US1918975A - Radio amplifier - Google Patents

Description

July 18, 1933. v L ND N 1,918,975

RADIO AMPLIFIER Filed April 18, 1950 INVENTOR Verna/7 file/Max;

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Patented July 18, 1933 came: STATES PATENT- OFFIC vEu-NoN n. LANIDON, or BOONTON, NEW JERSEY, ASSIGNOR TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA RADIO AMPLIFIER Application filed April 18,

In present-day practice, because of the necessity of imparting the desired characteristic of selectivity to radio-receiving circuits, the primary dndings on the radio-frequency transformers are designed with a minimum number ofv turns, and the loss in energy-transfer occasioned thereby is usually compensated by some form of regeneration.

,1 system of the above type will usually cause either excessive amplification at the high frequencies, or insufiicient amplification at the low frequencies, depending upon the constants of the circuit, thereby giving the circuit a poor response-curve characteristic.

In the same type of system employing a comparatively few turns on the primaries of the radio-frequency transformers, the problem of stabilizing the operation of the circuits by the well-known means of cancelling plate-grid feed back through the use of neutralizing condensers becomes m ore difficult, as it veryoften happens that, when the point of balance is reached for broadcast frequencies, oscillations wil occur at the higher frequencies, particularly those frequencies at which parasitic oscillations are prone to de-. velop. 7

Accordingly, it is one object of my inventic-n to provide a circuit, the frequency response curve of which is substantially uni- J form over the broadcast range of frequencies.

Another olject of my invention is to proide means for increasing the range of neutralization in the direction of the higher fro-'- quencies.

A further object of my invention is to provide means whereby a neutralizing means in radio circuit may be utilized for regenerative purposes. I

A further object of my invention is to pro I vide means whereby audio-frequency noises developed in the high-frequency circuits will not be transmitted. to the audio-frequency stages. 7 i

The means for accomplishing the above objects of my invention is disclosed in the 1930. Serial No. 445,291.

accompanying drawing, the single figure of I which is a diagram illustrating my improved receiving circuit, an explanation of which:

follows. 7

The system comprises a plurality of stages of radio and audioefrequency amplification,

the last stage of audio being a push-pull arrangement. My invention, however, may be used in various modified circuits, as will appear more apparent as the specific improve ments are pointed out in detail.

An antenna circuit 1 is shown coupled,

both inductively and ,conductively, to the in-' put circuit 3 of the first radio-frequency stage, the output'or plate circuit of which comprises a pair of coils 5 and 7, bridged by a resistor 9, the connection to .one of said coils being variable. I 1 i The resistor, 9 functions as a potentiometer means for controlling the input to the coil. To prevent undesirable changes in the plate potential of the first radio-frequency tube, when the variable contact on sistor 9 is'shifte d, I insert the coil 5', as described above, this coil serving as a'radiofrequency choke in the output'circuit, but, at the same time, providing a relatively lowresistance path. for the direct current in'the plate circuit. V v 1 The-above coil 7 is inductively coupled to the input circuit 11 of the second radiofrequency stage and, in similar manner, the output circuit 13 of this stage is inductively the control recoupled to the input circuit 15 of a third of the radio-frequency stages (preferably the 7 I second amplifier) between the cathode 31and the plate circuit 13, as shown, together with a resistor 83 of approximately 5000ohms in the plate circuit of the same amplifier, a

marked increase in the range of effective neutralizatlon is obtained. The circuits become more stable than heretofore at the higher frequencies at which parasitic oscillations are prone to develop. The presence of the condenser in the circuit, as shown,'renders the plate circuit a capacitative load on thetube, thus causing a degenerative feed back through the grid-plate capacity to counteract any tendency of parasitic oscillations to develop. 1 1

Not only have the above desirable results been obtained by the use of the capacity-29 and the resistor 33 but I have found, in addition, that the neutralizing condenser 21 of the third amplifier, in conjunction with its corresponding feed-back coil 27. functions as a regenerative feed-back circuit to further amplify the incoming signals.

In combination with the above, I have provided,in the output circuit of the last radio frequency amplifier-,a novel circuit arrangement which constitutes the coupling means between'said amplifier and the detector stage. This coupling between said amplifier and said detector comprises a resistor 35 of approximately 10,00050,000 ohms in parallel with a choke coil 37 of high L/C ratio, with dimensions of approximately .002 henries and low distributive capacity. Condensers 39 and 41 are inserted in the plate-grid connection between the resistor 35 and the choke 37 and between the choke 37 and the grid43, as illustrated, said grid being grounded through a high-resistance grid leak a5. Condenser 39 is not critical, any value from 2005,000 micro-microfarads being sufiicient while condenser 41 should have a value of approXi-' mately 250 niicro-microfarads, as it serves in the capacity of grid-leak condenser for the detector tube. r

The elements comprising the above cou pling and arranged as described, favor-the lower frequencies of the broadcast range, particularly by reason of a resonant condition existing between the choke, the plate-filament capacity of thepreceding radio frequency tube, the grid-filament capacity of the de--- tector andthe various stray capacities in the 50 coupling, whereby a lowfrequency peak is produced. This coupling, without the condenser 29 and resistor 33arrangement described above, is sufficient to stabilize the circuit at the higher frequencies, in which case, the neutralizing circuit involving the neutralizing condenser 21 will continue to act as a regenerative circuit. I Y

It is believed that the above stabilizing effeet at the frequencies above the broadcast range is a result of the distributed capacity of the choke acting as a balancing or neutralizing concienser at those frequencies in the radio frequency amplifier inwhich' it located', r

, However, the best results are obtained in the neutralizing effect with the combination of the two arrangements in the same system, as they appear to exert some influence upon each other to promote a greater degree of stabilization of the circuits.

Additional beneficial results are obtained from the above described coupling. Because of its favoring the lower radio frequencies, it either completely or partially flattens the response curve, in that the high frequencies, which might have been over-amplified through the first two radio frequency stages are now under-amplified through the third radio frequency amplifier. As might otherwise be expressed, the low-frequency currents whicli'might have been under-amplified in the first stages, are overeamplified in the last stages, whereby a substantially fiat. characteristic response curve may be obtained.

In addition-to the above, it has been found much easier to isolate a fixed-coupling arrangement, as described, than the ordinary tuned-transformer coupling, with the consequent result that overall feedback and audio pick-up are reduced or entirely eliminated, thereby further increasing the stability of the system.

A plurality ofxfixed couplings of the type described above may be used in a multi-stage circuit't'o advantage, where the ratio of the responses at high and low radio frequencies make this arrangement desirable.

. In a receiver employing a fixed coupling, as disclosed by me, ahead of the detector tube, as shown, if the choke 37 were omitted or if the resistor 35 were replaced by the choke, then variations in B. voltage, arising from poor regulation at the source or induced lowfrequ-ency currents in the output circuit,

would be fed directly to the grid 43 of the detector tube, resulting in a flutter or audio-frequency tone. \Vith the choke connected as sllOWn, those low audio-frequency noises or tones are bypassed and fail to reach the grid of the detector tube, thereby pre venting the disagreeable results outlined above. The above assumed changes, it will be noted, produce the ordinary resistance and impedance couplings well known in the art, in view of which it might be said that my coupling, in addition to its other features, comprises a new and useful improvement over prior well known coupling arrangements.

The detector-tube output comprises the primary of an audio-frequency transformer which is preceded by a low-pass filter comprising radio frequency choke {i7 and capaci-ties 49 and 51 for radio-frequency-current components existing in the output circuit of the detector.

{The secondary of the transformer consti tutes the input circuit of the first audioampli'fier designated by the reference numeral 53, the output of which is amplified system 5'? disclosed by me, and may be applied to a voltage divider comprising the I'BSISbOI" 59 from which desired B-potentlals may be tapped. An independent coil 61 is provided as a source of alterna-ting-current supply to the heaters of the radio frequency detector and first audio tubes all of which are preferably of the alternating current type, although direct current tubes may be used in lieu thereof. Another coil 63 is disclosed as providing alternating current for the filaments of the tubes used in the push-pull stage which are preferably of the power type; e. g. UK 171. The resistors 65, 6'7 and 69 constitute means for impressing a desirable negative grid bias on the radio frequency tubes, the first audio and the power tubes, respectively. Suitable capacities are provided for by-passing the various potentials.

Thus, I have disclosed, in detail, a radioreceiving system capable of functioning to attain the objects of my invention. Various modifications may bemade, such as shifting the various features of my inventionto any of the other stages of the system or utilizing them in analogous circuits, without departing from the scope. of my invention, and I, therefore, do not desire to be limited to the specific details disclosed except in so far as is necessitated by the prior art and the appended claims. i

I claim as my invention: 1. A high-frequency circuit comprlsmg amplifying means and a detector, means in combination with said circuit for neutraliz ing feed-back Within the broadcast range of frequencies, and means, for increasing the frequency range of neutralization of said circuit, said means comprising the combination of a capacitative load on saidamplifying means and a fixed resistance-choke coupling between said amplifying means and said detector.

2. A high-frequencycircuit comprisinga plurality of amplifying stages, a detector stage, means in combinationwith said circuit for neutralizing undesirable feedback at broadcast frequencies, and means for increasing the frequency range of neutralization in said circuit comprising the combination of a capacitative load in the output circuit of one of said amplifying stages and a resistance-choke coupling between certain of said stages.

3. A high-frequency circuit comprising a thermlonie device havin an 1n ut and an output circuit associated therewlt means cooperating therewith for neutralizing feed- 1 back in said circuits at broadcast frequencies,

said output circuit comprising coupling means of low distributive capacity for extending the range of neutralization above that of the broadcast range.

VERNON D. LANDON.

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