US2285895A - Automatic amplification control - Google Patents

Description

June 9, 1942. s. M. BROWN 2,285,895

AUTOMATIC AMPLIFICATION CONTROL Filed June 15, 19:59 Z'Sheets-Shet 1 ffilmlll u WI w 3% g :1: f

His Attorney.

, Jun 9, 1942.

AUTOMATIC AMPLIFICATION CONTROL Filed June 13, 1939 2 Sheets-Sheet 2 M, BROWN 2,285,895

6 eorge M. B rown,

Patented June 9, 1942 AUTOMATIC AMPLIFICATION CONTROL George M. Brown, Schenectady, N. Y., minor to mpany, a corporation of General Electric Co New York Application June 13, 1939, SerialNo. 278,854

6 Claims. (01. Iva-1n) This invention 'relates to electron discharge amplifier circuits.

It has been a problem in the art of communication by modulated carrier waves toobtain a maximum amount of modulation at all times in order to use the carrier wave most efllciently. It

. is desirable to amplify signal waves with which a carrieris to be modulated in such a way that the-average modulation depth approaches 100 per cent as nearly as possible whileretaining the fidelity of the signal waves. The design of my amplifier is such that signal waves are amplified linearly up to some arbitrary amount which modulates the carrier wave at an arbitrary depth, such as 90 per cent, while for-signal waves of greater level than this arbitrary. value-the."

amplification is reduced so that the output of my amplifier increases very slowly above this ar- "bitrary value., It is accordingly an object of my invention to provide an improved and simplified audio amplifier or conventional construction. which is adapted to supply amplified signal energy to any utilization circuit and at the same time controls the amount of amplification in responseto the relation of the amplified wave to an arbitrary level.

In certain types of amplifiers,such as those used with transmitters of frequency modulated waves, or phonograph recording apparatus and the like, it is desirable to provide means responsive to peaks of the signal waves of either polarity to control the amplification of the amplifier. In

the case of frequency modulated waves the avail-- able band width of the receiver is definitely limited and the frequency modulated waves transmittecl must not exceed this band width in their frequency changes. In phonographic recording invention; and Fig. 215 a circuit diagram illus- .trating a modification of the circuit of Fig. 1. Referring to Fig. l, a suitable source of audio frequency waves is represented by aninpi1ttransformer l0, across the secondary of which a voltage dividing resistance l I is connected. The primary of the transformer It may be suitably supplied with audio frequency waves from a micro phone or a phonograph pick-up, or the like. One end of the resistance ii is connected to ground and is also connected through a cathode bias resistor I 2 and a by-pass condenser ii to the cathode of a pentagrid tube II. The movable contact of the resistance H is connected to the 3 first grid of tube It, which serves to amplify voice waves applied through transformer it. The anode of the tube II is connected through a resistor and a resistor IE to a suitable source of operating potential. The second and fourth grids of the-tube II are screen grids and are connected through a resistor H to a point between the available groove width must not be exceeded on either side. Many communication systems have similar limitations. It'is therefor an additional object of my invention to provide improved amplifying apparatus continuously adjusted in amount of amplification in response to the'level of peaks of alternating voltage of either polarity.

resistors 15 and 16 in order to provide the screen grids with theproper biasing potential. These two screen grids are. connected by a condenser Hi to the cathodeoi! tube i4. i9 connects the point between resistors i5 and [5 with the cathode of tube It. The iii'th grid of tube I4 is connected directly to the cathode and acts as a suppressor grid.

Voice waves applied to the first or control grid of the tube H through the transformer it produce corresponding variations in anode potential of the tube i4 due to the variations in voltage drop through resistor l5. These variations in voltage are coupled by a condenser 20 to the control grid of a duplex diode triode'tube 2|. The

cathode of the tube 2| is connected to ground through cathode biasing resistors 22- and 23 in series. A grid resistor is connected from the and 23 to provide proper bias The features of my invention which I believe to be novel 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 obiects and advantages thereof may best be understood by ref,-

nection with the accompanying drawings in which Fig. 1 is a circuit diagram illustrating schematically an audio amplifier embodying my anode of the tube 2lislconnected through a -resistor 26 to a suitable source of operating p05 erence to the following description taken in congrid of tube 2| to a point between resistors 22 potential for'the grid. The resistor 23 has a sliding contactwhich is connected to ground and is adapted to vary the efiective portion ofv the resistor in the cathode circuit. Resistors 22 and 23 areshunted. by a by-passing condenser 25.; The

tential.

The grid of tube 21, which is supplied with voltage variations by condenser 20, causes voltage variations on the anode of'tube 2| due to the voltage drop through the resistor 2C. These anode voltage variations are coupled through a condenser 21 to theprimary'of a transformer Another condenser 28; The'other end or the transformer primary is connected to the-cathode tube 2|. The ends accuses the fact that the secondary or transformer 6f the. secondary of transformer 23 are 'conp nected respectively to the grids-of apair 0! tubes 29 and 30 arranged in push-pull relation. The

secondary of the transformer 23 has a center tap connection which is grounded and isalso pass condenser connected through a resistor 3| shunted by a by- 32 to the "cathodesoi' the tubes 29 and 30 The anodes of the'tubes 29 and are-connected respectivelyto the ends;of the primary of atransformer 32. A center tap con? nection of the primary of transformer 33 isconnected to a suitable source ofanjode potential.' The secondary of the transformer 33 is adapted may be poled in either direction.

It should also be. noted that, when values of voltage appear on the diode anode. of tube 2|,

suflicient to be rec fi d, that anode is prevented from risin to positive voltages corresponding to oped across resistor such values although it does follow the voltage wave as it swings in the negative direction. An effective negative biasing voltage is thus devel- 35 and'isapplied through the resistor 36 to the third grid of the tube N. This negative biasing. voltage reduces the gain of tube H rapidly so that the output from transformer as rises only veryslightly even with a to supply amplified audio. waves to any suitable device. This device may, for example, be the modulationstage in a radio transmitter, withwhich my invention is particularly useful.

"A typicalplate modulated transmitter, as illustrated, may include a source .60 of operating potential for-a carrierwave power amplifier device great increase in the input to the transformer L0. The critical value at whichthis. reduction in gain begins to occur may be adjusted by means of the sliding contact on the variable resistor 23. By movement sistance between the cathode of tube 2| and the ground is variedwith the result that the positive voltage of the cathode of tube 2| with re- G l. Thenegative terminalof the source is grounded and'the positive 'terminalis connected through-the secondary of transformer 33 and through a tuned circuit 82 to the anode of the device il, whosecathode is grounded. A earrier wave source 63 impresses'acarrler wave upon the device Six-to be amplified therethrough byan amount governed by the operating potential supplied through transformer 33'. The amplified carrier wave in the tuned circuit 52 'is' radiated from an antenna 64.

The anode of tube 29 is coupled through a condenser 34 to a conductive impedance 35,

which is'shown or convenjeneeas a. pure resistanc'e' This resistance, is grounded at its other end. Volta e variations of this anode therefore spect to ground may be adjusted; For any particular adjustment of the positive voltage of this cathode thereduction in gain of the tube [4 begins to occur when the level of the peaks of voltage on the diode anode is equal to the positive voltage of the cethode of tube 2|.

The proper adjustment of my amplifier for use me radio transmitter is such that the output of transformer-33 rises linearly in accordance with the-input to transformer up to a critical e I 33 is sumcient per cent., Any other. suitable .depth may be appear. across'the resistance- 35 A diode anode of the'tube MT is. connected to a point between condenser and resistance 35.' v

The cathode of tube 2| remains at a substantially constant positive potential with respect to* ground. due to resistor 22 and the variable resistor 23. The diode anode o f'the tube 2| there-- tore allows current topass only when its voltage through a. conductive impedance 33, which .isv "shown as a pure resistance, to the thirdgrid of rises to positive values greater than the substantially constant voltage existing-across res istor' 22- and variable resistor 23.

Whenever such high positive values'of voltage Q are rectified through the'diode anode, a continuousvoltage is developed across resistor 35.

The voltage across this resistor 35 is supplied the 'tube 14. This third grid s connected through a condenser 31 to ground' This con- *-denser is'ot sufficiently large value that voltage variations which occur at relatively high, speed 'do not affect the third gridoi the tube II. The

mentary transients.

value at which the output from the transformer V to modulate the carrier wave with a' predetermined depth, such, for exam le, as 90 chosen as desired. ,The' operation of my ampliher then prevents any great rise in output from transformer 33 above thisvalue and accordingly It has been found that some music and most speech, when translated into electric wave en-. F 50 has greater peak amplitudes of one polarity than of the other. Sincethe phenomenon called over-modulation withthe accompany g radiaresistor 36 in conjunction with .the condenser- 31, in fact, .forms a filter; which has a'time contlon or spurious signais is due principally to fiat-' tening of the carrier envelope on the zero axis rather than by extension of the maximum carvrier amplitude beyond'the level at twice the amplitude of the carrier when unmodulated, it is desirable that such higher peaks-of a signal wave produce the positive modulation peaks or maximum carrier amplitudes and that. the smaller peaks of opposite polarity produce the negative modulation peaks of the carrier. The provision of such operation allows more energy to be radistant. of the order 'oI-a fifth of a second. This time constant must be such as to prevent the automatic control embodied inmy invention from 'taking effect so quickly as to smooth out variations of the speech wave. However, the filter ated for a given signal wave, assuming maximum permissible modulation, with a corresponding increase in emciency. It is desirable, therefore, to

- secure such'operation, to pole the-output from circuit-must respond witli suflicient speed to bias the third grid of tube on the occurrence of peaks in the voice wave.

It should be noted that positive peaks of voltage on the anode of tube 29 may correspond transformer v33 toinsure that the peaks of higher polarity produce positive modulation peaks. It

is, of couIse first desirable to polethe input to I transformer ",jrom the microphone or the like,

- so thatv therednction of amplification either to" maximum or minimum amplitudes'of thecarrier, or 'in other. words, to either positiveor negative modulation peaks. This is due to initiated in t0 e l w r. peaks ofthe s gnalwavesw Since there is an of this contact the amount of reunavoidable from one side of. thepushrpnllcircuit including transformer It as in Fig. l.

'sistor 24 is connected directly to ground.

tubes 29 and 30 to the third grid of tube l4 through condenser 34 and resistor 38, there may be some ,feedback which shouldbe minimized by condenser 31. This feedback may be further reduced by including additional low pass filters in cascade with the one formed by resistor 33 and the capacitor 31. In the design of this particular circuit, one should connect the, condenser 34 to that anode of the push-pull stage which creates degeneration, in case there is any remaining feedback. This is desirable to obviate any possibility of oscillation.

A very convenient way of minimizing feedback is by the use of a bridge circuit in which audio voltages of o'pposite'phase are fed from the respective tubes 29 and 30 to the third grid of tube I4. In Fig. 1 a condenser 41, a-resistance 48,

and a switch 49 are connected in series between" the anode of tube 39 and the third grid of tube l4. The condenser 41 should be substantially identical with condenser 34 and resistance 48 with resistance 36. With the switch 49 open the operation of the circuit is as has been described. When the switch 49 is closed an audio voltage is supplied to the third grid of tube l4 in such condenser 34 and resistance 36 from the anode of tube 29 is substantially canceled.

A switch 50 and a resistance 9| are connected in series between ground and a point between condenser 41 and. resistance 43. The resistance 5| should be substantially identical with resist: ance 35. complete cancellation of the audio voltage feedback to the third grid of tube I4 is attained, since 1 the bridge is more nearly perfectly balanced.

In certain cases, for example, in frequency modulation or in phonographic recording where it is desirable tolimit amplification in response to signal peaks of either polarity, it maybe convenient to bias the third grid of tube 14 with a bias voltage derived from signal peaks of either polarity which are greater than a ilxed voltage. In such cases a switch 52 connecting the other diode anode of the duplex tube 2| to-a point between the switch 50 and theresistance 5| may be closed. Sucha connection, of course, retains the advantage of' the bridge circuit described. This other diode anode develops a biasing voltage on the third grid of tube I4 in the same way as the first diode anode with its coupling condenser 34 and resistances and 36. with separate sources of bias voltage may be substituted for the diodes in tube 2|. This ar- .rangement maybe better understood by considering the "principles of operation of the circuit illustrated .by Fig. 2.

Referring to Fig. 2, certain parts of the 'circuit illustrated thereby are identical with those shown in Fig. 1, and like parts have been givenphase that the audio voltage fed back through i voltage variations by condenser 20, causes similar voltage variations on the anode of tube 38 due to the voltage drop through resistor 26 which is conected between the anode of device 33 and a suitable source of operating potential. The voltage variations across resistor 26 are coupled as in Fig. 1 to a pair of tubes 29 and 30 arranged in push-pull relation. The anodes of these tubes are connected to the output transformer 33.

A pair of'voltage dividing resistances as and 40 are connected across a suitable source of direct current. The sliding contact of the resistance 39 is connected through a conductive impedance 41, which is shown for convenience as a pure resistance, to the cathode of. a diode element of a tube 43. Similarly the sliding contact of the resistance 43 is connected through a conductive impedance 42, also shown as a resistor; to the cathode of a second diode element of the tube 43. The cathode of the first diode is connected through a condenser 44 to the anode of tube 30. The cathode of' the other diode is connected through a similar coupling condenser 45 to the anode of tube 29.

The anodes of these two diodes are connected together and to ground through a resistor 46.

, These anodes are also connected through resistor When the switch is closed a more 36 to the third grid of tube H, which is coupled by the condenser 31 toground. 1

In operation, under the condition when no signal is being amplified, the cathodes oithe tube 43 are maintained at positive. potentials with respect to ground by amountsequal to the voltage from-the sliding contact of the respective resistances 39 and--40 to ground. The'anodes of tube 4.3 are maintained at ground potential by resistor 46. a

.'When signals are amplified by the arrangement, the voltage variations of the anodes of tubes 29 and.30 are impressed on the. cathodes of tube.43. These voltage variations due to signals'make the cathodes 0! tube 43 less positive during certain portions .of the signal wave.

When these cathodes become negative due to a signal of greater level than that. necessary to overcome the respective positive bias voltages produced by resistances 39 and 40, the tube 43 Separate diodes 1 passes current and the anodes ,of tube-43 become negative with respect to ground. This produces a similar negative voltage on the third grid of tube 14 and reduces the amplification thereof in a manner like that of the circuit of Fig. 1.

It shouldbe noted particularly that tube 29 amplifies signal voltage swings of one polarity and tube 30 those of the other.- Hence, if the signal peaks of either polarity increase more than predetermined amounts, the tube 43 passes current in the corresponding diode element and the amplification of tube I4 is reduced. It should also be noted that the predetermined levels at which the two diode elements pass current may I be separately adiusted by the sliding contacts of shunted by by-passing condenser 25 connects the a cathode of tube 33 to ground. It should be noted that resistor 23 is omitted, and that the grid rethe'two' resistances 39, and 40. They are preferably adjusted in proportion to the magnitude of alternating voltage on-thg" respective anodes of tubes 29 and 30 for the particular type Of signal being amplified.

It may be desirable only to limit the signal peaks of either polarity to the sam value. In

such case, the resistance 40 may be omittedand the resistor 42 connectedtothe sliding contact of resistor 39. It is, of course,.obvioi i that one diode element may be omitted withiits corresponding resistor, condenser and'voltage "divid The grid of tube as, which issupplied with

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