US2200519A - Thermionic amplifier circuits - Google Patents

Description

May 14, 1940- w. s; PERclvAL IHERMIoNIc AMPLIFIER cIncuITs File-d sepi. 11., 19375 INVENTOR W. s. FERC/VAL ,QZ WL, ATTORNEY Patented May 14, 1940 UNITED STATES PATENT oFi-icE 'rIIERMIoNIo AMPLIFIER CIRCUITS Application September 11, 1937, Serial No. 163,446

In Great Britain September 15, 1936 3 Claims.

This invention relates to coupling circuits for thermionic amplifiers and is particularly concerned with circuits intended to pass wide bands of frequencies as required' in television systems.

It is necessary for filters used in superheterodyne vision receiving circuits to be capable of transmitting a frequency pass band of several megacycles and the use of complicated filter circuits presents difficulties due to the self or distributed capacities of the inductance coils used in the lters.

The object of the invention is to provide a simple form of band pass filter circuit which mitigates difficulties due to the self or distributed capacity of coils and is easily built and adjusted.

According to the present invention a circuit arrangement includes a band pass filter or a coupling circuit in which the leakage inductance of a coil acting as an autotransformer constituting an element in said ilter or coupling circuit, is utilized in conjunction with a condenser or other capacity to provide a tuned circuit. The inductance of said coil in the case of a band-pass filter provides the inductance'of the shunt arm of a half-section, and the leakage inductance of said coil acting as an autotransformer provides the inductance of the series arm of the same halfsection.

In a particular circuit arrangement according to the invention the inductance of said coil in conjunction with capacity provided by a condenser or constituted by the inter-electrode capacity of a thermionic valve or valves forms the shunt arm of a half-section of a band pass lter, the series arm of which is formed by a condenser and the leakage inductance of said coil acting as an autotransformer, said condenser being connected to a tapping point in said coil.

In a thermionic valve amplifier circuit arrangement embodying the invention and capable of amplifying a range of frequencies of the Width employed in televisionl systems, the shunt arm of a half-section of a band pass filter is constituted by an inductance coil and the inter-electrode capacity of one or more valves in said circuit, and

the series arm of said filter is constituted by a condenser and the leakage inductance of said coil acting as an autotransformer, said condenser being connected to a tapping point in said coil.

In order that the invention may be more clearly understood and readily carried into effect, a known form of circuit and forms of circuit einbodying the invention will now be described by way of example with reference to the drawing in which:

Fig. l shows a known coupling circuit,

Fig. 2 adds to Fig. 1 lthe feature of a band pass lter, v

Fig. 3 shows a circuit embodying the invention,

Fig.V 4 shows a modified form of circuit embodying the invention.

Referring to the known circuit shown yin Fig. l of the drawing, the grid and plate capacities of the valves I and 2 are represented by a single condenser 2c shown in dotted lines.

This can be considered as a half-section of an ordinary low-pass filter terminated simply by a resistance R and with an additional capacity equal to that of the half-section at the other end. This, as has been shown in the specification of`my co-pending patent application Serial No. 1263??, filed February 18, 1937, now Patent No. 2,156,656. is the condition for a flat response when used in conjunction with a valve, such as a screenedgrid valve, with a high internal impedance'.

With this arrangement f-mT'z 0R t where f is the cut-off frequency of the filter and R, C and L are the resistance, capacity (2C) and inductance respectively shown in the drawing.

In the band pass lter case shown in Fig. 2, the inductance L1 is designed to resonate with the grid and plate capacities of the valves I and 2, again represented by the condenser 2c. A condenser Cz and inductance L2 are designed to resonate at the mean frequency fm of the pass band.

With this arrangement, as before,

l fz-WCR where f is the pass band of the corresponding complete lter, and C2==1L2C where The arrangement shown in Fig. 2 has been found to operate fairly satisfactorily, but suffers from the disadvantage that the effective shunt capacity is increased andthe pass band consequently decreased by the capacity of the coil Le to earth. An additional disadvantage is that tle self capacity of the coil L2 cannot be neglected.

In an arrangement embodying the invention as shown in Fig. 3, both the disadvantages inherent in Fig. 2 are overcome and in addition, one coil is eliminated. Thus, referring again to Fig. 2, if the combination of inductance La,

capacity C2, and resistance R are tapped down the coil L1, it can be shown that the equivalent circuit is not changed providing that the inductance Lz Which is constituted by the leakage inductance of L1 and resistance R are decreased and the capacity C2 is increased by proportional amounts. The response of the stage of amplification will therefore remain substantially constant at all frequencies in the pass band. As the tapping point on the coil L1 is moved nearer to the earthed end a point is eventually reached at which L2 is constituted solely by the leakage inductance of the coil L1. The circuit therefore becomes as shown in Fig. 3 from which it will be seen that the physical coil L2 of Fig. 2 is omitted. If desired, separate coils may be employed for the two portions of L1.

The circuit Fig. 3 may very easily be adjusted as follows: The combination of resistance R and condenser C2 is disconnected from the coil L1 which is then adjusted to resonate with the interelectrode capacity 2C at the required mean frequency of the pass band. RC2 are then connected to the coil L1 and the resistance R is short circuited while C2 is adjusted until the desired shape of curve, for example a single or a double hump curve is obtained. It will be seen that the three adjustments specified may be made entirely independently.

In Fig. 4 an arrangement similar to Fig. 3 is shown but in this case an m-derived termination is employed in which m=0.65 approximately. An inductance L3 and condenser Cz are connected in series with an inductance L4 and condenser C4 in parallel. Thus, at the expense of some complication a still wider pass band can be obtained for the same stage again. If required, the coil L3 can be eliminated by connecting the condenser C3 to a tapping point down the coil L1. As the coil La is quite small however, this last step is not generally advantageous.

What is claimed is:

l. A thermionic amplifier circuit arrangement capable of amplifying a range of frequencies of the Width employed in television systems and including a band pass filter which comprises an inductance coil and a condenser connected to a tapping point on said coil, the shunt arm of a halfsection of the lter being constituted by said inductance coil and the inter-electrode capacity of at least one amplifying tube in said circuit, the series arm of the half-section of said filter being constituted by said condenser and the leakage inductance of said coil which acts as an autotransformer by reason of the connection of said latter condenser to said tapping point on said coil.

2. An amplifier circuit for the uniform amplication of a wide band of high frequencies, comprising a pair of discharge tubes and coupling means between the output of the rst tube and the input of the second tube which includes a high frequency coil in the output circuit of the rst tube and a series path including a condenser and a resistance connected across a portion of said high frequency coil, and an m-derived termination connected across said resistance, said termination consisting of series connected inductance and capacity connected in series with a parallel combination of inductance and capacity.

frequency of said band, and a series path inn cluding a condenser and a resistance connected between an intermediate point on said coil and a low potential point of the circuit, said condenser and the leakage inductance of the autotransformer constituted by the tapped coil forming the series arm of said half-section of the filter and resonating at the mean frequency of the band.

WILLIAM SPENCER PERCIVAL.

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