US2037753A - Amplifier - Google Patents

Description

.April 21, -.1-936.

l.. E. BARTON AMPLIFIER original Filed Jan. so, 1932 S EG .Ennuh UQ Patented Apr. 21, 1936 AMPLIFIER 'Loy E. Barton, Collingswood, N. J., osslgnoito Radio Corporation of America, a corporation of Delaware Original application January 90, 1932, Serial Nor 589,846. Divided and this application May 29, 1933, Serial No. 673,370. In Spain January 24.,

C l lsclaims.- (cl. ris- 1) static discharges have relatively little -eii'ect on My invention relates to amplifiers and, more particularly, to methods and apparatus for obtaining volume control in amplifiers of the type utilized in the reception of radio signals, and is a division of my copending application, Serial No. 589,846, filed January 30, 1932 for-Amplifiers, and assigned to the same assignee.

Many automatic volume control systems, as

heretofore devised, have been open to some objection because of the necessityfor providing` an additional thermionic tube to supply grid biasing potentials varying in response to fluctuations in carrier amplitude. Furthermore, by reason of vthe fact that volume control systems, in

' general, permit maximum amplifier sensitivity when no carrier is beingc'received, the background noise received when tuning from one station to another has been excessivefand unpleasant.

In addition, automatic volume. control systems that respond to modulation areopen to the objection that static, if strong,'causes`temporary paralysis of reception while the bias potential resulting from the static is being dissipated. At-

tempts at rendering the systems unresponsive to static involve the use of circuits having timeconstants within the audio range. AIf circuits having time-constants too short are used, low

audio frequencies are effected, if too long, the recovery from a static dischargel is slow.

The invention further contemplates the use of manual volumecontrol means in the said amplifying system, and with the automatic volume to Iprovide an improved volume control systenr thatshall not be open to the foregoing enumerated objections. l

- It is a further object of my invention to provide an improved tone compensated volume conr trol system for use in connection with audio frequency circuits, which is simple and. effective,

and involves. comparatively inexpensive circuitv elements. t l

Preferably, in practising my invention, I consolidate the functions of demodulation of a signal and the supplying of a volume control potential into a single thermionic device, thus eliminating the customary extra volume-control tube.

The circuit is so arranged that the automatic volume control does not reduce the volume in a' volume or sensitivity. y

Since automatic volume control responds marily to' carrier, low frequency modulation or peak modulation does notaffect the automatic volume control.

The manualvolume control means is located in an audio frequency circuit, "such as the detector output circuit, and is provided with tone signal attenuating circuits connected to suitably spaced taps thereon, to gradually eilect a desired pri? tone control for predetermined volume levels,

simultaneously with the adjustment of the volume. This. maybe designated as an automatically tone compensated volume .control system.

The novel features that I consider characteristic of my invention are set forth with particularity' in the appended claims. The invention itself, however, both as to its organization and its method of operation, 'together with additional objects and '.advantages'thereof, will ,best be understood from the following description of a -specifimembodiment' when read in connectionl with the accompanying drawing, which the single iigureis a diagrammatic view of a portion of 'an amplierprovided with volume control means according to my invention.

In common with other automatic volume con- -trol systems I provide biasing means,.for one or more amplifying tubes, responsive to the amplitude of an incoming carrier-wave.A In my improved system,` ,referring to' the drawing.' the biasing means includes a thermionic tube I,

having a cathode 3, a plurality of grids 5, l, and 9, and an anode I I. l tube commercially designated RCA-238, is vsatisfactory.

one of alegrias, preferably the grid s, oom- Y monly used as a screen grid,is connected through -a resistor I3 to a source I5 of positive potential.

.The grid 9, which serves as the input or control electrodef is` connected to th'ecathode through an inductor I1, -which may or may not be provided with a tuning condenser l I9, and a fixed condenser 2|.

Normal bias for the thermionic device I, sufficiently negative to enable it to function satisfactorily as a demodulator, is provided by connecting the control-grid 9 through a resistor 23,`

\to the junction 24 between the negative terminal of the high potential anode-supply source and' a resistor 25 interposed between the said terminal yand the ycathode of the device.

The potential dividers 25 and 26 acrossA the voltage supply furnish the necessary potentialto The screen grid pentode keep the grid negative with respect to the cathode.

The anode II of the thermionic device I is contype, having a cathode 33, a plurality of grids and 31, and an anode 39., i

The grid 35 .of the intermediate frequency amplifier tube is connected toLthe cathode thereof ythrough an inductor 4I, which may or may not vbe provided with a tuning condenser 43, but includes a by-pass condenser 45. The cathode 33 of the device is connected tothe junction point 24 between the cathode of the/demodu'lator tube I and the aforementioned resistor 25.,7 through a self-bias-resistor 41.

The grid 35 of the intermediate frequency amplifier tube is connected to the junction point 24 through a resistor 48 and the resistor 29 included in the anode`circuit of the Ademodulator tube. Normal negative grid bias potential for the intermediate frequency amplifier tube, therefore, is supplied by the drop across the self-bias resistor 41, the magnitude of the resistor being so chosen with respect to the plate and screen grid potentials vsupplied 'that the said drop'is correct for maintaining the proper normal bias.

In the operation of my improved system, signals impressed upon the demodulator tube from the previous amplifying stage are demodulated therein and appear as signals at audio frequency in the output circuit including the screen grid 5 and the resistor I3.' Since, as is well known to those` skilled in the art, the output current. from the demodulator tube includes a component at the 70' upon frequency of modulation down to perhaps carrier frequency, the said component will be reimpressed between the anode and the cathode of the tube, by reason of the inductive couplingexisting between the output inductor 21 and an .inductor '49 connected in shunt relation to the -resistorl 3 insofaras the carrier frequency is concerned. A small condenser 50, offering high im- `pedance to audiofrequency, is included in circuit with the inductori49. The lowimpedance ,of the vcircuitf49-5II to carrier, and high impedance to audio frequency result -in relatively low percentage modulation ofthe carrier frequency induced in the inductor 21. 'I'his has an important practical result, i. e., only smallamounts of static frequencies are re-impressed on the detector I. 'I'he carrier, thereafter, is again rectified in thedemoduylator tube, giving rise to a uni-directional current which ows between the anode and the cathode of the device in the circuit including the output inductor 21, the resistor 29, and the resistor 25.

Because of the high impedance of the resistor I3- t' audio frequencies, the energy or signal transferred to circuit 21 has only a limited degree `Vof modulation as compared to the original carrier. A'Ihis wfeature results in an automatic volume control that is more or less independent ten cycles and'does not respond appreciably to static. The direction/of current ow in the rey sistor29 is such that` the end thereof connected to the grid .of the intermediate frequency amplier tube acquires `a negative potential with respect to the end which is connected to the cathode and, as a consequence, the bias potential applied.

to the said intermediate frequency amplifier becomes more negative than normal, the increase in the negative direction being proportional to the amplitude of the carrier wave impressed upon the demodulator tube.

It should be noted that the anode I l, of the tube I is, at all times, maintained somewhat negative with respect to the cathode thereof by reason of the drop across the resistor25. This is a desirable feature since it prevents the automatic volume-control from functioning until a signal of predetermined volume is reached.

The self-bias rheostat 41 controls the maximum sensitivity and it may, with advantage, be manipulated during tuning to minimize background noise between stations. Preferably the resistance of the rheostat is high, of the order of y3000 ohms which may be adjusted to a predetermined minimum value for maximum sensitivity.

As hereinbefore explained, my improved auto- 'matic volume control so functions that the gain in the amplifier, forall signals above a .certain predetermined minimum value, is inversely proportional to carrier amplitude. Suchbeing the case, further meansare required for controlling the sound loutput, from a. loudspeaker (not shown) to'meet different requirements.

It is also desirable that, at low sound intensities, the high frequencies shall be somewhat more attenuated than thelow frequencies.

Laccordingly, interpose a coupling network, for

obtaining automatically compensated volume' control, between the resistor -I3 included in the output circuit of the detector tube I and the input terminals of an audio-frequency amplifying,I

tube 5I. The audio frequency amplifying tube may be ofthe equipotential cathode type having an anode 52, a grid 53, and a cathode 55.- A selfbias resistor 1,51v shunted by a condenser 59, is interposed between the cathode 55 and ground and the grid 53 is' connected to ground. through a condenser 6I for additional manual tone-control purposes hereinafter tol be explained more 'in detail.1 y

The coupling network includes the resistance element 63 of a potentiometer, one end of which is connected to the grid 5 of the detector tube 'through a stopping condenser 65 and a resistor 61 and the other end of which is connected to ground through a grid-leak 69. 'I'he resistor 61 and a by-pass condenser 10 serve to eiectively attenuate the carrier frequencyiand prevent it from appearing on the grid of the tube 5I Since the resistor 61 offers high impedance to the c'arrier frequency,lit is compelled yto take a path to ground including the inductor 49 which offers vlow impedance to thet carrier frequency. The

junction between the resistance element .of the potentiometer and the grid leak is connected to `the cathode through a condenser 1I which acts as a hum filter.

Inlorder that the potentials, at high frequencies, developed across the potentiometer resistance element 63, shall be progressively attenuated as the ymovable arm thereof vis moved down, in the direction giving reduced volume, separated points on the element are by-passed to the cathode over paths including individual condensers 15 and 11, individual resistors 19 and 8|, and the condenser 1I, previously mentioned.

It should be clear, from 'a consideration of the foregoing. that the low frequency potentials imthe grid'and the cathode.

them, are attenuated less than the high frequencies, thus giving the desired effect. The degree of compensation depends upon the resistance between taps on the resistor 63 in relation to the condensers 15 and 11 and the resistors 19 and 8|. If furtherattenu'ation of the high frequencies is wished for, independently of the position of the potentiometer arm 13, a resistor' 83, interposed between the arm and the grid of the tube may be varied to control the shunting action of the condenser 6| connected between A very small condenser 84 may be interposed between the resistor 63 and the arm 13, if extremely high frequencies are to be compensated to some extent. y,

By way of example, the resistor or impedance element 63 may have a resistance of 500,000 ohms and may be, and preferably is, wholly constituted bythe resistance element of a potentiometer device. The condensers and 11 may havev values'of .0025 and .025 micofarad, respectively, y while the resistors 19 and 8| may have values of 100,000 ohms and 10,000 ohms, respectively.

In order that the disclosure of my improved automatic volume control system shall be vcomplete, I am giving below a listl of other more important circuit constants:

Resistor Ohms Condenser Microfa'rads 13- 50,000 to 100,000 000750 29 50,000 to 200,000 6l 0. 0001 67 20, 65 1 '83 0-1 megohm 70 .0002 84 0001 Although I have chosen a 'specic embodiment of my inventien for purposes of illustration, many modifications thereof will be apparent to those skilled in the art to which it pertains. For example for a different degree of compensation,

different values of certain of the circuit come ponents as above given may bechanged. Particular reference is had to the circuit components -15 and 11 together with the limiting resistors 19 -having terminal ends, a plurality of fixed spaced taps thereon between and spaced from said'ends', means including electrical impedance elements Aproviding separate shunt connections between each of said taps and one end of said resistor,

and means'providing at least one variable output tap connection movable along said L.resistor between said ends. and successively past said first named taps.

. 2. In an audio frequency volume control cir-l cuit to which signals of substantially constant l average amplitude are applied, a single variable lvolume control means comprising an. impedance f-f`element1having terminal ends connected with said circuit and spaced taps between said ends, and means in circuit separately between one terminal and each of said taps for attenuating signals Within certain portions of the audio frequency range, said means thereby being effective with variations in volume, and being connected in shunt relation to portions of said impedance element through said taps vand said one terminal.

3. In an audio frequency signal transmission circuit, means for supplying audio frequency signals thereto 4at substantially-constant average amplitude, a potentiometer device having an impedance element connected in shunt with. said circuit, an output circuit connected with one ter-j diate the ends of said impedance element, said tap points being spaced apart along said impedance element progressively to provide attenuation progressively with decrease in volume by adjustment of said contact.

4. In an audio frequency signal transmission circuit, a poteniometer device having an impedance element connected in shunt with said circuit,

an output crrcuit connected with one terminal P and the variable contact of said potentiometer device and a plurality of signal attenuating circuits connected each with 4'said one terminal and a separate xed tapl point on said impedance element, said tap points being spaced apart progressively along said impedance element to progressively attenuale signals with decrease in volume by adjustment of saidcontact, and at least one of saidcircuits including a condenser and a resistor connected in series, thereby to attenuate signals in the -higher audio frequency range.

5. In an audio frequency signal transmission circuit, a potentiometer device having an impedance element connected in shunt with said circuit. an output circuit connected with one terminal and the variable contact of vsaid potenliometer device, and -a plurality of separate signal attenuating circuits conn'ected each between said one terminal and a separate fixed tap point intermediate the ends of said impedance element, said tap points being spaced apart progressively along said impedance element and each of said signal attenuating circuits having a lower impedance to a certain range of audio frequency signals than the impedance of that portion Aof the potentiometer devicewith which it is effectively connected in parallel.

6. In combination, a potentiometer device com- A,prising anI electrical impedance element and a contact movable along said impedance element to'provide a variable connection therewith, said impedance element having a series of taps spaced progressively between its terminal ends, a plurality of separate signal attenuating frequency responsive means each connected with one of said terminal ends and with a separate one of said taps, automatic volume control means for supplying audio frequency signals of substantially constant amplitude to the terminals of said impedance element, and an output circuit connectedto j said movable contact and one terminal end of said element, whereby movement of said contact toward said termina-l end provides volume red uction and progressive signal attenuation.

7.121 combination. a potentiometer device cornh prising an electrical impedance element and a contact movable along said impedance element to provide a variable connection therewith, said impedance element having taps spaced progressively between-its terminal ends, a plurality of tap points betweenand spaced from the of said resistance element. t

11. In an audio'frequency amplifier system, an4

separate signal attenuating means each connected with one of said terminal ends and with a separate one of said taps, thereby progressively to attenuate signals vas the contact is moved in the direction of said one lterminal end, an audio frequency signal circuit connected with the terminal ends of said impedance element, and an audio frequency output circuit connected with said one of the terminal ends of said impedance element and the movable contact.

8. In combination, apotentiometer device com-' prising 'an'electrical impedance element and a contact movable along said impedance element to provide a variable connection therewith, said impedance element having spaced taps between its terminal ends, a plurality of signal attenuating means each connected with one of said terminal ends and .with a separate one of said taps,

an audio frequency signal circuit connected with the terminal ends of said impedance element, an audio frequency output circuit connected with said one of said terminal ends of said impedance eement and the movable contact, and independent additional tone control means connected in said output circuit between said contact and a high potential end of said impedance element.

9. In combination, a, potentiometer' device comi prising an electr/ical impedance elementv and a` contact movablealong said pedance element to provide a variable connect n therewith, said impedance element having spaced taps between'I its ends, a plurality of signal attenuating means each connected with one kof saldends and with a separate one of said taps, an audio frequency signal circuit connected with the terminal ends of said impedance element, an audio frequency output circuit connected with said one of said terminal ends of said impedance element and the movable contact, and a high frequency lter insaid audio frequency input-circuit comprising a shunt coupling impedance, a series impedance in one side of said circuit, and a condenser connected in parallel with said circuit following said impedance'in the directionof signal transmission.

10. In an audio frequency amplifier system, an audio frequency amplifier device having an output circuit, a second audio frequency amplifier device having an input circuit, and a volume control coupling network 'interconnecting said circuits for progressively attenuating signals in cer-v tain portions of the audio frequency range with decrease in volume, said network comprising a potentiometerresistor having a resistance element connected in shunt to said output circuit,

a movable Acontact and one terminal connected with said input circuit, anda plurality of signal attenuating means connected to provide separate circuits between said one terminal and spaced terminals audio frequency amplier devicehaving an output circuit, a second audio frequency amplifier device having an input circuit, vand a coupling network providing combined volume and tone control, interconnecting said circuits and comprising a potentiometer resistor having a resistance element connected yin shunt to said output circuit, and a plurality of signal attenuating device having an input circuit and a coupling l network interconnecting said circuits and comprising a potentiometer resistor having a `resistance element connected in shunt to said output circuit, a movable contact and one terminal connected with said input circuit, and a plurality of signal attenuating means connectedv between saidone terminal and'spaced tap points thereon, and a separate variable tone control means connected with said input circuit.

13. In an audio frequency amplifier system, an

audio frequency amplifier device having an output circuit, a second audio frequency amplifier device. having an input circuit and a couplingnet- V'work interconnecting said circuits and comprising a potentiometer resistor having a resistance element connected in shunt to saidfoutputcircuit, and a plurality of signal attenuating means connected between said one terminal and spaced tap points on said resistance element, said signal attenuating means each comprising a condenser and a resistor connected in series with each other,

and a separate compensating circuit connection between the output circuit and the input' circuit,

said connection including a coupling condenser" of relatively low capacity. y

, 14. In an audio frequency volume control circuit, a volume control potentiometer device having a resistance element provided with high potential and lo'w potential input terminals at its ends, a tap continuously movable along said element between said ends, a signal output circuit connected with said movable tap and the low potential terminal, a signal input circuit connected with said high and low potential terminals, said element having a series of'taps spaced fromsaid high potential terminal and from each other, and a separate tone compensating circuit includinga react-ance element connected between each of said last named taps 'and the low potentialterminal of said first named element. i l

15. In an audio frequency signal `transniission circuit, a potentiometer device having an impedance element connected in shunt with said circuit, an output circuit connected with one terminal and the variable contact of said potentiometer device, and a plurality of signal attenuating circuits connected eachwith said one terminal and a separate fixed tap point on said impedance element, said tap fpoints being spaced apart progressively along said impedance element vto progressively attenuate signals with decrease in volume by adjustment of\ said contact, and at least one of said circuits including a condenser and a resistor connected inv series, said circuit being connected with a tap point whereby it becomes effective in advance of said other circuits in the attenuation of said signals.. LOY E. BARTON.

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