US2266168A - Amplifier - Google Patents

Description

FIG.

Dec. 16, 1941.

T. H. CR-ABTREE AMPLIFIER Filed NOV. 22, 1938 2 Sheets-Sheet 1 [NI/EN TOR -7'.H.CRABTREE BY ATTORNEY Patented Dec. 16, 1941 AMPLIFIER Theodore H. Crabtree, New Brighton, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N.;Y.,. a corporation of New York Application November 22, 1938,' Serial No. 241,805

5 Claims.

This invention relates to amplifiers, particularly to amplifiers for sound recording systems and the object of the-invention is an amplifier capable of use with-recorders of different types.

Many of the recorders commonly used require -merely that the driving amplifiers shall be capable of providing the desired output level'and of transmitting a wide band of frequencies-at uniform efliciency. The requirements for an amplifier to drive recorders of the feedback type are however much more stringent so that an amplifier for general recording purposes must be capable of being modified in several important respects with minimum loss of time.

According to this invention, when the amplifier is being used to drive a recorder of the negative feedback type, one or more stages'may be used exclusively for amplifying'the energy fed back to the input of the remaining stages on which the signals to be recorded are impressed.

In this v manner a very high degree of negative feedback is obtained with the result that the recorder may have a fiat response over a very wide range of frequencies and amplitudes.

Switching means or strapping connections are provided in various portions of the circuit so that when the recorder to be driven is not of the feedback type, the feedback portion of the amplifier may be cut out or connected in such a manner as to increase the gain-of theamplifier.

When sufficient negative feedback is used to obtain a recorder characteristic which is fiat over a very wide frequency range it is very difficult to keep the phase shift around the feedback loop small enough to prevent the feedback from becoming positive at high frequencies. According to a feature of this invention, in order to increase the stability of the amplifierat high frequencies, less feedback is used than is required'for pro ducing a fiat response down to the lowest frequenciesin the band of interest. The low frequency deficiency is then corrected by making the signal amplifier more efiicient in this portion of the range. This increased eificiency .is conveniently obtained without additional circuit elements by a proper proportioning of the coupling impedances between the feedback and signal amplifiers. 7

Alternatively, or as an additional precaution against instability, positivefeedback at frequencies above the band of interest may be decreased or eliminated by shunting the feedback path with acondenser of such small capacity as to have only slight-elfect on transmission within the band of interest but a relatively low impedance for the 55 higher frequencies which tend to produce instability. I The output stage of the amplifier preferably comprises one or more self-biased pentodes of the beam tube type. 'Applicant has found that with i circuits commonly used for these tubes the average screen current increases considerably with increasing applied signalvoltage. The resulting increase in cathode current increases the grid bias and reduces the plate current. This reduction of plate current not only reduces the-output capacity of the amplifier but also results in the tube operating on a portion of its characteristic where'the harmonic distortion is greater than it is foroperation at maximum plate current.

An important feature of this invention consists in so regulating the potential applied to the screen'grids of the output tubes that the screen currentvaries with the applied signal in such a manner that the cathode current, which is the sum of the screenand plate currents, is substantiallythe same for all inputs up to full signal 'condition. 7

These and other features of the invention will 5 be more clearly understood from'the following detailed description and the accompanying drawings'in which:

Fig.1 is a circuit diagram of an amplifier according to this invention; 1

Fig. 2 shows-aportion of the circuit of Fig. 1 rearranged to illustrate more'clearly thefeature ofselectively amplifying the lower frequencies; and

Fig. 3 shows-the response curves of a typical feedback recorder.

- In Fig. lthe amplifier shown comprises a main amplifier ll operating from recording bus-bars I2, I 3 which areenergized at a suitable level from any desired input circuit such,'for example,- as a pick-up microphone l4-and preliminary. amplifier l5. The output of the amplifier II is impressed on the driving coil l6 of a disc'recorder ll which may be of the type shown in Patent 2,161,489, granted to L. Vieth et a1. June 6, 1939.

The coil I6 of the recorder drives the-stylus 18 to cut arecordand simultaneously generates in the coil l9 a voltage proportional to the velocity of the stylus. This voltage is fed back over the conductors 20 to the input of the negative feedback amplifier 2lthe output of which is impressed on the input of amplifier ll along with the incoming signals from the recording bus-bars l2, l3 and reduces the resonant response of the recorder to a fiat characteristic in the manner which is in general now well understood-in the art.

Coming now to a more detailed description of the circuit, the signals from the recorder bus-bars are impressed on the primary of the transformer 22 the divided secondaries of which are shunted by resistors 23, 24 and 25, 26 respectively. The portions of the secondary potential existing across resistors 24 and 26 are impressed on the grids 21 and 28 of the push- pull tubes 29 and 30 over the circuits including switch 3|, resistor 32, switch 33 and conductor 34 and switch 35, resistor 36, switch 31 and conductor 38 respectively.

The tubes 29 and 35 are self-biased by the in- 7 -again operating with reduced capacity and individual resistors 39, 40 and 4|, 42 andrals'o by' the common resistor 43, resistors 40, 4| and '43- being by-passed for signal currents bythe' condenser 44. The outputs of tubes 29 and 33 are resistance-capacity coupled in the well-known creased harmonic distortion as in the case where th'efscreen potential regulation is poor.

In 'applicants circuit the screen grid is at the potential of thepoint 58 which is determined by the potential drop in resistor 59 due to the sum of the screen currents and the current in resistor manner to the beam tubes 45 and 46 which are p self-biased by the common cathode resistor '82, and the output of these latter tubes drives the recorder IT as already explained. Internal negative feedback connections, 41 and 48 extending from the plates of tubes 45 and 45 to the oathodes of tubes 29 and 30 to reduce the noise and distortion of the amplifier and to reduce the effective output impedance of the beam tubes to a value which more nearly matches the primary impedance of the output transformer 49 in the manner well understood in the art.

Since at times other recording systems may be operating from the bus-bars I2 and 13 over circuits such as 58, it is necessary to prevent the negative feedback energy impressed on the inputs of the tubes 29 and 30 from being reflected back through the transformer 22 to the bus-bars. This is accomplished by making the value of resistors 23 and 25 of the order of ten times the value of resistors 24 and 26 so that thecircuit back to the bus-bars presents a high impedance to reflected currents. While this portion of the circuit also attenuates the incoming signaLthat is not a serious objection in systems of the type shown'since the energy level at the bus-bars is ordinarily quite high. v

The amplifier is energized from the alternating current supply 5! by means of the; usual rectifier 52 and filter network 53, the tube filament circuits (not shown) being connected to suitable taps on the winding 54 of the transformer 55. It will be noted however that the screen grids 55 and 51 of the beam tubes and 46 are connected to the point 58 between resistors 59 and 59 which comprise a bleeder circuit across the output of the filter 53. The manner in which these resistors are proportioned to give improved operation of the amplifier will now be described. 7

Applicant has found that when beam tubes or other power pentodes are operated with a given plate supply voltage into a given load impedance, there are optimum screen grid and cathode bias voltages which give maximum power output and minimum harmonic distortion. In conventional beam tube circuits however, the screengrid voltage which largely determines the plate current, falls off so much at full signal that the plate current is materially reduced thereby decreasing the output capacity. and causing the tubes to operate at a point on the grid voltage-plate current characteristic, where the distortion in the output is considerably increased.

It is, of course, well known that the plate current in these tubes depends on the screen voltage to a greater degree than on the plate voltage and the screen grid is therefore usually connected to the voltage source through a resistor which limits 60. As the screen current increases with increasing signal the potential of point 58 will be decreased due to the larger potential drop in resistor 59 and by proper choice of the values of resistors 59 and 60 the screen potential can be made to vary in sucha manner that the increase in screen current from no signal to full signal compensates for the decrease in plate current thereby keeping the cathode current and grid bias for each tube constant.

In general this involves the use of resistors low enough to pass a bleeder current which is large as compared with the screen current. .For example, when using beam tubes of one wellknown type with a plate voltage of about 400 volts and screen voltage of about 210 volts at no signal, resistor 59 may be 5800 ohms, resistor may be 7000 ohms in which case the bleeder current i about 40 milliamperes and the total screen current of about 4 milliamperes. The procedure for determining the resistor values requiredin a given case is first to determine the grid bias necessary for the desired plate current at full signal with -a given screen potential, then measure the cathode current and determine the screen potential which gives the same cathode current at no signal. 7 Having determined the difierences between the screen potentials and screen currents at full signal and no signal, respectively, the value of resistor 59 necessary to produce this variationis determined by dividing the difference in the screen :Volttgfis by the difierence in the screen currents for the twoconditions. The value of resistor 60 is then readily determined since it must be the same fraction of the total bleeder resistance that the no signal screen voltage is of the output voltage of the filter. 7

An amplifier regulated in' this manner not only has higher maximum output'c-apacity and lower harmonic distortion than conventional circuits butit also has a further advantage particularly in circuits using negative feedback. When a negative feedback amplifier is energized from a rectifierwhich has poor regulation," sudden changes in load on the rectifier set up transients in the amplifier which cause blocking and moton-boating. The current supplied to the output stage of an amplifier comprises a verylarge proportion of the total load on the rectifier but in this circuit the current to the output stage is substantially constant. The output voltage' of the rectifier will therefore vary onlyslightly for large changes in signal level and the amplifier is consequently much more stable in its operation.

In Fig. 3 of the drawingscurve It] represents a typical undamped response characteristic of a recorder of the type indicated at IT in Fig. 1. It will be noted that such a recorder should be of the singly resonant type preferably with the resonant peak occurringslightly abovegeometric mean frequency :of the range .of interest. If, for example, the recorder is to have a fiat response for frequencies from 9000 cycles down to 30 cycles per second, this result may be obtained by using sufiicient negative feedback to reduce the recorder response to the line 6. .When such a large amount of feedback'is used .over such a wide frequency range, however, it is very difiicult to prevent the feedback from becoming positive for frequencies near and beyond the upper limit of the band and hence the recorder develops an irregular rising characteristic 62 and becomes unstable at these high frequencies. The resonant frequency of the recorder is usually fixed by other considerations within the range of about 900 to 1000 cycles so that the amount of feedback used can usually be considerably less than the value corresponding to curve 6| without impairing the fiatresponse up to the upper limiting frequency of interest. For example, in the case shown the response may be reduced only to the line 63 which is fiat over the upper portion of the desired range but definitely drooping above the limiting frequency and therefore much more stable. It will be seen, however, that reducing the amount of feedback to insure stability at high frequencies also results in a drooping characteristic-over the lower frequency range,

such as between 80 and 30 cycles per second, which is part of the transmission band.

In the circuit of Fig. 1 condensers 64 and 65 in the output of the feedback amplifier 2| are of such capacity that their impedances for all frequencies in the transmission band are low as compared with the impedances of resistors 24, 32 and 26, 36 shunting the inputs to the tubes 29 and 30 of the main amplifier ll. Under this condition as is well understood in the art, the transmission characteristic of the coupling between the feedback amplifier 2| and the main amplifier II will be substantially fiat over the transmission band. The condensers 64 and 65, however, as will be seen more clearly from Fig. 2, are also parts of shunt paths across the inputs of tubes 29 and 30 of amplifier ll. Condenser 64, resistor 66 and condenser 61 of the rectifier filter network form a path from the grid 21 of tube 29 to the grounded cathode and similarly condenser 65, resistor 68 and condenser 61 from a path across the input of tube 30.

Condensers 64 and 65 .will ordinarily be of the order of .02 microfarad and condenser 61 of the order of 8 microfarads so that the impedance of condenser 61 is negligible as compared with the other elements of the shunt paths. A signal impressed on the primary of transformer 22 will produce a potential across resistors 24 and 26 and these potentials are impressed on the shunt paths referred to above through resistors 32 and 36, respectively. Since the impedance of the condensers 64 and 65 increases with decreasing frequency the proportion of th signal potential applied to the grids of the tubes 29 and 30 will be progressively greater as the frequency decreases and the signal path will have a rising low frequency characteristic as indicated by the curve 69 in Fig. 3.

By proper choice of the values of the condenser and resistors this increase in the efiiciency of transmission may be made of the proper magniture to compensate substantially for the drooping characteristic of the undamped recorder at thes frequencies so that as shown the flat response withnfeedbackisf extended down to the lowest frequency-of "interest. It will be clear from the foregoing explanation that this required correction at low frequenciesxmay, be obtained without impairing the feedback Ecircuit transmission characteristic by usingcoupling condensers 64 and 65 of such capacity that their impedances over thefrequencyrange in which compensation is required is greater than the impedance of resistor 66 shunting the feedback amplifier to give. increased. signal transmission at low frequencies but less thanthe impedance of resistors '32 and 24 to keep the feedback transmission substantially constant.

If for any reason'it is impractical to make the amplifier sufficiently stable at high frequencies by reducing the amount of negative feedback usedand compensating for a drooping characteristic in. the manner just described, the resistors ll, 12 in the input of the feedback amplifier may be shunted by condensers l3, 14 of the order of .001 mfd. to .006 mfd. capacity. Such condensers are of such high impedance as to have a very small effect on transmission for frequencies within the band of interest but 10w enough in impedance, for frequencies above the transmission band where the feedback is posi tive, to reduce transmission around the feed back loop to a level which does not makethe amplifier unstable. 7

When the recorder to be driven is not of the feedback type the feedback amplifier may be readily disconnected by operating switches 33, 3|,-35. and 31 to their dotted positions. In this case energy cannot be fed back to the bus-bars and the. fullpotential across the secondaries of transformer 22 may therefore be impressed directly on the grids of tubes 29 and 30 through the switches 33 and 31. A part or all of the increased over-all gain due to the elimination of external feedback and to the use of the full voltage across the secondaries of transformer 22 may be utilized, if desired, to reduce distortion by increasing the internal negative feedback in the amplifier ll. The switch 10 in the position shown connects the cathode resistors 39 and 42 to ground through condenser 44 thereby'by-passing theisignal currents around resistors 40, 4| and 43 and leaving only resistors 39 and 42 effective to impress negativesfeedback voltages on I the gridsof tubes 29- andn30. By closing the switch 10.0n contact8l only resistor '43 is bypassed .and .the alternating potentials developed across resistors 40 and 4| are also impressed on the grids of tubes 29 and 30 thereby increasing the negative feedback and correspondingly reducing the over-all noise and distortion.

When the recorder used is not of the feedback type and more gain is tained satisfactorily in the amplifier I I, the feedback amplifier 2| may be used in the signal circuit. In this case switches 33 and 31 are left in the positions shown, switches 3|, 35 and 15 to 18 are moved to their dotted positions and switch 10 is operated to either position in accordance with the gain and distortion requirements of the particular case as described above. Signals applied to the transformer 22 are then fed over conductors 19, to the input of amplifier I I through switches 33 and 31.

It will therefore be seen that this amplifier is well adapted for use as a general purpose recording amplifier and that the circuit may be readily changed to meet the requirements of recorders of different type While the invenrequired than can be obtion has beendescribed for purpose of illustration with reference to a particular circuit it will be understood that it may be modified in various ways within the scope of the following claims.

What is claimed is: V 1. In an amplifying system, a source .of signals, a resonant receiving device, a signal amplifier connecting the source to the device, means for generating a voltage proportional to the response of the device, a feedback circuit including an amplifier for impressing the voltage on the input of the amplifier to make the response of the device uniform over a predetermined frequency range and a. condenser coupling the feedback amplifier to the signal amplifier and formingwith the output of the feedback amplifier a shunt across the input of the signal amplifier, the capacity of the, condenser being proportioned with respect to the impedance of the input of the signal amplifier and the output of the feedback amplifier so as to give a fiat frequency transmission characteristic in the feedback circuit and a non-linear frequency transmission characteristic through the signal amplifier to compensate for the characteristic of the receiving device below the frequency range in which the response is made uniform by the feedback circuit.

2. An amplifying system comprising a source of signal currents, a receiving device, a signal amplifier having an input circuit connected to said source and an output. circuit connected to the receiving device, a second amplifier having an input circuit and an output circuit connected to the input of the signal amplifier, means. for selectively connecting the input of the second amplifier either to the receiving device to feed back to the signal amplifier a voltage proportional to the response of. the device to reduce the distortion in the output of the device or between the source of signal currents and the signal amplifierto increase the gain of the system, and means for, producing negative feedback in the signal amplifier to utilize the increased gain of the system to reduce the distortion in the signal amplifier.

3. In an amplifying system, a source of current, a resonant receiving device, an amplifier having an input circuit connected to the source and an output circuit connected to the device, means associated with the device for generating electric currents of an amplitude varying with the response of the device, a negative feedback circuit connecting said means to the input of the amplifier for reducing the resonant response of the device-to a fiat characteristic over a predetermined frequency range, said circuit including reactive elementslof such value that the feedback becomesgpositive for frequencies above said range and tends-to. cause instability in the operation of the amplifier, anda condenser connected across; the feedback circuit of such small capacity as to have only slight effect on the transmission offrequencies within the range of reson'ant response of the device, but to substantially reduce-transmission of. the frequencies tending to cause instability.

4-. In an amplifying system, a-source of signal currents, a resonant receiving device, an amplifier having an input circuit, connected to the source and an, output circuit connected to the device,'means associated with the device for generating electric currents of anamplitude varying with the response-of the device, a circuit conducting thegenerated currents to the input of the amplifier in phase opposition to the signal currents and producing the maximum negative feedback action consistent with stable operation whereby the resonant response of the device is reduced to a substantially fiat characteristic over a frequency range extending from a frequency within the band to be transmitted up to the upperlimit of the ,band, means in said circuit for reducing transmission for frequencies above said range and, coupling elements between the circuit and the amplifier having a fiat transmission characteristic for the generated currents for selectively increasing the transmission through the amplifier for currents fromthe source of frequencies below said range and producing a fiat characteristic down to the lower limit of the band.

5. In an amplifying system, a source of signal currents, bus-bars connected to said source for distributing the currents, a signal amplifier having input and output circuits, a connecting circuit between the bus-bars and the input circuit and a feedback circuitconnected to the input circuit and carrying currents varying in amplitude with the output of the amplifier, said connecting circuit having a shunt impedance and a high series impedanceconnecting a small portion of the shunt impedance across the input of the amplifier, whereby said connecting circuit presents a high impedance between the feedback circuit and the bus-bars.

THEODORE H. CRABTREE.

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