US2508586A - Supercharged cathode follower circuit - Google Patents

Description

May 23, 1950 P. s. VENEKLASEN 2,508,585

SUPERCHARGED CATHODE FOLLOWER CIRCUIT Filed March 26. 1946 2 Sheets-Sheet 1 May 23, 1950 P. s. vENEKLAsl-:N

suPERcHARGED cATHoDE FoLLowER CIRCUIT 2 Shee's-Sneefrl 2 Filed March 26, 1946 CASE Patented May 23, 1950 UNITED STATES PATENT OFFICE SUPERCHARGED CATHODE FOLLOWER CIRCUIT Application March 26, 1946, Serial No. 657,159

4 Claims.

This invention relates to an improved cathode follower circuit for use especially as a preamplifier or impedance transformer of unity voltage gain, between a high-impedance voltage source and a load; also it is used for supplying, if necessary, a stable bias voltage to the high-impedance voltage source.

With a simple cathode follow-er, the largest input impedance which may be obtained is of the order of 100 times the Value of the grid resistor, unless one uses special high transconductance tubes whose properties are not otherwise suitable. Where, for example, it is necessary to connect the source to the cathode follower by means of a exible cable or other extension, the largest factor by which the stray capacitances of the cable or extension can be reduced by feedback from the cathode follower is of the order of U.

Thus, if the high impedance source is a condenser microphone, a high input resistance to the preamplifier is required to maintain a low lowfrequency cutoff, and any extension cable for the microphone must have a low effective capacitance if the sensitivity is not be decreased. To maintain satisfactory high input resistance one must use an undesirably high value for the grid resistor of a conventional cathode follower; the stability of operation thus suers.

In accordance with the invention, I have devised a supercharged cathode follower circuit in which the effect of feedback in increasing the input impedance is much greater, and in which, also, the performance of the cathode follower is less sensitive to the magnitude of the load.

In the accompanying drawings:

Fig. 1 is a schematic view illustrating the circuit of the invention in one simple form.

Fig. 2.is another schematic view illustrating another form of the circuit reference characters used in the first figure being primed and applied to corresponding parts; and v Fig. 3 is still another schematic View illustrating the circuit combined with additional circuit elements for one specific adaptation, parts referred to by reference characters in the other views having similar characters double primed.

Attention is directed to the simple circuit indicated in Fig. 1. In this figure a complete amplifier circuit specially suited to a condenser microphone is shown, having input terminals l and 2, to which the terminals of the condenser electrodes of the microphone are to be directly connected, the first being at ground and the second led directly to the grid of a first tube T1. The output of this circuit is at terminals 3 and 4, the latter being at ground and terminal 3 being connected direct- 1y to the cathode of a second cathode follower T3, and to an intermediate point in the cathodeto-ground resistance of the tube T1, with a capacitor coupling C1 to the cathode of the lastnamed tube. The grid return of the first tube is through resistor Re to the same intermediate point last named. In this way part of the cathode resistance becomes effective in the grid-toground return, and a comparatively small amount of resistance is required in the resistor Re, at the same time that feed back from the cathode followers T1 and T3 is made more than ordinarily potent at the input. While tube T1 is utilized as a cathode follower, it combines conventional amplifier action by the inclusion of a plate resistor RB and a coupling of the plate by capacitance C2 to the grid of a conventional amplifier tube T2. In general, the principle of operation of the circuit involves feeding a signal from a plate load RB, connected to the cathode follower T1, to additional amplifying means T2-T3, and then feeding back the amplified signal in proper phase to the cathode of the cathode follower T1 to increase the input impedance.

The circuit includes three tubes T1, T2 and T3. Tube T1, if its plate load RB were short-circuited, would constitute a conventional cathode follower with output appearing across terminals 3 and 4, as will be apparent from an inspection of the circuit arrangement of Fig. 1. The signal from T1 is, however, amplified there and fed capacita'- tively from the T1 plate to the T2 grid, and from the T2 plate by a direct lead without inserted impedance to the T3 grid. 'The latter tube is a conventional cathode follower, except in the manner in which it is connected to the cathodeto-ground and grid-to-ground resistances of tube T1. A Very high gain is therefore attained at output 3-4, representing the amplified potential from T2 and power gain of T3, supplemented by the cathode output of tube T1, with the tubes T2 and T3 constituting amplifying means. The resulting amplified signal is then fed back in proper phase directly from the cathode of tube T3 to the cathode of the tube T1. This feed back therefore becomes effective as added impedance in series parallel with the grid resistor RG and upper portion of the cathode resistance.

It is an important function of the device that with the parameters and voltages indicated in Fig. 2, or Fig. 3, the voltages derived at the two cathode follower outputs are equal, and for adjusting these a check biases switch may be used as in Fig. 3, to enable separate adjustment.

For this, adjustment of the output at tube Ts. T3 or T3" is suflicient, and this may be accomplished by varying the cathode resistance of tube T2, T2 orTz. The values indicated in Fig. 2 are appropriate for one specific application, but for other "purposes,V instruments, Vetc., dierent values Imight be chosen; see for instance, Fig. 3,

Y hereinafter described.

Analysis of the circuit shown in Fig. 1 gives the following expression for the inputimpedance y,

' and 11p is the plateiresistance of' T1. 'For a'triode,

,where .Re can .easily vbe made `very muchlarger )than rp, the ratio ,is .easily approached. However, this cannot .he .donegfora pentode. Therefore, while `the-circuit promises -less improvement .for a triodefarconsiderable improvement .can be achieved .when 511i isfarfpentode. 4-Ivt should be .borne in mind'that .-,u ,for a-.triode .maybein :the l.ortierpf 20.,- while fonaipentode, ,u may be .about .41,000..

It is also shown thatfor T -1 as a pentode *Rafa* ARB Y Y Where .A yis .the amplification .of the interznediate .amplier I2 .and RB is the .resistor `,inthe .plate circuit -of 717.1.

:It is clear lthat if Ri .(Qr impedance, however contrived) .isltobe .large,.then.ARB must .be .made

' ,returning the cathode of ,tube .T3 to .the .point intermediately vof .the catho de-tc-ground .resistance and ,grid-to-ground resistance, .at .the first tube, .and Joy the .capacitorcouplng .of Lthe first pathodefto the last-mentioned return, as .described., and as .is embodied in al1 'the .a-lires Aof .the drawing. Y

The capacitance'from grid to .cathode will 'be reduced by the .Same 'factor .that the input resistanceis increased. Since thisY factor iis much ilarger than .can be obtained with a `simple cathode lfollower, the circuit has the advantage of permitting. the use of along extension cable beltween a microphoneand the preamplifier. VFur-- thermore, sufficiently large `values of .input impedan'ce :can be obtainedusing this circuit 'withjout using a `grid resistor .greater than 21 megohm. This contributes `greatly tothe stability of 'the 'bias voltages.

The inventionmay, for example, take .the form indicated in Fig.2, including a condenser micro- Y phone of 25 ,auf capacity used with a grid resistor of .5 megohm. The rcombination of 25 auf and .5 megohm without cathode follower action has a cutoff frequency (3 db. down) of 13,000 C. P. S. With the pentode cathode follower convention- :allyiused .theff'cutoff frequencyiisf reduced to 150 .'C. P. S. `When .the superchargeriis added, the system is flat to 20 C. P. S. It will thus be seen ithat in addition to other advantages mentioned,

the v.invention veiects an extension of the lower .limit of Jfrequencyresponse which may be utilized in a preamplifier. This makes it possible to take better' advantage of 'the high quality response ofthe condenser vmicrophone than would be the 'case where dependence is placed mainly on re- -sistors for Ithe impedance. To test the effect of :the .addition-.of va iiexible extension cable, a capacity of 1000 auf was connected from grid tocathode. The resulting insertion loss was 1 db. for frequencies below 110,000 C. P. S. The input :impedance :of :the supercharged V-scatl'r'odefchll'ower is therefore :increased :by a factor :of .fabout 230,0.

The supercharged ;catho de: follower .circuit flir- Vnishes an-excellent :basis ffor the design fafa-a sound Vpressure .meter :whose purposeis vtmgive za .direct .reading iof sound pressure level :when :the rsource is a condenser microphone and the final iloa'dfis an electronic voltmeter.

' In liig. 3 Ifhave .indicatelda schematicdiagram for :a .sound pressure Lmeter. 'The arrangement shown Vincludes the .supercharged ecatnod'e :fol-

lower circuit vsat .fthe .uppergletzside .ofithe iig-nreV Y -Cdentical with. Fig. .'2, except-that afhig'her plate :supply -voltage .is indicated), additional amplify- ;ing tmeanslat the :upper .fright-diend .side :of irthe ng-ure, and :internal power .suppl-y :means 'at the :lower '.sicle .of the :figure Y 1t 'will be .apparent that-several noi/rel features -are present..V -A regulated .power supply maybe included within the soundjpressuremeterandzthe 'entire .device :may "be A. `iC..-opera'.ted .and :self- .contained The .control .amplifier in hthe .regu- :lated power supply receives .its signal through a V`filter directly from-.the .point whose iloltagefis to .be controlled, inamely, fat :the base :of the fgrid 'resistor of lthe `first tube, 'which :voltage is :the e:bias on'the condenser microphone. Therefore, the condenser Apolarity v.bias is vfautornatically -maintained iand should require 1only .infrequent checking unless :tubes are :changedV .An additional .amplifier :has V:been included, (1.9 to 'provide, an .amount of'gain equal to fthe Vcorrection ?term which :is knormally required to convert `vol-tage output in lmicrovolts .to sound pressure levels (4-6 db); (2) to provide a '-.very'low output -impedance Vso as Vvto make the Lgain Lof the instrument Vsubstantiall.;T independent of load or Jcable length; (r3) "to isolate thersuperch-arger circuit lfrom load impedance; 14) to provide suliic'ien't output power to operate directly into filters. The entire instrument can be 'buil-t into a case "ap- Vprcximately 10" lx 6" -x 5. 'jVarious other applications and modifications maybe practised vin `keeping with the scope of 'the invention, as 'deiined by the appended claims.

It lwill be noted rfrom Fig.. 1 that'the parameters :have been 'so selectedfand combined that V"a vdirect 'coupling 1is 'made Apracti'cab'le. "between of an alternating current input. 'This enables the voltmeter to be calibrated in decibels or other units to ft various situations. At the same time that the high input impedance is achieved with small value resistors effective, as explained, the well understood advantages of low output impedance are retained.

Having thus described my invention, what I claim is:

1. A supercharged cathode follower preamplifier circuit of high input impedance and low output impedance for condenser microphones comprising first and second cathode follower tubes and an intermediate conventional amplifier tube, the first cathode follower and the amplifler having respectively first and second cathode resistances returned to ground a direct connection from the cathode of the second cathode follower to a midpoint of the cathode-to-ground resistance of the first cathode follower, a capacitor bridging the cathode end of said last-named resistance, the grid of the first cathode follower having a resistor to said mid-point, a plate potential source connected to the said tubes with interposed resistance plate loads for the first cathode follower and amplifier only, a capacitative coupling between the plate of the first cathode follower and the grid of the amplifier, a direct connection between the plate of the amplifier and grid of the second cathode follower, input leads direct to the grid of the first cathode follower and ground, and output leads respectively direct from the cathode of the second cathode follower and ground.

2. The structure of claim 1 wherein the parameters of the tubes are proportioned in values so that a common voltage level is independently maintained at the said mid-point of the said cathode-to-ground resistance of the first cathode follower and at the cathode of the second cathode follower.

3. The invention of claim 2 wherein the said cathode resistor of the amplifier is variable whereby correction of potential balance between the cathodes of the two cathode followers may be effected by variation of grid bias of the amplifier.

In a preamplifier system for a condenser microphone the circuit as set forth in claim l, .l-.Power supply therefor having grid-controlled electron tube amplifiers including a control tube having an anode, a cathode, and a control grid and means to automatically regulate the polarity bias across the said input leads comprising a connection between ground and said mid-point of said cathode-to-ground resistance of the firstnamed cathode follower in a control grid circuit of said control tube.

PAUL S. VENEKLASEN.

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

UNITED STATES PATENTS Number Name Date V2,270,012 Shepard, Jr. Jan. 13, 1942 FOREIGN PATENTS Number Country Date 515,158 Great Britain Nov. 28, 1939

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