425,553

PATENT SPECIFICATION

Application Date: Sept. 18, 1933. No. 25797/33.

Complete Specification Left: Sept. 18, 1934.

Complete Specification Accepted: March 18, 1935.

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PROVISIONAL SPECIFICATION

Improvements in and relating to Thermionic Valve Amplifiers

We, ELECTRIC & MUSICAL INDUSTRIES LIMITED, a British Company, of Blyth Road, Hayes, Middlesex, ALAN DOWER BLUMLEIN, a British Subject, of 32, Woodville Road, Ealing, London, W.5, and HENRY ARTHUR MAISH CLARK, a British Subject, of 119, Wynchgate, Southgate, London, N.14, do hereby declare the nature of this invention to be as follows:-

The present invention relates to thermionic valve amplifiers.

The invention is particularly concerned with amplifiers of the type adapted to supply substantial power to a consuming device or load, and it is an object of the present invention to improve the performance of such power amplifiers.

It is well-known that the amount of undistorted power which a thermionic valve amplifier can supply to a load, depends, among other things, upon the relationship between the output impedance of the amplifier, that is, the impedance seen looking towards the amplifier from the terminals of the load, and the impedance of the load itself. Generally, there may be said to be two optimum values of the ratio between the output impedance and the load impedance. When the load impedance is made equal to the output impedance, the load is said to be matched. This equality ratio of impedances is in some cases an optimum condition for the standpoint of frequency response of the combined apparatus. On the other hand, certain classes of thermionic amplifiers will give more output power, when the load impedance bears a definite ration, other than unity, to the output impedance of the amplifier.

Certain classes of load are inductive, and when such a load is required to operate over a wide frequency range, it will be seen that it represents a load of widely varying impedance. Further, in order that this load may give its correct frequency response characteristic, it is necessary that it be fed from a particular output impedance. For the purposes therefore of assessing the effect of such a load on an amplifier, it is usual to regard such inductive loads as equivalent to a load of a fixed resistive value equal to the output impedance from which they must be fed in order to give their correct frequency characteristic. That is to say, an amplifier which feeds an inductive load must be designed to provide the requisite power into a load which has the same impedance as the amplifier output impedance, i.e. is matched. That a non-inductive matched load is the most fair fixed representation of a purely inductive load operating over a wide frequency range, can be shown by considering the conditions that obtain with an inductive load at very low and very high frequencies. At a very low frequency the inductive load will have a negligible impedance, and if the matched resistive load simulating it be removed and be replaced by the inductive load of effective zero impedance, the output current of the amplifier will double; that is to say, any limitation due to the large output current causing overloading will be twice as sever in the case of the inductive load, as it is in the case of the matched load. Similarly at a very high frequency the inductive load impedance will be very high, and if the matched load representing it be removed and replaced by the very large inductive load the voltage across the load will be doubled, hence any limitation due to the large output voltage causing overloading will be twice as severe in the case of the inductive load as it was in the case of the matched load. It will be seen that the matched load produces equally favourable conditions for current and voltage as compared with the inductive load. Any other value of non-inductive load would produce either more favourable voltage o more favourable current conditions than it produces favourable current or voltage conditions respectively. It will be seen therefore that an amplifier which is intended to operate into an inductive load should be designed to produce the optimum undistorted power into a matched load, this load and the output impedance of the amplifier being both equal to the impedance from which the inductive device must be fed in order to give a correct frequency response characteristic.

It must be remembered that it is always possible to match a load impedance to an amplifier impedance or vice versa by means of transformer.

When an amplifier is feeding a purely resistive load where there is no requirement that the amplifier be matched to the load, the load impedance may be arranged so as to be the optimum from the point of view of operating the valve with maximum output or minimum distortion.

It is an object of this invention to provide means whereby a valve may be operated into a load which bears the optimum ratio to the valve impedance, from the standpoint of maximum output or minimum distortion, and which nevertheless presents a matched impedance to the load.

According to the present invention, a thermionic valve amplifier comprises a circuit adapted to produce feed-back in negative sense from the output circuit of the amplifier to a preceding point therein, the voltage fed back when the amplifier is connected to supply substantial power to a load being such a function of the current in the load circuit, or the voltage across the load, or both, that the effective output impedance of the amplifier bears a desired relationship to the effective impedance of the load, the load being chose to have an effective impedance which bears a desired relationship to the output impedance of the amplifier in the absence of feed-back.

Feed-back may be considered to be negative if it reduces the amplifier gain within the working frequency range of the amplifier, although it may increase the gain at extreme frequencies outside that range, due to phase shifts in the amplifier or feed-back circuits. Such phase shifts are practically inevitable in any amplifier which has not an infinite frequency range.

It can be shown that the effect of negative feed-back dependent in amount upon the magnitude of the current in the load is to increase the effective value of the output impedance of the amplifier. If, then, an amplifier has an output stage comprising a triode valve, the effect of negative feed-back dependent upon the current in the load is to increase the effective output impedance of the amplifier so that the output valve can be worked into an impedance greater than its own anode impedance and nevertheless be matched. Thus it is possible with a matched load to operate a triode into an impedance higher than its own anode impedance.

On the other hand, the effect of negative voltage feed-back, that is to say negative feed-back dependent in amount upon the magnitude of the voltage across the load, is to lower the apparent value of the output impedance. In an amplifier employing a pentode or the like output valve, negative voltage feed-back can thus be employed to reduce the apparent output impedance so that the output valve may be worked into an impedance less than its own anode impedance and yet the load is matched.

When both current and voltage feed-back are employed, the effect upon the output impedance of the amplifier depends upon the relative magnitude of the two feed-back voltages.

According, therefore, to a feature of the invention, a thermionic valve amplifier has an output stage comprising one or more thermionic valves such that optimum conditions, for example with regard to harmonic distortion, obtain when said stage works into an effective impedance greater than the anode impedance of said stage, and a negative feed-back circuit, the arrangement being such that for a load which has over the greater part of the frequency range over which the amplifier is designed to work, an effective impedance which is equal to the output impedance of said amplifier in the absence of feed-back, the magnitude of the feed-back voltage is dependent more upon the current in the load than upon the voltage across the load.

According to a further feature of the invention, a thermionic valve amplifier has an output stage comprising a thermionic valve such that optimum conditions, for example with regard to harmonic distortion, obtain when said stage works into an effective impedance less than the anode impedance of said stage, and a negative feed-back circuit, the arrangement being such that for a load which has over the greater part of the frequency range over which the amplifier is designed to work, and effective impedance which is equal to the output impedance of said amplifier in the absence of feed-back, the magnitude of the feed-back voltage is dependent more upon the voltage across the load than upon the current in the load.

It will be seen that the invention enables the output impedance of the amplifier to be increased or decreased by the use of negative feed-back, so that the impedance into which the output valve of the amplifier works may bear any desired relationship to the anode impedance of the output valve, while at the same time a condition of matching between the effective load impedance and the effective output impedance of the amplifier may be maintained. The required output matching conditions and a favourable condition of load impedance with regard to harmonic distortion can thus be simultaneously achieved.

In a further arrangement according to the invention, a thermionic valve amplifier has an output stage comprising a thermionic valve operatively coupled to a load, the effective impedance of which in the absence of feed-back is greater or less than the anode impedance of said stage, and a circuit adapted to introduce negative feed-back of such a magnitude that the effective output impedance of said amplifier is made substantially equal to the effective impedance of said load.

According to a further feature of the present invention, a thermionic valve amplifier has an output stage comprising a pentode valve adapted to be operatively connected to a load, negative feed-back wholly or mainly dependant upon the voltage across the load being employed to reduce the effective output impedance to a value which is lower than its value in the absence of feed-back.

In a thermionic valve amplifier having a triode output stage, according to a further feature of the invention a resistance is connected in series with the load into which the valve works, the potential difference developed across the resistance being fed back in anti-regenerative sense to the grid circuit of said valve, or of an earlier valve in said amplifier, the magnitude of the feed-back potential difference being such that the effective output impedance of the amplifier is increased to an apparent value 1.5 to 3.0 times the value in the absence of feed-back.

The invention finds a further application in the field of telephone repeaters, where the telephone line behaves, in effect, as a substantially resistive load. In order to reduce variations of attenuation due to mismatching, and, in the case of two-wire repeater systems, in order to provide a satisfactory termination impedance to the line so as to avoid reflexion loses, it is usual to match the output impedance of the repeater to the impedance of the line through a suitable transformer. By employing negative feed-back, the effective output impedance of the repeater may be increased, so that the repeater may be coupled to a load having a greater effective impedance. The ratio of the output transformer is so chosen as to match the effective output impedance of the repeater to the impedance of the line, so that the line is terminated by the required matched impedance, and at the same time, the output valve of the repeater works into an impedance greater than its own anode impedance. The restriction of harmonic generation so obtained is particularly useful in multiplex carrier telephony and telegraphy systems where inter-modulation between channels is undesirable.

According to the present invention, therefore in an amplifier adapted to supply power to a substantially constant resistive load such, for example, as a telephone line circuit, in which the impedance of said circuit is required to be matched to the output impedance of said amplifier, the effective value of the output impedance is modified by the use of negative feed-back, the modified output impedance being matched to the load impedance by means of a transformer or transformers.

It is found that the use of negative feed-back in thermionic amplifiers enables considerably greater power to be delivered, without exceeding a certain permissible amount of distortion, or, conversely, allow a given power to be delivered with less distortion. The advantages of impedance changing according to this invention may still be had in an amplifier the final valve of which is driven so strongly that considerable grid current flows; such amplifiers may be made to give reasonably distortionless output if considerable negative feed-back is employed to reduce the harmonic content. By arranging this feed-back in such a manner that the output impedance is suitably modified, the maximum power obtainable may be further increased.

In employing such an amplifier, considerable negative feed-back is used, this feed-back being dependent on both the voltage across the load and the current flowing into the load. By suitably adjusting the ratio of "current" and "voltage" feed-back, it is possible to modify the apparent output impedance of the amplifier so that with a load matched to this output impedance, the maximum available output power is limited equally by the anode potential and the anode current of the power valve becoming zero.

It should be noted that the invention is applicable to amplifiers for the amplification of both high and low frequencies.

The invention will be described as applied to the power output stage of a low-frequency amplifier adapted to supply power to a substantially inductive load such as a loudspeaker, for example.

The low-frequency input to a triode valve is applied to the primary winding of an input transformer, one end of the secondary winding of which is connected to the grid of the valve, while the other end is connected to a source of suitable grid bias voltage through a grid resistance of high value. The cathode of the valve is earthed, and its anode is connected to a source of anode voltage through a choke coil of high impedance, the negative terminal of the source being earthed. The anode is also connected to earth through a coupling condenser, the load, and a feed-back resistance in series. The junction of the feed-back resistance and the load is connected through a feed-back condenser to the junction of the secondary winding of the input transformer and the grid resistance. The coupling condenser and the feed-back condenser are both made of high capacity, so that their impedances over the working frequency range of the amplifier are negligible, and the value of the grid resistance is made large compared with the value of the feed-back resistance.

If in the circuit described above, the output valve has an anode impedance of Ra ohms and an amplification factor m , and the value of the feed-back resistance is R ohms, it can be shown that the effective output impedance Zm of the amplifier is given by the expression

Zm = Ra + (m + 1)R.

For all positive values of R, Zm is greater than Ra, so that if the effective load impedance is also equal to Zm, the effective load impedance will be matched to the effective output impedance of the amplifier, while at the same time, the valve will be working into an impedance greater than its own anode impedance. It is convenient when using a triode valve to make the term (m + 1)R about equal to Ra so that the valve works into an impedance approximately double its own anode impedance.

The circuit described above may be modified by connecting a high resistance across the secondary winding of the input transformer. The function of this resistance is to damp sharp resonances which may be formed by the leakage inductance of the transformer and the grid-cathode capacity of the valve. Resonances so formed may, be reversing the phase of the feed-back voltage, cause the valve to oscillate at very high frequencies.

In a circuit in which it is desired to obtain a feed-back voltage dependent upon the voltage set up across the load, the feed-back resistance referred to above is short-circuited, and the feed-back condenser is connected to a tapping point in a high resistance potentiometer connected across the load terminals. If the valve has an anode impedance of Ra ohms and an amplification factor m , then if the fraction of the potential difference across the load which is fed back is k, it can be shown that the apparent output impedance of the amplifier, Zm, is given by the expression

EQUAT. HERE

Thus when the amplifier is connected to a matched load of effective impedance Zm, the valve works into an impedance less than its own anode impedance.

Now it has hitherto been appreciated that the effect of negative feed-back in thermionic valve amplifiers is to restrict the generation of harmonics: the present specification shows that the same effect may be obtained by employing feed-back to vary the output impedance of the amplifier so that even when working into a matched load the output valve of the amplifier may operate into an impedance several times greater or less than its own anode impedance. In order to obtain a very large degree of harmonic reduction, it may be desirable to employ more feed-back than is necessary to vary the output impedance by the desired amount, the excess feed-back being employed, in a manner well-known per se, to suppress harmonics in the output circuit. The manner in which this suppression is obtained may be explained by saying that the feed-back voltage contains harmonics which would not otherwise be present in the input circuit; these harmonics are amplified and tend to cancel the harmonics in the output circuit, to which they are in anti-phase.

In an amplifier in which a large degree of feed-back is to be used, it is desirable to employ a combination of current and voltage feed-back in order to avoid making the output impedance too widely different from the anode impedance of the output valve. The amount by which the output impedance is changed, and the sense in which the change takes place, is then dependent upon the preponderance of one type of feed-back over the other. An amplifier of this type will now be described by way of example.

The oscillations to be amplified are applied to the primary winding of an input transformer, one end of the secondary winding of which is connected to the grid of a first amplifying valve while the other end is connected through a high grid-resistance to a source of grid bias voltage. The first amplifying valve is coupled in any known or suitable manner to a second amplifying valve which in turn is coupled to the grid-cathode circuit of a third amplifying valve, which is of the indirectly heated cathode type. The anode of this valve is connected to a source of anode voltage, and its cathode is connected directly to the grids of a pair of power output valves in parallel, and through a choke coil of very low ohmic resistance to a source of grid bias voltage.

The anodes of the power output valves, which are connected in parallel, are connected to a source of anode voltage through an output choke coil of high inductance and a feed-back resistance of low value, in series. The ends of the output choke are connected to the ends of the primary winding of an output transformer through two condensers, and the load is connected across the secondary winding. In the arrangement described, the effective load impedance is the impedance seen for the terminals of the output choke, looking towards the load. Connected in parallel with the output choke is a potentiometer comprising two resistances in series, the junction of which is connected through a condenser of high capacity to the junction of the secondary winding of the input transformer and the grid resistance.

Feed-back due to current in the load is obtained by means of the feed-back resistance in series with the choke, and feed-back due to voltage across the load is provided by the potentiometer. In the amplifier described, the output stage might consist of two "LS5" valves operated at an anode voltage of 285 volts with a steady anode current of 35 milliamps each. Under these conditions the anode impedance of each valve is 3550 ohms giving a combined impedance of 1775 ohms. The feed-back resistance may have a value of 53 ohms and the potentiometer may consist of resistances of 3090 ohms and 236,000 ohms, thus feeding back 0.0129 of the voltage across the load. Were a load of 1775 ohms employed, the voltage fed back from the resistance of 53 ohms would be a little over twice as great as the voltage fed back from the potentiometer across the load. The effect of this "current" feed-back being greater than the "voltage" feed-back, is to raise the apparent output impedance of the amplifier. In the case considered, the apparent output impedance might be raised to as high as 3700 ohms. Using feed-back and employing a 3700 ohms load (as seen through the output transformer), an amplifier built as described can be made to give a power output of 6.5 watts with only 1% harmonic content. The advantage of raising the apparent output impedance for such an amplifier, is that with a matched load the maximum output of the amplifier is equally limited by the anode potential tending to become negative and the anode current being reduced to zero.

Dated this 18th day of September, 1933.

REDDIE & GROSE,

Agents for the Applicants,

6, Breamís Buildings, London, E.C.4.

 

COMPLETE SPECIFICATION

Improvements in and relating to Thermionic Valve Amplifiers

We, ELECTRIC & MUSICAL INDUSTRIES LIMITED, a British Company, of Blyth Road, Hayes, Middlesex, ALAN DOWER BLUMLEIN, a British Subject, of 32, Woodville Road, Ealing, London, W.5, and HENRY ARTHUR MAISH CLARK, a British Subject, of 119, Wynchgate, Southgate, London, N.14, do hereby declare the nSubject, of 119, Wynchgate, Southgate, London, N.14, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:-

The present invention relates to thermionic valve amplifiers.

The invention is particularly concerned with amplifiers of the type adapted to supply substantial power to a consuming device or load, and it is an object of the present invention to improve the performance of such power amplifiers.

It is well-known that the amount of undistorted power which a thermionic valve amplifier can supply to a load, depends, among other thins, upon the relationship between the output impedance of the amplifier, that is, the impedance seen looking towards the amplifier from the terminals of the load, and the impedance of the load itself. Generally, there may be said to be two optimum values of the ratio between the output impedance and the load impedance. When the load impedance is made equal to the output impedance, the load is said to be matched. This equality ratio of impedances is in some cases an optimum condition from the standpoint of frequency response of the combined apparatus. On the other hand, certain classes of thermionic amplifiers will give more output power for given distortion, or less distortion for a given output power, when the load impedance bears a definite ratio, other than unity, to the output impedance of the amplifier.

Certain classes of load are inductive, and when such a load is required to operate over a wide frequency range, it will be seen that it represents a load of widely varying impedance. Further, in order that this load may give its correct frequency response characteristic, it is necessary that it be fed from a particular output impedance. For the purpose therefore of assessing the effect of such a load on an amplifier, it is usual to regard such inductive loads as equivalent to a load of fixed resistive value equal to the output impedance from which they must be fed in order to give their correct frequency characteristic. That is to say, an amplifier which feeds an inductive load must be designed to provide the requisite power into a load which has the same impedance as the amplifier output impedance, i.e. is matched. That a non-inductive matched load is the most fair fixed representation of a purely inductive load operating over a wide frequency range, can be shown by considering the conditions that obtain with an inductive load at very low and very high frequencies. At a very low frequency the inductive load will have a negligible impedance, and if the matched resistive load simulating it be removed and be replaced by the inductive load of effective zero impedance, the output current of the amplifier will double; that is to say, any limitation due to the large output current causing overloading will be twice as sever in the case of the inductive load, as it is in the case of the matched load. Similarly at a very high frequency the inductive load impedance will be very high, and if the matched load representing it be removed and replaced by the very large inductive load, the voltage across the load will be doubled, hence any limitation due to the large output voltage causing overloading will be twice as severe in the case of the inductive load as it was in the case of the matched load. It will be seen that the matched load produces equally favourable conditions for current and voltage as compared with the inductive load. Any other value of non-inductive load would produce either more favourable voltage or more favourable current conditions than it produces favourable current or voltage conditions respectively. It will be seen therefore that an amplifier which is intended to operate into an inductive load should be designed to produce the optimum undistorted power into a matched load, this load and the output impedance of the amplifier being both equal to the impedance from which the inductive device must be fed in order to give a correct frequency response characteristic.

It must be remembered that it is always possible to match a load impedance to an amplifier impedance or vice versa by means of transformers.

When an amplifier is feeding a purely resistive load, the requirement that the amplifier be matched to the load so as to obtain a desired frequency response does not exist, and the load impedance may be arranged so as to be the optimum from the point of view of operating the valve with maximum output or minimum distortion. It will be shown hereinafter, however, that is the case of certain resistive loads such as telephone lines, for example, it is desirable to match the amplifier to the load to avoid reflexion.

It is an object of this invention to provide new or improved means whereby the output valve of an amplifier may be operated into a load which bears the optimum ratio to the valve impedance, from the standpoint of maximum output or minimum distortion, and which nevertheless presents a matched impedance to the load.

For the purposes of this specification, a valve of the type in which, when the valve operates as a power amplifier for supplying power to a load, the ratio of the maximum anode voltage swing free of overload to the maximum anode current swing free of overload is less than the anode impedance of the valve, will be referred to for convenience as a valve of the pentode type. A valve which is of the type wherein the maximum anode voltage swing free of overload to the maximum anode current swing free of overload, when the valve operates as a power amplifier, is greater than the anode impedance of the valve will be referred to as a valve of the triode type. In the above definitions, a valve is regarded as being overloaded when the anode voltage or current is allowed to reach a value more than twice the static anode voltage or current respectively.

It can be shown that in a thermionic amplifier the effect of negative or anti-regenerative feed-back dependent in amount upon the magnitude of the voltage across the load is to lower the apparent value of the output impedance of the amplifier. In an amplifier employing as output valve a valve of the pentode type, negative voltage feed-back (that is feed-back dependent upon the voltage across the load) can thus be employed to reduce the apparent output impedance so that the output valve may be worked into an impedance less than its own anode impedance and nevertheless be matched.

Feed-back may be considered to be negative if it reduces the amplifier gain within the working frequency range of the amplifier, although it may increase the gain at extreme frequencies outside that range, due to phase shifts in the amplifier or feed-back circuits. Such phase shifts are practically inevitable in any amplifier which has not an infinite frequency range.

Furthermore, it is known that the effect of negative current feed-back, that is to say, negative feed-back dependent in amount upon the magnitude of the current in the load, is to increase the effective value of the output impedance of the amplifier.

It has been proposed, in an amplifier employing a triode valve, to provide negative voltage feed-back from the anode circuit to the grid circuit; simultaneously, feed-back dependent upon the current in the load is arranged to take place, and the magnitudes of the voltage feed-back and the current feed-back are made such that no change in the effective output impedance of the amplifier is produced by the combined feed-back.

According to the present invention, in a thermionic valve amplifier having an output stage which comprises a thermionic valve of the pentode type there is provided a negative feed-back circuit which is connected and arranged to feed back from the output circuit of said stage to the input circuit thereof a voltage dependent upon the voltage set up across the load impedance.

According to a feature of the invention, in a thermionic valve amplifier having an output stage comprising a thermionic valve of the pentode type, there is provided a negative feed-back circuit which is connected and arranged to feed back from the output circuit of said stage to the input circuit thereof a voltage dependent both upon the voltage set up across the load and upon the current in the load, and the arrangement is made such that, if it were used with a load which, by virtue of the feed back, were matched, the magnitude of the geed-back voltage would be dependent more upon the voltage across the load than upon the current in the load.

According to a further feature of the invention, in a thermionic valve amplifier having an output stage comprising a thermionic valve of the triode type, there is provided a negative feed-back circuit which is connected and arranged to feed back from the output circuit of said stage to the input circuit thereof a voltage dependent both upon the voltage set up across the load and upon the current in the load, and the arrangement is made such that, if it were used with a load which, by virtue of the feed back, were matched, the magnitude of the feed-back voltage would be dependent more upon the current in the load than upon the voltage set up across the load.

It will be seen that the invention enables the output impedance of the amplifier to be increased or decreased by the use of negative voltage, or combined voltage and current feed-back, so that the impedance into which the output valve of the amplifier works my bear any desired relationship to the anode impedance of the output valve, while at the same time a condition of matching between the effective load impedance and the effective output impedance of the amplifier may be maintained. The required output matching conditions and a favourable condition of load impedance with regard to harmonic distortion can thus be simultaneously achieved.

The invention finds a further application in the field of telephone repeaters, where the telephone line behaves, in effect, as a substantially resistive load. In order to reduce variations of attenuation due to mismatching, and, in the case of two-wire repeater systems, in order to provide a satisfactory terminating impedance to the line so s to avoid reflexion losses, it is usual to match the output impedance of the repeater to the impedance of the line through a suitable transformer. By employing negative voltage feed-back, the effective output impedance of the repeater may be decreased, so that the repeater may be coupled to a load having an effective impedance less than the value required for matching. The ratio of the output transformer, where it is necessary, is so chosen as to match the effective output impedance of the repeater to the impedance of the line, so that the line is terminated by the required matched impedance, and at the same time, the output valve of the repeater works into an impedance less than its own anode impedance. The restriction of harmonic generation so obtained is particularly useful in multiplex carrier telephony and telegraphy systems where inter-modulation between channels is undesirable.

According to the present invention therefore, in an amplifier which comprises a valve of the pentode type which is adapted to supply power to a substantially constant resistive load such, for example, as a telephone line circuit, it being required that the impedance of said circuit shall be matched to the output impedance of said amplifier, there is provided a negative feed-back circuit for producing negative feed-back dependent in magnitude more upon the voltage across the load than upon the current in the load; the modified anode impedance of said valve may be matched to the load impedance by means of a transformer or transformers.

It is found that the use of combined voltage and current negative feed-back in thermionic amplifiers enables considerably greater power to be delivered, without exceeding a certain permissible amount of distortion, or, conversely, allows a given power to be delivered with less distortion. The advantages of impedance changing according to this invention may still be had in an amplifier the final valve of which is driven so strongly that considerable grid current flows; such amplifiers may be made to give reasonably distortionless output if considerable negative feed-back is employed to reduce the harmonic content. By arranging this fee-back in such a manner that the output impedance is suitably modified, the maximum power obtainable may be further increased.

In employing such an amplifier, considerable negative feed-back is used and by suitably adjusting the ratio of "current" and "voltage" feed-back, it is possible to modify the apparent output impedance of the amplifier so that with a load matched to this output impedance, the maximum available output power is limited equally by the anode potential and the anode current of the power valve becoming zero.

It should be noted that the invention is applicable to amplifiers for the amplification of both high and low frequencies.

The invention will now be described by way of example with reference to the accompanying drawing, in which

Fig. 1 shows the output stage of an amplifier according to the invention, and

Fig. 2 shows a further amplifier according to the invention.

Referring to Fig. 1 the low-frequency input to the pentode output valve 1 of an amplifier is applied to the primary winding or an input transformer 2, one end of the secondary winding of which is connected through a coupling condenser 3 to the control grid of the valve, while the other end is connected to a point 4 in the primary winding 5 of an output transformer 8; the winding 5 is connected between the anode of the valve 1 and a point 6 in a source (not shown) of anode current, the negative terminal of which is connected to the cathode of the valve. The control grid of the valve 1 is connected to as source of suitable grid bias voltage (not shown) through a grid resistance 7 of high value. The screening grid of the valve 1 is connected to a point in the anode current source at a suitable positive potential, and the terminals 9 associated with the secondary winding of the output transformer 8 are connected to the load. The latter may be inductive, and may comprise the speech coil of a moving-coil loudspeaker for example, or it may be a resistive load such as a telephone line.

If the valve 1 has an anode impedance of Ra ohms and an amplification factor m , and if the fraction of the potential difference across the primary winding 5 of the transformer 8 which is fed back is k, it can be shown that the apparent output impedance of the amplifier, Zm, is given by the expression

EQUAT. HERE

 

This expression may be rewritten

EQUAT. HERE

where g is the mutual conductance of the valve 1, and it will be seen that if gk is large compared with EQUAT. HERE as is normally the case with a pentode valve, the expression is substantially independent of the exact value of Ra. This is an advantage since g is less dependent upon the amplitude of the applied oscillations than is the anode impedance Ra.

The amplifier is connected to the load by means of the transformer 8, the ratio of which is made such that the load is matched to the effective impedance Zm of the valve; it will be observed, however, that despite the condition of matching which exists, the valve works into an impedance less than its actual anode impedance.

When the load is a telephonic line, or like resistive impedance, the ratio of the transformer 8 is chose to match the effective impedance Zm of the amplifier to the characteristic impedance of the line. However, as explained above, when the load is inductive, the transformer is chosen to match Zm to an impedance equal to the output impedance from which the load must be fed in order to give its correct frequency response characteristic. The impedance which, for the purposes of matching is assigned to the load may best be found by trial and error.

In a modification of the arrangement shown in Fig. 1, a potentiometer resistance of high value is connected in shunt with the primary winding 5 of the transformer 8, and the end of the secondary winding of the transformer 2 remote from the control grid of the valve 1 is connected to a suitable point in the potentiometer resistance. In some circumstances, the output transformer may be omitted, and the load connected directly in the anode circuit of the valve; the feed-back potentiometer resistance is then connected in shunt with the load. In all cases a condenser may be connected in series with the potentiometer resistance.

Other methods of obtaining voltage feed-back will be readily apparent to those skilled in the art: for example, the feed-back voltage may be taken to the input circuit of a valve in the amplifier other than the output value, provided that the feed-back is always in the negative, or anti-regenerative sense.

Now it has hitherto been appreciated that the effect of negative feed-back in thermionic valve amplifiers is to restrict the generation of harmonics: it has been shown in the present specification that the same effect may be obtained by employing negative voltage fee-back to reduce the output impedance of the amplifier so that even when working into a matched load the output valve of the amplifier may operate into an impedance several times less than its own anode impedance. In order to obtain a very large degree of harmonic reduction, it may be desirable to employ more feed-back than is necessary to vary the output impedance by the desired amount, the excess feed-back being employed, in a manner well-known per se, to suppress harmonics in the output circuit. The manner in which this suppression is obtained may be explained by saying that the feed-back voltage contains harmonics which would not otherwise be present in the input circuit: these harmonics are amplified and tend to cancel the harmonics in the output circuit, to which they are in anti-phase.

In an amplifier in which a large degree of feed-back is to be used, it is desirable to employ a combination of current and voltage fee-back in order to avoid making the output impedance too widely different from the anode impedance of the output valve. The amount by which the output impedance is changed, and the sense in which the change takes place, is then dependent upon the preponderance of one type of feed-back over the other. An amplifier of this type will now be described with reference to Fig. 2 of the accompany drawing.

Referring to Fig. 2, the oscillations to be amplified are applied to the primary winding of an input transformer 10, one end of the secondary winding of which is connected to the grid of a first amplifying valve 11 while the other end is connected through a high grid-resistance 12 to a source of grid bias voltage (not shown). The first amplifying valve is coupled by means of a choke coil 13 and coupling condenser 14 to a second amplifying valve 15 which in turn is s8imlarly coupled by means of choke 16 and condenser 17 to the grid-cathode circuit of a third amplifying valve 18, which is of the indirectly heated cathode type. The anode circuits of the valves 11 and 15 include decoupling resistances 19 and decoupling condensers 20, and the control grids of the valves 15 and 18 are connected to a source of biasing potential through grid resistances 21 and 22 respectively. The anode of the valve 18 is connected to a source of anode voltage (not shown) through decoupling resistance 23, and its cathode is connected directly to the grids of a pair of power output valves 24 and 25 in parallel, and through a choke coil 26 of very low ohmic resistance to a source of grid bias voltage (not shown).

The anodes of the power output valves 24 and 25, which are connected in parallel, are connected to the source of anode voltage through an output choke coil 27 of high inductance and a feed-back resistance 28 of low value, in series. The ends of the output choke are connected to the ends of the primary winding of an output transformer 29 through two condensers 30, and the load (not shown) is connected across the secondary winding of the transformer 29. In the arrangement described, the effective load impedance is the impedance seen from the terminals of the output choke 27, looking towards the load. Connected in parallel with the output choke 27 is a potentiometer comprising two resistances 31 and 32 in series, the junction of which is connected through a condenser 33 of high capacity to the junction of the secondary winding of the input transformer 10 and the grid resistance 12.

Feed-back due to current in the load is obtained by means of the feed-back resistance 28 in series with the choke 27, and feed-back due to voltage across the load is provided by the potentiometer 31, 32. In the amplifier described, the output valves 24, 25 may be of the "LS5" type, and may be operated at an anode voltage of 285 volts with a steady anode current of 35 milliamps each. Under these conditions the anode impedance of each valve is 3550 ohms giving a combined impedance of 1775 ohms. The feed-back resistance 28 may have a value of 53 ohms and the resistance 31 may be 3090 ohms while the resistance 32 is 236,000 ohms, thus feeding back 0.0129 of the voltage across the load. Were a load having an effective impedance (seen through the output transformer) of 1775 ohms employed, the voltage fed back from the resistance 28 of 53 ohms would be a little over twice as great as the voltage fed back from the potentiometer 31, 32 across the load. The effect of this "current" feed-back being greater than the "voltage" feed-back, is to raise the apparent output impedance of the amplifier. In the case considered, the apparent output impedance might be raised to as high as 3700 ohms. Using feed-back and employing a 3700 ohms load (as seen through the output transformer), and amplifier built as described can be made to give a power output of 6.5 watts with only 1% harmonic content. The advantage of raising the apparent output impedance for such an amplifier, is that with a matched load the maximum output of the amplifier is equally limited by the anode potential tending to become negative and the anode current being reduced to zero.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:-

  1. A thermionic valve amplifier having an output stage which comprises a thermionic valve of the pentode type, wherein there is provided a negative feed-back circuit which is connected and arranged to feed back from the output circuit of said stage to the input circuit thereof a voltage dependent upon the voltage set up across the load impedance.
  2. A thermionic valve amplifier having an output stage comprising a thermionic valve of the pentode type, wherein there is provided a negative feed-back circuit which is connected and arranged to feed-back from the output circuit of said stage to the input circuit thereof a voltage dependent both upon the voltage set up across the load and upon the current in the load, and wherein the arrangement is such that, if it were used with a load which, by virtue of the feed back, were matched, the magnitude of the feed-back voltage would be dependent more upon the voltage set up across the load than upon the current in the load.
  3. A thermionic valve amplifier having an output stage comprising a thermionic valve of the triode type, wherein there is provided a negative feed-back circuit which is connected and arranged to feed back from the output circuit of said stage to the input circuit thereof a voltage dependent both upon the voltage set up across the load and upon the current in the load, and wherein the arrangement is such that, if it were used with a load which, by virtue of the feed back, were matched, the magnitude of the feed-back voltage would be dependent more upon the current in the load than upon the voltage set up across the load.
  4. A thermionic valve amplifier, which comprises in its output stage a valve of the pentode type which is required to feed power to a substantially matched load of substantially constant resistive value, wherein there is provided a negative feed-back circuit for producing negative feed-back dependent in magnitude more upon the voltage across the load than upon the current in the load.
  5. A thermionic valve amplifier according to claim 4, wherein the modified anode impedance of said valve is matched to the load impedance by means of one or more transformers of suitable ratios.
  6. A thermionic valve amplifier according to any of the preceding claims, wherein said output stage comprises a thermionic valve having its control grid so biased and connected that substantial grid current flows over a large part of the working range of output power.
  7. A thermionic valve amplifier substantially as herein described or as shown in the accompanying drawing.

Dated this 18th day of September, 1934.

REDDIE & GROSE,

Agents for the Applicants,

6, Breamís Buildings, London, E.C.4.

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Leamington Spa: Printed for His Majestyís Stationery Office, by the Courier Press. 1935.