US2135852A - Normalizing circuit - Google Patents

Description

Nqv. 8, 1938. D. v. SINNINGER NORMALIZING CIRCUIT Filed June 1'7, 195? Patented Nov. 8, 1938 UNITED STATES PATENT OFFICE 2,135,852 N ORMALIZIN G CIRCUIT Dwight V. Sinninger,

Chicago, Ill., assignor to Johnson Laboratories, Inc., Chicago, Ill., a corporation of Illinois Application June 17,

1 Claim.

narrow band of frequencies lying on either side of its nominal resonant frequency.

several serious problems which must be solved in the design of a satisfactory multi-stage amplifier. One of these difficulties is the inherent grid-plate capacitance of the vacuum tube.

grid-plate capacitance may be reduced substantially by the use of enough to contribute to the dency to oscillate of a multi-stage vacuum-tube amplifier, thus seriously limiting the useful gain or amplification which may be realized.

An object of this invention is to provide simple and inexpensive means for substantially eliminating the undesirable effects of vacuum-tube output capacitance in high-gain, multi-stage amplifiers.

amplifier.

An additional object of the invention is to provide means for substantially increasing the stable amplification which may be realized in a multi stage vacuum-tube amplifier.

1937, Serial No. 148,668

Although the arrangements of the invention are applicable to various types of high-frequency vacuum-tube amplifiers, they are especially useheterodyne radio receiver vacuum tube I might Dotted capacifixed capacitor 50.

Although coupling device 3 is shown to consist of two inductively coupled inductors l2 and I3 which are tuned by adjustable capacitors I l and I5, re-

suitable coupling tween vacuum tubes I and 2. For example, a coupling device employing three sented by dotted capacitor l6.

Although the major portion of the high-frequency current which flows in inductor I2 passes through tuning capacitor [4, some current flows also through by-pass capacitor 5 and output capacitance 4 of vacuum tube 1, which are effectively in series across tuned circuit l2--l4. Assuming that capacitor l were momentarily shortcircuited and capacitor I removed, it is evident that a portion of the circulating current of tuned circuit 8-9 would flow through capacitor 6 which is effectively in series with the output capacitance of vacuum tube 2. Such a common path for portions of the high-frequency currents flowing in the plate circuits of two successive amplifying vacuum tubes introduces serious feedback difficulties, with resultant instability and an unsymmetrical selectivity characteristic for the amplifier as a whole.

The invention provides means whereby the coupling which is common to circuits l2-l4 and 8-9 may be balanced out. The operation of the arrangement of Fig. 1 may be better understood by reference to Fig. 2, in which like parts are designated by like reference numerals. Capacitors 5, I, 9 and I0 form the four arms of a capacitance bridge, across one diagonal of which is connected inductor 8 and across the other diagonal of which is connected by-pass capacitor 6. Resistor II, in shunt with capacitor l0, has no appreciable effect upon the operation of the bridge, but is necessary to provide a direct-current path to the plate of vacuum tube 2. A choke coil may be substituted for resistor H.

The value of capacitor 5 depends upon the characteristics of vacuum tube 2 and thus is not readily adjustable. Capacitors 9 and I0 in series are in shunt with inductor 8, so that their values cannot be changed without affecting the resonant frequency of the output circuit of vacuum tube 2. In practice, capacitor is fixed at a suitable value and capacitor 9 is adjustable to-provide a means for aligning the circuit 8-9 at the desired frequency. Capacitor I may be given any desired value in order that the bridge may have the proper adjustment to eliminate the effects of undesired couplings between circuits l2-l4 and 8--9, and will require only a slight readjustment of capacitor 1.

In practice, it has been found that the degree of mis-adjustment introduced by vacuum tube replacement or by alignment of tuned circuit 89 is not sufficient to introduce serious feedback difiiculties in an otherwise properly adjusted system. It is within the scope of the invention, however, to provide means for re-adjusting the bridge if necessary, as for example by making capacitor 1 adjustable.

If the grid-plate capacitance l6 of vacuum tube 2 is assumed to be zero, the only coupling between the output circuit of vacuum tube 4 and the output circuit of vacuum tube 2 is that due to capacitor 6. This coupling may be balanced out by so choosing the value of capacitor 1 that the ratio of capacitors l0 and I is equal to the ratio of capacitors 9 and 5. This is the condition which would exist were additional means, not shown, to be employed to balance out the effect of grid-plate capacitance 16. When the bridge is balanced in this case, therefore, there is no common coupling or tendency toward feedback due to by-pass capacitor 6, and the system is stable.

In most cases, however, the grid-plate capacitance I6 of vacuum tube 2 is not neutralized, and thus provides a coupling between circuits 12- and 8-9 which either aids or opposes the coupling due to capacitor 6, depending upon the polarity of inductor l3 with respect to that tance l6 aids that of capacitor 6, the bridge must be unbalanced in the proper sense and degree to nullify the sum of the two couplings. This may be accomplished by so choosing capacitor 1 that the ratio of capacitors l0 and 1 is enough smaller than the ratio of capacitors 9 and 5 to compensate for the combined undesired couplings. The constants given below for an illustrative embodiment of the invention apply to a case of this type.

If the coupling due to grid-plate capacitance I6 opposes but is less than the coupling due to capacitor 6, the bridge must be unbalanced to compensate for the difference between the two couplings. This may be accomplished by so choosing capacitor 1 that the ratio of capacitors l0 and I is sufiiciently greater than the ratio of capacitors 9 and 5 to compensate for the coupling which remains. If the bridge constants are suitably chosen to meet this condition, the system will be free from instability due to coupling between circuits |2--l4 and 8-9.

By way of example of one successful embodiment of the arrangements shown in Fig. 1 of the drawing, the following constants are given for a system designed to operate at a frequency of 460 kilocycles:

Vacuum tubes 1 and 2 Capacitors 4 and 5 Capacitor 6 Capacitor 7 Capacitors 9 and 14 Type 6K7.

12 micromicrofarads.

0.05 microfarad.

0.005 microfarad.

104 micromicrofarads (approximate setting). Capacitor 10 0.01 microfarad. Resistor 11 800 ohms.

Inductors 8 and 12 It will be understood that these are approximate values and that the scope of the invention is in no way limited to embodiments employing the same or even closely similar values. A difierent operating frequency or the use of different types of vacuum tubes may require changes in some of the constants, and such changes are both contemplated and within the scope of the invention. 7

Having thus described my invention, what I claim is:

In a high-frequency amplifier, a first vacuum tube having a plate and a plate circuit, said plate circuit including a parallel tuned circuit and a source of plate potential shunted by a capacitor; a second vacuum tube having a plate and a plate circuit, said last-mentioned plate circuit including a parallel tuned circuit comprising an inductor shunted by an adjustable capacitor and a fixed capacitor in series, said fixed capacitor being shunted by a resistor; a connection from the junction of said adjustable capacitor and said fixed capacitor to the positive terminal of said source of plate potential; and a capacitor connected between ground and the junction of said inductor and said resistor.

DWIGHT V. SINNINGER.

1.0 millihenry.

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