US2169837A - Low frequency amplifier - Google Patents

Description

'Aug 15, 1939. CARL-ERIK GRANQVIST 2,169,837

LOW FREQUENCY AMPLIFIER Filed March 29, 1938 INVE TOR CAR "ERIK GRAN 51' ATTORNEY Patented Aug. 15, 1939 UNITED STATES LOW FREQUENCY AMPLIFIER Carl-Erik Granqvist, Stockholm, Sweden,

or to Hazeltine Corporation,

Delaware Application March 29,

assigna corporation of 1938, Serial N0. 198,625

In Sweden September 3, 1937 '7 Claims.

This invention relates to low-frequency amplifiers and. particularly to audio-frequency amplifiers stable in operation in modulated-carried signal receivers.

In modulated-carrier signal receivers, and particularly in superheterodyne receivers for shortwave reception, it is frequently difficult to obtain stable operation. A superheterodyne receiver, tuned to a station in the shortwave por- 10 tion of the frequency spectrum, may go into self-oscillation if there is a certain amount of detuning. A feedback of energy from the audiofrequency portion of the receiver to the local oscillator may cause the frequency of the local oscillator to vary about its normal frequency. If the receiver is correctly tuned, this variation of the local oscillator frequency results in a reduction of the input to the audio-frequency channel of the receiver for either an increase or a decrease in the frequency of the local oscillator from its normal value. Thus, the resulting frequency which is applied to the audio-frequency channel is twice that which causes the oscillator frequency to vary, and self-oscillation of the receiver is not sustained. However, with any mistuning, a variation in the intermediate frequency about its mean value results in a decrease in the audio-frequency input voltage for one direction of the frequency shifts and an increase for the other. The receiver may then go into self-oscillation. This form of instability manifests itself as a very annoying low-frequency tone, consisting of about 10 cycles per second for the signal type of receiver, and may easily destroy the usefulness of its received program. The abovementioned phenomenon may also occur in modulated-carrier signal receivers provided with automatic frequency control. The carrier Wave of the station being received may become very 40 weak so that the automatically operated frequency control does not function properly. When thereafter the carrier wave increases in strength and the automatic frequency control system becomes effective, a certain detuning may exist in the receiver circuits before the tuning is corrected by the automatic frequency control system and this detuning may be sufficient to start the periodically recurring blasts of sound.

The primary cause of these annoying low-fre quency tones or sound blasts is that the impedance of the source of space current is not sufiiciently small at the low frequencies at which the phenomena occur. Since the impedance of the source of space current is, in practically all receivers, larger at low frequencies than at higher ones, the amplification of low-frequency currents in the audio-frequency portion of the receiver introduces a low-frequency voltage component in the anode voltage supplied to preceding tubes of the receiver. As the impedance of the source of space current increases with decreasing frequency, this reaction increases. The amplification of the audio-frequency amplifiers is, however, simultaneously reduced, so that the reaction passes through a maximum at some low frequency in the audio-frequency range. If a component of the alternating voltage introduced in the anode voltage supply at this frequency is in phase with the input voltage to the audio-frequency portion of the receiver, the operation may become unstable and self-oscillation may occur. If this component of voltage is out of phase with the input voltage to the audio-frequency portion of the receiver, the resultant input voltage to the audio-frequency portion of the receiver is reduced at this frequency, causing distortion and unsatisfactory operation. By means of carefully designed filters in the space current circuits of the several tubes of the receiver, it is possible to eliminate the above-mentioned disturbances, but the elements for such filters are bulky and expensive. It has also been proposed, in condenser-resistance coupled audio-frequency amplifiers, to reduce the size of the coupling condensers between the several stages. This method is not practical in all cases, however, for the reason that such a change causes a. considerable reduction in amplification at the lower frequencies, resulting in objectionable frequency distortion within the audio-frequency range.

It is an object of the present invention, therefore, to provide an improved low-frequency amplifier which is stable in its operation.

It is a further object of the invention to provide an improved audio-frequency amplifier for a superheterodyne receiver which is stable under all operating conditions.

In accordance with the present invention, a low-frequency amplifier is designed so that alternating components of voltage in the space current supply, caused by alternating space currents of one or more of the amplifier tubes, are such as to cause minimum reaction on the input to the amplifier through preceding tubes coupled to the amplifier. Specifically, in accordance with the invention the amplifier is designed so that the alternating current components of the space current supply are displaced 90 degrees from the input voltage to the low-frequency portion of the system at the frequency at which the receiver reaction is greatest.

In the preferred embodiment of the invention, the receiver is of the superheterodyne type and comprises two stages of condenser-resistance coupled audio-frequency amplification. The constants of the low-frequency portion of the receiver and the power supply, or both, are so proportioned that a phase displacement is introduced in the alternating component of the anode voltage at the frequency of greatest reaction, the

alternating anode voltage component being displaced 90 degrees from the signal input to the audio-frequency channel of the receiver. The high-frequency portions of the receiver cause practically no displacement at this low frequency except the ordinary phase reversals in the preceding tubes.

For a better understanding of this invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing, Fig. l is a circuit diagram, partly schematic, of a complete superheterodyne receiver embodying the present invention; Fig. 2 is an equivalent circuit diagram of the source of space current supply of Fig. l at low frequencies; Fig. 3 is a vector diagram illustrating the phase relationship of voltages at various points in the audio-frequency amplifier of the receiver; and Figs. 4 and 5 are graphs showing certain operating characteristics of the audiofrequency portion of the receiver.

Referring now more particularly to Fig. 1, there is shown schematically a complete superheterodyne receiver of a conventional design embodying the present invention in a preferred form. In general, the receiver includes a radio-frequency amplifier III, having its input circuit connected to an antenna II and ground I2 .and its output connected to a frequency changer or oscillator-modulator I3. Connected in cascade with oscillator-modulator I3, in the order named, are an intermediate-frequency amplifier I4 of one or more stages, a detector and A. V. C. supply I5, an audio-frequency amplifier I6 of two stages, and a sound reproducer IT. A bias derived from A. V. C. supply I5 is supplied over conductor I5 to one or more stages of amplifier II], oscillatormodulator I3, and one or more stages of intermediate-frequency amplifier I4 to maintain the input to detector I5 within a narrow range for a wide range of received signal intensities.

It will be understood that the various circuits just described, other than the audio-frequency amplifier I6, may be of conventional construction and operation, the details of which are well known in the art, rendering further description thereof unnecessary. Considering briefiy the operation of the receiver as a whole and neglecting for the moment the arrangement of this invention presently to be described, a desired received signal is selected and amplified by radio-frequency amplifier I0, converted to a modulated intermediatefrequency carrier in oscillator-modulator I3, amplified and selected by intermediate-frequency amplifier I4, and rectified by detector I5, thereby deriving the audio frequencies of modulation. The audio frequencies of moduation are, in turn, amplified by audio-frequency amplifier I6 and reproduced by sound reproducer II. The A. V. C. bias derived from source I5 and applied to radiofrequency amplifier I8, oscillator-modulator I3, and intermediate-frequency amplifier I4 maintains the input to detector I5 within a relatively narrow range for a wide range of received signal intensities.

Referring now more particularly to the parts of the system involving the present invention, there is provided in Fig. 1 a two-stage audiofrequency amplifier including tubes I8 and I8. Audio-frequency signal voltages are supplied from the output terminals A and B of detector I5 to the input circuit of tube I8 through the coupling condenser I9 and bias resistor 28. The output circuit of tube I8 is coupled to the input circuit of tube I8 through the coupling condenser 2! and bias resistor 22, its output circuit, in turn, being coupled through an audio-frequency transformer 23 to sound reproducer II. A source of space current supply is provided for the receiver comprising a rectifier tube 24 supplied from a convenient source of alternating current through a transformer 25. A filter circuit comprising a series inductance element 26 and shunt condensers 21 and 28 is interposed between the output circuit of rectifier 2 1 and the space current circuits of tube I8, including transformer 23; of tube I8, including series-connected resistors 29 and 38 having their common terminal grounded for high-frequency currents through a condenser 3|; and of the preceding tubes of the receiver, supplied from the terminal C. Screen and grid-bias potentials are supplied to the tubes of the receiver in a conventional manner.

In considering the operation of the circuit just described, it will be seen that the alternating space current of tube l8 passes through the filter 2B2'I-28, building up alternating voltages thereacross which are greater at the lower audio frequencies. In order to avoid interfering alternating voltages of low frequency in the anode voltage supply of the several tubes of the receiver, the filter 262'I-28 and the rectifier 24 should possess a very small impedance to currents of these frequencies. The impedance of this filter is, however, considerable at low frequencies, which results in objectionable variations in the anode-voltage supply. At such low frequencies, the impedance of the space current supply shown is approximately the equivalent of a condenser 32 in parallel with a resistor 33, as shown in Fig. 2, the reactance of element 26 being negligible at these low frequencies. Therefore, condensers 2'! and 28 can be replaced by a single equivalent condenser 32. The impedance of rectifier 24 and of the secondary winding of transformer 25 is practically a pure resistance at these low frequencies and may, therefore, be represented by resistor 33.

The space current of tube I8 is filtered through resistance 29 and condenser 3i, the anode resistor 39 having a resistance considerably smaller than the resistor 22 so that the alternating voltage developed across resistor 38 is substantially opposite in phase to the alternating grid voltage applied to tube I8. The relation of the alternating voltages in amplifier I6 is shown in Fig. 3. In this analysis it is assumed that the filter 29, 3| is effective substantially to suppress alternating voltages from the anode-voltage supply, The output voltage of detector I5, that is, the audiofrequency voltage between the terminals A, B, is indicated by the vector 61, this voltage being resolved into two components e10, representing the voltage drop across condenser I9, and em, representing the voltage drop across resistor 20. The voltage cm is applied to and amplified by the tube I8, the output voltage of this tube being 180 degrees out of phase with respect to em, although for the sake of simplicity it is indicated in Fig. 3 by vector ez in phase with em but with a negative sign. The vector 62 is resolved into two components, -e2c, representing the voltage drop across condenser 2|, and c2a, representing the Voltage drop across the resistor 22. The voltage -2R is applied to and amplified by the tube I8 to the voltage 63 in Fig. 3. Owing to the fact that the anode circuit of tube I8 does not consist of a pure resistance, the alternating anode voltage ex is not in phase with the space current of the tube and consequently is not in phase with the voltage e2R. Since the operation of the audio amplifier is being analyzed for low frequencies, the inductance in the output circuit of tube I8 can be disregarded However, the capacitive reactance of the filter circuit 252i28 is effective to produce the above-mentioned phase shift of the vector es. The anode circuit of the preceding tube it also effectively adds resistance to the output circuit of tube l8. This resistance, however, may be disregarded inasmuch as it is considerably larger than the resistance of the source of direct current supply. The anode voltage 63 is retarded in phase somewhat relative to the voltage "62R. The voltage as may then be resolved into two components en, the voltage developed across transformer 23, and ez, the voltage developed across the filter 25-2l28, The voltage ez is always located within the quadrant indicated in Fig. 3 by the angle a and its phase relationship in this quad-- rant for a given frequency is determined by the size of the equivalent elementts 32 and 33 of Fig. 2. The voltage ez appears between the point C in the anode supply conductor for preceding poitions of the receiver and ground; that is, between points A and C. Inasmuch as the voltage 62., after being fed back to preceding portions of the receiver, again appears across the terminals A, B, either with no shift in phase or dislaced 180 degrees by the preceding tubes of the receiver,

it is the component of this voltage in phase with the voltage 61 which causes the receiver to go into oscillation, As the magnitude of the voltage which is fed back varies under different operating conditions, instability may easily occur when it exceeds a critical value. This is especially true in superheterodyne receivers for short-wave re ception, but oscillation will also occur in other kinds of low-frequency amplifiers.

As a rule, the frequency at which this selfoscillation occurs is very low. It is not sufiicient to provide such a phase shift of this voltage that it is fed back to the points A, B with a component in phase opposition to the voltage 81 so that there is obtained a negative reaction which prevents oscillation. Under these conditions, in a superheterodyne receiver having even a small amount of mistuning, the frequency modulation in the oscillator causes an amplitude modulation to be fed back to the detector, which, if in phase opposition to the detector input, will cause distortion. Therefore, in accordance with the present invention, the voltage ez which is fed back topreceding portions of the receiver is caused to be displaced degrees in relation to the signal voltage (21 impressed on the audio-frequency amplifier it at the frequency which causes maximum disturbance in the receiver.

In order to explain more clearly the'fun'ction and importance of a correct dimnsioning of the phase-displacing elements, the impedance Z of the source of space current supply, the audio-irequency amplification characteristic k0 of a conventional receiver without low-frequency reaction of the above-mentioned type, and the audiofrequency amplification kr of the receiver with low-frequency reaction of the above-mentioned type are shown in Fig. 4 as a function of frequency When the impedance Z of the source of space current increases with decreasing frequency, the low-frequency reaction increases. The normal amplification of the receiver, as shown by curve k0, is, however, simultaneously reduced and the resultant amplification curve is as indicated in Fig. 4 by the curve In. Within a certain frequency range f1-f2 the amplification approaches infinity; that is, if there is a component of feedback voltage ez of this frequency which is in phase with the vector 61, the receiver goes into selfoscillation. On the other hand, if there is a component of this voltage in phase opposition to the vector e1, amplitude distortion will result, Therefore, in accordance with this invention, the design of the receiver is modified so that, at this frequency, the feed-back voltage ez is 90 degrees out of phase with the voltage 21 impressed on amplifier l5 and undesirable operating characteristics are avoided, the amplifier having approximately its normal characteristic 700.

In Fig. 5 curve 51 shows the phase displacement of the signal caused by the combined effect of the coupling circuits comprising condenser l9 and resistor 20 and condenser 2| and resistor 22 as a function of frequency. Curve ,62 shows the displacement of the feed-back voltage in the space current filter 26-2l--28, or its equivalent comprising elements 32 and 33 of Fig. 2 Elements i9, 26 and 2t, 22 are usually similarly designed and the total phase displacement of these elements is thus twice that through either one of them. At very low frequencies their combined displacement approaches degrees as shown by curve 51. placement of the feed-back voltage eifected by the space current filter is at high frequencies and approaches 90 degrees as shown by the curve 62. It was assumed in the description with respect to Fig. 4 that the instability of the kind mentioned occurs between the frequencies of f1 and f2, which'freqencies are also indicated in Fig. 5. The geometric mean value of frequencies f1, and i2 is represented by in. In accordance with the present invention, the receiver constants are so designed that the diiference in the phase displacement of the signal voltage effected by coupling elements i9, 20 and 2!, 22 and that due to the space current filter elements is 90 degrees at the frequency in. This proportionment may be had by adjusting either or both of the phase angles c1, 52 (by adjusting elements i9, 26, 21, 22, or 32', 33, respectively) to the correct values at the critical frequency in. With such a design, instability no longer occurs within the range between f1 and is. For frequencies lower than f1, instability will not occur due to the reduced amplification as shown by curve 760, and for frequencies above f2, instability will not occur due to the low value of impedance Z at these frequencies.

In many amplifiers and other apparatus, it is not convenient to alter the elements included in the amplifier in order to obtain the desired phase displacement and, consequently, complete stability. In such cases an auxiliary filter of proper design may be-placed between the tube an:- odes and the source of space current, designed as hereinbefore described.

While the description of the invention has; been given in a preferred embodiment utilizing a resistance-coupled amplifier fed from an alternating current supply, the invention is equally applicable in other low-frequency amplifiers for direct current as well as for alternating current. In case of transformer coupling in the amplifier, for instance, the phase displacement of the transformer itself may be utilized and the direct current supply source may be bypassed by means of resistors and condensers in 75 order to obtain the desired phase displacement Within the critical frequency range.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a signal-translating system comprising a high-frequency signal-translating circuit including at least one vacuum tube, a low-fre quency amplifier comprising at least one vacuum tube and having an input circuit adapted to be coupled to said high-frequency signaltranslating circuit, a common source of space current for said tubes, said source having a substantial impedance within the range of said lowfrequency amplifier across which a feed-back voltage is developed, and the reactive constants of said amplifier and said source being so proportioned that said feed-back voltage is displaced approximately 90 degrees with respect to the input voltage to said amplifier at the frequency at which said feed-back voltage is greatest.

2. In a signal-translating system comprising a high-frequency signal-translating circuit including at least one vacuum tube, a low-frequency amplifier comprising at least one vacuum tube and having an input circuit adapted to be coupled to said high-frequency signaltranslating circuit, a common source of space current for said tubes, a filter network coupled between said source and said tubes, said filter having a substantial impedance within the range of said low-frequency amplifier across which a feed-back voltage is developed, and the reactive constants of said receiver and said filter being so proportioned that said feed-back voltage is displaced approximately 90 degrees with respect to the input voltage to said amplifier at the frequency at which said feedback voltage is greatest.

3. An audio-frequency amplifier for a modulated-carrier signal receiver comprising, a plurality of coupled vacuum tubes, a common source of space current for the tubes of said receiver, a filter network coupled between said source and the tubes of said receiver, said filter having a substantial impedance within the audio-frequency range of said amplifier across which a signal feedback voltage is developed, and the reactive constants of said filter being so proportioned that said signal feed-back voltage is displaced approximately 90 degrees with respect to the signalinput voltage to the first tube of said amplifier at the frequency at which said signal feed-back voltage is greatest.

4. An audio-frequency amplifier for a modulated-carrier signal receiver comprising, a plurality of vacuum tubes, impedance means coupling said Vacuum tubes having a phase-shift characteristic variable with frequency, a common source of anode supply for the tubes of said receiver, a filter coupled between said source and the tubes of said receiver, said filter having a substantial impedance within the audio-frequency range of said amplifier across which a signal feed-back voltage is developed and having a phase-shift characteristic varying with frequency oppositely to that of said coupling means, and the reactive constants of said filter and said coupling means being so proportioned that the difference in said phase shifts is approximately 90 degrees at the frequency at which said signal feed-back voltage is greatest.

5. An audio-frequency amplifier for a modulated-carrier signal receiver comprising, a first vacuum tube having input and output electrodes, a resistor-condenser network coupled to the input electrodes of said tube, a second vacuum tube having input and output electrodes, a resistorcondenser network coupling the input electrodes of said second tube to the output electrodes of said first tube, a common source of anode supply for the tubes of said receiver, a filter comprising series inductance and shunt capacitance coupled between said source and the tubes of said receiver, said filter having a substantial impedance within the audio-frequency range of said amplifier across which a signal feed-back voltage is developed, and the reactive constants of said coupling means and said filter being so proportioned that said signal feed-back voltage is displaced approximately 90 degrees from the input voltage to said first tube at the frequency at which said signal feed-back voltage is greatest.

6. An audio-frequency amplifier for a modu- U lated-carrier signal receiver comprising, a first vacuum tube having input and output electrodes, a resistor-condenser network coupled to the input electrodes of said tube, a second vacuum tube having input and output electrodes, a resistorcondenser network coupling the input electrodes of said second tube to the output electrodes of said first tube, said resistor-condenser networks having a combined frequency-dependent phase shift approaching 180 degrees as a maximum at 1 low frequencies, a common source of anode supply for the tubes of said receiver, a filter comprising series inductance and shunt capacitance coupled between said source and the tubes of said receiver, said filter having a substantial impedance within the audio range of said amplifier across which a signal feed-back voltage is developed and having a phase shift dependent on frequency and approaching a maximum of 90 degrees at high frequencies, the reactive constants ,1

of said networks and said filter being so proportioned that the difference in said phase shifts is approximately 90 degrees at the frequency at which said signal feed-back voltage is greatest.

7. An audio-frequency amplifier for modulatedcarrier signal receivers comprising, a plurality of coupled vacuum tubes, a common source of space current supply for the tubes of said receiver, said supply source having a substantial impedance within the audio-frequency range of said amplifier across which a signal feed-back voltage is developed, said impedance being a minimum at high frequencies and the amplification of said amplifier being a minimum at low frequencies, whereby said signal feed-back voltage is a maximum at an intermediate frequency, and the reactive constants of said amplifier and said source being so proportioned that said signal feed-back voltage is displaced approximately 90 degrees from the signal input to said amplifier at said intermediate frequency.

CARL-ERIK GRANQV'IST,

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