US2228862A - Distortion correcting device - Google Patents

Description

Jan. 14, 1941. H wEssELs 2,228,862

DISTORTION CORRECTING DEVICE Filed June 2, 1937 L040 INPUT 1 AMP.

INVENTOR HERMA NN WESSELS BY v? ATTORNEY Patented Jan. 14, 1941 PATENT OFFICE DISTORTION CORRECTING DEVICE Hermann Wessels, Berlin-Oberschoneweide, Germany, assignor to Allgemeine Elektricitats- Gesellschaft Application June 2, 1937, Serial No. 146,011 In Germany June 6, 1936 5 Claims.

In the art of distant radio communication in which amplifiers are used the latter should have in general a rectangular voltage level, that is to say, the amplification should be constant over a certain frequency range and drop steeply at the limiting range. In order to attain this course of the amplification the existing curve of the amplifier system should be made to conform as nearly as possible to the curve as it ought to be suitable distortion correcting devices.

The present invention relates to a distortion correcting device which is provided in the grid circuit of an amplifying tube and which renders it possible to obtain a nearly constant degree of amplification in the range of the frequency concerned, the amplification decreasing quickly when a certain frequency has been attained.

The invention is illustrated by way of example in the accompanying drawing in which Figure 1 is a circuit of an amplifier utilizing an arrangement for reducing distortion in accordance with the invention; Figure 2 is an axial section through a particular form of input transformer utilized in Figure l; and Figure 3 shows a series of amplification curves which will serve to explain the invention.

Referring now to Fig. 1 Us denotes an input transformer, the secondary side of which is composed of two coils 2 and 3. I denotes the primary side or coil of said transformer. Connected up in parallel with the portion 3 of the secondary coils 2-3 is a condenser C. V denotes the amplification tube. A resistance W is connected between the grid of this tube and the transformer secondary 23.

It is essential for the intended action of the distortion correcting device that the coil 3 have a large stray flux with respect to the coil 2, as Well as to the coil I. I attain the necessary large stray flux by arranging the coils I and 2 one over the other as shown in Fig. 2, in which the coil 2 is wound upon the coil I. This arrangement provides a fixed coupling between the coils I and 2 and since the winding 3 is located laterally with respect to said other two windings l and 2, the necessary high stray fiux is attained.

Fig. 3 represents a group of curves which show under varying conditions the dependency of the degree of the amplification upon the frequency ,f. The curve a shows the course of the degree of the amplification of the circuit connection of the kind illustrated in Fig. 1, but with omission of the resistance W. It will be seen that at a certain definite frequency, resonance due to the stray induction of the coil 3 and the condenser C takes place. It is desirable for the purpose in view to choose the resonance frequency in proximity to the limit of the frequency range which is to be amplified. At this resonance frequency the am plification attains its maximum value. Besides, a minimum takes place at another resonance place so that the device can also be used for the suppression of a frequency band. The curve b shows the variation of the amplification with frequency when only the resistance .W is provided, that is, 10 with omission of the condenser C and the transformer Us. However, when the circuit connections of the kind shown in Fig. 1 are used, then the curve 0 is obtained, provided the individual members constituting the device are suitably di- 15 mensioned. This curve shows that the degree of the amplification over the frequency range intended to be transmitted is nearly constant, but drops sharply at the limit of the transmission range. This drop of the curve is brought about by the subdivision of the secondary coil in the manner described above.

The results obtained with the invention may be explained further in the following manner. The windings I-2 provide amplification at the lower frequencies. As the frequency increases to a point where the amplification would otherwise begin to fall off the tuned winding 3 begins to help out and indeed at its resonant frequency would cause the high peak of curve a. This peak is due to the coil 3 resonating with C and also with K, the inherent input interelectrode capacty. Hence by inserting the resistance W the resonant action is damped and by proper choice of W curve 0 can be obtained.

It would also be possible to put resistance W in series with the condenser C but the position shown is better because in addition to the damping action the combination of W and K acts as a sort of potentiometer that gives the grid a lessening proportion of the total applied voltage as the frequency increases. This accounts for the drooping of the curve b in Fig. 3 at the high frequency end of the range.

The tight coupling between windings I and 2 and the primary resistance prevents resonant effects from coil 2, while coil 3 is so loosely coupled that the primary resistance has relativeiy little damping effect on the coil 3. Above the natural frequency of coil 3 the voltage reverses in phase which accounts for the big drop in gain as the two transformer voltages then buck each other.

If the number of turns of the coil 3 is chosen smaller than that of the coil 2, a steeper drop of the curve a is obtained. In like manner, the curve can be influenced within certain limits by varying the resistance W.

I claim:

1. A circuit for obtaining substantially uniform amplification over a Wide frequency range and sharp cut-01f at the higher limiting frequency of said range, comprising a vacuum tube amplifier, a transformer having a continuous secondary winding connected to the input of the amplifier, a condenser shunting a portion of said secondary Winding, the shunted portion of said secondary being disposed relatively to the transformer primary and to the unshunted secondary portion to provide sufficient stray reaotance which forms with the shunt condenser a circuit resonant to a frequency near the limiting frequency of the range to be amplified.

2. A circuit according to the invention defined in claim 1 wherein a resistor is included in the input to the amplifier in series with the secondary Winding, the efiect of said resistor being such that constant amplification without resonant peaks in the amplification-frequency characteristic of the circuit is obtained.

3. A circuit for the amplification of signals without distortion comprising an electron discharge device, an input transformer the secondary of which is connected to the input of the discharge device, a condenser shunting a portion of said transformer, the unshunted portion of the transformer being disposed concentrically around the transformer primary, and the shunted secondary portion being disposed adjacent to and coaxially with respect to said transformer primary.

4. Coupling means comprising a transformer having primary and secondary windings, the primary winding and a portion of the secondary vnnding being disposed to provide close coupling therebetween, the primary winding and the remaining portion of the secondary winding being disposed to provide loose coupling therebetween, and a condenser connected in shunt to said last mentioned portion of the secondary winding which is loosely coupled to the primary winding, said condenser adapted to resonate with the leakage reactance of the shunted portion at a predetermined frequency.

5. Coupling means comprising a transformer having a primary winding and two serially-connected secondary windings, the primary and one of the secondary windings being wound concentrically one with respect to the other to provide close coupling therebetween, the other secondary winding being Wound coaxially with respect to the primary to provide loose coupling therebetween, and a condenser connected in shunt to said other secondary winding. and adapted to resonate with the leakage reactance of the shunted winding at a predetermined frequency.

HERMANN \VESSELS.

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