US2068112A - Amplification and selectivity control circuit - Google Patents

Description

Jan. 19, 1937. RUST 2,068,1 12

AMPLIFICATION AND SELECTIVITY CONTROL CIRCUIT Filed Aug. 20, 1955 mow mmumw (HANGER Mean/us 0F a/as\ Mmm/s a; 41m: Sal/22:5

INVENTOR. NOEL M RUST ATTORNEY.

Patented Jan. 19, 1937 UNITED STATES AMPLIFICATION AND SELECTIVITY CONTROL CIRCUIT Nol Meyer Rust, Chelmsford, England, assignor to Radio Corporation of America, a corporation of Delaware Application August 20, 1935, Serial No. 36,964 In Great Britain August 15, 1934 1 Claim.

This invention relates to thermionic valve circuit arrangements suitable for use in radio receivers and in like apparatus.

More particularly the invention relates to ther- 5 mionic valve circuit arrangements for carrier frequency operation and of the kind wherein amplification and selectivity are increased by the use of reaction.

The principal object of the invention is to provide an improved high frequency thermionic valve circuit arrangement of the reaction type whereby very high selectivity and stability is obtained with the use of comparatively simple apparatus.

Another important object of the invention is to provide a stable simple high frequency thermionic valve circuit arrangement of the reaction type wherein it is possible to obtain, either manually or automatically, control of the selectivity and gain in such a manner that when the gain is at a maximum the selectivity is at a maximum and when the gain is at a minimum the selectivity is at a minimum. As is well known selectivity control of this nature is of considerable practical advantage for radio receivers, for in general maximum selectivity is required when the receiver is operated to pick up signals from a remote or weak transmitting station-that is to say, when the receiver gain is at a maximum.

According to this invention, a carrier frequency thermionic valve circuit arrangement of the reaction type comprises a plurality of tuned valve amplifiers in cascade, and there is inserted in series with the anode-cathode space of one of said amplifiers, an impedance, preferably a tuned impedance from which feed back voltage is taken,

said feed back voltage being superimposed upon voltage to a preceding valve. Preferably, means are provided for adjusting the steady bias of the 4 preceding valve to which the feed back voltage is applied, and the inserted tuned impedance is on the cathode side of the valve with whose anodecathode space it is in series.

Although not limited to its application thereto the invention is particularly suitable for use with, and gives maximum advantage when applied to, the intermediate frequency amplifiers of superheterodyne receivers.

. The invention is illustrated in the accompanying drawing which shows diagrammatically one embodiment thereof.

Referring to the drawing the intermediate frequency amplifier of a conventional superhetero- 5 dyne receiver (the rest of the receiver is not shown) comprises at least two fixedly tuned amplifier valves I, 2, in cascade. These valves are preferably radio frequency pentodes (and are shown as such) though other types of valve may be employed. In the description which follows 5 it will be assumed that radio frequency pentodes are in question.

Input signals from the frequency changer (not shown) of the superheterodyne receiver are applied, for example through a suitable coupling 10 condenser E, to the innermost or control grid 4 of the first radio frequency pentode l, and to this grid is connected one end of a parallel tuned input circuit L1 C1. The other end of this parallel tuned circuit is connected to an adjustable tap- 15 ping point 5 upon a potentiometer resistance 6, one end of which is connected to the negative terminal I of the common source (not shown) of the anode supply for the receiver, the other end of the resistance 6 being connected to the nega- 20 tive terminal 8 of a source (not shown) of bias potential whose positive terminal is connected to the terminal 7 which is grounded. The screen grid 9 of the first pentode l is positively biased, as in the usual way, through a lead it from a source, 25 not shown, and the outermost or suppressor grid II is connected to the cathode l2 also as in the usual way. The anode I3 of the first pentode is connected to the positive terminal i l of the source of anode potential through a parallel 3 0 tuned circuit L2 C2 which is identical with the tuned circuit L1 C1, and the said anode i3 is also connected through a coupling condenser i 5 to the innermost or control grid [6 of the second. pentode 2. This grid I6 is connected to the cathode of valve 2 through a resistance [8 in series with a negative bias potential source l9, and this connection provides a partial anchoring of the said grid to the cathode as regards high frequency potentials. Though theoretically it is better to anchor the grid Hi to the cathode ll it is often more convenient, in practice, to anchor the said grid to ground. Such anchoring of the grid I6 to ground would introduce negative feed-back. from anode to grid in the valve 2, but this is of small importance since such negative feed-back would be overwhelmed by the overall positive feedback from tube 2 to tube I. The screen grid 20 of the pentode 2 receives positive potential as in the usual way.

The suppressor grid 2| is connected to the cathode ll also in the usual way. The anode 22 of the pentode 2 is connected to the positive terminal of the source of anode potential through a parallel tuned circuit L3 C3 which is identical with the two parallel tuned circuits already mentioned. The anode end of this parallel tuned circuit is coupled to succeeding amplifiers, not shown-for example, to further intermediate frequency stages or to a demodulating detector--for example, by a condenser 23. The cathode Il of the second pentode is connected to the negative terminal I of the anode source through a fourth parallel tuned circuit L4 C4 which is identical with the three parallel tuned circuits already referred to, the cathode end of this fourth parallel tuned circuit being connected also to the cathode I2 of the pentode l. A Icy-pass condenser 25 is connected between the terminal 5 and the movable point 5 on the potentiometer resistance 6.

Since the two cathodes I2, H, are connected together and to one end of a feedback circuit constituted by the parallel tuned circuit L4 C4, feedback potentials will be applied to the oathode l2, and accordingly such feedback potentials will alter the grid-cathode potential of the pentode I in such manner as to assist input signals and provide additive reaction. If the control grid I6 of the pentode 2 is anchored relative to its associated cathode, and not relative to ground, the feedback potentials will not affect the second valve, though, as above stated, even if the grid it is anchored to ground, such negative feedback as is thereby produced will be swamped. All four parallel tuned circuits are identical, and for an intermediate frequency amplifier designed to operate at a million cycles per second a value of 2 microhenries for the inductance and .012 microfarads for the capacity in each of the four parallel tuned circuits have been found to give satisfactory results.

L4 C4 functions as the impedance disposed in the space current paths of tubes l and 2. Across this impedance Li C; is developed the feedback voltage. The small alternating current component of the space current of tube I develops across L4 C4 at resonance, a voltage which is impressed on the input electrodes of tube I in degenerative phase. The alternating current component of the space current of tube 2 is larger due to amplification, and by virtue of the phase reversal due to the resistance coupling, there is developed across L4 C4 at resonance a larger voltage which is impressed on the input electrodes of tube I in regenerative phase. It can be shown that increasing the product of the gains of tubes I and 2, or the impedance of L4 C4, will increase the regenerative feedback. Hence, by increasing the gain of tube I the regenerative feedback across L4 C4 increases, and accordingly increased selectivity results. Such gain increase, by shifting tap 5 to decrease the negative bias of grid 4, is employed when receiving weak signals; during weak signal reception increased selectivity being desired.

For intermediate frequency amplifiers designed to operate at two million cycles per second, the following values have been found satisfactory in each casez-inductance value 2.53 microhenries and capacity value .0025 microfarads.

With an arrangement as above described, it is possible to adjust the circuit constants in such manner that when the movable tapping point 5 is at, or near, the relatively positive end I of the potentiometer resistance 6, the self-oscillation point is just about reached, and in an arrangement so designed very high selectivity and gain will be obtained if the potentiometer tapping point be moved slightly in the negative direction from the self-oscillation point. By moving the tapping point still more negatively both gain and selectivity may be reduced.

In place of using a potentiometer to give manual control in this way, an equivalent result, namely alteration of the D. C. grid bias upon the control grid of the first pentode, may be obtained automatically in dependence upon received signal strength by any known automatic volume control circuit connected and arranged to provide a D. C. bias potential depending upon received signal strength. Control can also be exercised by varying the bias potential on the second grid, or alternatively by leaving the grid bias potentials fixed and varying the feedback impedance by means of a variable shunt resistance 3!! across the circuit L4 C4.

The following part theoretical analysis will assist in an appreciation as to the reasons for the satisfactory operation of a circuit as above describedz-It is known that if an amplifier system be so arranged that a fraction of the output voltage E be fed back to the input side, then, if e be the input voltage and the amplification factor, and if the fraction of the output voltage fed back be B,

that is to say, the effective amplification There are known reaction type circuits where in, in order to secure stable amplification and relative freedom from distortion, the co-efiicient B is made negative; that is to say, the feedback voltage is made of such sense as to reduce magnification. It is to be noted that in a circuit according to the present invention, and as above described, B is made positive, but the adjustments and design should be such that the product 3 is less than unity, for when this product becomes equal to unity the oscillation point is reached. So long, however, as [LB is below unity the circuit is stable. If B is much less than unity, the effective amplification is only slightly increased by reaction, and it will be noted that in the circuit described B can be increased up to the limit set by the oscillation point; this increase being effected by moving the adjustable tapping point on the potentiometer towards the relatively positive end of the potentiometer resistance, the factor a being thus altered and the factor B remaining constant.

The four parallel tuned circuits in the hereinbefore specifically described embodiment of the invention are all sharply tuned and since they are identical the factor B will be equal to onehalf. With the third and fourth mentioned parallel tuned circuits L3 C3 and L4 C4 identical that is to say with B=.5 the oscillation point is reached when l=2, for with these values of a and B the product ,uB unity. Preferably in carrying out this invention the circuit constants (including the bias upon the second pentode) are so adjusted that :2 or thereabouts, and B is selected at .5 or thereabouts.

A. practical advantage of this particular selection design lies in the fact that with these values of mu and B the ratio of inductance to capacity in the tuned circuits is very small as compared with the ratio normally used in radio receiving circuits. For example, whereas in an ordinary receiver circuit for 1,000 kilocycles 160-200 microhenries is considered a normal value of inductance for a tuned circuit, the inductance in each of the tuned circuits in the specifically described embodiment of this invention may be, as stated, only 2 microhenries and the capacity .014 microfarads. This low inductance-capacity ratio contributes largely to the smooth and satisfactory working of the arrangement and because the tuning capacities are relatively large, effects due to stray capacity and residual Miller effect capacities are much reduced. It is also possible to design the circuits to have a high so-called Q value (resistance divided by inductive reactance), and as this Q value determines the rate at which the amplification decreases for alteration of the frequency on either side of that frequency at which maximum amplification occurs (resonance), good selectivity is obtained. Even if there were no feedback, the initial conditions would be such as to result in good selectivity.

Although in the specific embodiment of the invention above described and illustrated the circuits L1 C1, L2 C2, L3 C3, and L4 C4 are all identical this is not an essential feature. For example, a convenient and stable embodiment is one wherein the circuit L4 C4 is shunted by a suitable resistance and in some cases the said circuit L4 C4 may be replaced by a resistance. It is owing to the fact that best results are obtained with the use of large condensers in the tuned circuits that the invention is most advantageously applicable to intermediate frequency amplifiers operating at frequencies of the order of 1,000 to 2,000 kilocycles.

In this connection it may be noted that it is very difficult with known arrangements to design an intermediate amplifier of the ordinary type and working at about 1,000-2,000 kilocycles which will have sufiicient selectivity to give two channel separation in a broadcast receiver with the present spacing of broadcast transmitters in accordance with international agreement. With a circuit as herein described, however, it has been found possible to obtain two channel separation quite easily and in fact in many cases single channel separation can be obtained. These facts together with the large gain which is obtained at maximum selectivity make the invention very advantageous for use in superheterodyne receivers. The invention is, however, not exclusively limited to superheterodyne receivers nor indeed to fixedly tuned amplifiers, for the described circuit arrangement may be modified to constitute a tunable amplifier by making the tuned circuits adjustable as to their natural frequency and gang-controlling them.

What is claimed is:

A carrier frequency thermionic valve circuit arrangement of the reaction type comprising a first valve having a carrier frequency tuned circuit in its control or input grid circuit and a carrier frequency tuned circuit in its anode circuit, a second valve having its control or input grid coupled to the anode of said first valve and a carrier frequency tuned circuit connected between its anode and a source of anode potential therefor, said second valve having an inserted impedance connected in series between its cathode and the negative terminal of the source of anode potential therefor, said negative terminal also constituting a point upon the grid circuit of the first valve whereby voltage set up across said inserted impedance is fed back to the control or input grid of the first valve, means for adjusting the control or input grid bias of said first valve, said inserted impedance being a tuned circuit substantially identical with the other three tuned circuits.

NOEL MEYER RUST.

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