US2325933A - Wide range amplifier - Google Patents

Description

Augs, 1943. y A w BARBER 2,325,933

' wrm; RANGE AMPLIFIER Filed Nav. 10. r193,9

INV'M ,y

Patented Aug. 3, 1943 WIDE RANGE AMPLIFIER Alfred W. Barber, Flushing, N. Y.,

Boonton Radio Corporation,

assignor to Boonton, N. J.

Application November 10, 1939, Serial No. 303,723

The present invention concerns thermionic vacuum tube amplifiers and in particular methods of and means for providing a. very Wide frequency response and low distortion in such amplifiers.

One object of the present invention is t provide a very Wide, essentially constant frequency response in a therinionic vacuum tube amplifier.

Another object is to provide reduced distortion in a wide frequency responsive amplifier.

Still another object is to provide a very iiexible output coupling system in a wide range amplifier.

A still further object is to provide a simple and effective volume control means in a wide frequency range amplifier which has no appreciable eiect on the frequency 'response or distortion of the amplifier.

These and other objects of the invention will be evident from the detailed description of the invention given in connection with the drawing.

In thepast, various means -have been employed to extend the frequency range of amplifiers and to reduce the distortion generated. A resistancecoupled amplifier seems best suited to a wide range frequency response. The high frequency response of a resistance coupled amplifier is limited by the shunt capacity due to tubes and wiring across the effective tube plate loads.

Decreasing the load resistors extends the frequency range but eventually a point is reached at which the load resistors are so low that no gain is produced. I have found that inductance coils of proper size may be utilized to greatly extend the upper frequency limit of resistance coupled amplifiers. For instance, I have been able to extend the frequency range from a former upper limit of 500,000 cycles to 1,000,000 cycles up to 5,000,000 to 10,000,000 cycles.

Distortion in thermionic vacuum tube amplifiers is due to the curvature of the plate current characteristics.

Push-pull amplifiers have been used in which the curvature of one tube balances the curvature of the other tube. It has not been found possible to do this in a resistance coupled amplifier. I have found it possible to so operate an amplifier of several stages that adjacent cascade stages balance out harmonics in a manner similar to push-pull but in a single-ended resistance coupled amplifier.

In providing a volume control a simple potentiometer is desirable but a very low value of total resistance is necessary to maintain a constant frequency characteristic. I have found that an 2 Claims. (Cl. 179-171) electrolytic condenser may be used to feed the volume control in which case the device is sultable for a wide frequency range-amplifier. By placing two amplifier stages after the volume control constant harmonic balance conditions are maintained.

By employing degeneration at low frequencies and relaxing it at high frequencies, I` have found that an added improvement in the high frequency response is produced.

In the amplifier output I have found it possible to provide a direct capacity-coupled output, a transformer output and a resistance attenuator output selectable at will by means of a multipoint switch. A level indicator across the output 'is also provided.

The figure of the drawing shows an amplifier embodying one form of the present invention.

In the figure is shown a four stage thermionic vacuum tube amplifier utilizing the four tetrode tubes I, 2, 3 and 4. A high-side input point 5 and low-side input point 6 is provided. The lowside point 6 is connected to ground G while the high-side point 5 is coupled thru condenser l to grid I0 of tube I. Tube I includes cathode 9, control grid Cathode 9 receives a, bias thru cathode resistor I3 by-passed for all frequencies to be amplified uniformly by condenser I4. Grid I0 is returned to ground G thru resistor 8. Screen grid II receives a bias from power supply IIO thru dropping resistor II and is by-passed to ground G by condenser IG. Plate I2 is energized from power supply IID and loaded to produce an output voltage by resistor I8 in series with inductance I9. Resistance I8 in series with inductance I9 forms an impedance having a rising characteristic with frequency. This compensates partially or up to a certain frequency the shunting effect of the plate capacity of tube I which is across. this load. If the inductive impedance is made equal to resistanceJS at the frequency at which the shunt capacity has an impedance equal to the resistance the total effective impedance will rise slightly with frequency equalling 1.4 times the resistance at this frequency. If the inductive reactance is made equal to one-half the resistance the total impedance will be equal to the resistance up to this limiting frequency.

The output Vacross load I 8..I9 is applied to grid 25 of tube 2 thru blocking condenserZIl and compensating inductance 22 shunted by damping resistor 23. Tube 2 is shown as a tetrode having cathode 24 heated by means not shown, contro1 -grid 25, screen grid 26 and plate 21. lCathode 24 I0, screen grid'II and plate I2.`

is by-passed by condenser 29. Screen'grid 28 is energized from power supply I| thru dropy ping resistor 3| and across by-pass condenser 30. The signal voltage applied to grid 25 `is derived from the plate yvoltage of tube lbut it is reso- `nated at the high end of the desired frequency band by 'means of inductance 22 which forms a series resonant circuit with the input or grid capacity of tube 2. This resonance lifts the upper end of the frequency response as much as ten times in a practical case. The resulting response peak may be too sharp to exactly complement the rest of the amplier response which case it may be made broader by shunting inductance 22 with a damping resistor 23. Grid 25 is maintained at the desired average potential by means of grid resistor 2| connected as far as the direct current circuit is concerned between grid 25 and ground G. Tube 2 has a load circuit connected between plate 21 and plate power supply ||0 consisting in resistor 33 in series with inductance 32. The values of resistance and inductance may be the same as for tube I or in case a wider frequency band is to be covered the inductances for both tubes ymay be chosen to over compensate their respective stages the rise in one stage gain lling in the valley in the gain of the other stage.

The signal voltage across plate 21 is fed to the following tube 3 thru blocking condenser 35,

frequency gain of the stage. In order to minimize the effect of grid capacity on the frequency characteristic of the volume control potentiometer 38 it is made a relative low resistance. I have found that a resistance of the order of 3000 ohms is satisfactory for a at frequencyI response up to 5,000,000 cycles. In order to supply low frequencies to this volume control alarge coupling condenser is needed. I have found that 16 microfarads is suitable for condenser 35. With the low grid resistance supplied by potentiometer 36 I have found an electrolytic condenser quite suitable for capacity 35. Placing condenser 35 on the grid side of coil 34 helps to improve the frequency response by supplying a more constant capacity into which coil 34 may. resonate than if grid 39 alone were utilized. The effect of grid capacity of grid 39 varies with the setting of the volume control while the capacity to ground of condenser 35 remains fixed. Volume control resistor 36 supplies damping to'inductance 34 so that additional damping may not be required.

Tube 3 is a tetrode including cathode 38 heated by means not shown, control grid 39, screen grid 40 and plate.4|. Screen grid 40 is energized from power supply ||0 thru dropping resistor 45 and across by-pass condenser 44. Cathode 38 receives a bias due to the cathode current drop in resistor 42 by-passed by condenser 43. Plate 4| is energized from the power supply ||0 thru the load resistor 41 connected in series with compensating inductance 46 chosen to give flat or rising characteristic as desired and as described in connection with tubes I and 2. Signal voltage is fed to grid 52 of the following tube 4 thru coupling condenser 48 and series resonating inductance 49 and across grid resistor 50. Choke 49 may be shunted to broaden the resonant peak or the peak may be pushed to a higher frequency where it becomes less noticeable due to the losses in the other amplifier stages.

The lastl tube is a tetrode as shown by tube 4 having cathode 5| heated by means not shown, control grid 52, screen grid 53, and plate 54. Cathode 5| receives a bias from cathode current owing in resistor by-passed by condenser 56. Screen grid 53 receives a bias from power supply I I0 thru resisto'. 58 and across by-pass condenser 51. Plate 54 'receives its current from power supply ||0 connected thru a plate load consisting of resistor 59 shunted by a seriescircuit includ- -ing inductance 60, inductance 6| shunted by resistor 62, and inductance 63. Signal voltage to feed an output attenuator or other device is taken off at the junction between inductances 60 and 6| thru coupling condenser 64 and series resonating choke 65.

Choke 63 is a large inductance choke providing a large impedance at the ,low frequency end of the frequency range to be amplified. I have 'found an iron cored choke having an effective inductance of 30 henrys to be suitable. Inductance 6| is a low distributed capacity ratio or intermediate frequency choke for providing a high impedance thru the middle of the frequency band to be amplified and on up into the high frequency end of the range. I have found that an air-cored choke having an inductance of 250 millihenrys and a distributed capacity of 2-3 microfarads is suitable for the purpose. Inductance 6| is shunted by a resistor which dampens the series resonance of inductance 6| with the distributed capacity of inductance 63 keeping the effective series impedance from falling below a few thousand ohms at any frequency between the extreme ends of the vfrequency range. I have found that resistance 62 should be of the order of 10,000 ohms to accomplish' this. Inductance 60 acts as a series resonator with capacity across the output side as previously described. Resistor 59 dampens various resonances in the system preventing peaks and valleys in the response. I have found that resistor 59 should be of the order of 10,000 ohms.

The output system is very flexible and it is modified as desired by means of a three-gang tap switch including moving contactors 66, 61 and 68 actuated by a single knob. Contaotor 66 is connected to a voltmeter which measures various voltages in the output system. Contactor 61 switches the signal voltage from output tube 4 to a direct circuit to output binding post |04,

across auto-transformer` |05, or across a resistance type attenuator. Contactor 68 connects output binding post |04 to the full output voltage, to various taps on auto-transformer |05, or to various taps `on the resistance type attenuator.

When voltmeter contactor 66 is on any one of taps 59, 10, 1|, 12, 13 or 14, the voltmeter measures the signal voltage at the output binding post |04. When voltmeter contactor 66 is on any one of taps 15, 16, 11, 18 or 19 the voltmeter measures the signal voltage across the inputto the resistance type attenuator. When the input contactor 61 is on tap 8|, contactor 68 is on tap 92 and the full unattenuated signal output of tube 4 is fed to the output binding post |04. When the input contactor 611s on any of taps 82, 83, 84, or 86 the output of-tube 4 is placed across auto-transformer |05 and output binding post |04 is Iconnected to oneof taps 93, '94, y95, 96 or 91 on auto-transformer |05. I have found turns 'ratios on transformer |05 which matched im- |02 on the attenuator made up ofresistors |06,

IUT, I08 and |09. I have used voltage ratios of 10 to 1 for each attenuator step. A second output binding post |03 connected to the ground G or low side of the system is also provided.

It will beseen that volume control 36 is located after the rsttwo amplifier stages of the system comprising tubes I and 2 and before the last two stages comprising tubes 3 and 4. This operates the four amplifier stages in two stages of two tubes each'in which each set of two -tubes operates at fixed gain. This is done in order to minimize distortion. The signal voltage is shifted 180 degrees in each amplifier stage and distortion especially even harmonics generated in tube I is opposed by similar distortion `generated in tube 2. Similarly distortion generated by tube 3 is opposed by distortion generated in tube 4. I have found that in tubes 3 and 4 that a very good cancellation of distortion can be aifectedover the full range of ampiier output levels by proper choice of tubes, operating voltages and plate resistors. I have found that if tubes are chosen which will operate over simil-ar curvature in their plate currentcharacteristics are used for stages 3 and 4 that substantial harmonic cancellation may be produced at an intermediate output level and that while a slight increase in distortion takes place for greater or less levels that much' l improvement is produced over conventional lampliiers. I have found that the following oonstants give cancellation of 2nd harmonic at about one-third full voltage output Where maximum rated voltage output across a 1000 ohm load is 30 volts.

The cathode resistor of tube 4 is eectively not quency response, I have used a variable degenerative circuit. If the cathode resistor of tube 4 is by-passed by a small condenser 56 the stage will degenerate .at low frequencies but at the high frequency end of the frequency range the bypassing will become eiective sincreasing the gain of th'e stageand thereby compensating an otherwise falling gain-frequency characteristic.

'I'he electrolytic coupling condenser 35 may be used provided the resistance of grid resistor or volume control 36 is low compared to the leakage resistance of the condenser. If the resistance of.

resistor 36 is' small compared to the leakage resistance of condenser 35 divided by the amplication factor of tube 3, the drop across resistor 36 due to condenser leakage current will have no ap-l preciable ell'ect on the net bias on grid 39.

While only'one form of the various features of the present invention has been shown and deby-passed to lowerthe gain of this stage for distortion cancellation.

As an added means of improving the high frescribed, many modifications will be apparent to those skilled in the art within the spirit and scope of the inventionfas set forth in the appended claims.

What is claimed is:

1. In a thermionic vacuum tube amplifier. the

combination of, a plurality of thermionic vacuum tubes associated with a pluralityof compensating circuits vto produce substantially constant gain and low distortion over a wide range offrequencies including, a volume control potentiometer preceeding two final tubes, a degenerative cathode circuit connected to the nal tube for reducing distortion to substantially that generated by the preceding tube to provide substantial cancellation of distortion generated by saidtwo tubes; and inter-tube coupling impedances including series and shunt compensating inductances to equalize the gain of said two tubes over a wide frequency range to extend the cancellation of said distortion over said wide frequency range.

2. Ina wide band amplifying circuit, the 4combination of, four cascaded amplifier stages embodying at least one thermionic vacuum tube in each stage, means forvarying the overall gain of said ampliiler while keeping the generated distortion substantially constant including, a gain control device connected between the second and third stagesfor varying the gain ofthe circuit while keeping the gain of the rst two and last two stages equal, for maintaining balanced distortion generated by each of said twovstages.

.AI-FRED W. BARBER.

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