US2217839A - Wide band amplifier - Google Patents

Description

Oct. 15, 1 a. 1.. GRUNDMANN WIDE BAND AMPLIFIER Filed Feb. 28, 1939 PICTURE f r 12.5 MC.

REL H 7'/ ME RESPONSE PICTURE SOUND Fuvcutor Gustave L. Grundmann Patented Oct. 15, 1940 UNITED STATES TT FlCIE to Radio Corporation tion of Delaware of America, a corpora- Application February 28, 1939, Serial No. 258,991

4 Claims. (Cl. 179-171) My invention relates to wide band amplifiers and particularly to intermediate frequency amplifiers for television receivers or the like Where rejection of a signal such as sound in a closely adjacent channel is desired.

It is an object of my invention to provide an improved amplifier of the above-mentioned type in which there is a sharp cut-on? on the high frequency side of the intermediate frequency amplifier pass band.

The invention is based upon the amplifiers described and claimed in Patent 2,207,796, issued July 16, 1940, entitled Band pass amplifiers and assigned to the Radio Corporation of America. In this patent there is described an amplifier circuit or I. F. coupling network which includes a rejection circuit tuned to reject sound signals or other unwanted signals on the low side of the I. F. amplifier pass band.

The present invention comprises a circuit designed for rejection on the high side of the I. F. amplifier pass band, the rejection being obtained in such a way as to obtain a pass band of maximum width while still obtaining the desired rejection. Maximum pass band width and sharpness of cut-ofi are obtained by employing a capacitive reactor for coupling primary and secondary circuits and so tuningthe primary circuit that a maximum voltage appears across the coupling reactor at a frequency just below the rejection frequency.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which Figure 1 is a circuit diagram of one embodiment of my invention,

Figure 2 shows selectivity curves for the circuit of Fig. 1, and I Figure 3is a circuit diagram of another embodiment of my invention.

Referring to Fig. 1, there is shown an intermediate frequency amplifier stage designed in accordance with my invention for use in a television receiver. This amplifier stage comprises an amplifier or detector tube l which may be of the usual screen grid type having a plate electrode 2 and a cathode 3 comprising the output electrodes. In the present example, tube l is assumed to be a first detector of a superheterodyne receiver. These output electrodes are coupled to the input electrodes of the amplifier tube 4 in the succeeding amplifier stage through my improved filter or coupling network. The amplifier tube 4 may be of the usual screen grid type having a control grid 6 and a cathode l which form the input electrodes of the tube. I r

. My improved filter circuit comprises a tuned primary circuit which is tuned by a condenser 9 in parallel with the output capacity oi the tube l or by the tube capacity alone, and a tuned secondary circuit comprising a secondary coil H which is tuned by a condenser i2 in parallel with the input capacity of the tube t orby the tube input capacity alone. The primary circuit in- 10 cludes a primary coil 8 and a rejector circuit It. The two tuned circuits arecoupled by a condenser l3 which is common .to the primary and v secondary'circuits. In order to provide a cer-. tain amount of damping, a resistor 54 may be 15 connected across the primary coil 8.

The condensers l6 and H are filter and blocking condensers, respectively, while the resistor 88 is a grid resistor through which biasis' applied to the grid 6. 20-

The rejector network I9 is tuned to resonate at a'frequency on the high side of the amplifier pass band whereby the rejector network will present an inductive reactance below the rejection frequency. Thus it; together with the coil 8, 25

may be resonated with the capacitive coupling reactance provided by the condenser l3.

The rejector circuit l9 employed in the circuit of Fig. 1 is of the same type as one of the rejector circuits described and claimed in my 30 above-mentioned patent. It comprises an inductance coil 2! in series with the primary coil 8 and couplingcondenser It, the inductance coil 2! being inductively coupled to the primary coil 8. This inductive coupling is in such direction as to provide a negative mutual inductance which, as is well known, is a capacity reactance.

The ,coil 2| is bridged by an inductance coil 22 and a condenser 23 connected in'series. The bridging-coil 22 preferably isprovided-With e variable magnetic core or the like for varying its inductance, whereby the rejector circuit l9 may be tuned to the desired frequency. A resistor 2? in series with the filter condenser it is connected between ground and-the junction I point of the primary coil 8 and the series rejectorcoil 25. The primary purpose OfythiS resistor is to balance out the effect of resistance in the parallel resonant circuit. It also provides a certain amount of damping for the two coupled tuned circuits. It" will be noted that theplate voltage for the" amplifier tube l is applied through a filter resistor 23, the resistor 24 and the primary coil 8.

It is assumedthat the sound I. F. signals 2.0- 55 companying the picture signals are on the low frequency side of'the picture intermediate frequency signals. These sound signals may be supplied to a sound intermediate frequency amplifier 26 by means of a circuit tuned to the sound carrier wave, this tuned circuit comprising inductance coils 21 and 28 shunted by a condenser 29, the coil 21 being coupled to the secondary coil H.

The selectivity characteristic of my improved circuit is shown in Fig. 2. The rejector circuit I!) has been tuned to the frequency fsZ by adjusting the shunting rejector coil 22, this frequency being the intermediate frequency for the sound signal in the next adjacent television channel. The sound circuit 21-2829 is tuned to the frequency fsl, this being the carrier frequency for the sound accompanying the picture signals which the amplifier is designed to pass.

It will be apparent that the sound signal having the intermediate frequency 152 in the next adjacent television channel is very close to the high frequency end of the picture pass band and that, at the same time, it is highly desirable to extend the high frequency cut-off point of the picture pass band as close to the frequency In as possible, while still obtaining the necessary rejection or adjacent channel selectivity, in order to obtain the maximum pass band width for the picture signals.

- In accordance with my invention, this desired sharpness of cut-off is obtained by making the coupling network series resonant from the plate 2 through the primary coil 8, the rejector circuit l9 and the coupling condenser l3 to ground at a frequency fr. This circuit resonance may be employed for giving a peak to the picture selectivity curve on this high side as indicated by the solid line curve for compensating for lack of high frequency response in other amplifier stages, or sufficient damping may be provided by means of the resistor 18 and/or other circuit resistance to damp this resonant peak and hold it down to the level indicated by the dotted line portion of the curve. In either case the high frequency cut-off is very sharp and the band width of the amplifier has been extended at the high frequency end a maximum amount.

The cut-off characteristic of the amplifier is sharp due to coil 8 and the effective inductive reactance of the rejection circuit l9 tuning to series resonance with coupling condenser I3 at or near the high frequency end of the pass band of the amplifier circuit. This resonance increases the voltage introduced into the secondary circuit and thereby produces the sharp cut-off characteristic.

It should be noted that, for best operation, there are at least three requirements to be met in selecting capacity and inductance values for the several elements of my network as follows:

1. The network must be properly coupled and tuned to function as a filter having the desired pass band.

2. The rejector circuit must be tuned to the frequency of the signal to be rejected.

3. The circuit must be series resonant from the tube plate to ground at a frequency just below the rejection frequency.

It will be apparent that these several requirements are interdependent.

The proper design may be obtained by treating the circuit as a pair of coupled circuits having a coupling capacitor common to both primary and secondary. The effective primary and secondary inductances and coupling capacitor required to give the desired band width may be calculated by filter theory. It now remains to modify the magnitude of the primary inductance to some value such that the insertion of the series rejector will permit the primary to have the previously determined effective inductance at mid-band. The selection of values to be used in the series rejector circuit must make this circuit resonate at the desired rejection frequency fs2.

It now remains to be seen whether the resulting network is series resonant from plate electrode 2 to ground at the correct frequency fr. If it resonates at the wrong frequency, the LC ratio of the rejector circuit must be changed. By cut and try with the use of visual line up equipment the circuit can readily be made to resonate in the desired manner at frequency fr to give the desired extension of the pass band and the desired sharpness of cut off.

If desired, the circuit values may be calculated by means of simultaneous equations but the procedure outlined above is more satisfactory in practice due to lack of information as to the exact values of distributed capacity of the various circuit elements.

The selectivity curve for the sound circuit 2l 28-29 is indicated by the curve labeled sound. It will be noted that it provides a certain amount of rejection in the picture channel for sound signals accompanying the picture to be received, this rejection being indicated by the dip in the picture selectivity curve. However, this is insufficient rejection for the sound signal and in practice the necessary additional rejection at the frequency in is provided in a succeeding amplifier stage.

Another embodiment of my invention is illustrated in Fig. 3 where parts corresponding to those in Fig. 1 are indicated by like reference numerals. The principal difference between this circuit and the circuit of Fig. 1 is that a bridged-T rejector network of a different form is employed. It comprises an inductance coil 3! in series with the primary coil 8, the coil 3! being shunted by two condensers 32 and 33 in series. A balancing resistor 34 is connected between the junction point of the condensers 32 and 33 and ground. This bridged-T rejection circuit is of the same type as a similar rejection circuit described in my above-mentioned patent. However, as in the case of the rejection circuit employed in the circuit of Fig. 1, it is tuned to provide rejection above the pass band of the amplifier whereby it presents an inductance reactance within this pass band.

The circuit of Fig. 3 is adjusted as described in connection with Fig. 1, the circuit 3I3233 being series resonant at a frequency fr from the plate 2 of the tube I through the primary coil 8, the rejection circuit 3I-3233 and the coupling condenser i3 to ground. The selectivity curve for the amplifier of Fig. 3 is substantially the same as shown by the curve in Fig. 2.

If desired, instead of taking sound signal off the secondary coil it may be taken off the primary circuit by means of a circuit 36 tuned to the sound frequency fsl coupled to the plate 2 through a coupling condenser 31.

It may be noted that in certain designs the coil 8 may disappear as a distinct coil due to the presence of sufficient inductance in the rejector circuit.

By way of example, the values of certain elements in Fig. 3 have been indicated in ohms and micro-microfarads for a circuit designed to give rejection and to pass signals at the frequencies indicated in Fig. 2. The output capacity of tube l and the input capacity of tube 4 are approximately 10 mmf. and 12 mmf., respectively. The coils S, 3! and H, in the example given, are of 35, 12 and 35 turns respectively wound on onehalf inch diameter tubes or forms. Coils 8 and ii are of .007 inch diameter wire while coil 3! is of .013 inch diameter wire.

In one embodiment of my invention the coupling network may become a low pass filter. In this case the coupling condenser i3 is larger than the tube capacities and the capacities in parallel therewith, these capacities acting as terminating impedances for a filter section. The high frequency cut-off is made sharp in the manner previously described.

From the foregoing description it will be apparent that I have provided an amplifier which gives infinite rejection to undesired signal on the high frequency side of the amplifier pass band while extending the high frequency cut-off point of the amplifier pass band to a point very close to the frequency of the signal being rejected.

I claim as my invention:

1. A coupling network for an amplifier comprising a vacuum tube having output electrodes including a plate electrode and an amplifier tube having input electrodes including a grid electrode, said network comprising two coupled circuits, the first of said circuits including a tuned rejector circuit and a coupling condenser connected in series between said plate and a point at low potential for the frequencies being amplified, the second of said circuits including an inductance coil and said coupling condenser in series between said grid and said low potential point, said circuits being designed to function as a filter having a certain pass band, said rejector circuit being tuned to a frequency on the high side of and closely adjacent to said pass band, the circuit from said plate electrode through said rejector circuit and said coupling condenser to said low potential point being series resonant at a fre-, quency just below the frequency to which said rejector circuit is tuned.

2. A coupling network for an intermediate frequency amplifier comprising a vacuum tube having output electrodes including a plate electrode and an amplifier tube having input electrodes including a grid electrode, said network comprising a tuned primary circuit and a tuned secondary circuit, a condenser common to said tuned circuits for providing coupling therebetween, said circuits being so coupled and so tuned as to function substantially as a band pass filter, a rejector circuit in said primary circuit which is tuned to a frequency on the high side of and closely adjacent to ing output electrodes including a plate electrode 1 and an amplifier tube having input electrodes, means for coupling said output electrodes to said input electrodes, said means comprising a primary circuit and a secondary circuit each tuned to approximately the same frequency and so coupled as to have substantially a band-pass characteristic, the coupling between said circuits being provided by a condenser common to said circuits, a rejector network included in one of said tuned circuits, said rejector network including a parallel resonant circuit tuned to a frequency above but closely adjacent to said pass band whereby the rejector network presents an inductive reactance at frequencies within said pass band, said rejector network also including means for balancing out the effect of resistance in said parallel resonant circuit, and the reactance values in said primary circuit being such that the portion thereof from the low potential terminal of said coupling condenser to said plate is series resonant at a frequency just below said rejection frequency and at the high side of said pass band.

, 4. An amplifier comprising an amplifier tube having output electrodes including a plate electrode and an amplifier tube having input elec-- trodes, means for coupling said output electrodes to said input electrodes, said means comprising a primary circuit and a secondary circuit each tuned to approximately the same frequency and so coupled as to have substantially a band-pass characteristic, the coupling between said circuits being provided by a condenser common to said circuits, a rejector network included in said primary circuit, said rejectornetwork being of the bridged-T type including a parallel resonant circuit tuned to a frequency above but closely adjacent to said pass band whereby the rejector network presents an inductive reactance at frequencies within said pass band, said rejector network also including means for balancing out the effect of resistance in said parallel resonant circuit, and the reactance values in said primary circuit being such that the portion thereof from the low potential terminal of said coupling condenser to said plate is series resonant at a frequency just below said rejection frequency and at the high side of said pass band.

GUSTAVE L. GRUNDMANN.

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