US1893302A - Method of and means for controlling electric wave amplifiers - Google Patents

Description

Jan. 3, 1933. PETERSON 1,893,302

METHOD .OF AND MEANS FOR CONTROLLING ELECTRIC WAVE AMPLIFIERS Filed June 20, 1928 2 Sheets-Sheet 1 Fla. I

l/v I/EN 70/? EUGENE PETERSON Jan. 3,

E. PETERSON METHOD OF AND MEANS FOR CONTROLLING ELECTRIC WAVE AMPLIFIERS Filed June 20, 1928 2 Sheets-Sheet 2 Fla. .5

lNVE/V 70/? fl/GENE Pz'rmsaiv ATTORNEY Patented Jan. 3, 1933 UNITED: STATES EUGENE PETERSON, OF NEW YORK, 11131.,

PATENT .PDF'FI'CE 1 Assrdnon T BELL TELEPHONE'LABORAP TORIES, INCORPORATED, OF YORK, N. 1., A CORPORATION OF N YORK METHOD or AND MEANS For. oonrnotmnenrnormc WAVE AMPLIFIERS Application filed June 20,

This invention relates to a method of and means for reducing modulation efiects or distortion in amplifiers and controlling the gain thereof, particularlyin space discharge amplifying tubes connected in push-pull relaion.

Annplifiers employingsp ace discharge tubes are commonly operated with a C battery or other source of constant negative grid bias.

It has also been proposed to employ, in connection with space discharge amplifier tubes operating singly or in parallel, a grid condenser-and resistance combination for accumulating and maintaining a charge derived from the grid current, thereby providing a grid'biasing potential without the use of a C battery and proportional at every instant to the envelope of the wave to be amplified.

It has been proposed, further, to employ a grid condenser and resistance combination to, supplement a C battery, thereby providing added negative gridbias, when momentarily high potentials are impressed upon the 2 amplifier, to prevent distortion of the wave.

When a grid condenser and resistance combination is employed in conjunction with tubes operated singly or'in parallel, the amplifier may be operated without material 9 distortion provided the amplitude of the input wave does not exceed a more or less definite value. When, however, the amplitude of the input wave is increased beyond a certain value, the amount of distortion progressively increases. The same result has been observed in an amplifier including pushpull connected space discharge tubes having their grids biased with respect to. their cathodes by the application of a constant 9 negative voltage.

When, however, a grid condenser and resistance combination isemployed in conjunction with amplifying tubes connected in push-pull relation, the useful range of input values is materially extended. That is, the range of amplitude of the input wave which the amplifier can satisfactorily amplify without increasing distortion, is materially widened.

An object of the present invention isto de- 1928. Serial Nmesa'fla crease wave distortion due to changes in the input imp'edance'of a space discharge amplifying system whenv grid current fiows in its external input circuit.v

Another object is :to causc,in asubstantially distortionless amplification system, a decreasing gain when the value of the input increases beyond a predetermined value.

' Manytransmission systems in common use are limited by economic, engineering or other practical considerations to a restricted range of amplitude'values for the wave appliedto the input electrodes ofthe amplifier. The

feature of the invention whereby the gain of the amplifier decreases with the amplitude of the applied wave is particularly useful for compressing waves, having. amplitudes varying over a wide range of values, into a restricted range without material distortion of the wave form. 1

The invention may be more fully understood from the following detailed description when read in conjunction with the drawings, wherein V Fig. 1 represents anembodiment ofthe invention, 7

Figs. 2 and 3 show typical operating characteristics of amplifying systems heretofore proposed, and

' Fig. 4 shows corresponding characteristics of" an amplifier embodying the present invention. i p

In Fig. 1, space discharge devices 1 and 2 are shown connected in push-pull relation between an'input transformer 3 and an output transformer 4. An impedance network, consisting of a condenser 5 and a resistance 6 in parallel, s inserted 1n the common input of which, by virtue of sufiicient amplitude of the whole, is effective to produce grid current flow in either or both of devices 1 and 2 and hence through the external input circuit,

will, by causing a charge to be accumulated upon the plates of condenser 5, increasethe negative grid bias of both devices and thus tend to decrease the grid current. The grid current automatically adjusts itself until it is. exactly sufiicient to'counterbalance the leakage of the charge from condenser '5 through resistance 6 and thus the circuit maintains a constant grid bias as long as the amplitude of the applied wave remains unchanged.

Any change in the amplitude of the applied Wave will beimmediately followed by a variation of the grid bias until, at a slightly different value of grid current, a new condition of equilibrium is, reached or until grid current ceases to flow.

The fractional value of the portion of the applied wave effective to produce grid current depends inversely upon the ratio between the resistance of element 6 and the internal grid-to-cathode resistance of the space discharge-device. In a given case, by increasing the value of resistance 6 the portion of the wave effective on the grid can be made as small as may be desired, within practical limits. In this way the maximumvalue of grid current produced by applied Waves of the largest amplitude for which the amplifier is intended may be made negligibly small.

WVhen the amplifier is operated in the above described manner, the resulting fluctuations in the internal grid-to-cathode resistance of the tube may be maintained relatively small. Consequently, the wave distortion, or grid current modulation, introduced by the gridto-cathode resistance variations is minimized.

While the equilibrium value of the grid current in any specific case is determined primarily. by the relative values of resistance 6 and the; internal grid-to-cathode resistance of the device, the time required for the system to change from one condition of equilibrium to another is dependent upon the time constant, or product of' resistance by capacit for the impedance network comprising condenser 5 and resistance 6.

Fluctuations in the amplitude of the applied waves which take place over periods thatare long, in comparison with the time constant, will be fully responded to by the network and a complete and substantially instantaneous adjustment of grid bias will occur. Faster fluctuations are compensated larly,

less quickly and completely, or, if too fast, they are not compensated at all.

For the amplification of modulated carrier waves, a network having a time constant of a few hundredths of a second has been used with good results. i l

In any given case a suitable value for the time constant of the network 5, 6 may readily be found either by computation or experiment.

If desired, any one of a'p'lurality of suit able impedance networks may be used to couple input transformer 3 to devices 1 and 2. Asidefrom e'liiciently transmitting the applied wave to the input terminals of the dis-- charge devices, the'essential requirements for an alternative network'are that it provide a conductive path and additional electrostat c capacity between the terminals of the disa charge devices, unless the internal capacity is found to be already of a suitable value before insertion of the network. The values of leakage resistance and time constant for an alternative network are to be determined in accordance with t-he rules hereinbefore stated for network 5, 6.

v The process hereinbefore described, whereby the grid bias is adjusted in. response to changes in the amplitude of the applied wave When-the amplitude exceeds a certain moderate value, results in a change in the operating characteristic curve of the combined discharge devices which is beneficial to this operation as an amplifier. The nature of the change in the operating characteristic is shown graphically by comparing Figs. 2 and 3 with Fig. 4. i

In Figs. 2 to l, the operating characteristic curves 11 to 15 and 20 are plotted with output currents as the ordinates and applied voltages as the'abscissae. Curves A, B and C are applied sinusoidal waves plotted with values of time as the ordinates and applied voltages as the abscissee. The applied waves repre sented by A, B and C are of increasing amplitude in the order named.

The amplitude of wave A is such as to traverse the segment 00a of any characteristic 13 to which it is applied in'Figs. 2 to 4.- Simiwave B traverses segment 6-?) and wave C traverses segment cc. 'Ihe amplitudes of waves A, B and C in the respective figures are equal. i

Fig. 2 shows typical operating characteristics for a push-pull amplifier of the type heretofore proposed and now well known in the art, such, for example, as might be represented by the amplifier of Fig. 1, omitting network '56. 7

' Curves 11 and 12 are the characteristics of the-individual devices comprising the pushpull amplifier. Curve 13 is the combined characteristic obtained by algebraic addition of the corresponding ordinates of curves 11 and 12.

Segment 0-0; of curve 13 is substantially rectilinear. Therefore, as is well known, the operation of the device over the segment will result in amplification of the applied wave substantially without distortion.

Segments 6-?) and cc depart progressively from rectilinearity by including the curved portions shown at either side of the straight part of curve 13. Consequently the waves B and C will be amplified wit-h increasing amounts of distortion.

F ig. 3 shows typical operating characteristics for an amplifier comprising two discharge devices similar to those shown in Fig. l but connected in parallel instead of in pushpull relation and providedwith a network such as network 56 for supplying additional negative grid bases when waves of large amplitude are applied. Curvell is identical with'the similarly numbered curve in Fig. 2 and, as before, is the characteristic ofan individual discharge device.

- In curve 20, which represents the characteristic of two such devices in parallel, each ordinate is twice as large as the corresponding ordinate of curve 11. As before, the segment of the characteristic over which each applied wave operates is designated by small letters indicating the terminal points of the segment. WVith this circuit arranges ment there is a certain amount of distortion .when wave A is applied and theamount of the distortion progressively increases with increase of the amplitude of the applied wave.

Fig. 4: illustrates the manner in which the device of the invention permits the amplification of waves of increased amplitude without material distortion due to curvature of the characteristic.

On account of the additional grid bias provided by the operating characteristic of the network 56 the individual characteristics 11 and 12 are displaced horizontallyby an amount proportional to the increase in the amplitude of the applied wave and'are thus combined in a manner which depends upon the amplitude of the applied wave.

Characteristics 13, 14 and 15, which are efi'ective for applied waves A, B and C respectively, differ from each other in slope but are nevertheless all substantially rectilinear. Waves A, B and C are therefore amplified substantially wvithout distortion. Due to the difierence in the slope of the characteristics, the applied waves A, B and C, are all reproduced as output waves which are substantially equal in amplitude. In other words, in spite of the difi'erence in the amplitudes of the impressed waves A, B and C, the amplitudes of the waves repeated by the device are limited to a substantially fixed value. This is due to the fact that within a range of amplitudes sulficiently large to produce grid current, the gain of the amplifier is caused to vary inversely with the amplitude of the applied wave. While this is true also of the amplifier operating in accordance with Fig. 3, the operation of the amplitude limitingfunction in that case is accompanied by noted above.

A comparison of Figs. 2, 3 and A shows that the invention effects a material extension of the range of applied waves which can be amplified without distortion.

hat is claimed is: v x I 1. An amplifier of alternating current comprising a plurality of space discharge devices connected in push-pull relation, each of said space discharge devices having a cathode, an anode and a control electrode, input'and output circuits for said space discharge devices, means for supplying a substantially constant negative biasing potential to said control electrodes,'said means being of such voltage as to prevent conduction of grid current through said space discharge devices whensaid amplifier is actuated by alternating. current within an extended range, o1 normal operating intensities, and a grid leak and condenser in said input circuit for supplying a second biasing potential to said control electrodes responsive to grid current whereby said amplifier is prevented from causing appreciable amount ofgridcurrent modulation when actuated by alternating current of intensities in a second extended range of intensities beyond said first mentioned range. 2. An amplifier of alternating current comprising a plurality of space discharge devices connected in push-pull relation, each of said space discharge devices having a cathode, an anode and a control electrode, input and output circuits for said space discharge devices, means tor' supplying a substantially constant negative biasing potential to said control electrodes, said means being of such voltage as toprevent conduction of grid current through said space discharge devices when said amplifier is actuated by alternating current within an extended range of normal operating intensities, and a grid leak and condenser in said input circuit for supplying a second biasing potential to said control electrodes responsive to grid current whereby said amplifier is prevented from causing appreciable amount of grid current modulation when actuated by alternating current of intensities in a second eX- tended range of intensities beyond said first mentionedv range, said grid leak and con denser -combined having a value of time con stant which is large compared with the alternation periodof the current amplified.

3. An amplifier of alternating current comprising a plurality of space discharge devices connected in push-pull relation, each of said space discharge devices having a cathode, an

pronounced distortion, as

anode and a control electrode, input and out put circuits for said space discharge devices,

means for supplying a substantially constant negative biasing potential to said control electrodes, said means being of such voltage as to prevent conduction of grid current through said space discharge devices when said amplifier is actuated by alternating current within an extended range of normal operating intensities, and a grid leak and condenser in said-input circuit for supplying a second biasing potential to said control electrodes in response to and in proportion to an increase in the intensity of the alternating current beyond said above mentioned range of intensitieswhereby said amplifier is prevented from causing appreciable amount of grid current modulation when actuated by alternating current of intensities in a second extended range of intensities beyond said first mentioned range. a

4;. An amplifier of alternating current com prising a plurality of space discharge devices connected in push-pull relation, each of said space discharge devices having a cathode, an anode, and a control electrode, input and output circuits for said space discharge devices, means for supplying a substantially constant ne ative biasing potential to said control electrodes, said means being or" such voltage as to prevent conduction of grid current through said space discharge devices when said amplifier is actuated by alternating current within an extended range of normal operating intensities, and a grid leak and condenser in said input circuit for supplying a second biasing potential to said control electrodes responsive to grid current whereby a reduction is effected in the transmission gain of the amplifier when actuated by alternat ing current of intensities in a second extended range of intensities beyond said first mentioned range while the amplifier is prevented from causing appreciable amount of grid current modulation.

5. An amplifier of alternating current comprising a plurality of space discharge devices connected in push-pull relation, each of said space discharge devices having a cathode, an anode and a control electrode, input and output circuits for said space discharge devices, means for supplying a substantially constant negative biasing potential to said control electrodes, said means being of such voltage as to prevent conduction of grid current through said space discharge devices when said amplifier is actuated by alternating current within an extended range of normal operating intensities, and a grid leak and condenser in said input circuit for supplying a second biasing potential to said control electro es responsive to grid current whereby a reduction is effected in the trans mission gain of the amplifier when actuated by alternating current of intensities in a sec- 0nd extended range of intensities beyond said first mentioned range while the amplifier is prevented from causing appreclableamount of grid current modulation, said grid leak and condenser combined having a value of time constant which is; large compared with the alternation period of the current amplified.

6. An amplifier of alternating current comprising a plurality of space discharge devices connected in push-pull relation, each of said space discharge devices having a cathode, an anode and a control electrode, input and output circuits for said space discharge devices, means for supplying a substantially constant negative biasing potential to said control electrodes, said means being of such voltage as to prevent conduction of grid current through said space discharge devices when said amplifier is actuated by alternating current within an extended range of normal operating intensities, and a grid leak and condenser in said input circuit for supplying a second biasing potential to said control electrodes in response to and in proportion to an increase in the intensity of the alternatin currentbeyond said above mentioned range of intensities'whereby a reduction is effected in the transmission gain of the amplifier when actuated by alternating current of intensities in said second extended range of intensities beyond said first mentioned range while the amplifier is prevented from causing appreciable amount of grid current modulation. I

In witness whereof, I hereunto subscribe my name this 16th day of June, 1928.

V EUGENE PETERSON.

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