US2101525A - Linear amplifier circuit - Google Patents

Description

Dec. 7, 1937.

H. BARTELS 2,101,525

LINEAR AMPLIFIER CIRCUIT Filed Dec. 18, 1934 3 Sheets-Sheet l ATTORNEY Dec. 7, 1937. H. BARTELS 2,101,525

LINEAR AMPLIFIER CIRCUIT Filed Dec. 18, 1934 5 Sheets-Sheet 2 ATTO RNEY Dec. 7, 1937. H, BARTELS 2,101,525

LINEAR AMPLIFIER CIRCUIT BY ykg'g/mw ATTO R N EY i Patented Dec. 7, 1937 UNITED STATES PATENT OFFICE LINEAR AMPLIFIER CIRCUIT Germany Application December 18, 1934, Serial No. 758,044 In Germany March 21, 1934 8 Claims.

The present invention is concerned with a method designed to stabilize regenerative amplifiers especially for audio frequency amplification. It has been suggested in the earlier art to 5 connect amplifiers for audio or radio frequency regeneratively with a view to improve the performance of the tube. However, as far as applicant is aware this suggestion has so far failed of being introduced in practice in so far as audio l' frequency amplifiers are concerned for the reason that it was found that ampliers thus regenerated exhibit a tendency towards instability.

The invention will be understood by referring to the drawings wherein Figures l, 2, and are 15,l curve sheets used to show various characteristics of the circuits described in the specification and Figures 3 and 4 are embodiments of the invention using non-linear resistances.

The considerations which resulted in the pres- 20 ent invention are associated with the fact that the internal resistance of customary types of tubes such as triodes and screen grid tubes, particularly the latter, inside the Working rang'e predominantly used, decreases with increase of plate 25 current. If the relationship between plate voltage and plate current of a tube, for different values of a steady grid potential is ascertained, there is obtained a family of graphs or characteristic pattern of the kind shown in Fig. 1, the

30 curves there shown were obtained with a Telefunken tube of the type Rens 1374. The internal resistance Rz' at a certain definite point of the characteristics is defined by the angle of inclination of the tangent drawn at this point on the 35 characteristic in question and the same may be expressed in the form of an equation, thus:

die R1 dIa(Eg constant) 40 where Ea and Ia are anode voltage and current respectively and Eg is the grid voltage. Connecting the tube regeneratively, and if the plotting of the characteristics is then repeated, there are obtained the graphs shown by the broken lines in 45 Fig. l, which show that the lack of linearity of the characteristic has been aggravated by regeneration, and that the internal resistance varies from positive values through zero and then changes into negative values.

50 Now, as long as the sum total of the negative internal resistance of the tube and the positive load resistance is over zero, the amplier works under stable conditions. Fig. l shows that the value of the negative internal resistance of the 55 tube with rising plate current grows very rapidly. If, therefore, for the purpose of insuring maximum gain, the working point has already been fixed so that the two resistances, due to regeneration, neutralize almost completely, it will be seen that a slight growth of plate current 5 must give rise to instabilities. If an attempt is made to avoid instability by modulating the tube only slightly, and if only the lower end of the characteristic is employed, this means that the feedback is entirely useless. But as soon as it is tried to operate inside a -range where an appreciable improvement of the tube quality by regeneration exists, there arises the risk of that part of the characteristics being reached (as a result of fluctuations of the working voltage or of larger alternating current) where the value of the negative resistance of the tube is greater than the plate resistance, and this means the production of spontaneous oscillations by the tube.

If, for instance, the resistance has been so chosen that the straight resistance line passes through a point which is located in the neighborhood of the part of the curves shown by broken lines where the internal resistance becomes zero, while yet this resistance is still positive, there is a chance for the said straight line intersecting once more the broken-line curve, to be more precise, at a point which corresponds to a negative tube resistance. However, this means that the tube becomes unstable. The working point experiences sudden and abrupt shifts in such a fashion that the tube becomes self-oscillatory.

It has been suggested in the prior art to eliminate non-linearity of the characteristics by the aid of an amplitude-dependent feed-back. However, it would be erroneous to believe that a tube thus linearized would make it feasible to increase the regeneration further to such a point that an appreciable improvement of the tube quality might be seemed. Quite apart from the fact that an exact linearization by the aid of simple means with positive feedback is not attainable, this further fact must be kept in mind that linearization applies only for a very definite individual tube and then only for a very definite working state. The consequence is that linearization is lost as soon as there is a change in these presuppositions, and this in turn means that the range of instability may be reached again.

Now, according to the present invention stable operation of positively regenerated amplifiers is made feasible by that the working point is placed at a spot of the characteristics obtained for the regenerated tube where the characteristics giving the relationship between the plate voltage and the plate current have reversal tangents. A suitable form or trend of the family of characteristics of regenerated tubes may be obtained either by choosing suitable forms of construction of the tube itself, more particularly convenient forms for the electrodes or by the judicious choice of the Ypotentials applied to these electrodes or else again by suitable construction of the circuit elements which govern the size of the feedback.

A number of characteristics referring to different feedback coeiicients and which, according to this invention, are characterized by the presence of a tangent to a reversal, are shown in Fig. 2.

It will be seen that the internal resistance of the tube, at the reversal point, assumes the lowest positive and the highest negative value; hence, this very point distinguishes itself by a stable electric equilibrium. Now, as long as the tube, at this working point, is still stable, i. e., does not oscillate, any and every other point of characteristic will be so much more stable: on the ground that the amplification at such other point is lower than at the reversal point.

The production of the characteristics shall be explained by reference to Fig. 5. The family of characteristics which shows the interdependence of the plate current and the plate potential for different grid voltages refers to a screen-grid tube of a kind which roughly corresponds to Telefunken model Rens 1374. The following considerations are first based upon current feedback by way of linear resistances and furthermore upon non-linear resistances.

All graphs denoted by I are based upon the assumption that the feedback coefficient is constant, that is to say, that a voltage is returned to the grid circuit which is directly proportional to the plate current (fe-:K-ia, where er is the feedback Voltage, K the feedback factor, and ia the plate current). If, for instance, the circuit elements have been so chosen that, in the presence of a plate current Variation of 15 milliamp., the feedback potential is just 4 V., there results, starting from point A a characteristic IA. The points of intersection of the plate-current variations of 15 milliamps. in each case with the consecutive gridvoltage characteristics (differing in each case by 4 v.) result in points corresponding to the desired curve IA. If this procedure be continued for several distinct starting points B, C, D, there results a family oil graphsy IA IB Ic In, represented by solid lines. This family of curves I which corresponds to the family of graphs indicated by broken lines in Fig. 1 is unsuited for the objects of this invention since operations are unstable, as previously pointed out.

If, then, a non-lineanresistance through which current flows is used from which the feedback potential is tapped, then a family of curves is obtained of a kind as indicated by broken-line curves II. In this instance, the feedback factor K is no longer linear. Like feedback potentials, in the presence of larger `starting currents, result fl'om greater variations of the current. What may be inferred from` the straight resistance lines Rai and Raz is that the increase in amplification with the use of feedback according to curves II is still comparatively slight.

However, this increase in gain will be substantially greater with the use of characteristics of the kind shown in thegfamily of curves denoted by III which is obtained in accordance with the family of curves II. As can be seen from the straight resistance line Ras, for the same change in grid voltage, there is brought about a change of plate voltage which is more than twice as large.

Now, in order that the working characteristic may be made of a form so that it will present a reversal point inside the operating range, recourse may be had to various ways and means.l In the first place, tubes could be used which are furnished with cathode exhibiting saturation properties. This condition holds good, for instance, of all metallic cathodes, e. g., tungsten cathodes. What is meant here by saturation properties is the arising of a saturation current which shall be defined as the current iiowing over to the plate when. by suitably high external voltages the space-charge has been overcome, with the result that all electrons which may be given off from the cathode will actually reach the plate. Provided that the so-called saturation voltage is not exceeded, a variation of plate potential will then not occasion any change in the emission current. The Working point, with the use of feedback must be so chosen in the case of such tubes that it will, at least approximately, come to coincide with the reversal point of the short-circuit characteristic of the tube. In this instance, regardless of the size of the feedback, this point will retain its property of a reversal point, with the consequence that at that very point maximum gain will prevail. In other words, it is possible to push ampliiication at the working point close to the limit where whistling is produced, without the risk of self-oscillation being occasioned as a result of a possibly still higher amplification existing at another point. In the case of metallic cathodes, the trend of the characteristics is greatly dependent upon the heating or filament potential. It is, therefore, recommendable to stabilize and iix the saturation properties of the emission process by the use of one or more electrodes mounted at close proximity to the incandescent cathode and kept at a. constant, and preferably positive potential.

In the case of an electrode system as shown in Fig. 3 the heated cathode K is surrounded by two grids Gi-Gz which are maintained at a constant positive potential. The additional grid G3 serves to control the current owing towards the plate A. The function to be fulfilled by the said two inner grids Gi-Ge consists in that the rst grid G1 Whose positive bias is lower compared with grid Ge, governs the number of electrons permitted to fly therethrough, whereas grid G2 determines the rate of speed of the electrons as they leave the grid and enter into the control space of the tube.

If the cathode K is heated so strongly that it gives off more electrons than can be drawn away across the inner grid G1, there will always prevail an excess of free electrons, and the consequence of this condition is that fluctuations in the heating or lament potential will have no effect upon the quantity of the electrons being available for the control electrode. Inasmuch as it is feasible by suitably choosing the conditions of mutual controllance and more particularly that of the exit grid G2 to make conditions so that the influence of the plate voltage upon the number of emitted electrons is precluded, it will be seen that a discharge characteristic referred to the current leaving the grid electrode G2 evidences similar saturation properties as those observed in a metallic cathode.

Another chance to impart to the characteristic of a regenerative amplifier tube the desired shape consists in associating with the tube non-linear resistances. It has already been indicated above that it is well known to linearize a tube by the use of amplitude-dependent regeneration. In fact, there is no obstacle or objection to shaping such amplitude dependence or function in such a way that the resultant form of the characteristic will not be a straight line, but will, indeed, present a reversal tangent.

A suitable regenerative circuit scheme is embodied in the scheme Fig. 3. More precisely speaking, what is here involved is a so-called current feedback, in other words, a feedback in which the regeneration is proportional to the plate current. The resistance Rk whence the voltage is tapped which is returned into the grid circuit is so chosen that its current-voltage characteristic is a non-linear function, and is of such a shape that the characteristic of the regenerative tube has a trend as desired and distinguishes itself by a reversal tangent. The regeneration resistance Ric may consist, e. g., of a contact or crystal detector, a cuprous-oxide rectifier (i. e.,

generally a rectifier comprising a stopping layer) or a discharge tube. It is also possible to connect the resistance Ric in parallel relation to the secondary Winding of the transformer T included in the grid circuit.

Another method capable of insuring the desired shape of the characteristic is to connect in parallel to the tube a non-linear resistance from which the regeneration voltage may be tapped (this involving what has been called voltage regeneration). A circuit organization of this kind is shown in Fig. 4 schematically. In parallel relation to the tube V is a voltage divider comprising the componental resistances R21-Rp which comprises a non-linear resistance, say, again a rectifier of any desired kind the characteristic of which is so chosen that the discharge characteristic plotted in the state of regeneration represents the desired function with a reversal tangent.

What is claimed is:

l. The method of operating a regeneratively connected thermionic device with relation to its plate current-plate voltage characteristic so as to prevent instability caused by increases in the amount of energy fed back due to increases in input energy, which consists in operating the thermionic device in a manner such that its characteristic has a distinct point of inflection within the operating range and adjusting the thermionic device so as to operate about this point of inflection whereby the amount of energy fed back actually decreases with large inputs.

2. In a circuit including an electronic tube provided with an anode, a cathode and a control electrode, means for assuring saturation discharge effects in the operation of the cathode comprising a pair of auxiliary grids interposed between the cathode and the control grid and means for maintaining said auxiliary grids at xed positive pon tentials with respect to the cathode. 3. In an amplifier circuit adapted to be connected to a source of input energy, said amplier circuit including an electronic tube having an anode, a cathode and a grid electrode, said grid electrode being included in an input circuit of said amplier tube and said anode in an output circuit thereof, said input circuit being linked with said output circuit to faciiltate the transfer of energy from the output circuit to said input circuit for producing regeneration, means for operating the cathode of said tube under incipient saturation conditions to thereby impart characteristics to the tube such that its anode current-anode voltage characteristic has an inflection point and means for operating the regeneratively connected tube about said inflection point whereby the volts fed back per volt applied to the input circuit remains substantially coristant over a wide intensityrange of the input energy and decreases thereafter.

4. A system as described in the next preceding claim wherein the means for operating the cathode of the tube under saturation conditions comprises at least one auxiliary grid electrode interposed intermediate the cathode and the grid electrode of the tube and means for maintaining said auxiliary grid electrode at a positive potential with respect to the cathode.

5. A system as described in claim 3 wherein the means for operating the cathode for said tube under saturation conditions comprises a pair of auxiliary grid electrodes interposed between the grid electrode and the cathode of the tube and means for maintaining both the auxiliary grid electrodes positive with respect to said cathode.

6. In an amplier circuit adapted to be connected to a source of input energy, an electronic tube having an anode, a cathode and a grid electrode, said grid electrode being included in an input circuit of said tube and said anode in an output circuit thereof, said input circuit being linked with said output circuit to facilitate the transfer' or" energy from the output circuit to the input circuit for producing regeneration, said cathode being of the metallic type whereby it may be operated under incipient saturation conditions to thereby impart characteristics to the tube whereby its anode current-anode voltage characteristic has an inflection point, said tube being operated about said inection point whereby the amount of energy fed back through the linkage is maintained substantially constant over a substantial intensity range of input energy and decreases when the intensity of said input energy goes beyond said range.

7. In an amplifier circuit adapted to be connected to a source of input energy, an electronic tube having an anode, a cathode and a control grid electrode, said grid electrode being included in an input circuit of said tube and said anode in an output circuit thereof, said'input circuit being regeneratively linked with said output eircuit, means for operating the cathode of said tube under incipient saturation conditions to thereby impart characteristics to the tube such that its anode current-anode voltage characteristic has an inflection point, said means comprising a pair of auxiliary grid electrodes interposed between the control grid and the cathode of said tube and means for maintaining the auxiliary grid electrodes at diierent positive potentials with respect to the cathode.

8. A system as described in claim 3 further characterized by that there is provided a nonlinear load device in the output circuit of said tube, which load device affects the tube in a sense to increase the gain characteristics thereof when the amplitude of the input energy is so great that the tube gain decreases.

HANS BARTELS.

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