US2269249A

US2269249A - Controlled feedback circuit

Info

Publication number: US2269249A
Application number: US242885A
Authority: US
Inventors: Bruck Lothar
Original assignee: Telefunken AG
Current assignee: Telefunken AG
Priority date: 1937-12-16
Filing date: 1938-11-29
Publication date: 1942-01-06
Anticipated expiration: 1959-01-06

Description

1.. muck 2,269,249

CONTROLLED FEEDBACK CIRCUIT Jan. 6, 1942.

Filed Nov.- 29, 19 38 I 7 PRE-DETECTOR NEGATIVE FEED BACK mwszvrogg. THA R BRU CK ATTORNEK Patented Jan. 6, 1942 Lethar firiiclnjBcriin, Germany,;assignor to Tele- Q funken Gesellschaft fur Drahtlose Telegraphic m. i b. 11., Berlin, "Germany; a corporation .6!

Appucaiormma 29, 1938, Serial No. 242,885 1 InGe 'mauy December 16, 11937, r

.1" claims. ,(01. 250420) There has been disclosed 'anamplifier in which th'e'volumeor gain iscontrolledybyrchange of the grid biasin'gi potential in ,one, or several, an p'li fier tubes,-a-nd comprising-anti+regeneration or negative regeneration' means, and in which the fraction of the output potential which=is in negative feedback relation to the input end is made variable in such away --that it increases as the gain diminishes. The advantage; residingin this arrangement is that an undesiredflreduction of the negative feedback, and thus the impairment of the correction of distortion, with: decreasing gain is avoided. Conditionsmay be made so that grid of; tube V2. The amplifiedaudio alternat in'g voltage isjfed tot-he loudspeaker -S by way of thetransform'er T'; Wound on the said transthe valueyor degree, of the negative feedback which is' -given by the relationship between the negative feedback potential and the input poten* tial, and thus also the amount of correction or compensation of distortion, stay constant oreven decreasing with diminishing gain so that the volume control is boosted. In the particular ar rangement disclosed in theipast the outputmov tentialwas applied to a voltagedividen one arm of which contained a resistance having. =anegative resistance-current characteristic.- .,Thi-s 'resistance was inserted in the cathode lead of; the

controlled tube and was regulated by the plate direct current. Y

The identical effect, accordingto theupresent invention, is accomplished more simply by impressing the alternating,potential-serving for the negative ieedback uponthescreengrid of a gain controlled tube by way of a resistancewhiohis of l at least the same ma'g nitude as the inner resistance of the screen :grid to cathodepathi The operation and effect of the arrangement of the invention is predicated uponthe-fact that the inner resistance is connected .so as to be used a s part of the voltage divider across which the potential subject to-negative-ieedback is impressed. By the variation of-Qt'he grid biasing voltage the inner resistance 'is-regulated ins'uch a way'that in the presence of more negative biasing voltage, that is lower gain, the inner resistance becomes higher and thus the fraction of the potential in negative feedback also higher.

former is an additional;secondary winding W. One end of the latter is connected with --a source of-positivepotential. The other end thereof is associated by way -of-resistance -R1- with the screen gridof tubefvh Thepolarityof the said additional winding must be so-chosen that the alternating potential impressed upon the screen grid' is in phase opposition to the alternating potential acting uponthe control 1 grid; Resistance -R1 should be :at leastas high as, or preferably higher "than; the" inner resistance of the 7 screen gridto cathode path. In such case "the iraction or theoutput-potenti-al which is associated :with the screengrid in negative ,Ieedback is roughly proportional to: the inner resistance. of the screen grid to cathode path, onci-rcuit, and it variesin accordance with the inner-resistance;

in other words, it increases as the biasing potential-of the-control-gridbecomes more negative. The constants may be chosen in such away that the 1 ratio between the negative feedback potential and the input potentiaLinsidethe regulat- =ing range, will not diminishes the gain diminishes, but will,- ,on the contrary,- become veven higher. In the presentinstance, this means that automatic volume control is boosted by the change in negativeieedback. Another vfact .is that vwhen a powerful transmitter is received the negative regeneration, and thus the compensation both of linear as well as of non-linear distortion, becomes particularly high. a a

If the direct current voltage 1s impressed upon the screen grid by-way of a separate resistance An exemplified embodiment of the invention 1 is shown in the drawing. From'the load resistance R which, in series with the radio frequency choke coil L, is connected in parallel relation to the signal detector diode D of a radio receiver, both the audio potential as well as the negative direct current potential are impressed upon the control grid of the pentode tube V1. 1

The anode of this tube is connected by the usual resistance-condenser coupling with the control.

rather than by way of resistanceR- then'such separate resistance should be-chosen high in comparison with the inner resistance of the screen grid to cathode path of tube V1 to the end that, I

on the one hand,.a high over-all outer resistance may be obtained, and that, on, the other hand,

the-inner resistance may not be short circuited. An expedient plan is to insert inthe lead from the additional winding W to the screen grid,

frequency-dependent means in order that the negative regeneration efie'ct may be brought about in definite frequency ranges, more particularly and predominantly upon the medium range of tonal frequencies.

From the load resistance R the direct current voltage may be fed by way of an audio filter network to the radio frequency and intermediate frequency amplifier tubes of the receiver for the purpose of gain regulation, where automatic vol- 1. In a'signal amplifier of the type comprising a tube having at least a cathode, signal grid, screen grid and plate, a signal input circuit coupled to the signal grid, a signal output circuit coupled to the plate, means for feeding back to the screen grid from the output circuit a signal potential in degenerative phase with the signal potential applied to the signal grid, said feedback means comprising a potential divider including the screen grid to cathode resistance of said tube, and means for simultaneously increas ing the negative bias of said signal grid thereby decreasing the tube gain thereby to increase the magnitude of said resistance whereby the value of said degenerative potential is increased.

2. In a signal amplifier of the type comprising a tube having at least a cathode, signal grid,

screen grid and plate, a signal input circuit coupled to the signal grid, a signal output circuit coupled to the plate, means for feeding back to the screen grid from the output circuit a signal potential in degenerative phase with the signal potential applied to the signal grid, said feedback means comprising a potential divider including the screen grid to cathode resistance of said tube, and means for controlling the bias of said signal grid in a sense to decrease the tube gain thereby to increase the magnitude of said resistance whereby the value of said degenerative potential is increased.

3. In a.signal amplifier of the type comprising a tube having at least a cathode, signal grid, screen grid and plate, a signal input circuit coupled to the signal grid, a signal output circuit coupled to the plate, means for feeding back to the screen grid from the output circuit a signal potential in degenerative phase with the signal potential applied to the signal grid, said feedback means comprising a potential divider including the screen grid to cathoderesistance of said tube, and means responsive to signal amplitude increase at the signal input circuit for increasing the signal grid bias of said tube whereby said resistance magnitude is increased and thereby increasing the magnitude of said degenerative potential.

4. In a signal amplifier of the type comprising a tube having at least a cathode, signal grid, screen grid and plate, a signal input circuit coupled to the signal grid, a signal output circuit coupled to the plate, and means for feeding back to the screen grid from the output circuit a signal potential in degenerative phase with the signal potential applied to the signal grid, said feedback means including a resistive impedance whose magnitude is substantially equal to the screen grid to cathode resistance and means for varying the gain of said tube in such a manner as to vary the magnitude of said resistance sufficiently to adjust said degenerative potential value.

5. In a radio receiver provided with a detector having a modulated carrier input circuit and a modulation voltage output circuit, a modulation voltage amplifier tube of the screen grid type, means coupling the detector output circuit to the modulation amplifier input electrodes, an output circuit coupled to the output electrodes of the modulation amplifier tube, means feeding to the screen grid of the latter from the last named output circuit a modulation voltage in degenerative phase with the modulation voltage at the said input electrodes, said feeding means comprising a voltage divider which includes the screen grid to cathode resistance of said amplifier tube, and-means for simultaneously controlling the gain of said modulation amplifier tube in such a manner as to adjust the magnitude of said resistance and thereby control the value of the degenerative voltage.

6. In a radio receiver provided with a detector having a modulated carrier input circuit and-a modulation voltage output circuit, a modulation voltage amplifier tube of the screen grid type, means coupling the detector output circuit to the modulation amplifier input electrodes, an output circuit coupled to the output electrodes of the modulation amplifier tube, means feeding to the screen grid of the latter from the last named output circuit a modulation voltage in degenerative phase with the modulation voltage at the said input electrodes, and said feeding means comprising a voltage devider including a resistor whose value is at least as great as the screen grid to cathode resistance of the modulation amplifier tube and means for controlling the amplifier tube gain in such a manner as to adjust the value of said resistance over a substantial range of values.

7. In a wave repeater circuit, a tube provided with at least a cathode, an input electrode, an output electrode and an auxiliary cold electrode in the electron stream to said output electrode, a wave input circuit coupled to said input electrode and cathode, a wave output circuit coupled to the output electrode and cathode, means for applying to said auxiliary electrode wave potentials in desired degenerative phase relation to the wave potential applied to the input electrode, said applying means comprising a potential divider which includes the resistance existing in said tube between the cathode and said auxiliary electrode, and means for controlling the input electrode bias of said tube thereby simultaneously to control the magnitude of said resistance and the magnitude of said auxiliary electrode wave potential in inverse relation.

, LO'I'HAR BRUCK.