US2282382A

US2282382A - Amplifier system

Info

Publication number: US2282382A
Application number: US291032A
Authority: US
Inventors: Charles S Root
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1939-08-19
Filing date: 1939-08-19
Publication date: 1942-05-12
Anticipated expiration: 1959-05-12

Description

May 12, 1942. c, s, ROOT 2,282,382

AMPLIFIER SYSTEM Filed Aug 19, 1939 a ,14 fi Fig.2. 3+ :44, 5 II:

5 g i i g a6 rUW39 .qz J

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CONTACT Low 0N. VOLUME CONTROL RESISTANCE CONTACT AT TOP VOLTS on VOICE con. in a O O vours on volcscou. g 5 O 100 400 3,000 ,000 10,000 so 90100 400 4000 4500 mono Fig.5.

CONTACT AT TAP 26 VoLTS 0N VOICE con. S c O Inventor: Charles 5. Root,

y H is Attorney.

the amplifier in degenerative phase.

Patented May 12, v1942 OFFICE AMPLIFIER SYSTEM Charles s. Root, stramra, com. assignor to General Electric Company, a corporation of New York Application August 19, 1939, Serial No. 291,032

Claims.

My invention relates to amplifiers and more particularly to such amplifiers as are used in radio receivers for household reception, for example.

It has to do with degeneration circuits as used in such amplifiers and has for one of its objects to effect certain improvements therein with respect to the response characteristic of the amplifiers at low frequencies.

One of the objects of my invention is to provide means whereby the response of the amplifier may be reduced by degeneration to a very low value at low frequencies thereby to reduce certain undesired phenomena which occur chiefly at frequencies below two hundred cycles.

Commonly, in radio receivers, a condenser is connected in shunt with the audio signal channel usually in series with a resistance, for what is known as bass compensation. This condenser medium and high frequency currents through the system and thus accentuates the reproduction of low frequency currents. This produces a fuller and more pleasing tone from the output of the loudspeaker.

In accordance with my invention a switch is arranged to connect a feed-back voltage from the output of the amplifier across said condenser and this voltage is impressed upon the input to The condenser is so chosen that at high frequencies this potential is of inappreciable value but at very low frequencies it is large enough very greatly to reduce the response of the amplifier. In this way, the tendency of the loudspeaker to overload, for example, during reception of strong bass programs is reduced, as is also the response of the receiver to sixty-cycle hum currents which may be present as modulations on the received carrier wave. It also has the effect of reducing booming sounds in response to voices having a strong bass resonance, as well as to reduce the tendency of the receiver to howl or produce fluttering sounds due to acoustic feed-back between the loudspeaker and oscillator, such as occurs frequently during short wave reception.

A further object of my invention is to combine such a feed-back system with a system of the type described in my patent application Serial No. 221,319, filed July 26, 1938, entitled Feed-back circuit and which is assigned to the same assignee as my present application. This application is a continuation in part of that application. In this way certain additional effects are produced which will later be explained.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 represents an embodiment of my invention; Fig. 2 represents a modification thereof and Figs. 3, 4, and 5 represent certain characteristics relative to its operation.

Referring to Fig. 1 of the drawingI have shown therein, at I a diode, which may comprise the diode detector of an ordinary radio receiver. This diode is connected in series with a resistance 2 and the secondary winding of a radio frequency transformer 3. This transformer may comprise the last intermediate frequency transformer of the receiver, if the receiver be one of the superheterodyne type, or may comprise a transformer to which currents of the frequency received upon the aerial are applied, if the receiver be one of the tuned radio frequency type. The resistance 2 is the ordinary diode load resistance and is shunted by a condenser 21 having a-value suitable to by-pass currents of radio frequency and to give the combination of condenser 21 and resistance 2 a suitable time constant for production upon resistance 2 of a unidirectional potential having the desired audio signal modulations therein. These signal modulations are applied through a coupling condenser 4 to the diagonally opposite'terminals 5 and 6 of a Wheatstone bridge having arms I, 8, 9 and Ill. The arm III of this bridge may comprise the usual volume potentiometer of a radio receiver having a variable tap I I to which the control electrode of the first discharge device I2 is connected. The output from this discharge device I2 is supplied through a coupling condenser I3 to the input of a second electron discharge device I4 from which the output currents are supplied through an audio frequency transformer l5 across the voice coal l6 of a loudspeaker.

Anode operating potential is impressedupon theterminals l1 and supplied through a primary winding of transformer I5 to the anode of discharge device I4 and through resistance III to the anode of discharge device l2. Resistance IB-is the usual grid resistance connected between the grid of discharge device I4 and ground, and resistance 20 is the usual bias resistance connected between the cathode of discharge device I4 and ground, this resistance being short circulted for signal frequency currents by condenser 2|.

Potential from the voice coil I6 of the loudspeaker is supplied through

conductors

22 and 23 to a second pair of diagonally opposite corners of the Wheatstone bridge. The bridge is, of course, so balanced with respect to these currents, that the points 5 and 6 are at equal potential with respect to these feed-back currents and, of course, the

conductors

22 and 23 are at equal potential with respect to currents supplied from the detector to the bridge.

' voice coil is supplied to the grid of amplifier 12.

Hence there is no feed-back either degenerative or regenerative through this path which includes conductors '22 and 23. As the contact H is moved downward however, potential from conductor 22 is supplied to the grid, this potential increasing throughout the length of resistance l0.

Conductors

22 and 23 are connected properly to the loudspeaker voice coil to cause this potential to be degenerative. Thus when th potentiometer is adjusted near its lower terminals as would be the case during reception of strong stations, the amplifier is. highly degenerative thereby to' reduce reproduction of hum currents, distortion, etc., generated in the amplifier itself, whereas, when the potentiometer is near the upper end of the volume control resistance l0, as would be the case during the reception of a very weak station, this degeneration is removed thereby increasing the sensitivity of the amplifier.

The bridge may be so balancedthat this degeneration is removed before the contact ll reaches the upper end of the volume control and so that when the contact is at the upper-extremity of the volume control resistance III, the amplifier becomes regenerative thereby additionally increasing the sensitivity of the amplifier by reason of regeneration. I

Across the lower portion of potentiometer I0 is connected a shunt path comprising resistance 24 and condenser 25. This condenser is the usual bass compensation condenser, which is commonly used in radio receivers to by-pass, to some extent, the high and medium frequency currents thereby to accentuate the reproduction in the output circuit of the low frequency currents relative to the high and medium'frequency currents. This produces a somewhat more full and more pleasing tonal response from the loudspeaker. This increased response of the receiver to low frequency.

accordance with my invention, by supplying a feed-back voltage from the output circuit oithe amplifier l2 through resistance 2'! across condenser 25. This potential, which is thus supplied to condenser 25, is impressed upon the potentiometer I0 through resistance 24 and hence upon the grid ofthe amplifier in degenerative phase. Condenser 25 may have acapacity of about .002 microfarad so that at sixty'cycles its impedance is about. 1.5 megohmswhereas at 600 cycles its impedance is only 150,000 ohms. Thus this condenser effectively short circuits the feedback potential supplied through resistance 21 at higher frequencies above four or five hundred cycles, whereas at lower frequencies this potential is appreciable and increases with lower frequencies, thereby increasingly reducing the response of the amplifier at the lower frequencies.

The connection of resistance 21 as shown not only reduces the response at low frequencies by the production of degeneration but it has the further effect of partially shunting condenser 25.-

internal impedance of source 'of operating voltage connected between terminals l'l. Resistance 21 may be of the order of 680,000 ohms, resistance IQ of the order of 470,000 ohms, resistance l8 of small. These impedances thus comprise an efcurrents is highly desirable with respect to tonal 'in such receivers to provide a switch such as that fective shunt at low'frequencies across the condenser 25, whose reactance at cycles may be -of the order of 1,500,000 ohms and thus reduce its eflect of accentuating low frequency currents just as would be the case were the condenser short circuited by the closing of switch 26 but to a lesser extent.

The portion of the degenerative voltage on condenser 25 which is supplied to the grid of amplifier I2 is of course greatest when contact II is exactly upon the point 28 to which resistance 24 is connected. This portion, however, is reduced upon movement of the contact II in either direction. Thus upon movement of the contact ll from.the point 28 downward the degenerative voltage supplied through resistance 21 to the grid of discharge device I2 is reduced at the same time that the degenerative voltage supplied through conductor 22 to the grid of discharge device I2 is increased. The reduction of the indicated at '26 whereby. this condenser may be erative voltages to be supplied through conductor 22.

It is desirable, however, to take advantage of the improved tonal response produced by the condenser 25 and at the same time to arrange to avoid all of the undesirable effects mentioned above, by providing a switch for reducing bass response in a different manner. It has been found that this result may largely be secured. in

age produced by conductor 22 may be entirely removed when the contact H is at the upper 2 may be of 330,000 ohms and condenser 4 of .005 v point of resistance I0. The degenerative voltage ml. or 600,000 ohms at sixty cycles whereas the upper portion of resistance l0 may be of one megohm. This means that when contact II is at the upper end of resistance l9 approximately half of the voltage supplied through

resistances

21 and 24 to the potentiometer is still impressed senting this relationship when switch 29 i open.

Switch 26, ordinarily is not used in the system and is shown on the drawing only by way of illustrating the means for removing the bass compensation over which my present method is an improvement. Curve B of this same figure represents the frequency response characteristic with switch 29 closed. It will be seen that the response of the amplifier is very much reduced at frequencies below 200 cycles, by reason of this degeneration.

Fig. 4 shows corresponding curves with contact ll about one-fifth of the way up from the bottom of the potentiometer III, the lower portion of the potentiometer being also of one megohm. This would be the position corresponding to a low level of speaker output. It will be seen that even with'switch 29 in the closed position with degeneration applied the amplifier has a fairly large response even at so low a frequency as ninety or one hundred cycles. Thus switch 29 effects a noticeable reduction of low frequencies, but not enough to make the tone sound too thin as might occur because the ear does not hear low frequencies as well at low levels as at higher levels.

Fig. shows curves A and B corresponding with curves A and B of Figs. 3 and 4 but with the contact I lpositioned at the point 28 which is the midpoint on the resistance M. In addition, this figure shows curve C which indicates the response of the amplifier with switch 29 open and condenser 25 shorted by means of a'switch such as that indicated at 26 so that the condenser 25 is not effective to accentuate the low frequencies this being a very commonly employed means of removing the bass compensation. It will be seen that the accentuation of low frequency currents is somewhat reduced by'short-circuiting the condenser as indicated by curve C, but that a much greater reduction is attained, particularly at very low frequencies, by applying degeneration to the condenser through switch 29 instead, as indicated by curve B.

Fig. 2 represents a slight modification in my invention in which the purpose of switches .29 and 26 is served by the single switch element 30. In the position shown the degeneration from the output of the amplifier is supplied through resistance l9 and switch 39 to one terminal of the condenser. With the switch moved downward the broadened end 3| thereof bridges contacts 32 and 33 and thereby short circuits the condenser while resistance I9 is simultaneously connected to ground through resistance 9. The resistance I9 thus serves the purpose of the usual grid resistor in the grid circuit of discharge device 34. When switch 39 is moved to its upper position the short circuit is removed and this degeneration through resistance l9 and condenser 25 is also removed, the lower terminal of resistance l9 being grounded-at 35 by the switch.

Under this condition low frequency currents are accentuated by the bass compensation condenser 25 in the normal manner.

It will be noted that one advantage and difference of Fig. 2 over Fig. 1 is that it eliminates one resistor, the resistor 21 of Fig. 1. In Fig. 2, however, the total DC resistance from the grid of discharge device 3km ground, embodying resistors I9, 24, the lower half'of potentiometer l0 below the tap 28 and resistor 9, may be too large for satisfactory operation of discharge device 34.

In this figure the discharge device 34 is indicated as one of the beam type of output tubes now commonly employed in which electrodes 36 serve as focusing members for the beam of electrons which impinges upon the plate.

While in the above discussion of the invention I have mentioned particular values of the various circuit components, it will be understood that such values are subject to very wide variation and they are indicated only by way of a single example of such values as may be used. They are not intended as any limitation upon the present invention since any of these values may be varied through extreme ranges.

While I have shown particular embodiments of my invention it will be understood that I do not wish to be limited thereto since various modifications both in the circuit arrangement and in the instrumentalities employed may. be made.

without departing from the spirit and scope of my invention as set forth in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, an amplifier, two feed-back paths from the output to the input of said amplifier, and means to vary the amplification of said amplifier and simultaneously to vary the effectiveness of said two paths, the effectiveness of one of said paths being varied by said means to a greater extent than the other.

2. In combination, a signal channel including an amplifier arranged to amplify currents having frequencies extending over a wide range, a resistance in shunt with said channel, a condenser, means to connect said condenser in a path in shunt to said channel to bypass high frequency currents in said range, thereby to accentuate low frequency currents, and means to connect said resistance in a path in shunt with said condenser thereby to reduce said accentuation, said amplifier being included in said channel between said condenser and resistance.

3. In combination, a signal channel including an amplifier arranged to amplify currents having frequencies extending over a wide range, a resistance in shunt with said channel, av condenser, means to connect said condenser in a path in shunt to said channel to bypass high frequency currents in said range, thereby to accentuate low frequency currents," means to connect said resistance in a path in shunt with said condenser thereby to reduce said accentuation,

said amplifier being included in said channel between said condenser and resistance, and means to supply the voltage impressed on said condenser from said resistance through said connecting means to the input of said amplifier in degenerative phase whereby said accentuation of low frequency currents is reduced both by the short circuit effect of said resistance on said con denser and by the degeneration produced by said connection.

4. In combination, an audio amplifier, abridge adapted to have audio frequency currents to be amplified 1 supplied across diagonally opposite points thereof and having feedback currents from the output of said amplifier supplied across other diagonally opposite points thereof, the input to said amplifier being connected between one of said first mentioned points and avariable point on that oneof the two opposite arms which arm to said intermediate point whereas feed-' back currents supplied to the input due to said is on the side .of the bridge from which said feed- Y bridge are increased from one end ofthe arm to I the other.

5. In combination, an amplifier, two feedback paths from the output to the input of said amplifier arranged to produce degeneration, means adjustable over a range to vary the volume of signal supplied to said amplifier for amplification thereby, and simultaneously to vary the effectiveness of both of said paths, and means in said paths to produce greater response'of said.

amplifier at frequencies below a frequency intermediate in the range of frequencies amplified by said amplifier when said means is adjusted tosupply low volume of signal to saidamplifier and to produce lower response at frequencies below v said intermediate frequency than is produced at said intermediate frequency when said means is adjusted to supply a high volume of signals to saidamplifier.

6. In combination, an amplifier, two feedback;

paths from the output to the input of said amplifier arranged to produce degeneration, means adjustable over a range to vary the volume of signal supplied to said amplifier for amplification thereby, and simultaneously to vary the.

effectiveness of both of said paths, said means being effective to reduce the effectiveness of both of said paths when adjusted from an intermediatepoint of said range to increase the volume and said means being effective to reduce the effectiveness of one path and increase that of the other as said means is adjusted from said intermediate point to decrease the volume, and means included in said paths to proportion the voltage supplied therethrough to render the response of said amplifier in the neighborhood of sixty to one hundred cycles less than that in the range between four hundred and a thousand cycles when said means is adjusted at said intermediate point and at points to supply larger volume of signals to said amplifier and to render the response in the neighborhood of sixty to one hundred cycles greater than that between four hundred and a thousand cycles when said means is adjusted for low volume.

7. ,The combination, in an audio amplifier, of a bridge having signal voltage supplied across one diagonal thereof, means to supply signal voltage from the output of said amplifier across another diagonal thereof, the input of said amplifier being connected between a fixed point of said bridge and a point on an arm of said bridge suchithat saidvoltage from the output is supplied to said amplifier in degenerative phase, said point being variable along said arm for volume control, a resistance connected between the output of said amplifier and a point of said arm intermediate the range of variation of said variable point to supply additional degenerative voltage to said amplifier, and a base compensation condenser connected between the terminal of said resistance remote from said arm and the low signal end of said arm, said condenser being so large that said additional degenerative voltage is substantially ineifective'to reduce the amplification of said amplifier at frequencies above four hundred cycles except when said variable point is in the region of said point on said arm to which said resistance is connected.

8. The combination, in an audio amplifier, of Y a bridge having signal voltage supplied across one diagonal thereof, means to supply signal voltage from the output of said amplifier across another diagonal thereof, the input of said amplifier being connected between a fixed point of said bridge and a point on an arm of said bridge such that said voltage from the output is supplied to said amplifier in degenerative phase, said point being variable along said arm for volume control, a resistance connected between the output of said amplifier and a point of saidarm intermediate the range of variation of said variable point to supply additional degenerative voltage to said amplifier, and a base compensation condenser connected between the terminal of said resistance remote from said arm and the low signal end of said arm, said condenser being so large that said additional degenerative voltage is substantially inefiective to reduce the amplification of said amplifier at frequencies above one thousand cycles.

9. The combination in a voice current amplifier, of a volume control potentiometer, the input of said amplifier being connected between a low potential terminal and a variable point of said potentiometer, a resistance and a condenser connected across a low potential portion of said potentiometer, means to supply voltage from the output of said amplifier across said condenser and thence through said resistance to said po-- tentiometer to produce degeneration of said amplifier, said condenser and resistance being so proportioned that the output of said amplifier per unit of input in the region of sixty cycles is less than that in the region of two hundred and fifty cycles for all high volume adjustments of said variable'po'int.

10. The combination, in a voice current amplifier, of a volume control potentiometer, the input of said amplifier being connected between a low.

, potential terminal and a variable point of said potentiometer, a resistance and a condenser connected across a low potential portion of saidpotentiometer, means to supply voltage from the output of said amplifier acrosssaid condenser and thence through said resistance to said potiometer to produce degeneration of said amplifier, said condenser and resistance being so proportioned that the response of said amplifier in the region of sixty to one hundred cycles is less than that in the region of four hundred cycles for all high volume adjustments of said variable point and is higher than that in the region of four hundred cycles for low volume adjustments of said potentiometer;

- CHARLES S. ROOT.