US2264890A

US2264890A - Amplifier coupling circuit

Info

Publication number: US2264890A
Application number: US329501A
Authority: US
Inventors: Charles S Root
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1940-04-13
Filing date: 1940-04-13
Publication date: 1941-12-02
Anticipated expiration: 1958-12-02

Description

Dec. 2, 1941. c. s. ROOT AMPLIFIER COUPLING CIRGUI'I Filed April l 3,' 1940 Inventor:

m w R m 5. m s t b m A r s a h H y b Patented Eco. 2, 1941 OFFICE AMPLEIER COUPLING CIRCUIT Charles S. Root, Bridgeport, Conn, assignor to General Electric Company, a corporation of New York Application April 13, 1940, Serial No. 329,501

4 Claims.

This invention relates to amplifier coupling circuits and has for an object to provide an improved amplifier coupling circuit which may be readily adjusted to be either highly selective or less selective for a broad band of frequencies.

It is also an object of my invention to provide an amplifier coupling circuit having three cascaded tuned circuits in which the over-all selectivity is adjusted between a high and low value in an improved way.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 illustrates a portion of a radio receiver embodying my invention; Fig. 2 illustrates a transformer especially constructed for use in the receiver of Fig. 1, and Fig. 3 includes curves showing the over-all selectivity of the circuit shown in Fig. 1.

Fig. 1 represents my invention as applied to a radio frequency receiver. A transformer I impresses a radio frequency signal wave from a source, not shown, such as the output circuit of a radio frequency amplifier, upon a multi-grid electron discharge device H which acts as a converter to change the frequency of the incoming wave to a fixed intermediate frequency. The wave of this intermediate frequency is amplified by a second electron discharge device I2 and is transmitted to further portions of the receiver, not shown, including a detector and an audio reproducing system.

The local oscillation circuits of the converter include a transformer I3 and a tuned condenser M in shunt to one winding thereof. The windings of this transformer are connected respectively through coupling condensers l5, l6, and through ground to appropriate electrodes of the discharge ll. One of these electrodes, which acts as an anode of the local oscillator, is supplied from a source of operating potential, not shown, through a resistor i'l. Another of these electrodes. which acts as a control grid of the local oscillator is biased by means of a grid leak resistor is. The screen grids of the device II are supplied from the same source of operating potential through a resistor i9 and are shunted to ground through a bypassing condenser 20.

The transformer It! is tuned to resonance with the incoming radio frequency wave by a condenser 2!, which is mechanically connected with the tuning condenser I4 in a well known manner for unicontrol, so that, by heterodyne action betweenthe incoming radio frequency wave and the wave generated in the circuit I3, M a wave of fixed intermediate frequency is produced for any received signal. The secondary of the transformer II) is connected between the second control grid of the .device H and a source of grid bias potential, not shown, which is shunted to ground through a by-passing condenser 22'. The cathode of the device II is connected to ground through a cathode resistor .23 shunted by a by-passing condenser 24. The anode of the device, H is supplied from a source of potential, not shown, through a resistor 25 and through an inductance 26, which forms a resonant circuit with .a shunt connected tuning condenser 21. .A point betweenthe inductance .26 and the resistor 25 is icy-passed to ground by a high frequency Joy-passing condenser 28. Heterodyne action in the device U, as explained above, produces :a wave of intermediate frequency to which the tuned circuit 2.6, 21 responds, so that a wave of intermediate frequency appears therein whenever a radio frequency wave is transmitted through the transformer l0.

A coupling circuit for transmitting the intermediate frequency wave from the device 1] to the device I2 comprises three cascaded tuned circuits of which the circuit 26, .21 is the first. The second tuned circuit includes an inductance 29 magnetically coupled to the inductance 26 and a condenser 36 connected through a switch V3! in shunt .to the inductance 29. The third tuned circuit comprises an inductance 32 connected at one end to the control grid of the device l2 and .at the other end through a switch 33 to a suitable source of grid bias potential, not shown, which is shunted to ground through a by-passing condenser 34. This third tuned circuit also includes .a tuning condenser 35 connected between the control grid of the device I2 and ground. The inductance 32 is magnetically coupled to the inductance 29. I

As just described and as connected in the figure the three cascaded tuned circuits are .adjusted for maximum selectivity. To attain 'a maximum selectivity characteristic each pair of adjacent coupled tuned circuits must have about the same degree of coupling as .every other pair of adjacent tuned circuits in the group. The coupling between the non-adjacent, or remote, .ones of the cascaded tuned circuits is made a minimum. r

The cathode of the device I2 is connected to ground through a cathode resistor 36 shunted by a by-passing condenser 31. The screen grid of the device I2 is supplied from a source of potential, not shown, shunted to ground through a by-passing condenser 38. The anode of the device is connected through a tuned

circuit

39, 40 identical with the tuned circuit 26, 21 to a source of potential, not shown, which is shunted to ground by a by-passing condenser 4|. The tuned

circuit

39, 40 is the first of three additional cascaded tuned circuits forming a coupling arrangement similar to that between the devices H and I2. It need be described no further, since it is understood that it may be constructed and operated in an identical manner to the first coupling arrangement.

To adjust the coupling device for much less selectivity, so that a much wider band of frequencies may be transmitted, an inductance 42 is provided which is magnetically coupled closely to the inductance 26. One end of this inductance 42 is connected to that end of the inductance 32 remote from the control grid of the device |2. The switch 33 is connected so that, in its alternative position it provides a series connection for high frequency currents through the inductance 32, the inductance 42 and through the by-passing condenser 34 to ground. Upon operation of the switch 33 to connect the

inductances

32 and 42 in series, there is produced magnetic overcoupling between the devices II and I2, which provides a greatly broadened frequency transmission characteristic.

The inductance 42, providing over-coupling be- 7 tween the remote tuned

circuits

26, 21 and 32, 35, produces a desirable lessened selectivity characteristic for the coupling system comprising three cascaded tuned circuits. If over-coupling be produced between only one pair of adjacent tuned circuits of such a group, the selectivity characteristics for the system assumes a shape such that waves of some desired frequencies are transmitted with maximum amplitudes and waves of other desired frequencies with much less 1 amplitudes. My invention avoids such a selectivity characteristic and achieves a desirable characteristic having substantially uniform transmission over a relatively wide range of frequencies with the use of only one device for producing over-coupling.

It is preferred that the switches 3| and 33 be mechanically connected together for simultaneous operation so that, when the

inductances

32 and 42 are connected in series to provide overcoupling, the tuned circuit 29, is broken so as to be inoperative. Thus, in the upper position of the switches 3| and 33 the coupling arrangement comprises three loosely coupled cascaded tuned circuits, while in the lower position of the switch the coupling arrangement comprises only two relatively tightly coupled cascaded circuits.

Although the inductance 42 is preferred, as a means of providing over-coupling between the tuned circuit 26, 21 and the tuned circuit 32, a condenser 43 may be connected from the upper endof the tuned circuit 26, 2! through a switch 44 to the upper end of

thetuned circuit

32, 35. The magnitude of the condenser is made such as to produce a suitable amount of over-coupling such as was produced by the inductance 42. It is, of course, preferred, if this alternative scheme be used, to couple together mechanically the switches 3| and 44 so that they operate simultaneously, thus producing the same results as in the other arrangement. Both types of coupling may be employed, if it be desired.

In Fig, 2, a

transformer comprising inductances

26, 42, 29 and 32 is shown. This figure illustrates the manner in which this transformer may be constructed. The inductances are wound as illustrated upon a cylindrical coil form 45 made of insulating material. At that end of the coil form 45 nearest the inductance 32, three

connectors

46, 41, 48, are fastened. These connectors are adapted to cooperate with a widely used type of gang switch, which may comprise the switches 3| and 33. Disk-like fiber spreaders 49 and 5|] fit snugly around the coil form respectively between the

inductances

42 and 29, and between the

inductances

29 and 32 in order to prevent connecting wires from touching or 1ying near inductances which they cross. These spreaders aid in reducing unwanted capacity effects.

It will be observed that

coils

26 and 32 are spaced widely apart so as to reduce direct coupling between them, and that these coils are about equally spaced from intermediate coil 29 to produce coupling between

coils

26 and 29 equal to that between

coils

29 and 32. The coil 42 is positionednear coil 26 and must be made quite large to produce as much coupling between the remote tuned circuits as exists therebetween by reason of coil 29. Actually it is made larger than this to obtain an overcoupled fiat resonance curve for best fidelity of high notes when coil 29 is open-circuited. Of course if the circuit of coil 29 were allowed to remain closed, this circuit would act as an absorption circuit and produce an undesirable double humped resonance curve.

The curves of Fig. 3 show the relation between the difference between the side band frequency and the carrier frequency, plotted as abscissa, and the ratio of the output of the intermediate frequency channel of a receiver embodying my invention to the input to that channel, plotted as ordinate. The deviation of the side band frequency from carrier is plotted in kilocycles. Curve 5| shows the selectivity of the complete intermediate frequency channel of a receiver such as is illustrated in part by Fig. 1, wherein the switches 3| and 33 are in the upper position. It may be seen that selectivity is at a maximum, so as to transmit the usual audio frequencies tolerably well. Curve 52 illustrates the over-all selectivity of the same intermediate frequency channel when the switches 3| and 33 are in their lower positions. The selectivity of the re-- ceiver has been reduced greatly and the frequency band transmitted has been greatly broadened so as to provide much more faithful reception of audio frequencies, or in other words to provide for reception of much higher audio frequencies.

While I have shown a particular embodiment of my invention, it will, of course, be understood that I do not wish to be limited thereto, since different modifications may be made both in the circuit arrangement and instrumentalities employed, and I aim by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a coupling device for transmitting high frequency waves, the combination of at least three cascaded tuned circuits having a predetermined amount of coupling between adjacent ones of said tuned circuits and a minimum amount of coupling between remote ones thereof, means for producing a greater amount of coupling between said remote ones of said cascaded tuned circuits, means to open-circuit at least one of said tuned circuits intermediate said remote ones of said tuned circuits, and means for operating said coupling increasing means and said open circuiting means simultaneously to change the selectivity of said tuned circuits.

2. In a coupling device for transmitting high frequency waves, the combination of at least three cascaded tuned circuits having a predetermined amount of magnetic coupling between adjacent ones of said tuned circuits and a minimum amount of magnetic coupling between remote ones thereof, means for producing a greater amount of coupling between said remote ones of said cascaded tuned circuits, means to open-circuit at least one of said tuned circuits intermediate said remote ones of said tuned circuits, and means for operating said coupling increasing means and said open circuiting means simultaneously to change the selectivity of said tuned circuits.

3. In a coupling device for transmitting high frequency waves, the combination of at least three cascaded tuned circuits having a predetermined amount of coupling between adjacent ones of said tuned circuits and a minimum amount of magnetic coupling between remote ones thereof, means for adding capacitive coupling between said remote ones of said cascaded tuned circuits, means to open-circuit at least one of said tuned circuits intermediate said remote ones of said tuned circuits, and means for operating said coupling adding means and said open circuiting means simultaneously to change the selectivity of said tuned circuits.

4. A coupling device for transmitting high frequency waves, comprising at least three cascaded tuned circuits having a predetermined amount of coupling between adjacent ones of said tuned circuits and a minimum amount of coupling between remote ones thereof, energy transfer between the remote tuned circuits being provided within a band of frequencies through an intermediate one of said tuned circuits, means for open circuiting at least said one intermediate tuned circuit thereby to reduce energy transfer between said remote tuned circuits within said band of frequencies, means for producing a greater amount of coupling between said remote tuned circuits, said last means being substantially independent of frequency over a frequency range substantially wider than said band of frequencies, and means for operating saidcoupling increasing means and said open circuiting means simultaneously to change the selectivity of said tuned circuits.

CHARLES S. ROOT.