US1911096A - Input system for electrical amplifiers - Google Patents

Description

y 23, 1933- H. A. SNOW INPUT SYSTEM FOR ELECTRICAL AMPLIFIERS Original Filed July 1929 kw. 0 W 3 Wan/e L's/797% 2 1 m 62. dm-

Patented May 23, 1933 UNE'EEE) STATES FATENT FFICE HAROLD A. SNOW, OF MOUNTAIN LAKES, NEW JERSEY, ASSIGNOR, BY MESNE ASSIGN- IVIENTS, TO 35310 COEPOEATIGN OF AMERICA, F NEW YORK, N. Y., A. CORPORA- TION O1 DELAWARE INPUT SYSTEM FOR ELECTRICAL AMPLIFIERS Application filed July 5, 1929, Serial No. 328,130. "Renewed October 24, 1931.

This invention relates to input systems for electrical amplifiers and particularly to input systems for use with radio frequency amplifiers of the type commonly employed in radio receiving sets.

Input systems have been proposed in which the signal energy was impressed across a fixed condenser of relatively large capacity that constituted a series element in a condenser-tuned resonant circuit. When the fixed condenser has a value substantially greater than the maximum value of the variable condenser, the input system tends to equalize the voltage step-up or gain throughout the band to which the system may be tuned. Unfortunately, however, the fixed condenser can not be given such relatively large value as might be indicated from a consideration of only the gainwavelength characteristic, since the fixed condenser shunts the signal energy to ground and thereby reduces the voltage available across the fixed condenser.

An object of the invention is to provide an eflicient input system including a resonant circuit operating into the first radio frequency amplifier, the input system being of such construction and arrangement that the tuning characteristics of the resonant circuit are substantially independent of the antenna impedance.

A further object of the invention is to provide an input system of the type in which the signal energy is impressed across a fixed condenser constituting a series ele ment in a condenser-tuned resonant circuit, and in which the magnitude of the fixed condenser may be such as to produce a de sired gain-wwrelength characteristic without, however, acting as a by-pass for signal energy.

These and other objects of the invention will be apparent from the following specification when taken with the accompanying drawing in which:

Fig. lis a fragmentary circuit diagram illustrating one embodiment of the inven- 'tiOn;

Fig. 2 is a circuit diagram of a modified form of input system; and

Fig. 3 comprises curves representing the performance of the input system of Fig. 2.

In the drawing, the reference character C indicates the fixed condenser which forms a series element of the resonant circuit LC. As indicated in the drawing, the inductance L may be adjustable for the purpose of making the tuning characteristics of the first stage identical with those of the succeeding stages, but the tuning of the circuit over a band of frequencies is effected by means of the variable condenser C.

In accordance with the prior practice, the antenna lead was connected directly to the junction of inductance L and fixed condenser C but when the capacity of C was of the order of .001 Inf, the bypassing effect of the condenser was decided ly noticeable. So far as the gain-wavelength characteristic of the system was concerned, better operation would be effected if the fixed condenser were given an even greater value, but the by-pass effect precluded any substantial increase in the magnitude of the condenser.

In accordance with the present invention, the signal energy of the antenna or collector system A is impressed across the fixed condenser C by means of a stepdown transformer L L the turn ratio of the transformer being, for example, of the order of to 1.

Due to the transformer action, the apparent impedance of the fixed condenser C is approximately 100 times the actual impedance, and therefore the by-passing of signal energy must be substantially prevented even though the actual value of O is materially increased. For the broadcast band, the value of C may be .05 mf with a value of .0005 for the maximum capacity of the tuning condenser C.

Due to the high capacity of the coupling condenser C the tuning characteristics of the resonant circuit may be made substan tially identical with those of the succeeding stages and the condenser C of the first amplifier may be mechanically connected to the condenser or condensers G which tune the input circuits of succeeding stages. For example, when C is of the order of .05 microfarads and the inductance L has the same physical and electrical values as the secondaries of the inter-stage transroriners T, any error introduced by varying the tan ing capacities C and C, at the same rate w ll not exceed the usual tolerance permitted in sensitive receivers.

As shown in Fig. l, the interstage coupling comprises a transformer T and the tuning condenser Q, shunted across the secondary of the transformer. It will be apparent that other iuned interstage couplings may be employed.

As shown in Fig. :2, the step-down transformer across which the signal energy is impressed may take the form of an auto transformer comprising the coils L and I Although the invention. is not limited to any particular values for the circuit elements, the following data is given by way e'li example:

L ==1O0 turns of No. 38 enamel wire close wound on 4; inch diameter tube;

11 turns of No. 30 D. C. C. wire space wound on top of L As stated above, the operation of the input s stem is characterized by a flatten ng of the step-up characteristic and this action is particularly desirable since the usual types of tuned radio freipiency amplifiers have a decidedly higher gain at the shorter wave-lengths. .Vhile it is possible to construct amplifier stages having a relatively flat gain-wavelength charactcritic, the provision of an input system which reduces the step-up at the shorter waves gives greater freedom in the design of the amplifier.

The curves of Fig. 3 are plotted from data obtained with an input system of the type shown in Fig. i, the transformer being constructed in accordance with the data given above, and the condenser C having a value of .05 mt. The three separate curves A, B and C represent the step-up characteristics when the input system was associated with antenna structures in which the antenna capacity had tie respective values of approximately 100, 200 and 500 innit. It will e noted that, as comp red w: h the step-up ch racterisiics of the usual input systems, the present invention provides a substantially uniform step-up throughout the band of frequencies to which the resonant circuit may be tuned.

By an appropriate choice of the values for the elements of the input system, the step-up characteristic may be given a definite and predetermined form. For a particular value of the antenna capacity (1,, the leakage inductance of {he transformer and the value of the condenser C ,may be such that the leakage inductance and U, are resonant at the lower end of the wavelength band, (note curve C of Fig. 3), and

the leakage inductance and C are resonant at the upper end of the band (curve A, Fig.

Since the large value of condenser Cm renders the tuned circuit substantially independent of the antenna characteristics, the alignment of the ganged tuning condensers C, C will not be disturbed if a volume control resistance R is shunted across the condenser C Although the invention is particularly useful when employed as a coupling between an antenna and the first stage of a cascaded radio frequency amplifier, the fact that the tuning characteristics of the resonant circuit may be rendered substantially independent of the impedances out of which the amplifier works indicates other useful applications of the invention. The novel form of input system may, for example, provide a coupling between a section of a carrier wave transmission line and a repeater, or it may be employed as a coupling between the plate circuit of one tube and the input circuit of the following tube.

I claim:

1. A vacuum tube input system of the type including a series-resonant circuit comprising av fixed condenser, a variable condenser and an inductance, coupling means for impressing signal energy across said fixed condenser, and means for applying the voltage across said variable condenser to the input terminals of a vacuum tube, characterized by the fact that said coupling means includes a transformer effective to increase the apparent impedence of said fixed condenser.

2. In an input system for a radio frequency amplifier, a collector system, a tuned circuit comprising in series a fixed condenser, an inductance and a variable condenser, means for grounding the junction of said fixed and variable condensers, means for applying the voltage across said Variable condenser to the input terminals of a radio frequency amplifier, and coupling means for impresing the signal eneregy of said collector system across said fixed condenser, said coupling means con'iprising a transformer for increasing the apparent impedance of said fixed condenser.

3. In an audion innput system of the type in which signal energy to be amplified is impressed across av fixed condenser forming a series element of a condenser-tuned circuit, a connection from the low potential side of said fixed condenser to ground, and a step-down transformer having the secondary thereof connected across said condenser, the primary having one terminal connected to ground and constituting the impedance across which received signal energy is impressed.

4. The invention as set forth in claim 2, wherein the leakage inductance of'said transformer and the said fixed capacity are resonant at the upper end of the band of wavelengths to which said circuit may be tuned.

5. The invention as set forth in'claim 2, wherein the leakage inductance of said transformer and the capacity of said collector system are so related. as to be resonant at the lower end of the band of wavelengths to which said circuit may be tuned.

6. The invention as set forth in claim 2, wherein the leakage inductance of said transformer and the said fixed capacity are resonant at the upper end of the band of wavelengths to which said circuit may be tuned and the leakage inductance and capacity of said collector system are resonant at the lower end of said band.

7. In an input system for a vacuum tube, the combination with a tuned circuit com prising in series a fixed condenser, an inductance and a Variable condenser; means grounding the junction of said fixed and variable condensers and means for applying the voltage developed across said variable condenser to the input terminals of a vacuum tube; of coupling means for impressing upon said fixed condenser the signal energy to be amplified, said coupling means comprising a transformer for increasing the apparent impedance of said fixed condenser.

8. The invention as set forth in claim 7, wherein the leakage inductance of said transformer and the said fixed capacity are resonant at the upper end of the band of wavelengths to which said circuit may be tuned.

9. The invention as set forth in claim '7, in combination with a transmission channel connected across the primary of said transformer, and wherein the leakage inductance of said transformer and the capacity of the transmission channel are so related as to be resonant at the lower end of the band of wavelengths to which said circuit may be tuned.

10. The invention as set forth in claim 7, in combination with a transmission channel connected across the primary of said transformer, and wherein the leakage inductance of said transformer and the capacity of the transmission channel are so related as to be resonant at the lower end of the band of wavelengths to which said circuit may be tuned, and the said leakage inductance and said fixed capacity are resonant at the upper end of said band.

11. In combination, a source of modulated carrier energy, a multistage carrier frequency amplifier, each stage including an electron discharge tube provided with input and output electrodes, a resonant circuit connected between the input electrodes of the first of said amplifier tubes, said circuit comprising in series a fixed condenser and inductance, and a variable condenser, a resonant circuit connected between the input electrodes of a succeeding tube and including a variable condenser, means for simultaneously varying said condensers, and means for transmitting energy from said source across said fixed condenser comprising a transformer arranged to increase the apparent impedance of the fixed condenser.

12. In combination, a source of modulated carrier energy, a multi-stage carrier frequency amplifier, each stage including an electron discharge tube provided with input and output electrodes, a resonant circuit connected between the input electrodes of the first of said amplifier tubes, said circuit comprising in series a fixed condenser, a variable inductance, and a variable condenser, a resonant circuit connected between the input electrodes of a succeeding tube and including a variable condenser, means for simultaneously varying said condensers, and means for transmitting energy from said source across said fixed condenser comprising a transformer arranged to increase the apparent impedance of the fixed condenser.

13. In combination, a source of modulated carrier energy, a multi-stage carrier frequency amplifier, each stage including an electron discharge tube provided with input and output electrodes, a resonant circuit con nected between the input electrodes of the firstof said amplifier tubes, said circuit comprising in series a fixed condenser and inductance, and a variable condenser, a resonant circuit connected between the input electrodes of a succeeding tube and including; a variable condenser, means for simultaneously and similarly varying said condensers, and means for transmitting energy from said source across said fixed condenser comprising a transformer arranged to increase the apparent impedance of the fixed condenser.

14. In combination, a source of modulated carrier energy, a multi-stage carrier frequency amplifier, each stage including an electron discharge tube provided with input and output electrodes, a resonant circuit connected between the input electrodes of the firstof said amplifier tubes, said circuit comprising in series a fixed condenser and inductance, and a variable condenser, a resonant circuit connected between the input electrodes of a succeeding tube and includlng a variable condenser, means for simultaneously varying said condensers, and means for transmitting energy from said source across said fixed condenser comprising a step-down transformer arranged to increase the apparent impedance of the fixed condenser.

15. In combination, a source of modulatprovided with input and output electrodes,

a resonant circuit connected between the input electrodes of the said amplifier tube, said circuit comprising in series a fixed condenser and inductance, and a variable condenser, and means for transmitting energy from said source across said fixed condenser comprising a transformer arranged to increase the apparent impedance of the fixed condenser, said transformer consisting of a pair of coils connected in series.

16. In combination, a source of modulated carrier energy, a carrier frequency amplifier including an electron discharge tube provided with input and output electrodes, a resonant circuit connected between the input electrodes of said amplifier tube, said circuit comprising in series a fixed condenser and inductance, and a variable candenser, and means for transmitting energy from said source across said fixed condenser comprising a transformer having a turn ratio of the order of to 1 arranged to increase the apparent impedance of the fixed condenser, and a gain control impedance connected across said fixed condenser.

17. In combination, a source of modulated carrier energy, a carrier frequency amplifier including an electron discharge tube provided with input and output electrodes, a resonant circuit connected between the input electrodes of said amplifier tube, said circuit comprising in series a fixed condenser having a value of the order of 0.05 uncrofarad and inductance, and a variable condenser, and means for transmitting energy from said source across said fixed condenser comprising a transformer arranged to increase the apparent impedance of the fixed condenser.

18. In combination, a grounded antenna circuit, a stage of radio frequency amplification including a tube provided with input and output circuits, said tube input circuit comprising an inductance coil shunted by a variable tuning condenser, means for coupling said antenna circuit and said tube input circuit in such a manner as to render the tuning of said tube input circuit substan tially independent of the antenna characteristics, and a variable impedance having one terminal thereof connected to one side of said coil and its other terminal connected to the grounded side of the said antenna circuit.

19. In combination, a grounded antenna circuit, a stage of radio frequency amplification including a. tube provided with input and output circuits, said tube input circuit comprising an inductance coil shunted by a variable tuning condenser, means for coupling said antenna circuit and said tube input circuit in such a manner as to render the tuning of said tube input circuit substantially independent of the antenna characteristics, and a variable impedance having one terminal thereof connected to one side of said coil and its other terminal connected to the grounded side of the said antenna circuit, said impedance being connected across said variable condenser.

20. In combination, a grounded antenna circuit, a stage of radio frequency amplification including a tube provided with input and output circuits, said tube input circuit comprising an inductance coil shunted by a variable tuning condenser, means for coupling said antenna circuit and said tube input circuit in such a manner as to render the tuning of said tube input circuit substantially independent of the antenna characteristics, and a variable impedance having one terminal thereof connected to one side of said coil and its other terminal connected to the grounded side of the said antenna circuit, and a second tuned radio frequency amplifier stage, coupled to the output circuit of said first stage, including a variable tuning condenser in its input circuit, and means for simultaneously varying the variable condensers of both stages.

21. In combination, a grounded antenna circuit, a stage of radio frequency amplification including a tube provided with input and output circuits, said tube input circuit comprising an inductance coil shunted by a variable tuning condenser, means for coupling said antenna circuit and said tube input circuit in such a manner as to render the tuning of said tube input circuit substantially independent of the antenna characteristics, and a variable impedance having one terminal thereof connected to one side of said coil and its other terminal connected to the grounded side of the said antenna circuit, said antenna circuit and coupling means being desi ned to be non-resonant over asubstantial portion of the operating range of the tuning condenser.

22. In combination, a source of modulated radio frequency energy, an electron discharge tube provided with input and output electrodes, a resonant network, comprising an inductive path and a shunt variable capacitive path, connected between the input electrodes, a large capacity connected across a portion of said inductive path, and means for inductively coupling said source to said portion.

23. In combination, a source of modulated radio frequency energy, an electron discharge tube provided with input and output electrodes, a resonant network, comprising an inductive path and a shunt variable capacitive path, connected between the input electrodes, a large capacity connected across a portion of said inductive path, and means for inductively coupling said source to said

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