US2323211A - Method and means for automatic gain control - Google Patents

Description

` June 29, 1943. F.J.FALT1O 2,323,211

METHOD AND MEANS FOR AUTOMATIC GAIN CONTROL Filed May 23, 1941 Sheets-Sheet 1 l -ikf www f I l /0 1 /4 V2 /2 [/3 v n June 29, 1943. F, J. FAL-rico 2,323,211. A METHOD AND MEANS FOR AUTOMATIC GAIN CONTROL Filed Mayas, 1941 2 sheets-sheet 2 Femm J. FALTNO f Tn/Patented June 29, 1943 UNITED STATES PATENT OFFICE METHOD AND MEANS FOR AUTOMATIC GAIN CONTROL ll Claims.

This invention relates to improvements in automatic gain controls for vacuum tube amplifier systems.

The use of automatic means for controlling the gain or volume of an amplifier is well known in the art but all systems with which I am familiar have had certain disadvantages chief of which has been the considerable time lag in the action involved in the use of reactive networks of various kinds.

It is, of course, extremely desirable to make the control not only automatic but practically instantaneous in order to reduce the gain the instant the signal builds up beyond a predetermined level and thus prevent overloading of the output circuit, distortion, and other bad effects. Particularly is this action desirable in amplifiers for hearing aid work in which the greatest possible gain must, be present but in which a loud signal, if not instantly suppressed, would cause extreme distortion and possibly reach the threshold of pain for the user.

It is the primary object of my invention, therefore, to provide a method of. and means for, automatically and, to all useful purposes instantaneously, controlling the gain or volume in an amplifier circuit. Another object is to provide a gain control of this kind which utilizes certain principles of the operation of the vacuum tubes of the amplifier and associated circuits to provide the desired control without affecting the gain on weak s'gnals and without adding parts to the circuit. A further object is to provide an automatic gain control of extreme simplicity and which may be initially set up to limit the gain to any desired degree.

These and other more detailed and specific bjects will be disclosed in the course of the following specication, reference being had to the accompanying drawings, in which- Fig. l is a wiring diagram of a three tube amplifier circuit embodying my invention.

Ffg. 2 is a graphic showing of the progress of a wave through the three tube amplifier of Fig. 1.

Fig. 3 is a diagrammatic showing of the output curve of an amplifier connected in accordance with my invention, several possible output levels being indicated.

Figs. 4 through 7 are wiring diagrams of amplifiers embodying certain variations or modifications of the gain control means of Fig. 1.

My invention as disclosed herein is particularly adapted for use in audio frequency amplifiers employing screen grid pentode or tetrode tubes,

although it may, of course, be used in connection with amplifiers handling other frequencies, wherever feasible.

In such amplifiers, unless some form of gain or volume control is used, there is always the probability of overloading and distortion since the signals handled may frequently exceed the capabilities of the tubes and non-linear amplication will then result. Particularly is this true where the circuits and tubes are initially selected and set up to have high amplification for weak signal response. In hearing aid Work this distortion and overloading is especially noticeable due to the wide range of signal intensities handled and limitations as to operating voltages imposed by the necessary compactness and light weight of such units. The distortion is usually met with in the output stage but frequently occurs also in the preceding or driving stage, and may contain both even and odd harmonics. The even harmonic distortion may be largely eliminated by the use of push-pull output stages or other means, as frequently found in power or radio amplifiers, while the odd harmonic distortion, usually of the third order, may be tolerated to no great disadvantage since it will not be evident to the usual ear.

From the foregoing it may be assumed therefore that in practically all ampliers there is some distortion caused by overloading and I propose to employ this factor to serve as a control for the circuit to automatically reduce the gain and then cause the elimination of such distortion, at least to a large extent, in the output signal, I therefore deliberately introduce distortion in one or more stages of the amplifier and later largely cancel out the distortion after it has served its useful purpose, my system being thus what might be termed a distortion-dedistortion type of gain control system.

To introduce such distortion I propose to operate one or more of the vacuum tubes at a point on its static or gain characteristic curve at which distortion and suppression of the signal will occur above a certain input signal, this being accomplished by setting up the operating potentials and bias on the tube or tubes to the point where both plate and screen grid currents are affected by an increase in signal levelabove a predetermined maximum. In other words, the operating point is such that, on increasing the control grid voltage (large signal) the screen grid voltage does not rise in normal ratio to the control grid voltage, or does not rise above a predetermined value.

The effect of distortion thus entered by a large signal is to shift the direct current component of the signal in the output of the tube and thus cause a relatively great increase in the plate current drawn by the tube or tubes above the normal or idling value. I then cause this current to flow through a resistance common to both the plate and screen grid of one or more tubes and the iR drop thus secured is employed to autoa matically reduce or limit the screen grid voltage. l It ls well known in the art that the behavior of a screen grid amplifier tube is such that large plate load resistances may be used since the space charge in the tube, and hence the gain, will be held up by the screen grid element. Thus in the usual amplifier with high values of load resistance and with plate current variations having little, if any, effect on space charge conditions, maximum gain is secured. The importance of the operating potential on the screen grid element is thus evident since, as the instantaneous plate voltage drops on the positive half of each control grid voltage swing, the ratio of the screen grid voltage relative to plate voltage must increase in order to hold up space charge conditions and any deviation, particularly toward a lower value of screen grid voltage, will radically reduce the space charge and gain.

Briefly therefore my invention resides in the introduction of distortion in an amplifier to secure a shift in the direct current component of the signal, the use of the plate current variation thus secured to cause a voltage drop to the screen grid element or elements in the amplifier tubes, and the later cancellation of the undesirable values of the distortion to provide an output signal substantially a replica of the input signal.

Referring now more particularly to the drawings, I show in Fig. 1 thereof a circuit by which the aforesaid operations may be carried out. This circuit employs, in at least the preampliiying portions, three screen grid pentodc or tetrode vacuum tubes V1, V2 and V3 which in addition to the normal control grids, filaments, suppressor grids and anodes or plates, the screen grids designed at I0, il and I2 in the respective tubes, these elements service as aforesaid to maintain space charge condition during normal operation. The signal input, from a microphone, or other detection means, is indicated at I to the control grid circuit of tube Vl while the output from tube V3 is indicated at O. This output may be connected to a translator of any kind but to greater advantage, as will later appear, should be used to drive a further amplifying stage, or stages, of normal characteristics.

The amplifier circuit is of the resistance coupled type and has the usual control grid bias resistors I3 for each tube, interstage coupling condensers I4 and plate load resistances I5, I6 and I'I for the respective tubes V1, V2 and V3. These plate load devices, of high resistance, are connected to the source of plate potential, or B positive, at I8, and interstage decoupling may be provided by resistors I9 and by-pass condensers 20. The last tube V3 has its screen grid element I2 connected directly to B positive I8 through dropping resistor 2l and is by-passed by condenser 22, the voltage for this element being thus conventionally secured.

In accordance with my invention however the screen grid element I of the first tube V1 is connected through dropping resistor 23 to the plate of tube V2 so that the load resistor I6 for this tube V2 is common to its plate and t0 the out of phase with each other.

screen grid element ID. In similar manner the screen grid element II of tube V2 is connected to the plate of tube V3 through resistor 24, thus making the load resistor Ii common to the plate of V3 and screen grid of V2. Both screen grid elements I0 and II are shown as by-passed to ground by condensers 25, the use and value of which will be later discussed.

Operating potentials are thus correctly supplied to all tubes V1, V2 and V3 and to a certain extent, or upon signals below a predetermined maximum intensity, the amplifier thus connected will operate in usual fashion.

Referring to Fig. 2 the exact operation will now be described. In this illustration there is shown at A1, A2 and A3 the static or gain characteristic curves of the tubes V1, V2 and V3, respectively, as plotted against plate current and control grid voltage. These curves have the substantially linear midportions and each bend at one end B toward an upper saturation point and at the other end or foot C toward a lower cut-off point.

The rst amplifier tube Vl is initially chosen and operating potentials set so that it operates near the center of the linear portion of the curve A1 at an operating point PI as indicated. An alternating current input signal or wave W is also shown and is plotted as of such amplitude as to pass through tube V1 without distortion, receiving linear amplification thereby. When this wave W is superposed upon the curve A1 about an axis at the operating point P1 then the resulting output wave W0, due to the slope of the curve A1, will be seen to be linearly amplified by the gain of the tube. Operation thus far is normal.

The second tube V2 is however set, by preselection of operating conditions, to operate at an operating point P2 between the center of the linear portion of curve A2 and the lower bend C thereof. Now as the signal W0 is impressed upon this curve A2 about an axis coinciding with operating point P2 the output wave from tube V2, shown at W2. is seen to be suppressed upon one side X due to the overloading of the tube under such conditions. There results then a condition of distortion and a shift, as designated at S2, in the direct current component of the signal, and the magnitude of this shift is determined not only by the amplitude of the signal but upon the position initially chosen for operating point P2.

The third tube V3 is similarly preset to operate at a point P3 toward the lower bend C of its curve A3 and as the wave W2 is then transferred to curve A3 about the axis the resulting output Wave W3 is seen to be similarly nonlinearly amplified but suppressed upon the side Y opposite to that of wave W2. This is, of course, due to the fact that the tubes V2 and V3 operate 180 degrees There again occurs a shift S3 in the direct current component of the signal in the plate circuit of tube V3.

Referring now again to Fig. 1 the result of the direct current component shift S2, through the plate resistor I6 to tube V2, is to increase the current flowing through such resistor and accordingly cause an instantaneous voltage drop therethrough by the iR. formula. Due to my novel circuit, however, this voltage drop is caused to affect the operating voltage on the screen grid element I0 of tube V1 instantly reducing the voltage thereon and hence lowering space charge conditions and gain in the first amplifier. In similar manner the current swing (upward) through resistor I1 in the plate circuit of tube V3, caused by the direct current component shift S3, is caused to drop the voltage applied to the screen grid element II of preceding tube V2 reducing space charge and gain therein. There thus results a control of the gain through both tubes V1 and V2 directly responsive to the signal amplitude in tubes V2 and V3. Due to the out of phase operation of what might be termed the gain controlling tubes V2 and V3 the gain suppression or reduction takes place on both positive and negative halves of each cycle as should be clearly evident.

The effect of the screen grid by-pass condensers 25 is to by-pass second harmonics or alternating current components of the signal and the value of these condensers is somewhat critical since they have a tendency to hold up the screen grid voltage, causing some time lag in the automatic gain control operation. By proper choice of the capacity used this time lag may however, be reduced to a negligible value for audio frequency work.

Preferably the output wave W3 from tube V3 is fed to a further amplifier stage as represented diagrammatically in Fig. 2 by the gain characteristic curve A1. The operating point PA1 of this stage is placed on a linear portion of the curve so that the output wave W4 finally delivered is again substantially symmetrical and a relatively true replica of the input wave W. Further amplification may be used and, since each successive stage is out of phase with respect to the next, the additional linear amplification may by proper circuit design, be used to iron out remaining irregularities on both sides of the Waves.

The progress of a signal of lesser intensity through the amplifier is also shown in dotted lines in Fig. 2 and it will be evident that a signal below the predetermined maximum level will be amplified in usual manner and without introduction of distortion thus indicating the desired high weak signal response of the amplifier.

By proper choice of operating potentials the operation of the circuit upon signals above a predetermined level may be selected as desired. Thus the amplifier may be caused to flatten off at the proper level as indicated at D in Fig. 3 or it may be caused to have a rising or falling-off characteristic as represented at E and F. Actual tests prove further that the amplifier may even be caused to cut off sharply above a selected level as indicated at G.

The advantages and importance of such operation particularly appear in hearing aid Work since by levelling or cutting off the response above a predetermined maximum the possibility of a signal reaching an intensity such as to cause pain to the user is absolutely prevented. The circuit however, will find uses wherever rap'd and automatic control of gain or volume may be desirable.

The circuit shown in Fig. 4 is essentially like that of Fig. l employing three connected vacuum tube amplifiers V1, V2 and V3 with input and output I and O, and having screen grid elements I0, and I2 respectively. The circuit has grid resistors I3. interstage coupling condensers I4, plate load resistors I5, IS and I1, screen grid resistor 2| and by-pass condenser 22 for the last tube V3, and common B positive supply I8, all as previously described. In this case, however, the screen grid elements I0 and I I of tubes V1 and V3 are connected together and through a single dropping resistor 2`| are connected to the plate of the tube V3, being therefore common to the plate load resistor I'|, and have a single by-pass condenser 26. I

In this circuit the operation is similar to that heretofore described and differs only in that the distortion entered in tube V3, and the resulting shift in the direct current signal component, is used to control the voltage and space charge conditions of both screen grid elements I0 and in the tubes V1 and V2. Effective control of the gain of these tubes is therefore obtained and both tubes may be, like the first tube V1 in Fig. 1, set up to normally give linear amplification.

The circuit of Fig, 5 employs the same basic principles of operation but the distortion entering tube is an "auxiliary tube having no output connection such as to enter the distortion into the amplified signal. Here the first two tubes V1 and V2 are arranged to give linear amplification, from the input I, to the output O connected through condenser 28 to the plate of V2. Grid biasing resistors I3 and plate load resistors I5 and I6 are provided for V1 and V2 to provide amplification in usual manner. A portion of the output of the second tube V2 is also taken through condenser 29 to the control grid of the auxiliary amplifier tube V4, which may be a triode and has the grid resistor 30 and a plate load resistor 3| connected to the B positive supply I8.

The screen grid elements I Il and I| of tubes V1 and V2 are here again connected together and derive their operating voltage through a dropping resistor 32 connected to the plate of V4 thus making load resistor 3| common to both screen grids. A by-pass condenser 33 for the screen grids is again used.

In operation the tubes V1 and V2 will amplify and deliver signals to the output O, but will also deliver a part of such signals to the tube V1 which is so biased and set on its characteristic curve that, above a predetermined level, these signals will cause distortion. The resulting shift in the direct current potential will cause a voltage drop in resistor 3| to reduce and control gain in tubes V1 and V2, the screen grid elements of which are common to said resistor exactly as heretofore specified. In this case however the distortion entered by the auxiliary or control tube V4, being mainly of the second harmonic variety, may be by-passed to ground by the condenser 33 and thus does not enter the signal path through the amplifying tubes V1 and V2 and no further amplification, for distortion cancellation at least, is needed.

In Fig, 6 I show a further variation of the circuit of Fig, 5 in which the tubes V1 and V2 are connected exactly as described and have the same circuit elements designated by corresponding reference numerals. A third tube V5, also of the screen grid variety, is used but connections are so made that this tube serves two distinct functions. Thus the output of tube V2 is fed through coupling condenser 34 to the control .grid of tube V5 with the usual bias resistor 35 but the screen grid element 3S of this tube is connected through load resistors 31-38 to the B positive supply I8 with a decoupling or by-pass condenser 39 at the junction of the resistors. The screen grid thus acts as a plate and this portion of the tube operates as a. high-mu triode, the output O being taken from the screen grid through coupling condenser 4D. Nevertheless the electron flow toward the plate of the tube maintains and this plate is connected through a load resistor 4I to B supply I8. The screen grids I Il and of V1 and V2 are then connected together, with a by-pass condenser 42, and are connected to the plate of V5 through the dropping resistor 43.

In action a portion of the signal appearing at the erstwhile screen grid 36, now acting as a plate of the tube V5, is transferred by the electron flow to the true plate of the tube, due to the electron coupling, and if sufficient to overload this portion of the tube, will cause distortion and plate current surges in the resistor 4l. The voltage drop in this resistor will then reduce the voltage on the screen grids of tubes V1 and V2 to control the gain thereof according to the signal level. Here again the distortion is by-passed by condenser 42 to prevent reaction on the signal itself and in this circuit the single tube V5 is caused to operate not only as a signal amplifier but as a distortion entering or control tube.

All of the foregoing circuits have the desirable characteristics of controlling the gain of the first tube or tubes in response to signal level in later amplifiers and therefore provide control in earlier stages where most needed and in response to the relatively great amplitudes the signals have after preliminary amplification.

However, as shown in Fig. 7, each tube may be arranged to introduce distortion above a predetermined signal level to control its own gain. In such case the tube, indicated at V6 has the usual input and output connections I and O and a plate load resistor 44 of high value, the tube being arranged, as heretofore described, to work on a portion of its characteristic curve at which distortion will take place above the desired maximum level. The screen grid 45 is then connected directly to the plate through dropping resistor 46 and has the by-pass condenser 41. The resistor 44 being thus common to both plate and screen grid will eiiect the screen grid voltage in response to signal induced current variations in the plate circuit and thus will control the gain in the same manner previously described. The action must properly be used in two or more succeeding stages of amplification to remove distortion from the output signal and provide proper control as will be evident.

This circuit is similar in essential respects and in gain controlling action to those shown in my copending application Serial No. 333,998, for Automatic gain compensating means, filed May 8th, 1940, now Patent No. 2,282,649, issued May 12, 1942, to which attention is invited for comparative purposes.

Where signals of minute amplitudes are handled in a circuit such as shown in Fig. 7 the screen grid by-pass may be eliminated, the tube then acting as a pentode to signals of small amplitude and as a triode to signals of greater amplitude. Where the screen grid by-pass is eliminated in circuits such as Fig. 1, where control is had from a succeeding tube, then a regenerative-degenerative network must be used as shown in my application Serial No. 333,998.

It is understood that suitable modifications may be made in the structure as disclosed, provided such modifications come within the spirit and scope of the appended claims. Having now therefore fully illustrated and described my invention, what I claim to be new and desire to protect by Letters Patent is:

1. Gain control means for a vacuum tube signal amplifier, said amplifier having at least two tubes and one of said tubes being of the screen grid type, the other tube being connected and arranged to operate in such manner that it will Cal enter distortion on signals above a predetermined level and thereby will cause a large increase in its anode current, said anode current being passed through a load resistor of high value, and said load resistor being common with the screen grid of the said screen grid tube to supply operating potential thereto and thereby reduce said potential as such increase in anode current occurs, and means for by-passing components of such distortion from the screen grid to the common ground of the amplifier.

2. Gain control means for a vacuum tube ampliiier system having at least two screen grid tubes preceding a later amplifier tube, comprising load resistors arranged to supply operating potentials to the anodes of the tubes, the load resistor of the second screen grid tube being connected to supply an operating potential to the screen grid of the preceding tube and the load resistor of the said later amplifier tube being connected to supply an operating potential to the screen grid of the second tube, and said second tube and later amplifier tubes being arranged to operate in such manner that they will enter distortion or signals above a predetermined level and cause an increase in anode current fiowing through their load resistors to thereby lower the operating potential on the said screen grids.

3. In a vacuum tube amplifier system, at least two screen grid amplifier tubes having signal input and output circuits, an auxiliary tube connected to receive a part of the output signal and amplify same but to enter distortion when the signal level exceeds a preselected maximum, and means for supplying operating potential to the screen grids of the amplifier tubes under control of the auxiliary tube in such manner that this potential will be reduced as the auxiliary tube reaches a condition of distortion.

4. A multi-stage amplifier system having at least two screen grid tubes, the first of said tubes having signal input means and output to the control grid of the second tube, the screen grid of the second tube being supplied with operating potential to act as an anode and having an output signal connection, the anode of the second tube being adapted by the electron flow in the tube to receive a signal voltage and being operated in such manner that a condition of distortion and anode current increase will occur when the signal level exceeds a preselected value. and means for reducing the operating potential on the screen grid of the first tube when such distortion occurs at the anode of the second tube.

5. The method of controlling gain in a signal amplifier employing screen grid tubes in at least a part of its stages, which includes operating at least one tube under conditions such that distortion will occur on signals above a predetermined value to thus secure a shift in the steady direct current component of the signal, and to control space charge conditions by the screen grids of the screen grid tubes in direct ratio to the distortion and shift in said direct current component.

6. The method of controlling gain in a signal amplifier employing screen grid tubes in at least a part of its stages, which includes operating at least one tube under conditions such that distortion will occur on signals above a predetermined valve to thus secure a shift in the direct current component of the signal, and to control space charge conditions by the screen grids of the screen grid tubes in ratio to the distortion and sh'ift in said direct current component, and finally cancelling out such distortion.

7. The method of controlling gain in a signal amplifier having at least one screen grid amplifier tube with input and output connections and another tube connected to receive signal voltages from said amplier tube, which includes operating said another tube under such conditions that it will enter distortion and cause a shift in the direct current component of the signal when the signal amplitude reaches a predetermined level, controlling the operating potential upon the screen grid of the amplifier tube in accordance with the shift in direct current potential to decrease the gain as the signal reaches said level, and by-passing the larger portion of such distortion away from said screen grid.

8. In a vacuum tube amplifier system including a screen grid tube, said tube being adjusted to operate under such conditions as to enter distortion onsignals above a'preselected amplitude to thereby cause an increase in its anode current, a resistor arranged to carry said anode current, and the screen grid of the tube being connected to receive its operating potential through said resistor whereby said increase in anode current will automatically reduce the potential on the screen grid when the signal exceeds said preselected amplitude.

9. In a vacuum tube signal amplifier including a screen grid tube and another tube with a resistor of high value arranged as its plate load, said another tube being adjusted to operate under conditions such that it will cause distortion upon signals exceeding a predetermined amplitude and thereby cause an increase in its plate current drawn through said load resistor, and said load resistor being also connected to supply a variable operating potential to the screen grid of said screen grid tube in proportion to the variations in current drawn through said load resistor.

10. Gain control means for a vacuum tube ampliiier system having at least two screen grid tubes preceding a later amplifier tube, comprising load resistors arranged to supply operating potentials to the anodes of the tubes, the load resistor of the said later amplifier tube being connected to supply an operating potential to the screen grids of the two screen grid tubes, and said later amplifier tube being adjusted to operate under such conditions that it will enter distortion when the signal level reaches a predetermined value and cause an increase in its anode current through said load resistor to thereby reduce the operating potential on the screen grids.

11. In a vacuum tube amplifier system including a tube of the screen grid type, said tube being adjusted to operate under conditions such that it will enter distortion upon signals of a predetermined amplitude and thereby cause a shift in the direct current component of the signal in its anode circuit, a resistor arranged to carry said current, and the screen grid of the tube being connected to receive a variable operating potential from said resistor variable under control of anode current variations therein.

FRANK J. FALTICO.

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