US2288600A - Electrical device - Google Patents

Description

July 7,1942. J. P. ARNm, JR 2,288,600

ELECTRICAL nnvrcn Filed nec. 19, 1940 2 sheets-sheet 1 i L ho-14' ArroRNl-:Y

July 7, 1942. J. P. ARNDT, JR" y 2,283,600

ELECTRICAL DEVICE Filed new'. 19, 1940 f2 sheets-sheet 2 INVENTOR ./a/m fie/wr ./e.l

ATTORNEY transduction are now in common use. By this.

Patented July- 7, y1942 lUNITED ySTATES VPATENT OFFICE azsasoo y ELECTRICAL nnvrcn John P. Arndt, Jr., Euclid.,` Ohio, assignor to The Brush Development Company,- Cleveland, Ohio,

. a corporation of Ohio Application December 19, 1940, Serial No. 3Zll,773

12 Claims.

rI'his invention relates to electrostatic` apparatus and more particularly-to circuits" embodying electrostatic generating devices. The' invention relates more especially to means for reducing the flow of current from electrostatic `generators when the latter are used in combination with amplifiers. Speciiically, it relates to amplifier circuits of particular design used in combi-l nation with electrostatic 'signal generating devices.

Electromechanical transducers such as microphones, phonograph pickups and vibrationepickups employing the electrostatic principle of principle an interchange ofmechanical energy to electrical energy may take place through the medium of a piezoelectric body or, alternatively,` of an electrostatic field which may be supplied by an external polarizing source or by an electret. Such electrostatic fgenerators are com. monly used in combination with vacuum tube amplifiers. However, difllculties 'are often encountered with such combinations in obtaining `satisfactory low-frequency response because the lint-ernal impedance of electrostatic generators of the type mentioned is predominantly capacitive while the input impedance of the usual amplifiers is generally resistive. The impedance of the electrostatic generatingvdevice may (be low enough over a wide range of frequencies so that the major part of the generated voltage is'ap- 1 plied to the input of the amplier.` At frequencies below this range, however,`the capacitive impedance becomes relatively very high and a `large part of the Asignal voltage is then developed across theinternal impedance `oi the generator and very little is applied to the amplifier. In other words, the combination exhibits an unclesired low-frequency cutoi. In many cases it is ditllcult to, avoid having the cutoff within the freq/uency range which the combination should accommodae. This is because the grid resistance of the ampliiier must be increased to many megohnis if the cutoii is to be avoided and because most ampliiiers become unstable or display other undesirable effects when the input grid resistance is so increased. This diiiiculty has been encountered in the design of high output, sound-celLmicrophones when a plurality ofsound-cells of the type `disclosed intSawyer Patent No. 2,105,010 and Williams Patent No.'` 2,126,438, are connected in series to obtain increased output. The series connection results in a small capacity for the microphone and thus (ci. 11a-i) sistance for satisfactory low-frequency response,

To'avoid this situation and to permit the use of gridl resistance of moderate size, it has been necessary in` the past to connect in parallel a plurality of such series-connected groups oi' cells. Such construction is expensive and bulky. and introduces acoustical errors which arise from the large size of the assembly. As will be understood from the subsequent explanation, this invention largely avoids such diiiiculties. A slmilar problem has been encountered in the -past in the design of condenser microphones since the capacity of such a microphone is-relatively low. In order' to place the cutoff at a' suiciently low frequency, it has beennecessary lto use excessively large grid and polarizing circuit resistancesl likewise, the problem has been encoun-l tereg in devices such as the piezoelectric presto provide an ampliiier circuit which has less loading eect on electrostatic generating devices than conventional ampliers. L

It is `another object to provide a vacuum tube coupling circuit for coupling'a'high impedance generating device to a .relatively low impedance l load circuit.

It is a further object to provide amplifier' circuits of the'type ,described which 're 'particularly suitable for use in combination with piezo electric signal generating devices, thev cli-cuits 40 being characterized by'the'factthat they apply no biasing potential to the piezoelectric' device.

It is still anotherl object to 'combine' a? condenser microphonev with'an amplifier circuit /of the type described which is particularly suitable for use lin combination theret'vith,` the amplifier circuit being characterized by the fact 4that it uses a minimum number of elements.

Another object of this invention is to' provide a circuit which is especially adapted for use in l combination withI a plurality ofy electrostatic generating devices connected in series.

Other `objects will be' apparent from the iol'- `lowing description taken in conjunction with the i accompanying drawings. requires an excessively high amplifier input re- Figures .1, 2 and 3 illustrate embodiments of the invention which permit over-neutralization. complete neutralization, or under-neutralization of signal current where the term neutralization is understood to mean that condition of operation wherein no signal current flows from the generator. In these figures,

Fig. 1 illustrates the invention in connection with an otherwise conventional single-stage amplifier.

Fig. 2 illustrates the invention in connection with a two-stage cascade amplifier supplying signal voltage to a potentiometer.v

Fig. 3 illustrates the invention in connection with a single-stage amplifier having inverse feedback.

Figures 4, 5, 6, '7, 8 and 9 illustrate embodiments of the invention wherein one may obtain under-neutralization which approaches complete neutralization. In these figures,

Fig. 4 illustrates a modification of the invention which is particularly suited for use with a piezoelectric or electret signal generator.l

Fig. 5 illustrates a modification which is particularly suited for use with a condenser microphone or a similar electrostatic signal generator.

Figures 6, 7 and 8 illustrate embodiments of the invention wherein the application of directcurrent potentials to the signal generator is avoided. f

Fig. 6 illustrates a modification wherein a tapped battery is employed to avoid the said direct-current potential.

Fig. '7 illustrates a modification wherein the direct-current potential is avoided by means of a blocking network.

Fig. 8 illustrates a modification wherein the direct-current potential is avoided through the use fy a push-pull amplifier circuit.

Fig. 9 illustrates an embodiment of the invention analogous to that illustrated in Fig. but which substantially avoids the direct-current p0- tential which the latter applies to the signal generator. In this embodiment an additional amplifier stage is provided to supply the amplified neutralizing signal voltage.

Referring now to Fig. 1, a high impedance signal generating device, I, as for example, a piezoelectric microphone, applies signal voltage between the plate and cathode is connected a plate The.

supply battery 9 and the primary I0 of transformerIS. Battery II supplies bias potential for the grid 'I, the positive terminal being' connected to the cathode 6 and the negative terminal being connected to the grid through transformer secondary I2 and grid resistance I3. When connected as shown, the bias battery I I applies a direct-current potential difference between the generator terminals. Usually this will be too small to harm a piezoelectric device but, if desired, other bias arrangements may be used, some of which are illustrated in some of the succeeding figures of the drawings. Output from the amplifier may be taken between terminals I4 and I5 connected to the transformer primary or alternatively between terminals I4', I5 connected to the transformer secondary. l

When the various elements of this circuit are adjusted to have a particular relationship described below. the amplifier has very little or no resistive loading effect on the signal generator I. That is, the amplifier draws little or no4 signal current from the generator. The desired relationship is obtained by proportioning transformer I6 in relation to the characteristics of tube 5 so that over the frequency range where high input resistance is required, the instantaneous signal potential at terminal I4 is substantially the same as the instantaneous signal potential at terminal 2. When this relationship is obtained, little or no signal current flows from generator I through resistance I3 because both ends of the resistance have the same instantaneous signal potentials. Since electrostatic generator I supplies little or no current in this circuit, there is little or no voltage ldrop across thel internal impedance of the generator; or stated in another way, the effective input impedance of the amplifier has been raised to a very high value,

The cutoff frequency Fc in a circuit consisting of a generator having an internal capacity C, and a load resistance R is given by the expression ZWCR At frequency Fc the signal voltage across resistance R is 70.7% of the generated voltage.r When utilizing the above described circuit, the effective input resistance corresponds to R in the formula and it will be observed that since this resistance may be increased to a very large value, approaching infinity, the cutoff frequency of the circuit may be greatly lowered, approaching zero. Usually leakage across the generator terminals will prevent the attainment of a zero cutoff frequency. Preferably the inductance of the primary of the transformer should be sufficiently high to introduce little or no phase shift or rela-- tive attenuation down to the' lowest operating frequency. If this condition is not obtained, phase and amplitude correcting networks may be introduced. If a phase shift is present, there will be a difference of potential between opposite ends of resistance I3 even though the voltage drop across the secondary I2 of the transformer is equal to the input voltage applied between terminals 2 and 3. However, if the phase angleA is not too large, the said potential difference will be smaller than the voltage that would be applied across resistance I3 if the resistance were connected in conventional manner between grid and cathode and most of the benefits of the invention will still be obtained. In this connection it may be observed that if there is a phase difference, the effective input impedance of the amplifier will have a reactive component as a result.

If the transformer I6 is so proportioned that the signal voltage developed across the secondary I2 is greater than the signal voltage supplied between terminals 2 and`3, the flow of current through resistance I3 will be reversed with respect to the flow obtained whenresistance I3 is connected directly4 in shunt with terminals 2 and 3, or with respect to the flow of current when the` voltage drop across the secondary of the transformer is less than the signal voltageapplied between terminals 2 and 3. Under this condition the effective input impedance of the amplifier will -have a negative value and thev amplifier will tend to be unstable unless terminals 2 and 3 are shunted by a positive impedance of suitable magnitude vto make the combination positive. This effect may be used to neutralize leakage resistance across the terminals 2 and 3.

If a high input impedance is required at very that the voltage drop betweenit and terminal '22 `the tap is raised to a point higher than the above 'mentioned neutral tapping point, the input resistance vbecomes negative. If the various cou-85 lation of current through resistanceV I3 and 'as 'a aasaeoo low frequencies, it may be impractical to concapacity coupled to the grid and cathode of thesecond stage of the amplifier in conventional manner. The output of vtube I9 is developed across resistance 20 at output terminals 2I and lo 22. 'I'he signal voltage developed between terminals 2i and 22 is,in general, in phase with but larger in magnitude than the input signal voltage i applied via the terminals 2 and 3. in adjustable tap 23 is provided on resistance 2l and connected 20V to grid resistance I2. When tap-2l is so adjusted is substantially the same as the signal voltage applied betweenl terminals 2 and 3, there will be substantially no flow of signal current through 25 resistance I2 because under these conditions bothl ends of resistance I3 have the same instanta- Vneous signal potentials.- Tap 23 may be adjusted to vary the effective 4input impedance of the amy plifler. When itis placed at the lower end of re- $0 sistance 2U,.the input resistanceof the amplifier is equal to the magnitude of resistance Il. When pling and by-passing condensers in the amplifier are not sufilciently large they will introduce some phase shift which will prevent complete cancelresult, the effective input impedance of the sys- '40 tem will havej a reactive component. However, it is usually possible to maintain suitable hase relations down to lower frequencies th hen transformer coupling isemployed. For most aplplications,`complete neutralization of signal cur- 4s rent is not necessary and the circuit as shown is entirely satisfactory. However, if more complete neutralization is desired at very low frequencies, the amplier may be changed to the direct coupled type by omitting the coupling andV by-passim ing condensers and introducing suitable biasing batteries. When this is done, no low frequency phase shift or-attenuation can occur and subl stantially complete neutralization of input signal current may be obtained at all low frequen- 5s cies down to direct current.

In Fig. 3 the invention has been embodied in antinverse feedbaclrtype amplifier. `In this circuit the electrostatic signal generator I1 is'connected via terminals 2 and 3 to grid 1 and to sn the plate-cathode circuit at the`plate side of the plate-cathode impedance. This impedance consists of primary Il of transformer It together with whatever load impedance'. is conn ted 'across output terminals Il and I5 (or the reas flected value of the load if connected across terminals I l', I 5'). Thus the full output voltage is fecil back in opposition to the applied signal voltage so that only a small signal is effective between grid and cathode.` The action of s uch cir- 70 cuits in reducing the eiIective, output impedance and in reducing distortion is 'well known. 'The bias circit used in Fig. 3 is, however, unconventional. Battery II provides grid bias and is connected between grid and cathode through rei 1 and cathode l. As

sistance I3 `and transformer secondary I2. It also applies direct current to the generator, as

i in Fig. l; and other biasing means maybe used.

For example, a by-passed resistance in-the cathode circuit, as in Fig. 2, wouldbe satisfactory.

The transformer II may be so designed that the signal voltage appearing across secondary I2 is just equal to .and substantially in phase with the signal-voltage applied-between terminals 2 and' 3.

There will thenv be no now of signal current through resistance Il because both ends of that resistance have the same instantaneoussignal v poteltials. If the voltage appearing across secondary I2 is greater, the4 effective input impedance will benegative and if the voltage is slight- IyS less, the input impedance will have a high positive value. vided to adjust the lamount of secondary voltage introduced in-serie's with resistance I3, thereby to adjustv the .effective input impedance ofthe amplifier. If desired, the output of the amplifier unay be taken from secondary terminals I4' and .Il' rather than from the primary terminals Il, Il. It will be observed that the circuit of Fig. 3 is substantially the same as the circuit of-Fi'g. l

except that inverse feedback is .introduced into the former by connecting terminal l to the output circuit at terminal Il rather than directly to cathode. l l l In the foregoing5 circuits the. signaleurrent through the input circuit is reduced by introducing into 'the grid lbias circuit an output signal -which opposes the ilow of signal current, and

under some conditions this opposing effect may be made complete so that substantially no signal current flows. In the modifications shown in Figs. 4 to 9 inclusive, although the input signal current isreduced, and the effective input'impedance thereby increased, complete neutralizationof theinput signal current is not achieved. The reduction of signal current isjaccomplishedv by introducing an opposing output signal in series with the signal generator and the grid cathode circuit. The grid resistance is so connected in the circuit that the signal voltage which is applied to it is the difference between the generl ator terminal-voltage and the said opposingsignal,I with the result that only small signal current flows therethrough.

The modification illustrated in Fig. 4 is similar in some respects to that of Fig. 3 vbut the grid bias-.path is connected directly between grid that in this modification the signal current cannot be reduced to zero since a small signal drop across resistance I3 is necessary to actuate the control grid of the tube.` For this reasonthe input resistance ofL the amplifier cannot be raised izo-infinity as in the circuits of Figs. 1 to 3, but yet may easily be increased to a value many times greater thani the value of resistance I3..

Under some conditions, the feedback voltage may not be quite in phase with the grid cathode sig- A potentiometer could .be pro-V polnted out in connection is connected component.

nal voltage due to the action of the inductance of primary I0, and if this is the case, the input impedance of the amplifier will have a reactive Output'may be obtained from the secondary I2 of the -transformer at terminals I4', I5'; or across the primary I0 'at vterminals I4, I5, in which case the secondary could be omitted.

To further explain the action of the circuit, an expression for the effective input impedance will now be derived, and typical values substituted to better-illustrate the advantage of the invention. In the analysis some simplifying assumptions will be made, i. e., that the effects of tube, wiring and other stray capacitances are negligible; that the resistance of inductance I is negligible; that a negligible load is connected across the output terminals (for example, the grid circuit of a succeeding amplifier) and that thev grid cathode signal is sufliciently small that linear operation of the tube is obtained. The following notations will b e used:

i :signal current supplied to amplifier through terminals 2, 3

e :signal voltage applied betweenterminals 2 and 3 eL=signal voltage developed across transformer primary I eg=net signal voltage applied between gridl and cathode L=inductance of primary I0 R. :resistance of grid leak resistor I3 u :amplification factor of tube Rp=plate resistance of tube Z :input impedance of amplifier measured between terminals 2, 3.

From inspection of the circuit, four equations may be written:

The voltage eg is found by combining Equations 1 and 2:

1+R, +JwL The current i is found by combining Equations 4 and 5:

R 1 #1T +Je-L The input impedance is found by combining Equations 3 and 6:

ucoLRp which may consist of a group of cells connected in series with a net internal capacity C of .0001 microfarad. If a cutoff frequency of approximately 48 cycles per second is desired (wc=21rX48=-300) it will be found that an input resistance of Ohms LL :20 L :100 henries Substituting these in Equation '7 we find, at 48 cycles (rr-:300):

For a cutoff frequency of 48 cycles, Z (if a pur resistance) must be 33.3 megohms. Thus, neglecting the reactive component JSR, we iind Zigi 19- 19 Thus with the circuit of Fig. 4, a relatively high impedance microphone may be used with a practical value of grid leak resistance (i. e., only 1.75 megohms) whereas with conventional circuits an excessively high value of grid leak resistance would be required (33.3 megohms). The reactive component of the input impedance (neglected in above example) tends to neutralize the R megohms l .7 5 megohms capacitive reactance of the microphone and extend the cutoff to a lower frequency. Asimilar analysis may be applied to the other circuits of this invention.

In Fig. 5 theA circuit of Fig. 4 has been modied by replacing primary I0 of transformer I6 with resistance 24, which may have a value in the order of the resistance that would be used in the plate-cathode circuit of a conventional amplier using the same type of tube. Additional resistance 25 is connected between cathode and resistance I3 to provide grid bias for the tube.

The operation of this circuit is substantially the same as that of Fig. 4 except that it does not have the reactive input impedance or the low .frequency limitation imposed by the inductance I0 in Fig. 4 since no inductances are involved in the plate-cathode circuit. The direct current voltage-drop across the resistance 24 is applied to the terminals of the signal generating device I. This circuit is especially advantageous when used with signal generators of the condenser microphone type since the last-mentioned direct-current potential may be utilized to polarize the generator. Thus this circuit eliminates the additional resistance and blocking condensers normally used in the polarizing circuit of such devices, and provides an unusually high input impedance. Since the capacity of condenser microphones isI usually very low, this high input impedance is especially desirable as it has been diflicult in the past to obtain suflif cient-ly low frequency cutoffsl in conventional amplifier circuits for the reasons explained above. Although this invention is concerned primarily with the reduction of the loading effect of the grid leak resistance of the amplifier, it

. aasaeoo' Ishould be noted that in the circuits o'f Figs. 4

to 9, inclusive, the loading eiect o f the'gridcathode capacitance (and any shunting capacitance) is reduced in a similar manner. This maybe of importance when low capacity genrators such as condenser microphones are used. Ifa conventional polarizing circuit is used, all or apartof its resistance may `bearranged in shunt with the grid circuit resistance I3 over the operating frequency range so that the signal current through it will also be reduced in the same vmanner that signal. currentnthrough the grid resistance is reduced.

When a piezoelectric signal generator is substituted for the condenser type of generator, the polarizing voltage applied to terminals 2 and 3 .in Fig. 5 is .undesirable since piezoelectric devices may be damaged bythe application of direct-current voltages,"or the operation may otherwise be impaired by the resulting mechanical bias. y t

- Fig. 6 shows a modification of the circuit which eliminates the direct-current voltage between . terminals 2 and 3. In this circuit, terminal 3 is connected to -battery at a tap 26 thereonwhich has the same direct-current potential as terminal 2. The signal current impedance of batteryil is relatively low so that forsignal currents the terminal 3A is effectively connected: to the plate sidev of resistance 24. Accordingly, the operationl of the ampliiieris the same as that of the amplifier of Fig. 5. In Fig. .6, grid bias for the tube is obtained from battery II. It will be understood that Vother suitable bias means could be used, as for example, a cathode-circuit resistance as in Fig. 5.

5' and 30', respectively), are equal and the.tube's` same direct-current potential by employing a push-pull circuit. The resistances of the platey. cathode circuitsof the twotubes 5 and 5' (consisting of resistances 24, 25 and 30, and 24',25

`5 and 5 have similar characteristics so that their plate currents are equal. -For this reason there'is no direct-current difference of potential between the cathode ends of grid resistances I3 w 'and I3 and therefore no direct-current difference of potential between terminals 2 and 3.

` Bias for-the tubes is provided by' the voltage drop acrossv the plate- circuit resistances 25 and 25. The plate-circuit .signal voltage developed across resistances 24, 24' opposes the signal voltage ap plied to terminals 2 and 3 to reduce the signal current through grid resistances I3 and |35.I For this reason it will be understoodthat the cirl cuit oifers a high impedance at terminals 2 and 20 Fig. 'z shows another noaincation of the ctrcuit which avoids-direct-current bias on the generator. Its operation at signal frequencies is substantially the same as that of Figs. 4, 5 and 6. The direct-current 'voltage \drop across resistance 21 is prevented from affecting generator I,

which may be any suitable electrostaticr generating device,'by .introducingbetween terminal 3 and the plate side of resistance 21, the condenser 28 havinglow impedance to signal currents. To prevent leakage across condenser 23 from applying some direct-.current voltage between terminals 2 "and 3, resistance 29 is connected between terminal 3 and the cathode in shunt withl resistance 21. The resistance 23 should be large compared with the reactance of condenser 23 p over themseful frequency range, and preferably of the same order of Snitude as resistance 21. The leakage resistanc of condenser 23 should.'

- of course, be high com red with resistance 23. vIt will be seen that as far as direct current is concerned, terminals 2 and 3 are substantially at the same potential, while for signal frequencies,

the terminal '31s effectively-connected to the plate side of the plate-cathode impedance as in Figs. 4,'5 and 6. .Also at signal frequencies,

resistances 2.1 and 29 are connected eiectively inparallel. If one of these resistances is much smaller than the other, then the smaller one determines the signal lvoltage drop in the platecathode circuit. If the two resistances are nearly equal .in magnitude, theny the parallel combination must be used in calculating the signal voltage drop. If desired, the lower end of.

grid leak resistance I3 may be connected toan 65 than a limiting sense. Furthermore, the circuits adjustable contact arranged to slide, along resistance 23 to vary the eil'ective input impedance of the circuit.

In F18. 8 the 3 in the "same or similar manner as' a high lmpedance isoffered by the circuits of the foregoing ilgures. The output from this circuit may betaken between terminals 3|, 3|', or if a low output impedance is desired as in the case of the circuits of Figs. 3 to 7 inclusive, the output may' be taken between terminals I5 and 15a. In the latter case the resistances 30, 30' may be reduced or omitted, thus increasing the opposing signal voltage drops across resistances'24, 24.

Fig..9 shows another modiiication which is di-k drop across this resistance to obtain the desired increase in the effective impedance between terminals 2 and 3, an additional amplierstage, designated generallyby the character 33, is provided. ,The input signal for thisY stage is obtained from .the signal current drop across a resistance 34 in the plate circuit of first tube 5.

'Ihe output is taken from the secondary of transformer 35 whose primary is connected irf the plate circuit of the ampl'ier stage 33. A resistance` 36 is connected between the plate of stage 33 and cathode 6 of tube 5 through a blocking condenser 31 so that a part oi'k the amplified signal current produced by stage 33 is passed through resistance 32, thus increasing the signal voltage.

drop suillciently toprovide'the required input impedance magnification. The output of the, cirpentode. orscreenfgrid tubes may of course be cuit may be taken at terminals 38 and 33, or ternati ely at terminals I4and I5.

regolng illustrations ofy the invention represent only a few of the many circuits which maybe designed .around the basic vprinciples which have now been disclosed. They, should therefore be regarded in an exemplary'rather themselves maybe mod ed in their minor details without departing from the invention. For example, batteries and cathode-circuit resist-ances may be used interchangeably tp obtain the proper'` `grid'biasrelativeto the'cathode.- While triode tubes have been illustrated, other typessuch as substituted. Furthermore,v transformers may terminus if: are item at than;u

usually be used'in place of potentiometers in the output circuit in order to obtain the properneutralizing potentials if due regard is given to the inherent low frequency limitations of transformers. Those skilled in the art will appreciate, however, that when substitutions of the character here mentioned are made, it may be necessary to make slight alterations or readjustments in other elements of the circuit. If a two-stage amplier, such for example as shown in Fig. 2, were being used with satisfaction and it were desired to replace the amplifier with one having three stages. those skilled in the art will realize that the output of the three-stage amplifier will be approximately 180 out of phase with the generator signal so that it is necessary to use a transformer, or other phase reversing means, to obtain the proper polarity. In making these substitutions, however, new elements have been added and they may introduce additional phaseshifts. If this were the case and if complete neutralization were desired, suitable phase-correcting means should be coupled to the circuit to compensate for the shifting introduced by the transformer, and/or additional stage of amplification. y

The cathode heating circuits which are necessary to secure operation of the vacuum tubes have not been shown inthe drawings for the reason that such circuits and their requirements are fully understood by those skilled in the art and need no explanation. Batteries have been shown in the plate circuit to illustrate a suitable source of plate current, and it will be understood that rectiers, motor-generator units, and the like could be substituted for the batteries when due regard is given to the individual characteristics of the various devices.

The circuits of the invention have been shown to have a pair of output terminals which are not connected to other apparatus. It is pointed out, however, that in general use, the amplifier would be connected to other equipment, such as meters, additional amplifier units, loudspeakers, or motivated apparatus of many kinds. In making such connections, regard should be had to the fact thatin some circuits, there may be directcurrent potentials as well as alternating signal potentials at the output terminals. When such direct-current potentials should not be applied to the input of the succeeding apparatus', a blocking condenser should be inserted, or the voltage maybe confined to its own amplifier in other Ways. Due regard should be given to the input impedance of the succeeding apparatus when designing the circuit.

It has already been pointed out above that many of the circuits shown in the drawings do not apply direct-current potentials to the signal generator, or apply only small biasing potentials. Thus, the circuits of Figs. 2, 6, 7 and 8 do not apply direct-current to the generator, while the circuits of Figs. 1, 3, 4 and 9 apply small direct-cur-v rent voltages. The circuit o'f Fig. 5 applies a rather large voltage, however. Thisfeature of the circuits should be carried in mindvwhen selecting a circuit for use with a particular kind of signal generator. Thus, the condenser microphone requires a polarizing direct-current potential, so that the circuit of Fig. 5 is well adapted for use withl it. On the contrary, piezoelectric devices should not be subjected to biasing potentials; accordingly, the circuits of Figs. 1, 2, 3, 4,

6, 7, 8 and 9 are especially useful in combination with them. In Figs. 1, 3 and 4, the small directcurrent potential applied to terminals 2, 3 by bias battery Il could, if desired. be avoided by adopting a different' biasing arrangement as will be understood by those skilled in the art.

In view of the many modifications which may be made of the circuits here illustrated, it is apparent that the scope of the invention should not be determined from the specific examples which have been relied upon to explain the invention, but should instead be Vdetermined from the following claims. In the preceding specification and following claims, the term amplier is used to mean an amplifier device whose output is controlled by the input through the valve action of an electron discharge device.

Having now disclosed the invention, what I claim is:

l. In combination: an electrostatic signal generator adapted to supply a signal voltage; an amplifier which has an input circuit connected to said generator, and is capable of developing an output signal voltage at a pair of output terminals, said input circuit tending to load said generator by drawing signal current from the latter; and means for utilizing at least a part of the output signal voltage to reduce the flow of signal current in the input circuit, and thereby to reduce the loading effect of the input circuit.

2. In combination: an electrostatic signal generator adapted to supply a signal voltage; an amplifier which has an input circuit connected to said generator, and is capable of developing an output signal Voltage at a pair of output terminals, said amplifier including a tube having a grid and cathode, and a bias circuit adapted to bias said grid with respect to the cathode, said bias circuit tending to load the generator by drawing lsignal current from the latter; and

means for utilizing at least a part of the output terminal, and a bias circuit adapted to bias said grid with respect to the cathode, and extending between said grid and a point on the output circuitfso located on tthe latter that the signal potential difference between it and grid is less than the signal potential difference between the input terminals. g

4. In combination: an electrostatic signal genyeratoradapted to supply a signal voltage; an

amplifier having a pair of input terminals con nected to said generator, a tube having a grid and cathode, an output circuit in which is developed an output signal corresponding to the input signal, a grid-to-cathode leakagepath in shunt with said input terminals for biasing said grid relative to said cathode, and an input circuit for applying between said grid and cathode at least a part of the signal voltage applied at saidA input terminals, and means in said leakage path for utilizing at least a part of said amplified output signal to reduce the flow of generator signal current in the leakage path.

5. In combination: an electrostatic signal generator adapted to supply a signal-voltage; an amplifier having a pair of input terminals connected to said generator, a tube having a grid and cathode, an output circuit in-jwhich is developed anoutput signal corresponding to the input signal, a biasinglfcircuit between grid and,

minals,'and means in vsaid input circuit for utilizmaintaining the grid negative with-respect to the V cathode, said means including a high resistance ing at least a part of said amplied output sig- Y nal to reduce the 'flow oi' signal current in said input circuit. 1

6. In combination: an electrostatic signal generator adapted to supply a signal voltage; an amplifier having a pair of input terminals connected to said generator, and an output circuit'.- connected to a pair oi output terminals, said amplier being adapted to develop a. signal voltage at its output terminals, and including a tube having a grid connected to one input terminal and a cathode connected to the other input terminal, and a bias'circuit adapted to bias said grid with respect to the cathode, said bias circuit extending between said grid and a point on the output circuit so located on the latter that the signal potential difference between it and current path connecting the grid to the platedirect current path connecting said grid to the plate-cathode circuit at a point on the latter lying between the cathode and said impedance; and means for maintaining the terminals of the generating device at substantially the same direct current potentialy while applying at least a part of the said generator signal voltage betweenthe Srid and a point on the plate-cathode circuit which lies Abetween the plate and said impedance.

-ductance connected in series therewith between said plate and cathode, and means for maintaining the grid negative with respect to the cathode, said means including a high resistance direct current path connecting said grid to the plate-cathode circuit at a point on the latter 'lying between the cathode and said inductance;

and means for applying at least a part of the signal voltage of the generator between the grid vandv a point on the plate-cathode circuit which `lies between said plate and said inductance.

l0. In combination: an electrostatic generating device which provides a signal voltage; an ampliiler comprising: a vacuum tube which in-.

`cludes at least a' grid, a cathode and a plate, a battery, a plate resistance connected in seriesv withl the battery between said plate and cathode,

. and means for maintaining the grid negative cathode circuit at a point on the latter. lying betweenthe cathode and the impedance; and means for applying at least a part of the said generator signal voltage between the grid and the plate-cathode circuitat a point on the latter lying between the plate and said impedance.

8. In combination: an electrostatic generating device adapted to provide a signal voltage between a pair ofterminals; an amplifier comprising: a tube which includes a grid, a cathode and a plate;v a direct current source of plate current and an impedance connected in series with the latter between said plate and cathode; means for with respect to the cathode. said latter means including a high resistance direct current path connecting the grid'to the plate-cathode circuit at a point on the latter lying between the cathode and said plate resistance; and means for applying at least a part of the said signal voltage or the generator between the grid and the plate-cathode circuit at.a point on the `latter lying between the plate and said plate resistance.

1l. The combination as claimed in claim 1 0 wherein said generating device is a condenser microphone.

2. The combination as claimed in claim 10 wherein said ampller is of the push-pull type.

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