US2289752A - Wide band amplifier - Google Patents

Description

July 14, 1942. L. BRUcK ET AL WIDE BAND AMPLIFIER Filed March 11, 1959 I NV EN TORS ZOTHAR @QU'CK AND g0 TUXEN A TTORN E Y.

Patented July 14, 1942 r WIDE BAND AMPLIFIER Lothar Brick and Otto Tiixen, Berlin, Germany,

- assignors to Telefunken 'Gesellschaft fiir Drahtlose Telegraphic m. b. 11., Berlin, Germany, a corporation of Germany Application March 11, 1939, Serial No. 261,286

. In Germany April 27, 1938 1 Claim.

It is already known to provide in amplifiers for wide frequency bands regeneration for certain frequency ranges to be preferred, or pronounced. A disadvantage of such arrangement lies in the increase of. the non-linear distortion caused by the regeneration. The feedback favors the appearance of upper harmonics as well as combination oscillations whose frequencies are equal to the sum or difference of the original oscillations.

In accordance with the present invention the frequency range within which the regeneration is effective, is situated at one of the two limits of the frequency band to be transmitted and covers substantially not more than one octave, while for the frequency band adjacent the center of the frequency band to be transmitted, a counter-coupling is effective. This adjacent frequency range is to be at least so wide that it covers practically all combination frequencies appearing with noticeable amplitude, and eventually. upper harmonics of the oscillations of the feedback frequency range which fall within the frequency range to be transmitted.

The advantage of the present invention will be shown for the case wherein the regeneration is effected for the frequency range situated at the lower limit of the frequency band to be transmitted. In this case a part of the combination frequencies-namely the greater part of the difference frequencies-falls into the region of the lowermost frequencies which anyhow are not transmitted or reproduced. A comparatively small part of the combination frequencies, as well as of the harmonic frequencies, may be within the regeneration range. If the frequency range of the regeneration is small, 1. e. if it does not cover more than one to two octaves, the aforesaid frequency part is very small and, therefore, negligible. All higher combination frequencies (which is the greater part of the added frequencies) and harmonic frequencies, also the harmonic frequencies of higher order, come within the wide frequency range in which the counter-coupling already occurs, and, therefore, they are greatly weakened in view' of the countercoupling. Especially if the regeneration range does not exceed the range of one octave, i. e.. therefore, if it extends from 50 cycles to' 100 cycles, the frequencies of all upper harmonics and combination oscillations of two original oscillations of this range are either above 100 cycles whereby they are weakened by the counter-coupling, or they are below 50 cycles thus being practically not reproduced. I

The invention, therefore, affords an effective.

raising of the lowest frequencies of the transmission range in which an appreciable undesirable I lowering would occur without regeneration, without increasing the non-linear distortions. Addi tional tubes are thereby not required. A like advantage obtains if the range in which the regeneration takes place, is situated at the. upper limit of the transmitted frequency band. It is known simultaneously to feed back the high frequencies and to counter-couple the low fre with a double stage audio-frequency amplifierof a receiver. The audion-tube V1 is coupled with the end tube V: by the transformer T. The connection of the two anodes across the resistor R and condenser C provides a counter-coupling for the intermediate audio frequencies. The polarity of the transformer T is to be such that it does not cause a phase rotation for the intermediate audio frequencies, so that the alternating potentials at the two anodes are in phase opposition. For the low audio frequencies'from 50 to cycles, or 150 cycles the transformer T causes an appreciable droop of the transmission curve and at the same time a notable phase displacement The condenser C is so chosen that it does not represent practically a short-circuit for the intermediate audio frequencies while as regards the low audio frequencies it causes a weakening of the amplitude of the returned alternating potential as well as a phase turn. The design may also be such'that for a frequency of 80 cycles'the phase displacement caused by the transformer is 60 and. that produced by the condenser C is likewise 60 in the same direction, so that. in total for this frequency the phase of the counter-coupling is turned at The low audio frequencies below about 100 or cycles are thus retum-coupled, or regeneratively fed back, instead of counter-coupled. For the frequencies below 50 cycles the amplitude of the return-coupled oscillation has become so small however, that a notable feedback no longer exists.

The course of the return-coupled alternating potential in: relation to the alternating input potential at various frequencies is seen from Fig. 2. The voltage vectors indicated by the arrows pointing towards the right near the abscissa are to eflect a pure counter-coupling. With a decrease in the frequency, a phase displacement takes place below about 150 cycles and the amplitude decreases. The vector of the return-coupled potential next turns at 90 whereby the effective counter-coupling becomes lower, and then it turns again at about 30 so that for the frequencies below, for instance, 100 cycles for which the voltage vector has a component pointing towards the left, a regeneration takes place.

In the arrangement shown in Fig. 1, a reversing of the counter-coupling to provide regeneration is possible only in that also in the amplification channel proper a considerable phase displacement for the low audio frequencies takes place which is not sufficiently the case with resistance coupling, or coupling by means of a proper transformer. Furthermore, in the arrangement according to Fig. 1 for a frequency range not too narrow, there occurs a regeneration with a phase'displacement of about 90 which regeneration acts neither as a pure feedback nor as a purecounter-coupling. The degree of .the phase-pure regeneration has a very irregular pattern within the regeneration region.

A pattern of the returned-coupled potential will be more suited whereby the frequency range in which the 90 regeneration takes place does not exist at all (the voltage vectors thus lying approximately in the abscissa) or is very narrow, or whereby at least in this frequency range the amplitude of the retum-coupled potential is very small. Hence, the approximately pure countercouplipg should, possibly in a jump-like fashion,

pass above a frequencylimit, into an approximately pure regeneration below this frequency limit. Such pattern ofthe returned alternating potential is shown in Fig. 3.

An arrangement through which this problem pled alternating potential jumps according to Fig. 3 at about 120 cycles, suddenly at 180 such that the approximately phase pure counter-coupling is transformed into an approximately phase-pure regeneration.

A continuous passing of the phase through 90 does not occur. A phase turn in the amplifier channel proper is thereby not required. The degree of regeneration shows a rather uniform pattern in the feedback range, and can be set for the desired value in a convenient manner by properly choosing the resistors R1 and R2. The danger of self-excitation. in a certain frequency can thus be easily eliminated. There may be inserted eventually in the counter-coupling line, in addition, an ohmic resistor R; for the purpose of limiting the feedback and counter-coupling. The resistor R2 may be omitted.

It is, also, possible so to modify the arrangement that not only forthe verylow frequencies from about 50 to 100 cycles, but also for the very high frequencies of for instance 4000-8000 cycles regeneration takes place, while the entire intermediate audio range of about 100-4000 cycles will be counter-coupled. This can be achieved by means of the bridge circuit according to Fig. 5 likewise placed in parallel to the secondary side'of the output transformer T. 'Between the terminals of the secondary winding, which in the present case need not be tapped, there is placed again, on the one hand, the series connection comprising the ohmic-resistors Rrand R: and acondenser C', and on the other-hand, the series connection comprising a coil L and the two ohmic resistances R3 and R4. 0. represents a reactance only as regards the low frequencies, and L represents an inductance of appreciable value only can be solved in a substantially better way than is possible with the arrangement according to Fig. 1, is shown in Fig. 4. In this arrangement The cathode whose grounded lower end is connected with the tapping Z, andthe end at the cathode side is connected to the common point of the resistors R1 and R2 for obtaining a counter-coupling. The

condenser C is so chosen that for the intermediate audio frequencies it represents above about 150 cycles a short-circuit and for the lower audio frequencies, however, a resistance comparable .to the resistances of the ohmic resistors R1 and R0. The resistor R1 is chosen slightly higher than Ra whereby for intermediate audio frequencies the phase of the potential derived at the common point of said resistors is in the same phase with the alternating potential at the lower end of the secondary winding, while for the low audio frequencies the phase is approximately equal to the phase at the upper end of the secondary winding,

This bridge arrangement makes it possible to accomplish that the phase of the return-coufor the very high audio frequencies; R1 being chosen slightly higher than Rs, whileR; is slightly higher than R3. The potential tapped between the common point of R1, R2 and that of R3, R1 is applied to the input in any known manner such that for the intermediateaudio frequencies a a condenser C. of low capacity may, in addition,

he placed in parallel. to R4 and-which short-circuits R4 as regards the very high audio frequencies. In this case the coil L-may also be omitted.

Fig. 6 shows a circuit in which at the output of the receiver only asingle potential is tapped, but which is applied ,to the amplification channel across two paths having different dependence on the frequency, such that the said potential causes in one case a regeneration and in the other case a counter-coupling and this always for different frequency ranges. The one end of the secondary coil of the output transformer T is connected to a point having a fixed positive potential. At the other end an alternating potential is tapped and applied to the cathode of the preliminary tube V1 across the resistor R'1 in series to the parallel circuit comprising the resistor R: and the condenser C1. The resistors R'1 and R:

are chosen so high that the cathode of V1 will have a positive potential substantially lower than the potential which the secondary side of the transformer T may have. The condenser 0'1 is so chosen that only theintermediate audio frequencies and high frequencies above cycles will be returned with an appreciable amplitude, but not the lower frequencies, and thereby counter-coupled. The resistor R: placed in parallel to the condenser C1 takes care that as regards the limit frequencies the phase of the returned potential will not be substantially turned, so that the counter-coupling remains approximately phase-pure.

The upper end of the secondary winding, furthermore, is connected to the screen grid of the tube V1 across the series connection of the three resistors R's, R'4, R5, so that a regeneration takes place across this path The common point of the resistors R'4 and R is grounded across a comparatively large capacity C2 so that only the very low audio frequencies approximately below 120 cycles reach the screen grid efiecting at this place a regeneration. If desired, the series connection of the two resistors R; and R5 may be bridged by a condenser C3 whereby also for the very high audio frequencies a regeneration will be efiective' whereby the counter-coupling will be eliminated, or even exceeded, for these frequencies across the other path.

In the arrangement according to Fig. 6, the counter-coupling is eifected across a longer part of the amplifier channel than is the feedback, but covers completely the regenerated amplifier part. In some cases it may be suitable to carry through the regeneration only across a small amplifier part; for instance, across a single tube, while the counter-coupling is carried out across several tubes.

What is claimed is:

In combination with a wide frequency band transmission network provided with alternating voltage input and output circuits, a regenerative alternating voltage feedback path connected between the output and input circuits, a second feedback path devoid of any electron discharge tube constructed and arranged between said output and input circuits to impress alternating-

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