US2151795A - Thermionic amplifier circuits - Google Patents

Description

March 28 1939. w 3 PERCNAL 2,151,795

THERMIONIC AMPLIFIER CIRCUITS Filed Sept. 14, 1937 lNVENTOk w. s. PERCIVAL I W I ATTORNEY Patented Mar. 28, 1939 PATENT OFFICE THERM'IONIC ANEPLIFIER CIRCUITS William Spencer Percival, London, England, assignor to Electric & Musical Industries Limited Application September 14, 1937, Serial No. 163,758 In Great Britain September 24, 1936 2 Claims. 01. 178-44) This invention relates to coupling circuits in thermionicvalve amplifiers and is particularly concerned with coupling circuits intended to pass wide bands of frequencies. In television systems it is necessary for coupling circuits to be capable of passing frequency bands of the order of four megacycles in width.

In the specification of my co-pending patent application Ser. No. 126,317, filed February 18, 1937, it was pointed out that the highest value of frequency pass-band multiplied by amplifier stage gain can be obtained by allowing the shunt capacities .due to the grid and plate capacities of valves employed in an amplifier circuit to act as the shunt capacities of the sections of a filter terminated at one end and with an extra capacity equal to that of a half section at the other end.

Again, inthe invention set forth in the specification of my co-pending patent application Ser. No. 163,446, filed September 11, 1937, use is made of the leakage inductance of a coil forming an element of a coupling circuit designed to pass a wide range of frequencies, together with a condenser, to provide a tuned circuit forming a section of a filter. In a particular arrangement described in the specification of that application a condenser is connected to a tapping point in an inductance coil connected in shunt across the amplifier circuit, the leakage inductance of that coil forming the required inductance.

According to the present invention a coupling circuit for a thermionic valve amplifier designed to pass a wide band of frequencies is provided in which separate tuned sections of the circuit include inductance elements connected to tapping points in an inductance connected in shunt across the amplifier circuit, the inductance elements being coupled together so that the effect of the mutual inductance between the coupled inductance elements tends to neutralize the effect of the leakage inductance arising from the connection of the inductance elements to the tapping points in the shunt inductance. Preferably the coupling between the coupled inductance is adjusted to produce substantially complete neutralization of the said leakage inductance.

In order that the nature of the invention and the method of carrying the same into effect may be more clearly understood, a circuit arrangement to which the invention may be applied, together with explanatory circuits will now be described, with reference to the accompanying drawing, in which Fig. 1 shows a form of amplifier circuit forming a band-pass filter to which the invention may be applied and Figs. 2, 3 and 4 are explanatory diagrams.

Fig. 1 of the drawing shows a simple band pass amplifier circuit comprising valves l and 2. The input circuits (not shown) of valve l is connected to the control grid in the usual way. The output circuit, connected to the anode, comprises condenser C1 and inductanceLi, and coupling inductance L2 across-which the input circuit of valve 2 is connected. This input circuit includes the shunt branch including inductance L3, condenser C3 and resistance R1. A high tension supply is connected to the anode of valve I over high frequency choke L4, as indicated by the sign at the end of the lead, the lead being shunted by a suitable decoupling condenser C5 to prevent interfering efiects being set up through the supply. The plate-cathode capacity of valve I is also effective in the output circuit as represented by the shunt condenser 04 shown dotted, and the grid-cathode capacity of valve 2 is effective in the input circuit of the valve as shown by shunt condenser C2, also shown dotted. 'Ihe cathode leads of both valves include resistances R2 and R3 respectively with by-pass condensers Cs and 07 respectively, in accordance with wellknown practice.

A simple circuit of this form sulfers from practical disadvantages when the frequency passband to be handled is several megacycles wide and when the shunt capacities are small the self capacities of the inductance coils L1 and L3 seriously upset the response characteristic, and their capacities to earth materially increase the effective shunt capacities of the filter.

In Fig. 2 of the drawing the filter arrangement of Fig. 1 is represented more clearly, elements corresponding to those in Fig. 1 being indicated by like reference letters. In Fig. 2 the condenser C1 is shown as tapped down the coil L4 as in the arrangement described in the specification of application Ser. No. 163,446 already referred to and the inductance elements L1 and L3 are shown tapped down the inductance coil L2, the valves I and 2 of Figure 1 and associated immaterial elements C5, C6, C7, R2 and R3 having been omitted. The tapping down allows the values of inductances L1 and L3 to be reduced and even without the mutual inductance shown effects a certain improvement, since the self-capacities of the coils L1 and L3 are less serious owing to their smaller inductances, and the method is satisfactory if the leakage inductance of the coil L2 is small. However, at high frequencies high permeability cores which would be necessary in order to reduce the leakage are not very suitable owing to losses and the variation of permeability with frequency.

In the absence of mutual inductance between the coils L1 and L3 the circuit equivalent to Fig. 2 would be as shown in Fig. 3 where elements U2 and L4 are equivalent to the elements L2 and L4 of Fig. 2 and in which inductance Z represents the leakage inductance of coils L2 and L3. This leakage inductance is undesirable. To overcome this drawback, according to the present invention, coils L1 and L3 are coupled together. This, as is well known, increases the effective inductance of each of the coils so that they can be made smaller than they otherwise would be and thus have reduced self-capacity, and moreover, with this arrangement an apparent negative inductance, M say, is effectively inserted in series with the leakage inductance produced due to the auto-transformer effect between coils L1, L2 and. L3 and represented by the coil 1. The coupling between the coils L1 and L3 is represented by the dotted line M in Fig. 2. By adjustment of this coupling the value of M can be made equal to l which is thus neutralized and the resultant circuit is that of Fig. 4 which is the required standard band-pass filter. In this figure, elements L1, L'z, Us and U4 are the equivalent to elements L1, L2, L3 and L4 of Fig. 1.

The plate capacity of valve l and the grid capacity of the valve 2 may be approximately equal. This is not an essential requirement, however, because if, for example, the capacity at the unterminated end of the filter is low, the series resonant circuit comprising elements L1 and C1 can be tapped down the inductance L4 and a suitable change made in the value of the coil L1.

What I claim is:

1. A coupling network for connection between a pair of vacuum tube amplifiers and adapted to pass a wide band. of frequencies, comprising an inductance included in the plate circuit of one tube and a second inductance included in the grid circuit of the succeeding tube, a series tuned circuit connected between the first inductance and a tapping point on the second inductance, a second series tuned circuit connected between said tapping point on the second inductance and the low potential end thereof, said series tuned circuits being provided with inductance elements which are coupled together so that the efiect of the mutual inductance therebetween tends to neutralize the efiect of the leakage inductance arising from the connection of said inductance elements to the tapping point on said second inductance.

2. A coupling network for connection between a pair of vacuum tube amplifiers and adapted to pass a wide band of frequencies of the order of several megacycles, comprising an inductance included in the plate circuit of one tube and a second inductance included in the grid circuit of the succeeding tube, a series tuned circuit including a condenser and a coil connected between a tapping point on the first inductance and a tapping point on the second inductance, a second series tuned circuit including a condenser, a coil and a resistance connected between said tapping point on the second inductance and the low potential end thereof, the coils of said series tuned circuits being coupled together so that the effect of the mutual inductance therebetween tends to neutralize the effect of the leakage ini

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