US2759050A - Amplifier regulation circuit - Google Patents

Description

Aug. 14, 1956 LORD, JR 2,759,050

AMPLIFIER REGULATION CIRCUIT Filed April 26, 1952 1.0- HEATER VOLTAGE INVENTOR. a ARTHUR H. A, magi-f? 6pm 70 CATHODE Van-4&5

ATTQENEY United States Patent@ AMPLIFIER REGULATION CIRCUIT Arthur H. Lord, Jr., Houston, Tex., assignor to The Texas Company, New York, N. Y., a corporation of Dela- Ware Application April 26, 1952, Serial No. 284,661

3 Claims. (Cl. 179-171) This invention relates to circuits for electron discharge devices. More particularly it relates to circuits for stabilizing performance characteristics of discharge devices having thermionic cathodes.

Stabilization circuits have been used extensively in the past which by automatically adjusting the magnitudes of certain of the potentials applied to discharge devices, and without necessarily altering their performance characteristics as such, serve to compensate for changes unavoidably occurring in the magnitudes of others of the potentials or changes occurring in the efliciencies of the devices themselves. An example is an automatic gain control circuit in which a variable Mu tube used for amplifying R. F. signals whose magnitudes are not always the same because they come from different transmitters and/or are subject to atmospheric effects is feed-back-biased with the direct current component of the detected signal. Another example is a circuit including means for applying to the input of its discharge device negative feedback of some of the output signal for preventing excessive changes in the gain of the circuit with changes in the efliciency of its discharge device, which may occur due to aging, replacement, etc. Thus a type of circuit which provides negative feed back of some of the signal by the use of cathode degeneration, i. e., the cathode follower, has a gain which is very stable (at a value near to 1) independently of large changes in the cathode emission of its discharge device.

However, as alluded to above, these prior art circuits do not control, as such, any of the performance characteristics of their discharge devices. Instead, an A. G. C. circuit is actually controlled by, in the sense that its operativeness depends on, a particular characteristic for its discharge device, that is a grid-voltage vs. plate-current characteristic which is non-linear as in a remote cut-ofi tube; and signal negative-feed-back circuits neither particularly control nor are controlled by the tube characteristics, at least not in their behavior as voltage amplifiers, but rather are as completely independent thereof as it is possible to make them. Incidentally A. G. C. circuits have the disadvantage of not being operative without entailing objectionable distortion unless the input signals fed to their tubes are of small peak-to-peak amplitudes as compared to the grid voltage ranges between their cutoff and saturation potentials, and negative feed back circuits have the disadvantage that they can only be used where it is not necessary to attain the maximum possible amount of voltage gain.

However, there are many applications for which it is desirable to stabilize as such a performance characteristic of a discharge device, for example to stabilize its Eg/Ip characteristic against changes which can be brought about by variations in the power dissipated in its cathode heater. In as much as the resistance values which the cathode heater windings will have at various temperatures will remain relatively constant during the life of a tube, changes in delivered heater power are substantially always due to changes in the applied heater voltage. These 2,759,050 Patented Aug. 14, 1956 ICC changes may come about due to line voltage fluctuations, overloadings of power transformers which include heater secondary windings, chemical changes in A dry batteries, etc.

Typical applications in which stabilization of the Eg/Ip characteristic is important are: (l) in a limiteramplifier wherein pulses which drive a tube to saturation are amplified and clipped at what is intended to be a constant amplitude; and (2) in the linear amplifier section of a circuit for amplifying pulses, such as the output pulses of a radiation detector, preparatory to sorting them in accordance with difierences in their amplitudes. It will not be possible to maintain constant-amplitudeclipping or reliable separation of pulses if the Eg/Ip characteristics of the clipping or linearly-amplifying tubes as the case may be, are permitted to shift upward or downward or leftward or rightward (as they appear when conventionally plotted in rectangular coordinates), with one exception, in the case of latter tubes only, where they are almost perfectly linear over the entire operating region, for example because the tubes are biased to operate in only small central portions of these characteristics and the largest pulses applied to them are no larger than the grid voltage ranges over which these portions extend.

Accordingly it is an object of the present invention to devise circuits for stabilizing operating characteristics of thermionic discharge devices such as against variations due to changes in the heater power delivered to their cathodes.

It is a further object to provide circuits of the kind set forth above whether the discharge device is of the variable Mu remote cut-off type or of the more linear sharp cutoiT type.

In general these objects are attained by the use of circuits which vary the grid-to-cathode bias of a discharge device, in accordance with any heater voltage variations, to the end that the effectiveness of the grid as a potential barrier will be substantially constant despite the fact that the average initial velocities of the electrons emitted by the cathode will vary with each change in its temperature. The grid-to-cathode bias in question may be applied to either the grid or cathode electrode, while the other is maintained at some relatively fixed reference potential level, or it may be applied between these electrodes. Moreover, the bias potential in question may be provided by the use of a voltage divider connected across an A battery which provides direct current heater power or by the use of a rectifier-filter arrangement fed either from the heater current winding or some other winding of an A. C. power transformer whereby the bias which it provides will vary with fluctuations in the output voltage of the winding whether the fluctuations be due to line voltage fluctuations or fluctuations in certain variable loads imposed on the transformer.

In the drawing:

Figs. 1-3 are schematic diagrams of circuits embodying the present invention; and

Fig. 4 shows a family of Eg/Ip characteristic curves plotted in rectangular coordinates for the same tube for heater voltages which are respectively: normal; 13% below normal; and 21% below.

The circuit shown in Fig. 1 is that of a resistance coupled amplifier using an A battery to supply the heater power and a C battery to provide grid bias. The circuit comprises a triode 10 with a conventional output circuit including a load resistor 11 and a blocking condenser 12 and a conventional input circuit including a coupling condenser 13, (i. e., the blocking condenser of the preceding stage), a grid resistor 14 and a C battery 15. Heater power is provided by an A bat-.

battery cells or dry cells. In either case the output po- 3 tential provided by the battery 16 will be subject to fluctuations which can be very objectionable if the amplifier is to be used in a precision instrument instead of, say, a portable broadcast receiver.

Though it might not be apparent from experience with some devices such as portable radio receivers, the output voltage'provided by a battery under load, even if it is a low impedance A battery, is quite unstable. Certain chemical reactions occur within a cell, as current is drawn from it, which progressively increase its internal resistance until opposing recuperative changes caused by products of the reactions are large enough to keep pace with them. Until this time the output potential of the battery will drop continuously even though the load remains constant. Thus even a new battery is a fairly unstable potential source for a significant length of time after it is placed under load. Moreover this condition gets worse as the battery becomes older even though it is otherwise still a useful source of power.

The circuit is so arranged that at least part of the total grid-to-cathode bias is derived from the A battery 16 whereby it will change in magnitude with any change in the applied heater potential. To this end a voltage divider comprising resistors 17 and 18, at least one of which preferably should be adjustable, is connected in series across the battery 16 and their point of junction is connected to the cathode. Since the negative terminal of the battery 16 is grounded and the cathode is only connected to ground as to direct current through the resistor 18, the cathode will normally be biased somewhat positive with respect to ground at a value which will depend upon the voltage provided by the battery 16 and the division thereof effected by the voltage divider. There is nothing critical about the values, as such, of the resistors 17 and 18 though for each circuit and tube one particular (and therefore critical) ratio between their values will be more appropriate than any other for best operation according to the present invention. Of course, the resistors should not have such extremely low values of resistance that they will waste a substantial amount of power from the battery 16 nor should they have such extremely high values that the resistor 18 can substantially limit the cathode current of the tube or cause undesired cathode degeneration. Since the cathode currents of most tubes are very much smaller than their heater currents (e. g., 1 mil to 150 mils in the case of the 6AQ6 tube used in obtaining the curves shown in Fig. 4) it will be a very easy matter to simultaneously avoid both extremes. If desired it will of course be possible totally to eliminate cathode degeneration by the use of an A. C. by-pass for the resistor 18 such as the condenser 19 shown in Fig. 1.

The operation of the circuit shown in Fig. 1 and of the similar circuits shown in Figs. 2 and 3, may perhaps be best understood by referring to Fig. 4. As indicated therein the plate current of an amplifier tube having a fixed anode potential will sustain a noticeable drop for almost any value of grid bias within the useful range thereof when the heater voltage is reduced below its normal rated value. I have found that this behavior results from a diminution in the average initial-velocity of the electrons emitted by the cathode rather than in their number. As is known, in the operation of a gridcontrolled discharge tube a potential excursion of the control grid valves-through electrons (towards the anode) from a cloud thereof which accumulates between cathode and grid and which acts as a reservoir of electrons which can meet very substantial transient peak demands unless the tube is operating with its current limited by the cathodes emission as might be the case in a discharge device having a large ratio between the electroncollecting surface of its anode and the electron-emitting surface of its cathode, a large applied anode potential, little or no grid bias and quite a low cathode temperature. The reason why the anode current diminishes with each reduction in delivered cathode heater power despite the presence of the reservoir of electrons is that their average velocity goes down and thereby causes an increase in the eifectiveneess of the grid as a potential barrier to the fiow of current through it. In view of the foregoing it will be seen that the circuit shown in Fig. 1, as well as the similar circuits of Figs. 2 and 3 will automatically cause a reduction of the effectiveness of the grid as a potential barrier each time that there is a reduction in delivered heater power. In addition, and contrary to what one might expectin view of the reduction in the number of electrons (in the cloud) which also occurs with a drop in cathode temperature, I have found that the reserve supply of electrons will still be suflicient to sustain the slope of the Eg/Ip characteristic over all of its length. Thus an adjustment in the voltage divider 17-18 which causes it to have an appropriate stabilizing effect at one Eg point along this characteristic will usually prove to be substantially equally appropriate for all other Eg points therealong. As a result of this the circuit shown herein exhibits constant gain when the heater current is subject to variation, this being equally true for small signals which operate over restricted portions of almost any part of the Eg/Ip characteristic, large signals, such as pulses, which extend over most of the characteristic, and pulses which actually drive the tube to saturation or cut 0E.

The circuit of Fig. 2 is a modification of that of Fig. l in which the A battery is inverted in polarity so that the bias provided at the juncture of its resistors 17 and 18 is negative instead of positive to the end that the stabilizing component of the bias potential may be applied to the grid rather than to the cathode. Accordingly in this circuit the cathode return is simply made directly to ground. If desired, condenser 20 may be connected across the resistor 18 for decoupling purposes if the battery 16 serves more than one tube.

The embodiment of Fig. 3 is another simple resistance coupled amplifier circuit which differs from the circuits of Figs. 1 and 2 in that the cathode is heated with alternating current. In this type of circuit the delivered heater power will change with variations in the line voltage applied to the primary of the power transformer. Of course line voltage variations will also effect the value of the B+ voltage applied to the anode. However since potential changes which occur at the grid and cathode of the discharge device, including changes in the potentials of emitted electrons, are more critical than other potential changes, it may be quite advantageous to employ stabilization in accordance with the present invention even if the 13+ supply is entirely unregulated. And, obviously, there may also be applications in which it is desirable to employ both the present type of stabilization and regulation of the B+ supply. In effecting stabilization in the Fig. 3 embodiment a rectifier-filter arrangement 21 is connected across some secondary winding of the transformer, for example the main heater winding itself, or perhaps an idle 5 volt rectifier-heater winding, to provide a small adjustable bias voltage which varies with fluctuations in the line voltage. It is to be understood that since the voltage provided by this source will in effect be applied to the signal input of the tube 10 it should be well filtered to eliminate virtually all of its 60 cycle A. C. component. In addition if the bias supply is used for more than one tube in the same apparatus it may be advantageous to employ a decoupling capacitor 22 at each tube to avoid signal transmission from the input of one tube to that of another over the common biasing network.

Obviously where the fluctuations whose effects are to be avoided are ones which are attributable to line voltage fluctuations, the rectifier filter arrangement 21 may be energized by a secondary winding of a separate small transformer especially provided for the purpose, such as a small filament transformer, and in fact it may be energized directly from the line.

However fluctuations to be avoided may also be attributed to overloading of a power transformer which is operating close to peak rating. Thus if considerable B+ power is being drawn because of, say, a fast pulse counting rate in a radiation detector, the core of the power transformer may begin to become saturated so that all of its leakage reactances are increased in value. In such a case the power delivered to the heaters of the apparatus would diminish as a result of the transient loading of the B+ supply. In such a case an A. C. powered grid bias supply such as that of the arrangement 21 may be utilized to stabilize each tubes Eg/Ip characteristic against the effects of the resultant fluctuations in cathode temperature. However in such a case the secondary winding which feeds power to the bias supply should be a part of the same power transformer which is energizing the cathode heater(s).

Where the fluctuations whose eifects are to be avoided are likely to include fast fluctuations, it will be advantageous for the rectifier filter arrangement 21 to have charging and discharging time constants which correspond in magnitude to the thermal lag of the cathode of the tube whose Eg/Ip characteristic is to be controlled. Otherwise it may produce a bias change before the temperature of the cathode has undergone a change which makes compensation desirable. Where, however, the fluctuations in question are slow, evening drops in line voltage as more lights go on, this precaution will not be necessary.

The data used in plotting the curves shown in Fig. 4 was obtained in actual tests in which the tube'used in the circuit was the triode section of a 6AQ6. When the circuit was modified as shown herein and the test then repeated, curves were obtained which were substantially coincident from end to end. These curves are not shown in Fig. 4 to avoid unduly complicating the drawing by a profusion of mutually-overrunning lines which would be diflicult to distinguish from one another.

Obviously many modifications and variations of the Invention as hereinabove set forth may be made without departing from the spirit and scope thereof and only such limitations should be imposed as are indicated in the appended claims.

I claim:

1. Electrical apparatus comprising an electric discharge device having at least two active electrodes including a thermionic cathode and a control grid, 21 uni-directional source of electric energy for heating the cathode, biasing means for establishing a predetermined potential between said grid and cathode, said biasing means including in series between the grid and cathode a primary uni-directional source of bias and a secondary uni-directional source of bias, said secondary source of bias comprising a portion of a potential divider shunted across said unidirectional cathode heater source, the ratio of said portion of said divider to the total thereof being so selected that the total bias compensates for any variation in cathode emission due to variations in the uni-directional heater source, and the absolute magnitude of the resistance of said divider being so selected as to minimize current flow from the uni-directional heater source while permitting adequate direct cathode operating current to flow.

2. The apparatus of claim 1 wherein the secondary bias derived from the potential divider is applied to the grid-cathode circuit between the cathode and a point of ground potential.

3. The apparatus of claim 1 wherein the secondary bias derived from the potential divider is applied to the grid-cathode circuit between the grid and a point of ground potential.

References Cited in the file of this patent UNITED STATES PATENTS 1,888,467 Mueller Nov. 22, 1932 1,901,986 Ranger Mar. 21, 1933 1,958,998 Hentschel May 15, 1934 2,029,033 Perkins Jan. 28, 1936 2,431,306 Chattarjea et a1 Nov. 25, 1947 2,541,198 Brenholdt Feb. 13, 1951

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