US2424847A - Amplifier circuit - Google Patents

Description

J. G. PRENTISS AMPLIFIER CIRCUIT.

Filed Oct. 4, 1943 mvzmoa JOHN G. PRENTISS BY M ms; ATTORNEY Patented July 29, T947 AMPLIFIER CIRCUIT John G. Prentiss, Berwyn, 111., assignor to Zenith Radio Corporation, a corporation of Illinois Application October 4, 1943, Serial No. 504,958

2 Claims.

This invention relates to a high gain amplifier, and particularly to such an amplifier as may be found useful in small radio or hearing aid apparatus. It is desirable to make radio and hearing aid apparatus of small size and with the least number of parts for purposes of convenience in carrying and economy in manufacture. In such apparatus, input signals must usually be amplified by a relatively fixed amount for accomplishing the purposes of the apparatus. Also, it is usually desirable to make the amplification of the apparatus relatively independent of the magnitude of input signals. The desirable qualities of smallness are better realized when the amplification of each stage is high and when the number of stages is a minimum.

It is therefore an object of this invention to provide an improved small high gain amplifier stage and particularly one which produces linear amplification.

In the use of hearing aid apparatus, the user must bear a relatively great expense for replenishing the power supply. As a general rule, the more electron discharge devices used, in the hearing aid circuit the greater is the drain on the power supply. Also, it is desirable to incorporate a tone control in the hearing aid apparatus and this is usually accomplished by sacrificing some of the overall amplification or gain realized in the hearing aid circuit; in general, a sacrifice of overall gain or amplification is reflected as an increased drain on the power supply.

Therefore, another object of this invention is to provide an improved amplifier circuit having a tone control therein and requiring a minimum number of electron discharge devices.

Another object of this invention is to provide an improved high gain circuit requiring a small amount of space current.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, both as to its organization and manner 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 Figure 1 shows an improved hearing aid circuit incorporating a high gain amplifier circuit which embodies my invention; and

Figs. 2, 3 and 4 show a portion of the apparatus of Fig. 1 in various operating positions.

In Fig. 1 there is illustrated an improved hearing aid circuit, incorporating a high gain amplifier including electron discharge device It] and a second amplifier including electron discharge device M. The two amplifiers successively amplify signals from microphone II and impress such amplified signals on sound reproducing device E2 in linearly amplified form. A switch l3 for effecting tone control is interconnected with the device [0.

Sound waves impinging on microphone H are transformed into electrical variations in the main control electrode circuit of discharge device Ill. The microphone ll may, for example, be of the piezo-electric type as illustrated, or, because my amplifier produces such great gain, it, may be of the magnetic type. The device Ill greatly amplifies electrical variations produced by microphone H and the amplified electrical variations are further amplified by electron discharge device M before being applied to the sound reproducing device l2. One of the important features of this invention resides in the fact that sound waves impinging on microphone I l are reproduced by sound reproducing device H2 in greatly amplified form and the degree of amplification is substantially independent of the magnitude of sound waves impinging on microphone I l or signal voltage applied to the main control grid H of device l0.

Microphone I I, which produces electrical variations in response to sound Waves impinging thereon, is connected between the main control grid I5 and the grounded filamentary cathode of discharge device it); and a grid leak resistance 16 is connected in parallel circuit relationship with microphone l l to bypass continuous current flowing around microphone l I between control grid l5 and the cathode of discharge device Iii. Substantially no grid current flows through resistance l6, because the grid I5 is at a negative potential with respect to the cathode of device Ill, such negative bias potential being provided by connecting grid l5 through resistance 16 to the grounded negative terminal of source 4i] across which are connected the opposite terminals of the filamentary cathode of device It]. It is to be understood, of course, that other conventional means may be used to establish a suitable continuous operating potential between grid I5 and its cathode and such means may be a battery or a voltage drop produced by space current flowing through discharge device l0. Resistance l'l, much smaller than resistance 1 6, is also connected in parallel to microphone ll through switch 13 for a different purpose than resistance It as is described hereinafter.

Electron discharge device It) is of the pentagrid 3 type in commercial use and may, for example, is of the type commonly known as the 1R5. The particular elements of this device ill, however, are connected in a linear high gain amplifier circuit in a novel manner not heretofore known This invention is not limited to the use of a discharge device of the type commonly known as the IRS, but within the scope of my invention any discharge device which performs equivalent functions is suitable. Large gain is realized when device H3 is connected in the manner hereinafter described and such large gain is substantially independent of signals applied between the main control grid l5 and cathode of discharge device 10.

In general, device I is connected so as to be effectively two amplifiers in cascade with regeneration between the two amplifiers. In addition to the main control grid l5, discharge device H) has what is termed a second control grid IS, a suppressor grid [9 connected to the oathode,'a main anode 2ll, and a pair of screen electrodes 2| and 22 on opposite sides of the second control grid l8.

Operatin continuous potentials for device la; are supplied from a voltage source 23 whose negative terminal is grounded and whose positive terminal is connected to the main anode 23 of discharge device [0 through a series circuit including adjustable voltage dropping resistance 24 and output coupling resistance 25. Electrodes 2! and 22 are connected together and are maintained positive with respect to the cathode of device Ill by connection to the positive terminal of voltage source 23 through the series circuit including voltage dropping resistance 24 and coupling resistance 26. The continuous operating potential of the second control grid H3 is stabilized by connecting it to ground and the cathode of discharge device If! through resistance 21, which also serves. as a coupling resistance for audio frequency current as described hereinafter;

.When alternating current signals are impressed between the main control grid l and cathode of discharge device l0, substantially all of the alternating output voltage appears across the out-putcoupling resistance 25, a bypass capacitance 28 of low reactance being connected between the grounded cathode of discharge device Island the lower terminal of resistance 25 removed from the main anode 2d. Potential variations on electrodes 2| and 22 due to an alternating voltage applied between main control grid l5 and'icathode Of discharge device Ii! are impressed on the second control grid l8 through a coupling capacitance 29 connected between electrodes 2| and 22 and grid l8.

Therefore, alternating voltages applied directly to control grid I5 and indirectly to control grid 18 cause alternating output signals to appear across resistance 25, which output signals are then applied-to the grid circuit of another linearly amplifying discharge device l4. Coupling capacitance 3| and input resistance 32 are connected in series. and the series circuit formed thereby is connected in parallel circuit relationship to the series circuit formed by output coupling resistance 25 and low reactance bypass capacitance 28. Capacitance 3! is of relatively low reactance and serves essentially, as .a means for blocking-the flow of continuous current from source 23 toresistance 32. The alternating voltage developed across resistance 32; through condenser M is applied between the first or control grid and cathode of discharge device 14 so as to control the space current therein, which current normally flows due to the fact that voltage source 23 is connected between the plate and cathode of discharge device l4 through the primary winding 34 of an output transformer 35.

Discharge device I4 is preferably of the pentode type having its suppressor grid connected to the cathode and with the voltage source 23 connected between its screen grid and cathode through a voltage dropping resistance 36. The screen grid is maintained at constant potential in the presence of signals of frequency corresponding to audio frequencies by means of low reactance bypass capacitance 31, which is connected between the screen grid and grounded cathode of device l4.

Alternating voltages developed across input resistance 32 are amplified linearly by discharge device M, and appear across the secondary winding 39 of transformer 35 which is connected to impress those amplified voltages on the sound reproducing device l2 which is connected across the terminals of secondary winding 39.

The filamentary cathodes of discharge devices H1 and M are preferably heated by current flowing therethrough as shown in Fig. 1. In such case the cathodes of discharge devices l0 and I4 are connected in parallel circuit relationship to voltage source 40.

The circuit thus far described is especially useful as a hearing aid circuit and, because of the high gain obtained by the use of discharge device |0,; only two discharge devices requiring small space current; are necessary for good performance, even though a tone control described in later paragraphs is provided. Because of the high gain obtained from the use of discharge device In, a highly efiicient and useful hearing aid circuit is provided which requires a small current drain from voltage source 23. This is particularly true when, as in this instance, resistances 24, 25- and 26 are relatively large.

The volume of signals reproduced by speaker l2 may be controlled by adjusting volume control resistance 24. An adjustment of resistance Zlcauses 'no substantial nonlinearity between the intensity of input signals applied to grid 15 and the intensity of signals developed across resistance 25 over a large range of input signals. This volume control circuit and features thereof are described and claimed in the co-pending application of Gilbert E. Gustafson, Serial No. 518,071 filed January13, 1944, and assigned tothe-same assignee as the present application. r

The. tone of signals reproduced on sound reproducing device I2 may be controlled by connecting resistance [1 and capacitance 38 in the hearing aid circuit thus far described. In general resistance l!- which may be connected in parallel circuit relationship to piezo-electric microphone It servesto reduce the intensity of the low'frequency signal components, and capacitance 30 which may be connected between the electrode 2| and cathode of discharge device I6 serves to reduce the intensity of high frequency signal components. Different types of tone may be produced corresponding to thefour positions of the tone control member l3 shown in Figs. 1-4. That is,-tone control member I3 is a short circuiting; member of suchshape that in its clockwise movementiti assumes. positions whereby; (1-) as in Fig. lithe: capacitance alone is connected in the hearing aid circuit and high frequencies only are suppressed, and (2) as in Fig. 2 when both resistance l1 and capacitance 30 are connected in the hearing aid circuit both some of the high and some of the low frequencies are suppressed, and (3) as in Fig. 3 when resistance I! alone is connected in the hearing aid circuit only some of tho low frequencies are suppressed, and (4) as in Fig. 4 when neither the resistance I! nor the capacitance 30 is connected in the hearing aid circuit neither the low notes nor the high notes are affected.

As mentioned previously, discharge device l operates as two amplifying stages combined in the envelope of one discharge device with regen eration between the two amplifying stages. The first stage may be considered to comprise a triode section including the cathode of discharge device IO, main control grid l5 and electrode 2|, which electrode operates as the anode of a conventional triode. The second amplifying stage operates as a pentode and includes control electrode l8, screen electrode 22, suppressor electrode I9 and anode 20.

Measurements substantiate the theory that, in effect, there are two amplifying stages in the envelope of discharge device l0. That is, the gain measured between cathode and electrode 2| was found to be of the order of Ell with the device adjusted for good operation, and the gain measured from electrode 2| to anode 2i! was of such magnitude that, when multiplied by the gain measured between the cathode and electrode 2|, an overall gain was determined corresponding to the measured overall amplification of the discharge device ID.

The overall gain of discharge device I6 is preferably adjusted by adjusting the amount of regeneration in discharge device I0. It is desirable to make the amount of regeneration such that linear operation of device I0 is assured consistent with as high gain as possible. For purposes of analysis, the two stages may be considered to be equivalent to two separate discharge devices, a triode and a pentode, connected in cascade, wherein the regeneration may be symbolized by a negative resistance of the dynatron type connected across the anode load of the triode.

The regeneration which comes into being is believed to be due to the negative transconductance from the second control grid l8 to the screen electrodes 2| and 22. This negative transconductance effect is produced in accordance with the following considerations: The total cathode space current of device In is substantially independent of the voltage applied to the second control grid l8, which controls the distribution of current between electrode 2| on one side and electrode 22 and anode 20 on the other side. Since the two electrodes 2| and 22 are connected together, substantially equal and opposite effects are produced on the electrodes 2| and 22 and on anode 20 by a change of the voltage on the second control grid l8. That is, if electrode I8 is made more negative, more space current tends to flow to electrode 2| and less to electrode 22 and anode 2|]. But the decrease in current to electrode 22, which shares its current with anode 20, is less than the increase in current to electrode 2 I. Therefore, the net current to electrodes 2| and 22 increases with negative potential in electrode l8, and current to anode 20 decreases. Consequently, the transconductance between second control grid l8 and anode 20 is positive, and the transconductance between second control grid 8 and connected electrodes 2| and 22 is negative. When, as in this instance, the current flowing to connected electrodes 2| and 22 produces a voltage drop across external resistance 26 in response to variations impressed on the main control grid l5, at least a portion of that voltage drop is fed back to the second control grid l8, through coupling capacitance 29 for further controlling the electron stream in discharge device In regeneratively by voltage on electrode I8. That is, in Fig. 1, it appears as though a negative resistance effect, which causes regeneration for signals of audio frequencies, occurs between connected electrodes 2| and 22, second control grid I8 and the cathode of device l0, considered as a group, since coupling condenser 29 is of low reactance for signals of audio-frequency.

In the particular amplifier circuit shown in Fig. 1, when properly adjusted, the increase in triode gain in discharge device It! due to regeneration is about 70%. It is possible, however, to increase the regeneration to such an extent that a gain of about 2,000 is obtained in the triode section alone. This condition is not satisfactorily stable.

In general, any means that causes a change in transconductance between the control electrode l8 and anode 20 of device Ill may be used to control the negative transconductance between control grid 8 and connected electrodes 2| and 22 so that the amount of regeneration in discharge device I0 is also changed. That is, when the positive transconductance between grid l8 and the anode 20 is high, the negative transconductance between connected control grid l8 and grids 2| and 22 is also high.

For that reason, the amount of regeneration is adjusted by adjusting the anode output resistance 25. When anode resistance 25 is decreased below a critical resistance, the transconductance between the control grid I8 and anode 2|) is increased so that device lll breaks into oscillation, due to the corresponding increase in negative transconductance between control grid l8 and connected grids 2| and 22. The signal voltage developed across resistance 25 is, of course, equal to the signal current flowing therethrough multiplied by the resistance of resistance 25. By making resistance 25 high the transconductance of the device is lowered but the voltage developed across resistance 25 tends to remain constant.

It is desirable to make the resistance 25 as large as possible, consistent with high gain, not only for suppressing oscillation in device II] but also for decreasing the current drain from voltage source 23. Also, resistance 25 is made large so as to assure linear amplification of high intensity signals applied between the grid 5 and cathode of device Iii. Since a reduction of anode to cathode conductance takes place when the anode potential is reduced low enough to cause operation along the curved portion of the anode voltage anode current characteristic in some distion between the triode and pentode sections of discharge device ii] is to adjust the continuous operating potential of connected grids 2| and 22, for example, by adjusting the resistance 25. In general, the smaller the resistance 26, the more discharge device in approaches a condition of self-oscillation. When resistance .26 is made :large, 'spajce current in electrodes 2| and 22 is reduced.

=Regenerationin discharge device it is also affected by the amount of electron emission from the filament or cathode. It isnot uncommon for to prolonged current draintherefrom, the overall amplification of device ID tends to increase and thus a compensation is provided for the loss of amplification resulting from the decrease in voltage of source 23 in prolonged operation of the hearing aid. Under certain conditions a change in voltage of the heating source 150 may cause a condition of self-oscillation. Variation invthe circuit behavior due to change in heating source voltage 30 is minimized greatly When resistances 25 and 2.6 are suitably large,

.A possible explanation for the increase in regeneration when filament emission is decreased may perhaps be that there is a reduction inemission so that all electrode. potentials rise. Since large anode and screen electrode resistances are used to force operation of the pentode part of the discharge device on a steeply sloping part of the anode voltage and anode current characteristic, when anode and screen potentials rise, the dynamic transconductances of both anode and screen electrode increase with a consequent. increase in gain and regeneration. The negative transconductance between the grid I15 and anode 20 is increased and correspondingly the regeneration in the triode stage is increased. When resistances Z and 26 are properly adjusted for high gain consistent with low current consumption in accordance with principles discussed herein, reduction of voltage of source 30 has appreciably no effect on operation of device It over a large range of change of voltage of source til.

Under certain conditions of adjustment, the resistances 24 and 25 may have such relative magnitudes that. the electrons constituting the space charge are attracted to connected electrodes 2i and 22 and anode 2c in such proportion that the respective transconductances are substantially unaltered as the space charge is reduced when the filament emission is decreased. This has been observed when resistance 25 is of the order of 709,000 ohms and resistance 25 is of the order of 80,000 ohms, device It being a 1R5.

In ademonstration to show the maximum overall gain obtainable by using device Iil, the plate load resistance .25 was reduced so that it was almost small enough for the production of self-sustained oscillations in'discharge device H3. With 100 microvolts input, the output voltage was 1.4 volts. This corresponds to an overall g ain-of lei-n00 in-one discharge device. Such gain is usually not conveniently usable because of the greater difhculty ofproportioning resistances 25 and 23 to avoid self-sustained oscillationsas the voltage of source 46 drops.

The circuit-shown inFig. 1 has much-advantageover conventional regeneration circuits. In general, in order to produce regeneration-insultable manner in an electrical circuit, it is necessary that an output signal of an amplifier be transferred back to the input circuit of the ainplifier through circuits which shift the phase of the signal by a total amount approaching 360 or a multiple thereof including the phase shift in the amplifier. This necessitates either carryingthe phase shifting operation over two tubes, one'or which shifts phase or using a transformer for phase inversion. Systems incorporating such means are inferior in stability and frequency range to the amplifying system as disclosed herein incorporating a single discharge device with a resistance load, because such systerns must all transfer signals through at least two filter meshes in the feedback loop to obtain the 360 phase shift through the entire feedback loop, while the present arrangement requires but one 'm'esh including resistances 2-6 and 27 and condenser 29 to complete the entire feedback loop for shifting signals 360; 01', if it be preferred, 0. That is, there is no phase shift desired in translation of signals through the single mesh comprising resistances "26 and 27 and condenser 29, signals on grids 2i and 22 being effective to produce regeneration when impressed in phase on grid Ill. The regenerative effect is correspondingly constant over a frequency band limited only by the signal transferring ability for which the mesh including resistances 2:5 and 27 and condenser 29 is adjusted.

When a signal of increasing instantaneous intensity is applied between main control grid l5 and the cathode of discharge device ill, on instantaneously decreasing current correspondingly flows through resistance 25. That is, the effective transconductance between grid 15 and anode 28 is negative in character. This is in line with the theory that device [0 includes two amplifying devices in cascade wherein, as is well known, the effective transconductance between the grid of the first device and the anode of the second device is negative in character.

'One of the important features of the present invention resides in the fact that the instantaneous intensity of current flowing through resistance 25 varies substantially linearly with the instantaneous intensity of a signal applied between main control grid I5 and cathode of discharge device lil. In other words, the effective transconductance between control grid 15 and anode 20 is substantially constant over the range of intensity of signals applied between grid [5 and its cathode.

When the resistance 25 is increased, the amount of regeneration in discharge device I!) is decreased, the amount of space current flowing to anode 28 is reduced, and consequently the overall gain i decreased. Also, when the resistance 26 is increased, the amount of space current flowing to screen-e21 and 22 isreduced, and the overall gain is decreased. When resistance 25 or resistance 26 is made small the space current flowing through the corresponding resistances 25 or 26 is increased and, the overall amplification increases to a point where the amplifying circuit changes abruptly intoa state of self-oscillation. Just before such self-oscillation begins, amplification is great, there is enhanced linear or frequency distortion, enhanced instability and the anode voltage is not sufficient to accommodate signal voltage changes, so that signals so greatly amplified are distorted. Such adjustment is undesirable. By making the resistance 25 very large. as taught for high gain and low current consumption, regeneration is reduced sufficiently that the device It is highly linear and yet has great gain for amplification of small signals. It is thus seen that, in. general, when the amount of space current flowing in device H) is reduced, the amount of gain is reduced, and conversely, when the gain is increased, the amount of space current flowing in device in is increased also. It is understood, of course, that such space current flow in varying degree is the same as varying current drain from source 23.

Adjustment made in increasing the size of resistance 25 to cause more linear operation and to suppress self-oscillation and reduce discharge current is quite different from that present in known self-oscillation circuits having a resistance in the anode circuit. That is, in the amplifyingoscillating circuit of a multivibrator, having a resistance in the anode circuit, self-oscillation is suppressed when such resistance is made smaller, whereas in the arrangement shown in- Fig. 1 selfoscillation is suppressed by increasing resistance 25.

With a constant output voltage across resistance 25, the relationship between intensity of input'voltage applied to main control grid 15 and the intensity of such constant output voltage becomes more linear as the resistance 25 is increased. When resistance 25 is so increased, a smaller amount of output signal current through resistance 25 is necessary to produce such constant voltage output, and regeneration in device I is suppressed suitably, thus affording more linear operating conditions. Consequently, when plate coupling resistance 25 is increased, compensation is introduced for nonlinear conditions prevailing when resistance 25 is small.

Since the resistances 25 and 26 influence regeneration and linear operating conditions of device IO, and since they are connected in separate series circuits, including the common voltage source 23, their relative sizes influence the operating conditions of device l0.

When resistance 25 is increased, the overall transconductance of device l0 tends to decrease; and it is then desirable to have resistance 26 relatively small, consistent with linear operating conditions, so as to provide a compensation for the loss in gain due to increase in resistance 25.. That is, when the ratio of resistance 25 to resistance 26 is increased, the overall gain of device In tends to be less affected by a change in resistance 25 even though a greater and more linear voltage amplification is produced when resistance 25 is increased. That is, when resistance 26 is small compared to resistance 25, greater linearity is achieved than when the resistance 25 is comparable to resistance 26. Also, when resistance 26 is small compared to resistance 25; variations in heating voltage source 46 have little effect on the operating conditions of discharge device III.

In a practical embodiment of the present invention, the various circuit elements described herein had the following properties: discharge device I 6-1R5 R. C. A. type, resistance l6-5 megohms, resistance I'I470,000 ohms, resistance 24400,000 ohms, resistance 25--680,000 ohms, resistance 2668,000 to 82,000 ohms, resistance 21-10 megohms, resistance 32-4.? megohms, condenser 28-.1 microfarad, condenser 29100 micromicrofarads, condenser 30-.02 microfarad, condenser 3l-.001 microfarad.

While I have shown and described the particular embodiments of my invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention in its broader aspects, and I, therefore, aim in the appended claims to cover 10 7 all such changes and modifications as fall within the true spirit and scope of my invention.

I claim:

1. An amplifier circuit including an electron discharge device having an anode, cathode, first control electrode, second control electrode and at least one grid electrode, said grid electrode being interposed between said first and second control electrodes, a source of anode operating potential for said electron discharge device, said source having positive and negative terminals, a capacitor connected between said second control electrode and said grid electrode to couple voltage changes therebetween, a first resistance connected between said grid electrode and said positive terminal of said source, second and third resistances connected between said first and second control electrodes, respectively, and said cathode electrode to properly load said control electrodes, and an anode load resistance connected between said anode electrode and said positive terminal of said source, said electron discharge device having a normal range of anode load resistances, said amplifier circuit tending to oscillate when said anode load resistance lies in said range, said anode load resistance being chosen to have a magnitude greatly in excess of the magnitudes in said normal range of anode load resistances, whereby said amplifier is prevented from oscillating.

2. An amplifier circuit including an electron discharge device having an anode, cathode, first control electrode, second control electrode and at least one grid electrode, said grid electrode being interposed between said first and second control electrodes, a source of anode operating potential for said electron discharge device, said source having positive and negative terminals, a coupling network connected between said second control electrode and said grid electrode to couple voltage changes therebetween, a first resistance connected between said grid electrode and. said positive terminal of said source, second and. third resistances connected between said first and second control electrodes, respectively, and said cathode electrode to properly load said control electrodes, and an anode load resistance connected between said anode electrode and said positive terminal of said source, said electron discharge device having a normal range of anode load resistances, said amplifier circuit tending to oscillate when said anode load resistance lies in said range, said anode load resistance being chosen to have a magnitude greatly in excess of the magnitudes in said normal range of anode load resistances, whereby said amplifier is prevented from oscillating.

JOHN G. PRENTISS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,228,080 Herold Jan. 7, 1941 2,342,492 Rankin et al Feb. 22, 1944 2,226,561 Herold Dec. 31, 1940 2,287,280 Terman June 23, 1942 2,235,817 Freeman Mar. 25, 1941 2,262,916 Boucke 2 Nov. 18, 1941 2,214,614 Hunt Sept. 10, 1940

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