US2370399A - Electrical circuits - Google Patents

Description

Feb. 27, 1945. 5 GQQDA'LE 2,370,399

ELECTRICALL CIRCUIT Filed July 14, x942 RESPONSE FREQUENCY I INVENTOR A E. 0. GOODALE ATTORNEY Patentedfeb. 27 19.45

UNITED STATES PATENT OFFICE I :racm'rs Elmer Dudley Goodale, 'Bayside, Long Island N. Y., 'assignor-ito Radio Corporation of Amer:

.ica, a corporation of Delaware imentary s,

1942, Serial No. 450.888

This invention relates to thermionic amplifiers, and more particularly, to thermionic am'plifiers requiring substantially uniform amplification and substantially linear phase characteristic of a very wide band of frequencieasuch as are used in television and video systems.

, In many applications, it is necessary to have thermionic amplifiers capable of amplifying frequencies of a very wide band, and at the same time to insure that the relative phase positions of the currents to be amplified are maintained after amplification. Uniform amplitude of a,

wide band of frequencies cannot be achieved without some form of compensation due to the inherent anode-to-cathode capacity and control grid-to-cathode capacity. These inherent capacities of the thermionic amplifier, while relatively small, being only on the order of tens of micromicro-farads, nevertheless become important at frequencies above 0.1 meaacycle.

The usual practice to obtain uniform amplification is to insert compensating inductances of low values which tend t resonate with the inherent tube capacities,-and consequently. to maintain the input and output impedances of the thermionic tubes in the amplifier. However, useof such inductances may result in an impedance characteri'stic which oscillates about a mean value in the region of the resonant frequency of the compensating inductance and tube capacity, and such an oscillatory impedance characteristic is accompanied by a phase characteristic which is no longer linear.

By my invention, I overcome the shortcomings of previously known compensated amplifiers by first of all providing a monotonic impedance characteristic up to a predetermined limiting frequency, o ether with a phase characteristic which is substantially linear throughout the useful band of frequencies to be amplified.

In accordance with my invention, I use a form of series-shunt-peaking" circuit in which the shunt arm comprises an inductance in series with ,a parallelly connected resistor and condenser,

"these conditions, it is possible-to. obtain substantially constant amplification and'substantial- 1! linear phase characteristic over a band of frequencies 20% greater than is possible byusing other forms of compensating or "peaking networks. 6 Moreover, it is possible to utilize the distributed capacity of one or more of the circuit elements to provide the necessary relationship among the elements to give uniform amplification and substantially linear phase characteristic. ample, in a television transmitter where a water cooled resistor is used in series with the inductance in the shunt arm, the distributed capacity across the water cooled resistor, by suitable design, may be made to have the proper value to is meet the conditions necessary for uniform amplification. D

The benefits of my inventiontarise from the careful proportioning of the values of the circuit parameters used with respect to each other takmg into account the values of the tube distributed capacities, as well as the stray capacities ofthe circuit elements to ground.

Accordingly, the main object of my invention is to provide a new and improved thermionic amplias and substantially linear phase characteristic over a, wide band of frequencies.

Another object of my invention is to provide a thermionic amplifier which" is compensated to 30 give substantially uniform amplification and 1 linear phase characteristic over a given wide band of frequencies with increased over-all gain.

Another object is to provide a thermionic amplifier having a load circuit comprising a shunt :5 arm of an inductance connected in series with a parallelly connected resistor and capacity and a series-arm comprising an inductance to provide substantially constant impedance over a wide band of frequencies.

Still another object of my invention is to provide a thermionic amplifier in a television transmitter in' which the distributed capacity of a water cooled resistor is so chosen and'utilized to provide uniform amplification and-substan- 45 tially linear phase characteristic.

, Other objects-of my invention will become apparent to those skilled in the .art upon read ing the following detailed description in conjunction with the drawing.

In the drawing.

Figure 1 shows schematically a circuit of a V thermionic amplifier embodying my invention;

Figure 2. shows schematically an equivalent electrical network of the circuit arrangement as shown in Figure 4:

For exfier having substantially uniform amplification V acteristic over a predetermined frequency band.

' cuit elements, it is necessary that the elements q Figure 3 shows in graphical form the response curve of an amplifier embodying my invention, compared to the response curve of an amplifier having a different form of compensating network;

Figure 4 shows schematically a modification of my invention embodied in Figure 1; while Figure 5 shows details of a water cooled resistor.

Turning now to Figure 1, I have shown, for purposes of describing my invention, a two-stage thermionic amplifier comprising a first thermionic tube 5 having a cathode 1, a control electrode 9 and an anode ll. Input signals are supplied to the terminals l to impress the signals between the cathode 'land the control electrode 9. A bias battery 3 serves to provide the proper operating characteristic of the tube 5, A'battery 4 supplies the anode II with operating potentials through the resistor l5 connected in series with the inductance IS. The resistor is is shunted by condenser l1, and I have shown, in dotted lines, the distributed anode-to-cathode capacity as the condenser I3. I

The second thermionic amplifier 29 is capacityresistance coupled through the condenser 23 and resistor 25 to the anode circuit of the tube 5, through the inductance 2|. There is thus impressed between the cathode 3i andthe control work or a load circuit of the amplifier shown in Figure 1. In Figure 2, the condenser C1 has the value of the'distributed capacity l3 between the anode II and the cathode I of tube 5, plus the stray capacity to groundof the circuit elements up to L2. The condenser C2 of Figure 2 has the value of the capacity 26 representing the electrode capacity between the control electrode 31 and the cathode 3|, plus the stray capacity to ground of the elements associated with the control electrode. The resistor R of Figure 2 represents the resistance l5, while the capacity C: of Figure 2 represents the condenser ll of Figure l. The inductances L1 and L2 of Figure 2 represent the inductances l9 and 2!, respectively, of Figure 1. Since the condenser 23 and resistor 25 are used for coupling purposes, these can be neglected as far as the high frequency response is concerned, since the condenser 23 has a very large value and, consequently, its impedance is so low'as to comprise effectively a short circuit in the connection'between the inductance 2i and the control electrode 33. Likewise, the resistor 25 has such high resistance that it is effectively an open circuit between the control electrode 33 and the cathodeil, so that these two elements may be neglected.

It can be shown that the generalized impedance squared function is as follows:

where Z is the impedance of the network look ing in at the terminals 50, R is-the element independent of frequency, a, b, d and m are constants, and a" is equal to aiCR, where or represents the angular velocity of the currents to. be amplifled and C is equal to C1+C'2.

Equating now the like powers of the polynomials in the denominator and numerator, the following simultaneous equationswill be obtained.

or distributed capacities l3 and, 28. the amplifier may be given aflat response characteristic together with a. substantially linear phase char- The frequency band will be on the order of 20% greater than obtainable in other forms of amplifiers. l

In order to correctly proportion the various cirbe sochosen as to provide a substantially monotonic impedance characteristic. Such a characteristic may be obtained by writing downthe generalized impedance function, which will gen-'1 erally be a quotient of two polynomials, squaring the impedance function and orders of terms in the numerator and denominator to each other, and solving for the coemcient of the polynomials from the simultaneous equations obtained by equating the like terms of the polynomials. The derived coeflicients will thendetermine the proportions between the circuit parameters necessary to obtain proper im-.

pedance characteristic.

In Figure 2, I have shown the equivalent nct 7 6.5 then equating like finity.

Attention is called to the met that meo term in the denominator mustbe equal to zero, which requires at least one of the coefllcients equal to zero if the impedance characteristic is to be constant over all ranges of frequency out to in- From this, it is clear that-the circuit arrangement cannot-provide uniform amplification over an infinitely wide band of frequencies,

but the solutionvcan be restricted to giving uniform impedance characteristic upto a predetermined frequency by making the value of a term negligible. The solution will then be monotonicand the imper'ance will be substantially cohstant. The values above tabulated for the coefflcients a, b, d and 1 give a solution-in which the 3" term can be safely neglected without dis- ,turbing the condition of uniform amplification senses and substantially linear phase characteristic for all values of a: up to 2.2.

From these values the magnitude and relationship of the circuit arrangements may obtained since C'R wCR=2.15 for .l'db. deviation By substituting the above determined values for.

the coefiicients, it will be noted that L1 equals .53 L2 and C: equals 1.22 C1.

In view of the fact that L1 and L: are elements be readily which are added to the circuit, the achievement distributed capacity of the resistor may be made Y equal to that shown by the condenser i1, and consequently no additional capacitor is necessary.

.A conventional water cooled resistor is shown in Figure 5 and may be used as resistance element .I! in Figure 1 where power demands and dissipation requirements are such as will make.v

.the use of a water cooled resistor necessary or desirable. The resistor includes a ceramic tubular member Cl, the outside surface of which is coated -with a resistance material. Surrounding this ceramic tube is a glass tube orcylinder 62 and the ends of the ceramic cylinder 80 and glass cylinder 82 are closed by means or metallic caps ll. The caps are maintained in position by a bolt which passes through the center of'the ceramic tube 80 while a nut 88 is threaded on the bolt it to maintain the desired pressure. Naturally, gaskets may be used to prevent leakage or thecooling fluid. The caps 84 are provided with inlet and outlet tubes "and 12 respectively and since the caps 84 are in contact with the ends of the ceramic tube '0, an electrical current may be conducted to the resistor 69 by way of lead conductors 18. Water or an appropriate coolin: liquid or medium is circulated through the space between the ceramic tube 80 and the outer tube It. The heat produced by current fiow over the resistance element is readily carried away. Naturally, in the construction 0! such a resistance, a predetermined distributed capacity exists between the two end plates 04. This distributed capacity, i! of proper value, may replace the condenser il shown in Figures 1 and 4. The water cooled resistor-shown in Figure 5 is only by way of example since various other types of water cooled resistors may be used.

In certain amplifiers, it may be found that the ed resistor II and condenser il are'now in shunt to the control electrode circuit of the tube 29, and that the series arm comprising the inductance M is connected directly to the, plate so that the distributed plate capacity 5| acts as the terminatins element of the network, while the distributed control electrode cathode capacity 53 is substantially in shunt to the shunt arm of the correcting network. Since it has been assumed that the distributed capacity BI is larger than the distributed capacity 63, the relationship that Ca=1.22 C1 can still be met to provide the uniform amplification above described.

In Figure 3, I have shown the response characteristic of an amplifier embodying my inventiori in the curve 43, which curve is only. 1 down from unity response at predetermined fre- .quency. For comparison purposes, I haveshown the response characteristic 4| of a compensated amplifier using ordinary series-shunt peaking known to the art, which is down 1 /2% on a generalized frequency scale of 1.00. It will be noted that my new amplifier deviates from unity response-on a generalizedirequency scale at 1.23. In other words, the band width for the sameamplificatlon has been increased 23% by my new circuit arrangement.

It will be appreciated that, while I have shown various power-supplies as batteries, these may be derived from rectified current, and that the thermionic tubes, which are shown merely conventionallyior. illustration only as triodes, may be any other typeof thermionicamplifiers such as'pentodes.

From the above description, it, or course, will be apparent thatmany'and varied modifications from the general principles described and out:

a: the invention may be made without departing linedhereinabove, and I, therefore, .believe myself to be entitled to make any and all oi these modifications such as would suggest themselvestothose skilled in the art to which the invention relates, provided, of course, that such'modificahereinafter appended claims.

Having now described my invention, what I claim is: a

1. An amplifier comprising a first thermionic,

tube having at least acathode, control electrode -and an anode, an inductance connected in series with a parallelly connected resistor and condenser connected between said anode and said cathode, a second thermionic tube having at.

least a cathode, control electrode and anode, and a second inductance connected between the anode of said first thermionic tube and the control electrode of said second thermionic tube.

2. An amplifier comprising a first thermionic tube having at least a cathode, control electrode and an anode, an inductance connected in series with a paralielly connected resistor and condenser connected between said anode and said cathode, a second thermionic tube having at least a cathode, control electrode and anode, a second inductance connected between the anode of said first thermionic tube and the control electrode of said second thermionic tube, said first named inductance having a'value substantially equal'to .53 of said second named inductance,

3. An amplifier comprising a first thermionic tube having at least a cathode, control-electrode and an anode, an inductance connected in series with a water cooled resistor, said series inductance and resistance being connected between the anode and cathode of said tube, a second thermionic tube having at least a cathode, control electrode and anode, a second inductance connected between the anode of said first thermionic tube and the control electrode of said second thermionic tube, said first named inductance having a value substantially equal to .53 of said second inductance, and the distributed capacity across said water cooled resistor being chosen to have a value equal to substantially one-tenth the sum or the inter-electrode capacity between the anode and cathode of the first thermionic tube and the inter-electrode capacity between the control electrode and cathode of the second thermionic tube.

4. A television transmitter amplifier comprising a first thermionic tube having at least a cathode, control electrode and an anode, a seto .53 of said second inductance, and said distributed capacity having a value equal to substantially one-tenth of the sum of the stray capacity between the anodegnd cathode of the first thermionic tube and the stray capacity between the controlelectrode and cathode of the second themiioni'c tube.

- .--'E1MER DUDLEY 'GOODALE.

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