US2120823A - Coupling means for thermionic valve circuits - Google Patents

Description

' June 14, 1938. E. c. WHITE COUPLING MEANS FOR THERMIONIG VALVE CIRCUITS Filed April 8, 1936 "VI EH70)? F. L. WH/TE 57' ATTORNEY mon the feature that the coupling to the next sociation with the first valve.

Patented June 14, 1938 COUPLING MIEAN S FOR THERMIONIC VALVE CIRCUITS Eric Lawrence Oasling White, Hillingdon, England, assignor to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Application April 8, 1936, Serial No. 73,310 In Great Britain April 3, 1935 4 Claims. (Cl. 179-171) The present invention relates to coupling valve is so arranged as to introduce a measure of means for thermionic valve circuits. compensation for the increase in effective anode It is the object of the present invention to proload at low frequencies. The values which the vide improved coupling means capable of transcoupling components should have in the triode 5 mitting with substantially uniform attenuation case can be readily arrived at by calculation in oscillations covering a wide range of frequencies well-known manner. including zero frequency or direct current. It is an object of the present invention to pro- A thermionic valve circuit is known in which vide improved coupling means which are parthe anode of a screen grid amplifying valve, the ticularly suitable, for example, for coupling thercathode of which is at earth potential, is conmionic valve amplifying stages in cascade. '1 nected to the positive terminal of a source of our- The present invention provides a circuit arrent having its negative terminal earthed, rangement comprising a first thermionic valve through two resistances in series, a decoupling coupled in cascade to asecond thermionic valve, condenser being connected between the join of in which the anode load impedance of said first these resistances and earth. Of the two resistvalve, together with the anode impedance of ances, that connected to the anode serves as the that valve, can be represen OVeI' a o n anode load resistance of the valve, while the other range of frequency down to and including zero acts as a decoupling resistance. The decoupling frequency by a pure resistance R in series with an condenser becomes less effective as the frequency impedance Z, in which the anode of said first of the oscillations applied to the control grid devalve is coupled to the Control grid Sa d S nd 20 creases, with the result that the effective anode valvethrough a oupling imped and in load impedance increases and the lower frequenwhich a leak resistance R is connected between cies are accentuated. the control grid and the cathode of said second In multi-stage amplifiers, the input circuit of a valve, if desired through a suitable source of negsucceeding valve is usually coupled to the valve ative bias potential, characterized in that, in or- 25 referred to in the preceding paragraph by means der to ensure that the ratio of the amplitude of of a condenser and a resistance connected in sean oscillation applied to the control grid of said ries between the anode and cathode of the first first valve to the amplitude of an oscillation set valve, the input to the second valve being taken up at the control grid of said second valve is subfrom the ends of this resistance. The last-named stantially uniform throughout said Working 3 condenser reduces the lower frequencies and the range, it is arranged that the relationship non-uniformity of response due to the effect of I I the decoupling condenser is therefore decreased. If the decoupling resistance and condenser have values represented by R2 and C2, the anode load 15 Substantlally sa'fllsfied- 35 resistance a value represented by R3, and the The P Of the anode load i p ance of the other condenser and resistance values reprefirst Valve Which may be represented by an imsented by C'z and R's, it is known to select these Pedence Z sually comp o 01 more devalues such that R3C2=R3C2; in these circum- Coupling condensers- 40 stances, substantially uniform amplification can Fu t r features of present V ion Will 40 be obtained over a considerable range of low freapp from the following pt on and. apquencies, but such an amplifier is not, of course, pend d cl ims.

capable of amplifying direct current. The invention will be further described, by way It is also known that, in the case in which the of example, with reference to the accompanying first valve is a triode, a similar compensation for schematic drawing, in which 45 the effect of the decoupling condenser can be 010- Fig. 1 illustrates an embodiment of the inventained; since, however, a triode valve differs from tion in which the first valve is a screen grid valve, screen grid and like valves in that the anode cur- Figs. 2 and 3 are circuits which are equivalent rent varies markedly with variations in anode to that of Fig. 1, voltage, the relationships between the Values of Fig. 4 is an equivalent circuit of an arrange- 50 the coupling elements are relatively more commentof the kind illustrated in Fig. 1, but employplex. In the triode case, the dispositions of the ing a triode first valve, and

coupling elements are the same as in the case of a Fig. 5 illustrates an arrangement employing screen grid valve, and the two cases have in comtwo decoupling resistances and condensers in as- Referring to Fig. 1, a screen grid valve V1 has its anode connected through anode load resistance R2 and decoupling resistance R2 in series to the positive terminal of a source B of anode current, the negative terminal of which is returned to the cathode of valve V1. Decoupling or by-pass condenser C is connected between the join of resistances R2 and R3 and the cathode of valve V1.

The anode of valve V1 is coupled to the control grid of a second valve V2 through a condenser C"2, the latter being provided with leak resistance R's. 'Resistance R2 is connected'in shunt with condenser '2, and a resistance R1 is connected between the cathode of valve V2 and the negative terminal of source B. Oscillations to be amplified are applied to input terminals I;

valve V2 acts as a so-called cathode follower valve, and the output of the device is taken from terminals 0.

In the equivalent circuit of Fig. 2, the resistance R and the impedance Z by which the anode 'load impedance of valve V1 may be represented,

the impedance Z and the resistance R are given by the following expressions:

R Rg

R( R+Z) v R+R|Z+Z where ia. is the change of current in the plate circuit of valve V1 for unit change in the instantaneous potential of the control grid of valve V1.

If now WIN Thus for a current is which, for a constant input, is constant over a range of frequency, V is also constant.

It will be noted from Expression (II) that V has its maximum value when m is much greater than 1, and R is accordingly preferably made much greater than, say several times, R.

Now in the arrangement of Fig. 1, the impedance Z is constituted by a resistance R2 shunted by a condenser C2, and in order that the relationship (I) above shall be satisfied, the coupling condenser C2 and coupling resistance R2 are iven the values and mR2 respectively. In these circumstances,

R2C2 is equal to the time constant C2R2, and it can readily be shown in View of this equality that The static positive bias on the control grid of valve V2 due to its conductive connection to the anode of valve V1 may be neutralized by a suitable choice of the value of R4, or means such as a bias battery or the like may be provided for biasing the cathode of valve V2 to a suitable positive potential.

As has been pointed out, the invention is not limited to cases in which the first valve V1 is one of the screen grid type; when this valve is a triode, the elfect of its anode impedance as a shunt across the anode load impedance must be taken into account, and the manner in which this is achieved Will be described with reference to Figs. 3 and 4. Fig. 3 is an equivalent circuit of the arrangement of Fig. 1, the resistances RA, RB and the capacity CA being given by the expressions must be substantially satisfied, where Rv is the anode impedance of valve V1. The way in (III) which the equivalent circuit of Fig. 4 may be transformed into a practical circuit of the kind illustrated in Fig. 1 need not be discussed in detail, since transformations of this kind are wellknown to those versed in the art; similar transformations are discussed in Transmission circuits for telephone communication, by K. S. Johnson, New York, 1925. It is only necessary to point out here that owing to the fact that the anode impedance Rv of the first valve V1 must be taken into account in the triode case, as indicated in Expression (III), the expressions relating the various circuit elements will diifer somewhat from the expressions for the tetrode case.

Reference is now directed to Fig. 5, which shows an arrangement of the kind shown in Fig. 1, but modified by the use of an additional decoupling resistance R5 shunted by a further decoupling condenser 0:. The condenser C2 may be regarding as effectively in shunt with both resistances R2 and R5.

The circuit of Fig. 5 can be redrawn in the form shown in Fig. 2, and in order that 7 should again equal an additional coupling condenser C'5 and a further coupling resistance R5 in shunt are provided, resistance R's being arranged in series with resistance R2, and both resistances being shunted by condenser '2.

In order that the relationship shall be satisfied, condenser 0'5 and resistance R's are given the values and mRs respectively, so that As in the arrangement of Fig. 1, it is arranged that the following time constant equalities also hold:

The arrangement of Fig. 5 may be modified by the use of a triode first valve, the method of calculating the values of the circuit elements being similar to that outlined with reference to Figs. 3 and 4.

The invention is not limited to the arrangements described, and many modifications thereof within the scope of the appended claims will occur to those versed in the art.

I claim:

1. A circuit arrangement comprising a first valve having a cathode, a control grid, a screen grid and an anode, a second valve having a cathode, a control grid and an anode, a source of potential difference having its negative terminal connected to the cathode of said first valve, a load resistance and a decoupling resistance connected in series between the anode of said first valve and the positive terminal of said source, a decoupling condenser effectively in shunt with said decoupling resistance, a leak resistance connected between the control grid and cathode of said second valve through a source of bias potential, and connected in shunt between the anode of said first valve and the control grid of said second valve, a coupling resistance and a coupling condenser of such values that the time constant of said load resistance and said decoupling condenser is substantially equal to the time constant of said coupling condenser and said leak resistance, and the time constant of said decoupling resistance and decoupling condenser is substantially equal to the time constant of said coupling resistance and coupling condenser.

2. A circuit arrangement according to claim 1, wherein a further decoupling resistance efiectively shunted by a further decoupling condenser is connected in series with said first-mentioned decoupling resistance, and wherein a further coupling resistance efiectively shunted by a further coupling condenser is connected between the anode of said first valve and the control grid of said second valve, the said further coupling resistance and condenser together having a time constant substantially equal to that of said further decoupling resistance and condenser.

3. In an amplifying circuit capable of transmitting with substantially uniform attenuation oscillations covering a wide range of frequencies including zero frequency or direct current, a pair of electronic tubes connected in cascade, each of said tubes being provided with at least an anode, a grid electrode and a cathode, a source of space current for said tubes, said source having a positive terminal and a negative terminal, means for impressing the oscillations to be amplified between the grid electrode of the first tube and the cathode thereof including a pair of input terminals adapted to be connected to a source of signal energy oscillations, a connection between the anode of the first tube and. the positive terminal of the source including a load impedance and a filter impedance in series, a connection between the negative terminal of the source and the oathode of the first tube whereby the anode of the first tube is maintained at a positive potential with respect to the cathode thereof, a filter condenser conneced between the cathode of the first tube and a point of said first named connection intermediate the two impedances, means connecting the anode of the second thermionic tube to the positive terminal of the source, means including a resistor element connecting the oathode of the second tube to the negative terminal of the source, a circuit for transferring energy from the first tube to the second tube comprising a connection between the anode of the first tube and the grid electrode of the second tube, said connection including a coupling condenser shunted by an impedance element, a leak resistance element connected between the grid electrode of the second tube and the cathode thereof, and means including a pair of output terminals for connecting a utilizing device across the resistor element which is connected between the cathode of the second tube and the negative terminal of the source.

4. A circuit arrangement as described in the next preceding claim in which the following relationship is substantially satisfied:

where R2 ERIC LAWRENCE CASLING WHITE.

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