US2238457A - Amplifier circuit organization with means to compensate nonlinear distortions - Google Patents

Description

April 15, 1941.

W. VON WERTHER ET AL AMPLIFIER CIRCUIT ORGANIZATION WITH MEANS T0 COMFENSATE NONLINEAR DISTORTIONS Filed NOV. 20, 1936 Fig :l

Inpuf Impedance Load lrfapedance Fig .2

COHVCVSIOH Nefwork AAAIAAAA Loaded Cable or Lme 'INVENTORS 1 WALTER V0 WERTMEA SEQ/Mk0 AA LC/fHOFF BY v ATTORNEY Patented Apr. 15, 1941 I I AMPLIFIER .cIItoUIT ORGANIZATION WITH MEANS T0 COMPENSATE NONLINEAR pIs- TORTIONS Walter von Werther, Berlin, and Gerhard Kalckhoif, Schonwalde, near Velten, Germany, assignors to Siemens & Halske, Aktiengesellschaft, Siemensstadt, near Berlin, Germany Application November 20, 1936, Serial No. 111,794 In Germany November 18, 1935 .3 Claims. (01. 179-171) In a multi-stage amplifier thenon-linear distortions caused in the various stages may be come pensated if the grid potentials are impressed upon two tubes of the amplifier with a phase displacement of 1'80 degrees. This phase angle, in capacity-coupled amplifiers, is obtained for certain frequencies inherently in the circuit scheme, while in the case of transformer-coupled amplifiers it may be insured by choosing suitable polarity for the windings.

According to U. S. Patent #2,0 22,49 6, with a view to insuring as complete a compensation as possible of the non-linear distortions, the matching ratio, i. e., the quotient of load resistance and inner resistance of the tube is chosen for the first or input tube lower than for the second tube or power tube. Choosing dimensions as stated is predicated upon this presupposition that the two tubes are so operated and controlled that overloading would occur first in the end or power tube. But this rule is satisfied in nearly all amplifier arrangements,

In the drawing, Figure 1 illustrates in schematic form a circuit diagram utilized in describing certain features of the invention; and, Figure 2 is a detailed showing of an amplifier incorporating the features of the invention.

Now, amplifier arrangements of the kind disclosed in said patent are improved according to this invention by that the input tube V1 has a resistance or resistance combination A as its load which simulates the load resistance B of the power tube V2 in proper phase relation for all frequencies of the transmission band or range (Fig. 1).

The invention starting from and being predicated factor (non-linear distortion) may be caused to disappear it is moreover necessary that the ratio between the values of load resistances at all frequencies have the same optimum value. The size of this ratio is entirely governed by the nature of the tubes that are used, and is a function of the tube characteristics and the relationship of the modulation percentages.

If the attempt to insure phase imitation of the load resistance of the power tube is attended with dificulties, and this may be the case if the load comprises a Pupinized cable, a filter, or some other artificial line or network having a complex impedance characteristic, then, according to another object of this invention, 'a'conversion network is inserted between the output transformer of the power stage and the load resistance. By the aid of the said network the impedance con- 1 nected with the output transformer of the power tube is rendered real and constant. In calculating the load resistance of the input tube all that is necessary is to take into consideration such a shift of the phase as is occasioned by the output transformer of the power tube.

In order that the blur factor compensation may be made still more complete and perfect, it

' is necessary according to this invention to provide a voltage divider arrangement between the two tubes by the aid of which themodulation percentages of the-two tubes are so-set that the resultant blur factor of the amplifier cascade will attain its lowest value (fine regulation). This step is based upon the fact that theblur factor of the tube is dependent upon its modulation,

While it is true that voltage dividers between two tubes are known in the prior art, they have in the past been utilized only for an adjustment of the gain or amplification factor rather than to minimize the blur factor or the non-linear distortion. The gain, however, contradistinct from what is true of the prior art of using voltage divider schemes, is not to be changed. It is therefore advantageous to so change the voltage to be applied to the preceding or input tube that the gain of the aggregate arrangement stays stable. A device which makes it feasible to adjust the minimum value of the blur factor at any desired time is valuable in actual practice for the reason that the elements of an amplifier, especially the tubes as well as the load resistance associated with the end or power tube, are subject to frequent changes occasioned by operation and practical use of the arrangement.

Instead of, or conjointly with, the said regulation of the ratio of modulation percentages, the invention has the further object to so adjust the operating point in one of the tubes by variation of the grid biasing potential that the resultant blur factor will be decreased to its minimum value. Inasmuch as for different working points thecurvatures of the characteristics will differ from one another, it will be apparent that in this manner it will also be possible to insure a change of the resultant blur factor.

The ways and means as just mentioned for acting upon the blur factors are of practical value for all amplifier schemes whenever the desideratum is to diminish the blur factor by the adoption of suitable circuit schemes or dimensions. For the working conditions in nearly all instances will cause certain changes in the non-linear properties of the amplifier which must be compensated if a minimum amount of blur factor or non-linear distortion is to be insured.

Fig. 2 shows an exemplified embodiment of the basic idea of the invention. The end or power tube of the amplifier cascade comprising the two tubes V1 and V2 has a" load which consists of the impedance B, say, a Pupinized cable or line. Between the said load resistance and the output (1:1) transformer AU is inserted the conversion network U. The latter is chosen of such dimensions and value that the ensuing impedance prevailing at the secondary winding of the output transformer willbe constant and real.

The load resistance of the input tube V1 comprises the ohmic resistance R, the inductance L and the capacity C. The resistance R corresponds essentially to the resistance associated with the secondary winding of the output transformer, whereas the inductance L. and the capacity C serve essentially to take care of the phase angle shift occasioned by the output transformer AU. The capacity C, as will be understood, may wholly or partly consist of existing circuit or tube capacitances.

The point of connection of the grid circuit of the power tube with the plate resistances of the input tube is made variable or adjustable so that the above-mentioned regulation of the blur factor minimum may be accomplished by a change of the modulation percentage ratio.

What I claim is:

1. In a multi-stage amplifier, a pair of cascaded electron discharge devices of the type which introduce distortion in operation, an impedance load output circuit for one of said tubes including reactance which causes a phase angle displacement of the output energy, an impedance load output circuit for the other of said tubes, said last named output circuit being diminished so that it causes a phase angle displacement of the energy fed therethrough which is substanitally equal to the phase angle displacement caused by said first named output circuit, a resistor element connected across said second named output circuit, an input circuit for said first device,v and adjustable means for coupling said input circuit across any desired portion of said resistor to thereby provide for adjusting the relative amountv of'distortion produced in said devices.

' 2. An arrangement as described in the next preceding claim characterized by that the relative amount of distortion produced in said devices is adjusted to such a point that the distortion produced in one of said devices is substantially neutralized in the first named impedance load circuit by the distortion produced by the other device.

3. In a multi-stage amplifier, a pair of cascaded electric discharge devices of the type which introduce distortion in operation, individual similar impedance load circuits connected to the respective outputs of the tubes, means for coupling the input of the second tube to the output of the first tube comprising a potentiometer resistance connected across the impedance load circuit of the second tube and means for connecting the input circuit of the second tube across any desired portion of the resistance to provide for adjusting the relative amount of distortion produced in said tubes.

WALTER VON WERTHER. GERHARD KALCKHOFF.

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