or bias line common for both tubes, the plate current 29 of the tube I6 which may be designated as IbA, increases, and as the grid becomes positive, grid current designated as IcA, flows according to curve 40. As the signal reverses, Ib. decreases and as it passes the common bias line, IbA becomes zero and plate current Ibis of the other tube I1 increases according to the curve 30. Therefore, each tube functions over one-half cycle, While the other is practically idle.

It should also be noted that the entire output power must be transferred from only one side of the output transformer primary during each half cycle. Therefore, the load impedance applied at 26, which either the tube I6 or the tube I1 is working into, is calculated as if only one tube is supplying the total power from one side of transformer primary 21, but in calculating plate dissipation, each tube functions for one-half the time sothat the total plate loss is divided between the two tubes.

The plate current input to the output tubes resembles a true full-wave rectified current, the

frequency of which is double the frequency of the signal. Therefore, the power input to the plate of the output tube is:

0.637 IpmEb: power input in which 1pm: peak plate' current (alternating current) .f Eb: plate or anode supply voltage (direct current) 0.637 1pm: Ib or average plate current The power output may be represented as:

0.707 Epmx 0.707 Ipm=power output (2) in which Epm=maximum voltage (alternating current) delivered to the load. The eflciency of this amplifier is given by:

If Epm approaches Eb as a limit, then the e'iciency becomes 78.5 percent for half-sine-Wave outputs.

The output power from (2) is:

As was noted above, the plate current is not limited as in the class A amplifier, so that a load resistance for maximum power output is such a value that the limit of emission is approached. The minimum instantaneous plate voltage Ehm and the allowable plate dissipation y are also factors which determine the load resistance.

The load resistance for this amplier is-calculated as with a class A amplifier which is:

E,... Im (6) The peak plate current at a point on the desired loadv curve, which is within the permissible distortion limit, is used to calculate the output power of two tubes operating as a class B audio =R load resistance amplifier. This value of peak plate current and the corresponding loadl resistance are used in Equation (4) to obtain the power output of two tubes operating as a class B audio amplifier.

5 The input plate power is calculated by using Equation (1) and the plate circuit efficiency by using Equation (5).'

'I'he input resistance of the amplifier is indefinite, as with a class A amplifier with the grids driven positive, but a minimum -value can be obtained from the grid current curve 40 of Fig. 2, being in the present example, point 4I for a maximum positive signal voltage swing. 'I'his may be used to indicate the input series resistance that may be permitted for a given degree of distortion. The minimum input resistance is calculated from the maximum grid current and also the voltage swing required for this current. However, the slope of the grid current curve must also be considered because if this resistance represented by the slope of the curve is low compared with the effective grid series resistance, considerable distortion will result. This is especially true if the operating bias is considerably negative as indicated.

Referring now to Fig. 3, the alternating current component of the anode or plate current Ip, the alternating current plate voltage Ep, and the alternating signal voltage Eg, applied to the grids of the output tubes are plotted as ordinates against time, in curves 44, and 46 respectively.

The curve 45 for the plate voltage is shown with respect to the normal applied anode or plate potential En indicated by a line 41, while the cathode potential or zero axis is indicated by a line 48.

The normal negative grid bias potential with respect to the cathode line 48 is indicated by a line 49, about which as an axis, the curve 46 is drawn. One half cycle of the alternating cur- 40 rent component of the alternating current anode or plate current Ip is shown and is drawn with respect to a line 50 corresponding to the normal anode or plate current indicated in Fig. 2 at point 34 on curve 29.

45 These curves show the relation between the various instantaneous values of the alternating current components of the anode current, anode voltage, and grid voltage. y

The operating characteristics indicated by the 5o curves of Figs. 2 and 3 will now be considered in the design of the amplifierl shown in Fig. 1 as follows:

1. 'Ihe determination of the maximum anode current 1pm, the peak positive grid potential or signal swing Em, to obtain such plate current, and the minimum instantaneous plate voltage Ebm, required to cause the plate current to flow. The grid current and grid voltage may be estimated for the above conditions. Incidentally the above condition for minimum plate voltage is a condition where the space charge limitation and emission limitations are practically equal. 2. With the above data, which may be esti'- mated from'standard data sheets of characteristie curves of available tubes, the minimum input resistance Rg is estimated between the grid and the cathode at the point of maximum positive grid swing. The peak grid voltage swing is also estimated. 3. Knowing the voltage and current requirements of the input circuit, the next step is to provide a means in the input circuit for supplying the required grid current without appreciably affecting the voltage input wave, even though at times the grid current in the output stage may be zero or the input resistance may be very high.

By way of example, and as a preferred value which has been found to be desirable, the permissible input regulation may be chosen at ap-l proximately 10 percent, from which the eil'ective resistance in series with the ygrid of each output stage tube approximately may be one tenth `of the value of the minimum grid to cathode resistance, for maximum positive grid voltage swing in response to a signal wave. 10

4. Knowing this allowable grid series resistance and the-voltage required, the transformer or input device I8 is designed with a proper ratio for this purpose, and to permit the use of a tube in the driver stage that, without overloading, will 16 supply the necessary input signal voltage at the known impedance of -the tube, to the primary 28 of the input transformer I8 for the output stage.

The above procedure permits a grid swing of the output tubes to a point where the plate cur- 20 rent ,becomes limited by the space charge or by the emission. 'I'his is a feature ofimportance in permitting maximum power output from the amplifier. It is obvious that, if this grid voltage variation or swing and plate current swing are 25 obtained, platedissipation must be considered to prevent overheating of the plates. The latter may be controlled by the load resistance.

The above features may be used successfully to the limit or the output of the tubes, if a load re- 30 sistance is used, such that vwhen maximum plate current fiows, the voltage drop in the load resistance causes the plate voltage to drop from the value Eb (Fig. 3) of the plate voltage supply to the minimum value Ehm (Fig. 3) as determined 35 by emission and space charge. It has been found that such desirable loading of the output tubes yrequires that the load resistance be 'made lower than the plate resistance of the output tubes. This is a feature of the improved class audio 40 amplifier, which is highly desirable and effective in providing a maximum power output economically.

The following will explain the method of calculating the minimum input resistance of ea'ch 45 of the tubes in the output stage.

The slope of the curve 40 in Fig. 2 at the maxirnum swing point 4| is the minimum resistance of grid to cathode or the internal input impedance of one of the output tubes, which may be 60 called Rg.

The total input or grid voltage change or swing on each output tube is the swing indicated at 38 plus that indicated at 39, with a minimum resistance Rg. Rs, or the resistance of-the input 55 circuit in series with the grid of one output tube, must be appreciably lower than the resistance Rg, and is preferably of the order of one-tenth oi' Rg, as hereinbefore pointed out, although it is not limited thereto. However, no. shunt resist- 00 ance must be permitted except to improve the frequency characteristic and the coupling device in the input circuit is designed to provide, in itself, this desired effective input resistance in series with the grids of the output tubes when 65 coupled with the stage.

Assuming the transformer or coupling device I8 to have a turn ratio of N1 to Nz for primary and secondary, respectively; if the tube I2 in the 70 driver stage has a plate impedance, rp, o1' 3000 to 3500 ohms, then RL the load resistance in the driver stage output circuit I5, which in Fig. 1 is the primary 20 of device I 8, should be 30,000 to 35,000 and if in push pull, twice this amount 75 output circuit of the driver and the signal voltage across RL (primary 20 in mg.

lL. Il@

X maximum grid swing (38 and 39 in Fig. 2).

The following will explain the method used to determine the optimum load resistance Rp for the output stage.

Referring to Figs. 2 and 3, the maximum alternating plate voltage Epm delivered to the load across the primary 21 of the output device I9, or the maximum voltage decrease from the plate supply Eb (Fig. 3) is determined by the minimum allowable plate voltage Ehm which will cause the peak plate current to flow. This is dependent upon the space charge of the tube and is limited thereby. Ehm is then, the. minimum plate voltage necessary to obtain peak emission or as nearly peak emission as safe operation permits, the peak plate current being 1pm.

'I'he peak grid swing is such that Ipm flows with Ehm on the plate and the plate current must be proportionate to grid voltage Eg to the point 1pm, Fig. 3, as indicated at point 36 in Fig. 2, for example.

The load resistance Rp for each of the tubes I6 and I1 for the time it operates is:

ang,

If the available potential is such that the value of Eb is low compared to a certain selected value of Ehm, then it is desirable to use a lower value of Ehm so that Ip will be limited by the space charge rather than the emission.

The underlying thought in the above design is that of providing a class B output amplifier, with input and output circuits including suitable coupling means, whereby a maximum power output is provided without appreciable distortion of the signal wave applied to the driver stage in the input circuit.

In providing this amplier arrangement, in the input circuit there is interposed between the grid circuit of the output stage and the output circuit of the driver stage, a coupling Vmeans such as a step-down transformer so designed that a reflected series impedance Rs, over from the preceding driver stage, is of a lower value than the impedance of the grid circuit between the grid and cathode of each tube in the output stage, when said grid is at a maximum positive potential and drawing a maximum grid current.

At the same time, in the output circuit, a coupling means such as a transformer, is provided whereby the reected load Rp back into the plate circuit of the output stage is of a lower value than the internal impedance Tp of each of the output tubes, in series with the plate circuit.

Furthermore, the direct current resistance of the grid or input circuit of the output stage must be relatively low, together with the source of bias potential, to prevent distortion, because of voltage drop in the elements of the circuit. Also, in the output circuit of the output stage, the anode potential supply must have good regulation.

It will be noted that a low impedance grid circuit, as a part of the input circuit for the output stage, is provided without resorting to artiiieial loading means such as a shunt resistance, in connection with the grid circuit. 'I'he loading effect of such resistance upon the grid circuit and upon the driver stage, would seriously introduce distortion or introduce practical limits into the design, whereby the desired operating characteristics would not be obtained. Chief among the disadvantages of such a circuit would be the lowering of the applied signal voltage to the output stage and additional load upon the driver stage.

Stated in other words, an audio frequency amplifier is provided, in which a class B type of output stage, and a driver stage therefor, are connected in cascade between a source of signal potentials and a source of load for maximum power output, limited only by the space charge or emission of the output devices. The output stage includes electric discharge devices in balanced or push pull relation, and a coupling device, preferably a transformer, is arranged to couple the anode circuit of the driver stage with the grid circuit of the output stage.

The impedance ratio of the transformer is such that the current requirements of the grid circuit are met without over-loading the driver stage and introducing distortion, and furthermore, is of such an order that the anode circuit impedance of the driver stage, or operating impedance, reflected'over into each half of the grid circuit of the output stage in series therewith, is substantially less than the grid impedance of the electric discharge device connected with that half when drawing maximum grid current through the grid circuit.

Between the output stage and a load impedance there is provided an impedance changing or transformer means having an impedance ratio such that the load impedance, or operating irnpedance, reflected back into each half of the anode circuit of the output stage may be substantially less than the anode impedance of 4the electric discharge device connected with that half, and in series therewith, whereby the electric discharge devices in the output stage may be driven to the limit of their emission or space charge.

Viewed in still another way, the grid or input circuit, including the secondary of the coupling device orvtransformer, must be of low impedance so that the power or current requirements of the tubes through the grid circuit may be met without seriously distorting the wave shape of the signal applied to the driver stage, and the reected load back into the output circuit, or the output circuit impedance, must be kept ,low with respect to the internal impedance of the output tubes, whereby the tubes may be driven to the limit of their emisimpedance between certain points such as themid-tap 24 and a point 5I of Fig. 1 representing the input impedance Rs, must be substantially less than the impedance betweencertain other points l2 and Il, representing the grid impedance R.. 'I'he impedance R. is the reected .im-

. connected with that portion oi' the input circuit.

circuit 5l 01' Fig. 4, in

as pointed out hereinbefore, when it is drawing a maximum grid current. U

The entire input circuit may be reduced to the which terminals 56 represent the signal voltage applied to the output stage. This Signal voltage as indicated, will be the mu or amplification factor of the tube i2 multiplied by the value of the applied signal voltage at terminals I3, and by the turn ratio of transformer i8, Fig. 1.

The internal'grid to cathode impedance of the output tube I 6 for example. or theimpedance between points 62 and Il, is indicated -at l1 and is the value R. above mentioned. 'I'he reflected impedance of the driver stage output circuit, over in series with the circuit 22, is indicated at 59 and is the impedance between the points 6| and 24, as indicated. 'I'he circuit 64 of Fig. 4 is thus the equivalent of and serves to illustrate the fundamental values of the input circuit shown in Fig. 1.

Coupled to the output stage is the load impedance as provided between the terminals 26, and which when reflected back into the output circuit of the ampliner, appear in each half of the output circuit, such as between points 28 and.

59, for example. This reflected load impedance over into the output circuit is preferably relatively low with respect to the internal impedance oi' the corresponding output tube, such as the tube I6 for example, between a point 60 and the point 53, being the plate to cathode or internal anode impedance of the tube. 'I'his output circuit is represented in Fig. 4 by the circuit 6I in which the impedance 62 is the reflected load impedance between points 59 and 28, for example, in the output circuit 26, and the impedance 63 is the internal impedance 'of the tube between the points 60 and 59 as above pointed out.

The signal voltage delivered by the output tube is applied as indicated by the terminals 55 and is, as indicated, the mu or amplification factor of the output tube multiplied by the signal potential En applied from the driver stage to the output stage.

By way of example, if the tubes i6 and i1 in the output stage each have a mu or amplication factor of 20, and the signal potential applied by the driver stage isl 160 volts, then the potential available in the output circuit will be 20x160 or 3200 volts, oi' which, 1300 volts may exist between points 28 and 59 across the load, and the remainder of which, 1900 volts, may exist between points 60 and 53 or across the tube in the output circuit.

Assming now that an applied potential of 2000 volts or E is provided as indicated in Fig. 3, the 1300 volts or Epm will leave an instantaneous minimum potential Ehm on the plate oi' 700 volts. As hereinbefore explained, this last named voltage may be and preferably is, the minimum anode potential necessary to maintain a maximum ilow of plate current as determined by the space charge of the particular tube. At the same time, the impedance of the output circuit should be and preferably is such that a maximum plate current Ipm may flow with minimum instantaneous applied potential. It will thus be seen, as hereinbefore pointed out, that the power output is affected by the load impedance and by thespace charge of the tubes employed. It is also atlected by the emission limits of the tubes. and the load impechnce must be such that the platerv dissipation of the tube is not exceeded.

The values of the circuit elements ofthe input and output circuits of ldgs. 1 and 4 may vary in different installationspdepending von the character of work to be amounts of power to be transmitted, but in any case impedance R. must always be less than imf pedance R. and the impedance R@ must always be less than the impedance rp for maximum power output.

'I'he load impedance on each tube of the bal. anced tube arrangement, during the time it works, must be relatively low in order to obtain a desired power output, whilev at the same time it must be high enough to prevent a plate dissipationbeyond the rating of the tube employed. The load resistance is thus chosen that for a maximum grid excitation the normal plate dissipation may be reached and the minimum inthe space charge and current to reach values near the limit of the emission of the tubes. l

Through the medium of an amplifier of the type shown and described, an'l audio frequency power output may be obtained, which is many times larger than the output available from the same number of tubes in an ordinary class "A" or class B amplifier. This is chiefly for the distortion of the applied An amplifier of this character is therefore particularly weil adapted for use as a modulator in a broadcast transmitter.

By way of example, it has been found that 'it is possible to use a one K. W. output tube with class C operation for a one K. W. broadcast transmitter, and'modulate the plate circuit to 100 percent by two 350 watt tubes as a class B" audio frequency amplifier of this type, whereas otherwise with existing systems it is necessary to use a 4 K. W. tube operating as a class B high frequency amplifier for a one K. W. station with 100 percent modulation, with its associated input system. A broadcast transmitter having a modulator system of this character embodying the invention, is shown in Fig. 5, to which attention is now directed.

Referring to Fig. 5, a radio frequency powex amplier of the class "C type is indicated at 64 and includes a plurality of electric discharge amplifying devices 65 connected in parallel with a' tuned radio frequency input circuit 66, through a coupling device 61 and radio frequency input terminals 68. The output or anode circuit indicated at 69 is also parallel connected with the devices 65 and is coupled with a radiating system 10 through a suitable output coupling device 1I.

Audio frequency modulating signals are applied to the anode circuit 69 to modulate it, through a modulator 12, which includes a driver stage 13 and an output tween audio frequency input terminals 15 and the anode circuit 69.

stage 14 rascade connected, be-

The audio frequency input terminals 15 are suitably coupled to an input circuit 16 for the driver stage 13 by an audio frequency coupling device or transformer 11. Between the output circuit of the driver stage indicated at 18 and the input or grid circuit of the output stage indif cated at 15J, a second audio frequency coupling device is interposed and is designed to provide an impedance relation between circuits 18 and 19 substantially like that provided by device I8 lin Fig. 1, and for the same purpose.

Likewise between the output circuit of output stage 14, as indicated ,at.88, and the anode circuit 69, an output coupling device 8l is connected to provide a load impedance arrangement in the output circuit 88 substantially like that provided by device I9 in Fig. 1 and--for the same purpose. A radio frequency choke coil 82 and a suitable bypass condenser 83 serves as a filter to isolate the anode circuit 69 from the modulator 12.

It will be noted that in the output stage 14, the

electric discharge devices indicated at 84 are arranged in balanced relation between the input coupling device 80 and the output coupling device 8| in substantially the same manner as in the basic circuit shown in Fig. 1 in connection with devices I6 and l1. The driver stage, however, includes two devices 85 arranged also in balanced relation as distinguished from the single driver tube utilized in Fig. 1.

For greater power output, this arrangement in the driver stage h-as the advantage that the generated voltage from plate to plate in the output circuit thereof is doubled for a given input voltage between grid and cathode on the output stage, and the step-down ratio of the input coupling device 80 may then be increased by two, or an impedance ratio of four. At the same time the plate resistance of the driver stage in series with the primary or coupling device 80 is only increased by two. Therefore, the net gain may be one half of the eiective resistance in series with the grid, such for example as resistance Rs of Fig. 4.

In a modulator and a class C output ampliiier embodying the circuit shown in Fig. 5, tubes known as radiotrons type 'UV-211 have successfully been employed in the driver stage at 85 with tubes of the type UV-851 used at 84 in the class B modulator stage 14, while in the RF power amplifier 64, tubes of the UV-204-A type have been used at 65. These tubes are designated only by way of example, since their characteristics are well known and thereby the values employed in the design of the modulator may more readily be appreciated and understood.

With electric discharge devices of the types named, the coupling device 80 is preferably a stepdown transformer having a turn ratio of substantially 5 to 1, or 10 to l, to each side of the secondary. This provides an impedance ratio of substantially' 100 to l and with an internal plate impedance Rp in the driver stage of substantially '7000 ohms, thus providing a reflected impedance over through the coupling device 80, in series with the grid circuit 19, of substantially 70 ohms. As compared with the internal impedance Rg of the d-evices 84 in the output stage, of 800 to 1200 ohms, this impedance is relatively low, as will be seen. The impedance of 800 to 1200 ohms, of course, is measured at a maximum positive potential on the grid which in the present example was substantially 80 volts.`

Since the driver stag-e is a class A amplifier, and anode current is owing at all times in both halves of the coupling device 80, in calculating the reflected impedance over, the entire primary is considered in connection with one half of the secondary of the device 80. Thus, in the circuit of Fig. 4 applied to the circuit of Fig. 5, Rs or 58 is equal to substantially '70 ohms, while Rg is equal to 800 to 1200 ohms.

In the output circuit 88 of the class B stage 14, the calculated load over from'the load circuit 68 is provided by the power amplifier 84 and is sub'- stantially 2200 ohms. With an internal impedance of each of devices 84 or rp of substantially 1400 ohms the coupling or output device 8| is so designed that a turn ratio of 'one to 1.4 from each side of primary to secondary is provided, and an impedance ratio of one to two. The reilected impedance, Rp, of the load, over into each half of the output circuit 80 is then approximately 1100 ohms, which, as will be seen, is lower than the internal impedance of the output tube associated therewith.

With a modulator of the character described, connected as shown, an output of approximately 1500 watts is obtainable from the two tubes, which is more than sufficient to modulate to percent the 1000 watt station, provided by the remainder of the equipment including the RF amplier indicated at 64. It may be noted that in the modulator shown, the grids of the devices 84 have in practice been drivenl into the positive range to such an extent that relatively high values of grid current'were obtained. For the tubes indicated, this grid current at substantially 80 volts positive, on a positive signal swing reaches as high a value as milliamperes. As this current has to be supplied without an appreciable drop in the signal voltage delivered by the driver stage, it will be appreciated that the direct current resistance of the grid or input circuit 19 must be maintained at a relatively low value, otherwise serious distortion may take place. Likewise, the directA current resistance of the bias potential source must be kept low. It may further be interesting to note that the corresponding power output of the same tubes 84 in a class A type of amplifier and at a maximum output rating, is approximately 200 watts. Therefore, the greater power output from the modulator above described is more readily apparent.

As a practical consideration in the design of the modulator shown and described, the cost of tube equipment was found to be prohibitive when utilizing the usual modulator or class A amplifier. For example, to provide the same modulator power output as the two U17- 851 tubes in the amplier described, it was found that between 8 and 10 one K. W. tubes would be required in a usual class A amplifier, each tube being valued at several hundred dollars. It will, therefore, be appreciated that in theuse of an improved class B amplifier in a modulator such as that shown and described, a material saving in cost may be effected.

In order to simplify the diagram, the iilament or cathode supply circuits have been omitted. The source of cathode current supply, however, is indicated at 86 and is a generator driven by a suitable motor 81. The positive terminal of the generator 86 should be understood as being connected to all of the cathodes, although as above mentioned, the connection lead is omitted. The opposite or negative terminal of the generator is connected thru a common grounded supply lead 90 to all of the cathodes as indicated, and serves as a grounded cathode return lead for all circuits. In practice this connection is provided by making all connections to ground.

Anod or plate potentials for the amplifier tubes 6 and the class B modulator tubes 64 are supplied by a suitable high voltage direct current generator 9| thru a positive supply lead 92 having branches 93 and 94, in each of which branches is located a suitable ammeter 96 and 96 respectively. It will be seen from the diagram that the branch 94 supplies the anode circuit 69 in series with the output device 6|, while the branch circuit 93 supplies the class B modulator tubes 84 thru the usual push-pull circuit arrangement.

A generator 91 in series with generator 9| serves to raise the voltage supplied by the generator 9| to an initial value from which the required potential for operating the above named modulator and amplifier is raised by the generator 9|. Generators 9| and 91 are connected in series by a lead 99 and generator 91 is in yturn connected with the common ground lead 90 as indicated at 99, whereby the operating anode potentials are applied between the cathode and anodes of the tubes supplied by said generators.

A direct current generator, or generators, are employed in the system of the present example for thel reason that they provide good. regulation under a varying load condition imposed by' the operation of the class B amplifier 14. An anode supply source of this character also has a large power capacity.

In the supply of anode potentials to amplifiers of the above character, it will be appreciated that rectified alternating current may be em-.

ployed. However, certain precautions must be observed to provide good regulation. An alterhating current supply means of this character will be described hereinafter.

The generators 9| and 91 are also driven by a suitable motor |00, and on the same shaft therewith is indicated a third generator |0| which at its positive terminal is connected with the common grounded cathode return lead 90 and which at its negative terminal is connected thru a circuit lead |02 to branch supply leads |03 and |04 to supply a negative biasing potential to the class B modulator stage 14 and to .theradio frequency amplifier ,64. 'I'he branch lead |04 is suitably by-passed to the ground lead 90 by a by-pass condenser |05, while in the branch lead |03 for the class B modulator, a lter and voltage reducing means is interposed, lcomprising a series connected resistor |06, a lter choke coil |01 and a by-pass capacitor |08.

'I'he arrangement is such that the grid circuit 19 of the class B modulator 14 is isolated from the bias supply |0| by the filter means and the biasingpotential is reduced to a desired value. It will be noted that one side of the filter means is connected with the cathode return lead 90 thru a return lead |09 and that between the leads |03 and |09, adjacent to the filter, there is connected a bias voltage supply battery Il). This last is preferably a storage battery or other suitable means for the purpose o'f providing a stabilizing reservoir of extremely low resistance in the grid circuit return, or in the grid circuit of the class B modulator 14. To exactly balance the. anode currents of tlbes 84, an additional bias adjusting battery or bias means is provided in connection with one tube. In the present example this is a battery 89 in the grid circuit of one tube. n

plate circuit of theclass B As hereinbefore pointed out, it is essential that the grid circuit be of relatively low resistance and this also includes the source of bias potential, whereby variations in the amount of grid current drawn by the tubes 64may have no appreciable eilect upon the appliedsignal potentials to cause distortion. This precaution 4in the other portions of the transmitter is not as essential, hence in the present example a battery reservoir is utilized only in the position shown,

In the driver stage 13 any suitable bias supply means may be employed, and in the present example this may be a simple battery indicated at Likewise it should be noted that the source of bias potentials |0| and ||0 for the class B modulator stage is not limited thereto, but may be provided by any suitable means having a low .resistance relative to the resistance of the grid circuit to which it is connected and which is carrying the current drawn by the grid circuit 19.

It is of interest to note that in the circuit shown, the direction of grid current flow, as will be well understood, is in such a direction that it charges the battery ||0, tending to maintain itin a charged condition.

The anode or plate potential supply for the driver stage 13 is supplied by the generator 91 thru a tap connection lead I2 between it and the generator 9 I. A suitable choke coil in the connection lead ||2 which, together with a |13 is interposedJ by-pass condenser ||4, provides a filter for the y supply of anode potential to the driver stage 13. The anode potential or current supply lead 93 in the modulator stage 14 is also suitably by-passed by a condenser H5. In each case the by-pass connection is to the common grounded cathode supply lead 90.

It will be noted that ammeters 96 and 95'are respectively included in the plate circuit of the class C output amplifier 64 and in the output modulator 12. In the present example, with no signal being transmitted through the modulator, the plate current may be adjusted to substantially 40 milliamperes, while the plate current through meter 96 is sub- 'stantially one ampere, with tubes of the UV-85l type at 84 and tubes of the UV-204-A type at 65. A Variation in the current through the meter 95, in response to signals may be taken approximately as an indication of the modulation being applied to the amplifier 64, since the plate current increases proportionately with an increase in the value of the applied signals.

The plate potential applied to the vclass B" amplifier 14 and to the output amplier 64' through the supply lead 92 is approximately 2000 volts as delivered by the generators 9| and 91 .in series. The potential applied to the plate circuit of the driver stage 13 through the lead l |2 is approximately 1000 volts.

The filament potential supplied by the generator 86 is approximately 15 volts. Ihe biasing potential applied to the class C output amplifier 64 by the generator |0| is approximately 115 volts. The grid bias employed in connection with the class B amplifier stage 14, as provided by the battery I0 and the generator |0| vis substantially 80 volts negative, the voltage being reduced from that provided by the generator by the series resistor |06. The. biasing potential supplied by the source is that required to operate the devices 85 as normal class A ampliers.

The class C amplifier 64 is provided with the usual oscillator circuit in connection with the` tuned circuit 66, having a grid leak and condenser combination, I|6, in the grid lead and a suitable neutralizing condenser, ||1, connected with the plate circuit 69. It will be noted that in each of the individual plate leads there is connected a resistor I I 8. These are for the purpose of damping out parasitic oscillations in connection with the multiple connected tubes.

The operation is as follows: With the electric discharge devices energized by operation of 'the supply generators, radio frequency signals from a suitable source are supplied to the terminal 68 of the class C radio frequency amplifier 64 and are transmitted through the radiating system 'l0 as a carrier wave. The value of the signal strength may be measured by a suitable meter I9 in the radiating system.

Audio frequency signals for modulation are applied to the terminals 'l5 of the modulator 12 and are amplified in the successive stages 'i3 and 14. The amplified output is supplied to modulate the plate circuit 69 of the amplifier 64 through the coupling means provided by the output device 8|. Because of the operation of the devices 84 in the manner hereinbefore described, and because of the result of such operation as described in connection with Fig. 1, a large audio frequency power output is obtained.

While the anode current taken by the devices 84 is maintained at relatively low normal and` average values and while the applied anode potentials are within the normal rating of the devices 84, the power output is several times that obtainable from the same devices when used in an ordinary class A amplifier. This is for the reason that the impedance of the input circuit in circuit 19 is relatively loW, whereby the devices 84 may be driven far into their positive grid bias range without distorting the input wave. At the same time, in the output circuit a load impedance reected into the circuit 80 is such that the output of the devices 84 is limited only by the space charge or the emission. At the same time the impedance of the load is such that the emission limits of the devices 84 are not exceeded.

A transmitter of this character has the advantage that the radio frequency amplifier and the modulator may be substantially separate units as shown, and may be coupled for plate circuit modulation by an isolating coupling device such as a transformer. Furthermore, by the proper design of the input and output coupling devices of the output stage of the modulator, the amplifier devices employed therein may be driven far beyond their normal useful range of operation to produce a power output many times in excess of that ordinarily provided, thereby materially reducing the cost of manufacture and operation of a transmitter for a given power output. As class C radio frequency amplifiers are primarily plate circuit modulated and the class B amplifier of the present invention is adapted to operate in connection with various loads by the choice of a suitable impedance matching means, such as a transformer, the class B amplifier of the present invention is particularly well adapted for use in combination with the class C type of amplifier employed in transmitters.

Referring now to Fig. 6, a class B amplifier embodying the invention in its adaptation for battery operation is indicated at |20. This amplier is similar to that shown in Fig. l and includes an input device |2| and an output device |22 between which are connected a pair of balanced or push-pull amplifier devices or tubes |25 similar to devices I6 and of Fig. 1. The input device I2| is connected with a driver stage including an amplifier device |23.

Anode potentials for the two stages are provided by suitable sources indicated by terminals |26 and |21. Terminal |26 is a common cathode return terminal and grid biasing potentials are supplied between it and a third terminal |28. The grid biasing potential for the output stage |20 is obtained from a tap |29 on a series of resistors |30 connected between terminals |26 and |28.

Likewise, a biasing potential is applied to the driver stage or to the tube |23 from a tap |3| between series connected resistors 32 between the terminals |26 and |28. In this case the resistors |32 are of such values that they serve to operate as coupling resistors in connection with the device l23. The biasing potential applied to either of the stages connected with the source |26|28 are dependent upon the values of the resistors |30 and |32 and the position of the tap points |29 and |3|.

Audio frequency signals are applied to this amplifier thru the terminal |26 and a suitable grid input terminal |33 and a suitable coupling device such as a condenser |34. In actual practice, in a battery operated amplifier the terminal |28 is lthe B minus terminal, terminal |26 is the negative A and the positive 221/2 volt terminal of the B battery, and terminal |21 is the positive 180 volt terminal of the B battery. It will be seen that this provides a B battery voltage upon the anode circuits of the devices |23 and |25 of substantially 157 volts. With a battery type of tube or electric discharge device of the Radiotron UX 230 type, substantially 15 volts negative bias is provided at tap point |29 and with a normal plate current to each of the devices 125 of approximately .'7 milliamperes.

The conservation of power in a battery receiving set has always been sought for a successful and practical receiver. At the same time sufcient power output for a reasonable volume of sound has also been a desired feature. This is particularly true at present because of the almost universal use of alternating current operated power ampliers in even the most modest receivers. Heretofore. both features have not been capable of combination in a single receiver.

However, with ordinary tubes of the so-called battery type rated at very low output in an amplifier embodying the invention and connected substantially as shown in Fig. 6, a relatively high audio frequency power output may be obtained with relatively low battery drain. The characteristics of such an amplifier are `substantially as follows: 'Ihe amplifier employs tubes of the Radiotron UX 230 type, having a filament consumption of 120 milliamperes as against 260 milliamperes for a standard battery operated amplifier. The maximum power output is approximately 1.2 watts with approximately 7 milliamperes average fB battery drain with a signal being transmitted. The above standard battery amplifier has a normal output of about .35 watts and a B battery drain of approximately 16 milliamperes.

In response to an applied signal, the plate current of each tube increases as the signal potential increased in a positive direction upon its grid. Grid current begins to flow at about one volt positive and increases to approximately 1.5 milliamperes at 20 volts positive. The maximum plate current for this -grid voltage is approximately 25 milliamperes. Each tube alternately functions to take the load as its grid becomes positive, the operation in this respect being the of the driver stage reflected over in series therey mately milliamperes.

same as that described in connection with Fig. 1. The anode impedance rp of the devices |23 and |25 of the type above mentioned, is approximately 9000 to 10,000 ohms, while the input or grid impedance is approximately 12,000 to 14,000 ohms fora maximum positive grid swing. With these impedance values, the turn ratio of the coupling device |2| is one to .75 for each side of the secondary, or an impedance ratio to each grid of one to .45 step-down. This provides an input or grid circuit for the stage |20 which may be diagrammatically indicated as shown in Fig. 7 to which, along with Fig. 6, attention is now directed.

The input or grid circuit for the amplifier |20 is indicated at and includes the impedance with by the coupling device |2| and represented at |36, having in the present example a value of substantially 5,000 ohms. Across the circuit is the grid impedance Rg of each one of the devices |25 as indicated at |3'| and has a value of substantially 12,000 to 14,000 ohms. The signal potentials delivered by the driver stage to its outputvcircuit for transfer to the output stage may be indicated by potentials which may exist between input terminals |38 for the circuit |35.

The load impedance necessary for connection with the output circuit of the amplifier |20 for maximum power output is substantially 3750 ohms on each side, although it may be lowered in the present example to 2500 ohms for a maximum power output. However, substantially 4000 ohms, as shown, is believed to be more generally satisfactory and safer from an operating standpoint, as approximately this value insures against excessive plate current on peak signals. In other words, the load impedance becomes a limiting factor in the control of the plate current.

The ratio of the coupling device |22 is such that a 2.7 ohm output device may be employed at |24, and for this purpose a step-down ratio of 70.8 to 1. is provided. A loading device, including a. winding and a condenser in series therewith, as indicated, respectively, at' |39 and |40 is provided in the coupling device |22 -to lower the impedance in the output circuit at high frequencies because the output device |24 produces a high impedance at high frequencies.

The values of the load and plate impedance in the output circuit are indicated at |4| and |42, respectively, in a circuit |43, Fig. 7, representing one half of the output circuit of Fig. 6. In the present example, the value of the plate impedance |4| is substantially 9000 ohms, while that of the load indicated at |42 is approximately 3750 ohms as above mentioned. The output potentials provided by the output stage |20 may be considered as being applied at the terminals |44 and are dependent upon the mu of the tubes in 'the output stage. Hence, it will be seen that it is desirable to employ higher mu tubes in the power amplifier. l

It has been found that with devices of the character described in the present example, a peak swing of 20 volts positive in the grid of one of the devices |25, the peak plate current is approxi- This current drawn thru a load of 3750 ohms reduces the plate voltage to about 63 volts. A lower plate resistance serves to raise the minimum plate voltage and permits a greater plate current. It will be seen that, as hereinbefore pointed out in connection with the amplifier of Fig. 1, the relatively low load impedance above indicated is desirable in stant of about 35,000 ohms.

order that the power output may not be reduced by a limitation of the plate current by the space charge.

Themaximum gridcurrent depends upon the power available to drive the grids positive. The grid current forfa 2500 ohm load at a positive bias potential of 20 volts is approximately one milliampere, which represents a resistance for the in- Therefore, the voltage. from the driver stage must not drop appreciably when such load resistances are encountered. The minimum resistance as calculated from the slope of the current grid voltage curve is about 14,000 ohms as indicated above.

Since the output tubes are biased to nearly plate current cut oil', the plate current must increase With power output. 'I'his requires that the plate current supply source be adapted to provide a constant voltage, with good regulation. The power requirements for a battery operated amplifier of this character may best be understood from the fact that with an output of 1.2 watts the input power to the plate circuit is approximately 18.7 milliamperes at 158 volts or 2.96 watt for continuous sine wave output.` The plate efficiency for the average load atfull output is about for sine wave inputs.

It is possible and practical toA use tubes as class B output tubes in a battery receiver or amplier,l because the plate andl grid voltages are constant and regulation is a factor that need not be considered. The sensitivity of a receiver using two class B operative output tubesis equal to that'of the sensitivity of a receiver using .a normal battery operated amplifier. y'I'he power' capabilities of the class B amplifier using relatively small amplifier tubes is about three times that available from the normal larger and ordinarily much more powerful tubes of the Radiotron UX`231 type, for example. Furthermore, the average "B battery drain is approximately 12 to 15 milliamperes for a receiver using the class B amplifier described, as compared with a drain of from 25 to 30 milliamperes for the usual battery operated receiver. In each case, the "B" battery load is about 100% greater. 'I'he filament current drain is also substantially less than that-oi.' standard amplifier tubes employed for battery operation.

As a general comparison of the capabilities of f' a class B battery operated amplifier, it has been found that power output of tubes of the UX-230 type, connected in accordance with the invention as class B amplifiers, could be equalled only by resorting to the use of two tubes of a character such as the Radiotron UX-171- A type, connected in push pull as class A amplifiers. Such tubes, of course, would have excessive and prohibitive B battery drain and would require a storage type of battery for niament excitation.

Referring now to Fig. 8, a transmitter having a modulation system provided by a class B modulator embodying the invention, is shown in connection with an alternating current source of operating potentials.

In the transmitter shown in Fig. 8, the radio frequency system includes a crystal oscillator |45 followed by two cascade connected radio frequency amplifiers |46 and |41 which drive a class .C radio frequency output amplifier |48. The class C amplifier, like that shown in Fig. 5, is plate circuit modulated and for this purpose is provided with a plate current connection |49 in which is located an indicating ammeter and a radio frequency filter means including a choke coil |5| and a suitable by-pass condenser |52 adjacent to the amplifier.

Audio frequency modulation is applied to the circuit |49 from a class B amplifier |53 constructed after the manner of that shown in Fig. 1 and connected to the circuit |49 after the manner of that shown in Fig. 5. For this purpose, the amplifier is provided with an input coupling device |54 providing a low impedance input circuit and an output coupling device |55 providing a low impedance output circuit, between which are connected in balanced relation, the class B amplifier devices |56.

In common with lead |49 the anode circuit of devices |56 receives its operating current or potential from a supply lead |51, and biasing potentials are supplied by a suitable source |58. Audio frequency signals are supplied to the input coupling device |54 from a suitable source indicated at |59. This may be a speech amplifier as indicated. Direct current anode potentials are delivered to supply lead |51 from an alternating current source indicated by terminals |60, thru a suitable full wave rectifier 6| which includes a pair of cathode rectifier devices |62.

The cathodes of the device |56 and the devices |62 are heated from the source |60 thru a separate transformer |63 which permits the cathodes to be separately heated before the anode potential supply is excited. For this purpose, a separate switch |64 is aprovided for the cathode lheating transformer |63 and another switch |65 is provided for energizing the rectiiier |6|.

The positive and negative supply leads |66 and |61, respectively, from the rectifier are connected with the cathodes of devices |56 and with the positive supply lead |51. In one of the leads, preferably in the positive supply lead |66, is connected a choke coil |66 in association with a suitable by-pass condenser |69, whereby the desired regulation is obtained under load conditions imposed by the devices |56. The operation of the filter means provided by the choke coil |68 and by-pass condenser |69 will hereafter be considered.

The coupling device |55 should preferably be designed to couple the devices |56, which may be of the Radiotron UV--203-A type, as class B audio amplifiers for a maximum anode current of approximately 400 milliamperes for the type of tubes above mentioned, and operated at approximately 1000 volts. Such a loading permits approximately maximum output of the class B amplifier. Higher currents to the class C amplifier will result in an overloading of the class B tubes |56. It should be noted that during a peak modulation of 100% the input power to the plates of the class C amplifier |48 increases 50% so that the plate dissipation increases by the same percentage. However, the plate current to the class C tubes as indicated by the meter |50, will remain steady so that care must be taken not to exceed the tube rating.

If a maximum output is desired from the modulator |53, medium power tubes should be used in the output stage of the speech amplifier |59 to excite the class B tubes |56 thru the coupling device 554. l

It will be noted that a resistor |10 is employed in the anode circuit of each of the devices |56. These may be of a value of approximately 40 ohms, for example, and serve to prevent any high voltages because of high oscillations, or

surges in the plate circuits of the class B tubes. The radio frequency choke coil |5| and the condenser |52 prevent radio frequency currents from entering the transformer |55. The C battery |58 for the class B tubes, is adjusted to such a value that each tube draws a normal plate current of approximately 20 to 30 milliamperes for the type of tubes above mentioned, this being substantiallyv plate current cut off for such tubes. The battery |58 represents any suitable source of C bias having a low resistance as described in connection with the preceding embodiment of the invention, and is preferably a heavy duty type of B battery.

A meter |1| is placed in circuit with the anodes of the devices |56. As the maximum plate current to a class B audio frequency modulator or amplifier depends upon the power output, the variations of the meter |1| may be taken as a fair indication of the percentage modulation. It will lloe noted that the secondary of the output coupling device |5| is arranged in two sections |12 and |13,- whereby the sections may be connected in series, or in parallel as shown. The type of connection depends upon the type of tubes used in the class C radio frequency amplifier |48. For lower plate currents at higher voltages the series connection may be used, while for higher plate currents and lower voltages the parallel connection may be used.

Any change in plate voltage and current to the amplifier |48, or change in plate voltage on the class B amplifier |53, should be carefully studied to make sure that the modulator and class C amplier tubes are not being overloaded.

The above described system of modulation in connection with a class C" amplifier is of relatively low cost and of simple construction and has a relatively high radio frequency power output with 100% modulation. Furthermore, the use of the meters in circuit, as indicated, particularly the meter |1 I, provides a visible indication ofthe operation of the modulator and an indication of the percentage modulation.

It will be noted that the rectifier |6| or anode supply circuit of which it forms a part, is the usual type of full Wave rectifier. However, the filter reactor |68 is a special design to improve regulation, and the filament transformer |63 is separate from that of the rectifier |6| so that hot cathode mercury vapor tubes may be employed at |62 and heated before the plate supply switch |65 is closed. During silent periods of operation of the transmitter, the filaments of all of the tubes may be left on while the plate supply is turned off, thereby requiring a minimum time to start when placed again in operation.

With reference to the filter |68 and |69, the inductance |68 is preferably of the iron core type in series, as shown, with the rectifier source, and is tuned as by the condenser |69 to a frequency such as the second harmonic, being 120 cycles for a cycle supply circuit. As indicated in the drawings, there should be no filtering between the rectifier and the resonant filtering element because it is arranged to operate upon an appreciable alternating current component. Any addi-l it near toits saturating point, since a change of reactance for regulation purposes must be proportional to an increase in the load current. The tuning'condenser |69 may be omitted if the reactor is so designed that its impedance decreases vrapidly with an increase in direct current.

In operation, the regulation is aided somewhat by providing for a normal load current, or from 10 to 20% of the output current derived from the plate supply source. With this normal load current, the filter is tuned until the inductance oiiers a maximum impedance at the chosen frequency which reduces the output voltage to a minimum.

The choke coil is also'designed so that it tends to saturate as the load current is increased so that it tends to introduce a lower reactance in the circuit for greater load currents, while at the same time the change in reactancev detunes the circuit, which also tends to sharply reduce the impedance of the circuit yto maintain the tube voltage more nearly constant regardless of the tube current 'within the power range of the plate supply means. The condenser |14 functions as a normal filter condenser and should be relatively large to effectively by-pass the A. C. components -to currents to the class B modulator.

While the invention has been illustrated vand described for purposes of convenience in its present preferred application to audio frequency amplifiers generally, to battery operated amplifiers, and for the modulation of high power transmitters, it is obvious that it is not limited thereto, but

may be applied toany electrical apparatus requiring large quantities of fluctuating electric power,

.without involving costly and complicated equipment including a large number of amplifier devices and associated potential supply apparatus.

I claim as my invention:

1. An audio frequency amplier including, in combination, a pair of electric discharge devices each having an anode, a' cathode, and a, control grid, said devices being connected in balanced relation to each other and being connected to be biased to substantially anode current cut off, an input circuit of low alternating current impedance with respect to the grid impedance of each of said devices when drawing a maximum grid current, and an output circuit of low alternating current impedance with respect to the anode impedance of eachof said devices.

"2. An audio frequency amplifier including in combination, a pair of balanced electric discharge devices, an input circuit therefor and an output circuit therefor, said input circuit having a lower alternating current impedance in series with the input electrodes of said devices than the lowest input operating impedance of said devices effected by high signal amplitudes causing current flow to said electrodes, and said output circuit having a lower series alternating current impedance than the output impedance of said devices, by an amount sufcient to permit said devices to be driven in response to signals to the limit of their emission or space charge.

3. An audio frequency amplifier system including in combination, an input circuit therefor and an output circuit therefor, and means arranged for connecting between said circuits a pair of electric discharge devices in -balanced relation as an amplifier biased to substantially anode current vcut off, said means providing in said input circuit stantially to zero, an input coupling device in the input circuit adapted when connected with a driving source of signal potentials to provide an alternating current impedance in series with each half of the input circuit of a valuel substantially lower than, and having a direct current resistance of a value one-tenth of the value or less of the internal impedance of each of said electric discharge devices when drawing a maximum grid current, and an output coupling device connected with said output circuit having a coupling ratio such that when connected with a load source the series alternating current impedance provided thereby in each half of the output anode circuit may be substantially less than the internal or anode impedance of the device connected therewith.

5. In an audio frequency amplifier, means for connecting a pair of electric discharge devices in balanced relation and for biasing the same to essentially space current cut off, said means providing series input and output alternating current impedance in connection with the input and output electrodes of said devices of values lower than the lowest internal operating input -and output impedances respectively of said devices, whereby signal potentials may be applied to each of said devices, having maximum positive values and the anode current of each of said devices is limited only by the space charge or by the emission characteristics of said devices.

6. An audio frequency amplifier including, in combination, a pair of electric discharge devices each having an anode, a cathode, and a control grid, said devices being connected in balanced relation to each other and provided with means for biasing the same to'substantially anode current cut off, an input circuit of low alternating current impedance with respect to the grid impedance of each of said devices when drawing maximum' grid current, and an output circuit of low alternating current impedance with respect to the anode impedance of each of said devices,

said input circuit further being of relatively low direct current resistance, said output circuit alternating current impedance being of such low value, that the output anode current of each of said devices is limited by the space charge or emission characteristics thereof, and said anode circuit including a source of anode potental supply having a predetermined relatively high regulation.

7. An electric discharge amplifier having a balanced grid circuit and a balanced anode circuit, means for biasing said amplier substantially to anode current cut off, an input coupling means connected with said grid circuit providing aos-1,180

an alternating current impedance in series therewith of a value lower than the internal input impedance of said amplifier when drawing maximum grid current, and an output coupling device for an alternating current load circuit adapted to provide a load impedance, lower than the output impedance of output anode, whereby the output anode current of the amplifier is limited by the space charge or by the emission of the electric discharge devices therein and whereby a grid potential variation is permitted to a value whereat the anode current becomes so limited.

8. In an audio frequency amplier, a balanced electric discharge amplier device, an input transformer and an output transformer pertaining thereto, providing input and output circuit' alternating current impedance values therefor substantially lower than operating values of the internal input and output operating impedances of said device when drawing grid current.

9. An audio frequency ampliiier including in combination, a step-down input transformer and a step-down output transformer adapted to be connected with a pair of electric discharge devices of the vacuum tube amplier type in balanced relation thereto, the first named transformer providing a lower series input circuit alternating current impedance than the lowest input operating impedance of either of said devices, and the second named transformer providing a lower series output alternating current impedance than the output impedance of either of said devices.

10. In an audio frequency amplier, a balanced input and a balanced output transformer ar- .ranged to be connected with a pair of electric discharge amplier devices having input and output electrodes, each half of the secondary of the balanced input transformer having an operating alternating current impedance lower than the lowest alternating current impedance of said amplifier devices to be connected therewith between their respective input electrodes when drawing current in response to signals of high amplitude, and the balanced output transformer having a lower alternating current impedance in each half of its primary than the internal impedance of the amplifier device to be connected'therewith between its output electrodes and lower than is permissible for class A operation thereof.

11. An audio frequency amplifier including, in combination, a pair of electric discharge devices each having an anode, a cathode, and a control grid, means providing a balanced signal input circuit connected between said control grids and the cathodes of said devices, the alternating current impedance of said input circuit being substantially less than the input impedance of said devices when the grids are driven into the cxtreme positive range of grid potentials by the peak of the signal wave to be amplified, a balanced output circuit connected between the anodes and the cathodes of said devices, a loadg flected load impedance having a value lower than the output impedance of said devices, when' said grids are driven into said extreme positive range of grid potentials.

12. An audio frequency amplifier including, in combination, a pair of electric discharge devices each having an anode, a cathode. and a control grid, said devices being connected in balanced relation to each other, an input circuit for said devices of low alternating current impedance with respect to the grid impedance of each of said devices when the grids thereof are made positive with respect to the cathodes by the magnitude of the applied alternating current energy, a balanced output circuit connected between the anodes and the cathodes of said devices, a loud speaker, and coupling means linking said output circuit and said loud speaker for determining the alternating current load impedance of the loud speaker reflected over into the output circuit, said reflected load impedance having a value lower than the output impedance of said devices, when said grids are made positive by the magnitude of the applied alternating current energy.

13. A balanced electric discharge ampliiier system of the type in which the signal grids are biased to a point such that signal energy causes the said grids tooperate in a positive region that causes grid current to flow, having'an input circuit and an output circuit characterized by the fact that both the input and the output circuits have low alternating current impedance and direct current resistance of such values that the potential drop through said circuits resulting from iiow of signal current is substantially negligible in its effect upon the signal wave characteristics.

14. In an electron tube circuit, a pair of power output electron tubes, an electron tube driver for said power output tubes, an impedance adjusting transformer connected to couple said driver tube to said output tubes, said transformer having an impedance ratio whereby the driver tube output impedance reflected into the input circuit of said output tubes is substantially lower than the grid impedance of the output` lower than the anode impedance ofsaid power J output tubes when said grids are driven into said positive bias range by a strong signal.

LOY E. BARTON.

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