US2264018A

US2264018A - Signal amplification control system

Info

Publication number: US2264018A
Application number: US358620A
Authority: US
Inventors: Nelson P Case
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1940-09-27
Filing date: 1940-09-27
Publication date: 1941-11-25
Anticipated expiration: 1958-11-25

Description

{Nov. 25, 1941. CASE 2,264,018

- SIGNAL AMPLIFICATIQN CONTROL SYSTEM Filed Sept. 27," 1940 Z'SheetS-Sheet 1 l5 l4 I -x INTERMEDIATE- I msnuzucv AMPLIFIER OVC 086 ILLATOR- MODULATOR FIG. I.

ELL

INVENTOR V NELSON P. CASE BY a ' ATTORNEY Nov. 23, 1941.

N. P. CASE SIGNAL AMPLIFICATI ON CONTROL SYSTEM Filed Sept. 27, 1940. 2 Sheets-Sheet 2 INVENTOR jaLsom P. CASE B Y E N R O T A Patented Nov. 25, 1941 UNITED ST lCiE SIGNAL AMPLIFICATION CQN'I'ROL SYSTEM Nelson P. Case, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Delaware 8 Claims.

The present invention relates to an improved signal amplification control system and, more particularly, to such a system having an oscillation generator controlled in accordance with a predetermined characteristic of a signal to be amplified and controlling, in turn, the amplification of a signal amplifier in the system. While the invention is of general application, it has particular utility as a noise suppressor system for silencing a modulated-carrier signal receiver during intervals when the amplitude of received signals is below a predetermined. value and the invention will, therefore, be described in that connection.

It is desirable to incorporate in a modulatedcarrier signal receiver an arrangement for silencing the receiver during intervals when the signal input to the detector is below the predetermined level necessary for satisfactory reproduction, either because the received signal is near the prevailing noise level or because the receiver is considerably mistuned from a strong desired signal, in which latter case the reproduction of signals is also distorted by such mistuning.

"-I he prior art noise suppression systems gener ally have used a suppressor tube arranged as a direct current amplifier to amplify and apply to the amplification control electrode of a signal amplifier a control bias derived by rectification of received carrier signals. The polarity of the control bias is such that the amplifier is biased to cutoff, or blocked, during the intervals when the amplitude of the signal input to the detector is below a predetermined value. Noise suppression systems of this character have the disad S vantage that the range of control biases between that corresponding to the completely blocked and that corresponding to the completely unblocked condition of operation of the signal amplifier is relatively large so that received signals having a range of amplitudes which produce this range of control-bias potentials are translated by the signal amplifier with severe distortion due to the nonlinear characteristic of the amplifier in this region. A second and perhaps equally important disadvantage of such noise suppression systems arises from the fact that either the cathode of the signal amplifier or that of the suppressor tube operates at all times at a relatively high unidirec tional potential with respect t gro whereby 5 there is not only a loss of otherwise available operating potential for the signal amplifier, but additionally there results a strain on the insulation between the cathode and its heater, which normally is at ground potential.

To avoid the first-mentioned disadvantage of the prior art noise suppression systems previously described, it has been proposed that an electromechanical relay be used in conjunction with a suitable relay control amplifier, the relay being so arranged in the control system that the signal amplifier operates either with-full output when the amplitude of the signal input to the detector is above a predetermined value or is completely blocked when the amplitude is below the predetermined value. Such electromechanical relays must be designed to operate upon a very small current and, therefore, involve a relatively delicateand expensive construction. Among the disadvantages of such relays involving mechanical moving parts may be mentioned the fact that their operation is so sensitive as to be greatly impaired when the relay is subjected to external vibrations and mechanical shock, as occurs when the relay is used in mobile carrier-signal receivers. Mechanical relays have the further disadadvantage that their operation is impaired when the relay is subjected to corrosive atmospheres, for example, atmospheres adjacent a body of salt water. This necessitates that the, relay be enclosed within a hermetically sealed housing, thereby increasing the cost of the relay and add ing further disadvantages from the standpoint of adjustment and periodic servicing of the relay contacts.

It'is an object of the present invention, there fore, to provide a signal amplification control system which, while of general application, is particularly suited for silencing a modulatedcarrier signal receiver during intervals when the amplitude of received signals is below a predetermined value or when the receiver is mistuned a predetermined amount from a strong desired carrier signal and one which avoids one or more of the disadvantages and limitations of the prior art systems.

It is a further object of the invention to provide a signal amplification control system of simple and inexpensive construction wherein a predetermined charaoteristic of a signal to be amplified controls the operation of an oscillation generator, the output of which controls, in turn, the amplification of a signal amplifier.

In accordance with the invention, a signal amplification control system comprises an input circuit adapted to have signals applied thereto, a vacuum-tube amplifier coupled. to the input circuit, and a relaxation oscillator having a bias voltage-output amplitude characteristic which has an abrupt discontinuity at one value of bias voltage upon variation thereof in one sense and a second abrupt discontinuity at another value of bias voltage upon variation thereof in opposite sense. The system also includes means responsive to a predetermined characteristic of the applied signals for deriving a bias voltage varying with the signal characteristic and for applying the derived voltage to the oscillator, and means responsive to the output amplitude of the oscillator for controlling the amplification of the amplifier.

In accordance with a preferred form of the invention, a signal amplification control system of the type described comprises a relaxation oscillator having a bias voltage-output amplitude characteristic which has an abrupt discontinuity at one value of bias voltage upon variation thereof in one sense and a second abrupt discontinuity at another value of bias voltage upon variation thereof in opposite sense, each of the abrupt discontinuities dividing the characteristic into a relatively large output-amplitude portion and a relatively small output-amplitude portion. The system includes means responsive to a predetermined characteristic of applied signals for deriving a bias voltage varying with the signal characteristic and for applying the derived voltage to the oscillator, and means responsive to the small output and large output portions of the oscillator characteristic for respectively permitting normal amplification and reducing the amplification of the amplifier.

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

Referring now to the drawings, Fig. 1 is a circuit diagram, partly schematic, of a complete carrier-signal receiver embodying the invention; and Fig. 2 is a circuit diagram representing a modified form of the signal amplification control system of the invention, per se.

Referring now more particularly to Fig. 1, there is represented schematically a complete carrier-signal receiver of a conventional design embodying the present invention in a preferred form. In general, the receiver includes a radiofrequency amplifier l having its input circuit connected to an antenna-ground system H, l2 and having its output circuit connected to an oscillator-modulator l3. Connected in cascade with the oscillator-modulator I3, in the order named, are an intermediate-frequency amplifier M of one or more stages, a combined detector, automatic amplification control system and first stage of audio-frequency amplification l5, one or more additional stages of audio-frequency amplification l6, and a sound reproducer IT. The automatic amplification control system of unit I5 is coupled in conventional manner over the conductor indicated as A. V. C. to the input circuits of one or more of the tubes ofthe radio-frequency amplifier ill, the oscillator-modulator l3, and th intermediate-frequency amplifier M. A signal amplification control system l8 has its input circuit coupled to the automatic amplification control system of unit l5 and its output circuit coupled to the input circuit of the audio-frequency amplifier of unit I5 to control the amplification thereof in a manner presently to be described. 7

It will be understood that the various units just described may, with the exception of the signal amplification control system l8, be of a conventional construction and operation, the details of which are Well known in the art, rendering detailed description thereof unnecessary. Considering briefly the operation of the receiver as a whole, and neglecting for the moment the operation of the signal amplification control system l8 presently to be described, a desired carrier signal is selected and amplified by the radiofrequency amplifier l0, converted to an intermediate-frequency carrier signal in the oscillatormodulator I3, amplified in the intermediate-frequency amplifier l4, and detected by the detector of unit [5, thereby to derive the audio-frequency modulation components. The audio-frequency components are, in turn, amplified in the audiofrequency amplifier stage of unit l5 and in the audio-frequency amplifier l5, and are reproduced by the sound reproducer IT in a conventional manner. An automatic amplification control bias derived in unit I5 is effective to control the amplification of one or more of the units l0, l3, and I 4 to maintain the signal input to unit l5 within a relatively narrow range for a wide range of received signal intensities.

Referring now more particularly to the portion of the system embodying the present invention, the signal amplification control system l8 includes a source of oscillations comprising an oscillation generator l9 which may be of any wellknown type, but is shown as a single-tube multivibrator of the type disclosed in United States Letters Patent No. 2,203,519, granted June 4, 1940, to Madison Cawein. Briefly, the generator comprises a vacuum tube of the pentode type having its screen and suppressor grids directly coupled together through a condenser 22, the suppressor grid being provided with a grid leak 30. The screen grid and anode of the tube 20 are energized from a source of space current +B through

suitable load resistors

23, 24, respectively. A condenser 2| is connected between the anode and ground, the condenser being periodically and alternately charged through the anode load resistor 24 from the space current source +B and discharged through tube 20. A cathode-bias resistor 34 is included in the oathode lead to provide from space current of tube 20 a bias for the control electrode. The control grid of vacuum tube 20 is coupled through a time-constant circuit comprising

series resistors

25, 26 and a shunt condenser 21 to the adjustable contact of a voltage divider 29 connected across the output of the automatic amplification control system of unit IS. The higher audio-frequency components which appear in the output of the automatic amplification control system are coupled to the control grid of tube 20 through a condenser 28. A negative-bias potential is developed across the suppressor grid-leak resistor 30 when the tube 20 is generating oscillations; this potential is applied through a filter network comprising series resistors 3|, 32 and a shunt condenser 33 to the control grid of a vacuum tube 35. The tube 35 is connected to form the first stage of audio-frequency amplification of unit I5, its input circuit having audiofrequency signals applied thereto through a coupling condenser 38 from a volume control load resistor 39 of the detector of unit l5. A switch 36 is included in the cathode circuit of tube 20 and is preferably so mechanically connected to the adjustable contact of voltage divider 29, as indicated by the broken line 31, that the switch is opened upon movement of the adjustable contact to the limit of its travel corresponding to amplification control by control system [8 only on carrier signals of minimum intensity. Opening the switch 35 eifectively de-energizes the control system I8 and removes its control over the receiver output.

In considering the operation of the circuit just described, reference may be had to the aforementioned Patent 2,203,519 for a description of the detailed operation of the oscillation generator l9. Briefly, however, for purposes of the present description of the system operation it may be stated that the anode current of generator I9 is of pulse wave form and that the generator generates a voltage of saw-tooth wave form across the condenser 21 in its output circuit. During the time when this occurs, the oscillations of the anode circuit cause a potential of pulse wave form to be produced in the circuit of the screen grid which potential is applied through condenser 22 to develop across the suppressor-grid resistor 39 an alternating potential having small positive half-cycles and relatively large negative half-cycles. This particular type of generator possesses the important characteristic that the amplitude of the output sawtooth oscillations developed and the grid bias developed across grid resistor 39 by grid rectification are substantially independent of variations of anode potential and, insofar as the pres- I ent invention is concerned, may be said to be substantially independent of variations of bias applied to the input electrodes of the generator. The generator possesses the additional important characteristic that the generation of oscillations I;

is suddenly terminated when the negative-biasing potential applied to its control grid exceeds a predetermined magnitude and the generator remains inactive until such negative bias is reduced to a value lower than that at which the Therefore, the speed of control of generator 19 is controlled only by the efiective time constant of its input circuit comprising voltage divider 29 and the circuit elements 25, 2S, and 21', and the time constant of its output circuit comprising the circuit elements 3!, 33, 32, 3B, and 39. These time constants can be made so small that, to the ear, the operation is substantially instantaneous. It follows, from the definite on-off action of the amplification control system l8, that there is no transition period where generator I9 is partially operating. Since it is the presence of a transition region which causes distorticn at a critical carrier-signal intensity with conventional noise-suppressor systems, it is apparent that the amplification control system of the present invention is free from such distortion.

Obviously, there also can be no undesirable volume control effect when the bias on the first stage of amplification of unit l5 is either normal or far beyond cutofi. This further advantage of the present invention follows from the lack of a transition region.

The bias derived by the automatic amplification control system of unit !5 varies in magnitude directly with the intensity of received sig nals applied to the input circuit of unit I5. This control bias is applied as a negative bias to the control grid of vacuum tube 20 to modify the operation thereof. The automatic amplification control system of unit t5 thus comprises means responsive to the amplitude characteristic of signals applied to the input circuit of unit H: for deriving a bias voltage varying therewith and for applying the derived voltage to the generator I9 to modify the operation thereof Assume now that the intermediate-frequency carrier signals applied to the input circuit of unit 15 have an amplitude less than the predetermined value required to derive an automatic am plification control bias of sufficient magnitude to prevent generator I9 from generating oscillations. There is consequently developed across grid resistor 3% a relatively high negative potential. The average value of this potential is derived by the

filter

3l, 32, 33 and, upon application to the control grid of the audio frequency amplifier of unit 15, is sufficiently large to bias the latter beyond cutoff, thus causing the ampli iier to b1ock. Since the amplifier of unit I5 is biased not only to cutoff but, as stated, beyond cutoff, any relatively small variations of ampli--- tude of the generated oscillations caused by lesser values of the amplification control bias do not affect the blocked condition of the amplifier once such condition has been established. Conse quently, no audio-frequency signals are trans= lated by the audio-frequency amplifier under these conditions and the receiver is effectively silenced.

Now assume that the amplitude of the inter mediate-frequency carrier'sig-nals increases somewhat .above the predetermined value first as-' sumed. The derived automatic amplification control bias becomes correspondingly larger and is now sufiicient to terminatethe generation of oscillations by generator is. The potential developed across resistor 39 thereupon disappears and the audio-frequency amplifier of unit I5 now 1 operates with full output, thereby supplying audio signals to the audio-frequency amplifier i6 and sound reproducer ll. Reproduction of the received carrier signals continues until the amplitude of the intermediate-frequency carrier signals again decreases, as by fading or by reason of the detuning of the receiver therefrom, below the predetermined value at which generator I9 may oscillate. The generator l9 generally does not again oscillate, however, until a noise pulse or other pulse of energy is applied to its control grid through the condenser 28. When this occurs, the generator suddenly breaks into oscillation and a bias is immediately developed across the grid resistor til, thereby silencing the receiver output. The suppressor grid and its circuit ele merits including grid resistor Sii thus comprise characteristic which has an abrupt discontinuity at one value of bias voltage upon variation there-1 of in one sense and a second abrupt discontinuity at another value of bias voltage upon variation thereof in opposite sense, means responsive to the output amplitude of the oscillator for controlling, in accordance with the amplitude of generated oscillations, the amplification of the controlled amplifier, and means responsive to the amplitude characteristic of applied signals in either of two ranges of values for developing bias voltages in excess of the higher of the aforesaid bias-voltage values or less than the aforesaid other of the bias-voltage values and for applying the bias voltages to the oscillation generator I9.

As descriptive of the bias voltage-output amplitude characteristic, or simply the oscillator characteristic, of the relaxation oscillation generator I9, it may be said that its characteristic has substantially zero value for values of bias voltage greater than the smaller of the two biasvoltage values which cause abrupt discontinuities in its characteristic when the bias voltage Varies in the sense corresponding to decreasing values of the signal characteristic. Each of the abrupt discontinuities of the oscillator characteristic divides the characteristic into a relatively large output-amplitude portion and a relatively small output-amplitude portion. The means responsive to the output amplitude of the oscillator for controlling the amplification of the amplifier of unit I5 is responsive to the large output-amplitude portion of the oscillator characteristic for reducing the amplification of, or even blocking, the amplifier and is responsive to the small output-amplitude portion of the characteristic for permitting normal amplification of the amplifier.

The setting of the voltage divider 29, which determines the predetermined amplitude of the carrier signals necessary to terminate the generation of oscillations by generator I9, is preferably adjusted to silence the output of the receiver at or near the prevailing background noise level. By suitable selection of the value of resistance of the voltage divider 29, the carriersignal intensity at which the amplification control system I8 becomes operative to silence the output of the receiver may be any selected one in a wide range of intensities extending from very weak to very strong carrier signals applied to unit I5.

From the above-described operation of the invention, it will be apparent that the amplification control system of the invention has the important advantage that the generation of oscillations by generator I9, when once terminated by the control potential applied to its control electrode, will not start up again when the control potential is reduced unless noise is present and, even then, only when the noise peaks are considerably greater than the carrier-signal amplitude. This means that when, for example, the voltage divider 29 is so adjusted that a relatively strong carrier signal applied to unit I5 is required to terminate the generation of oscillations by generator I9, the generation of oscillations once terminated does not begin again until the strength of the carrier signal is well below the level of moderate noise. This permits the received carrier signal to be reproduced without interruption in the event that the carrier signal exhibits only moderate fading.

Since noise pulses or similar pulses of energy are necessary to shock the generator I9 back into oscillation when the carrier signal applied to unit I5 becomes weak or is absent, it follows that the extent to Which noise peaks can exceed the carrier-signal amplitude without silencing the output of the receiver is subject to a Wide degree of control by suitable choice of the values of the condenser 28 and resistor 25.

As illustrative of a specific embodiment of the invention, the following circuit constants are given for the embodiment of the invention shown in the drawings:

Vacuum tube 20 Type 6SJ7 Voltage-divider resistor 29 ohms 500,000 Resistor 23 do 68,000 Resistor 24 do 50, 000 Resistor 25 do 10,000 Resistor 26 megohms 2 Resistor 30 do 1 Resistor 3| do 3 Resistor 32 do 10 Resistor 34 ohms 220 Load resistor 39 megohm 1 Condenser 2I micro-microfarads 2000 Condenser 22 do 3000 Condenser 21 do 3000 Condenser 28 do 300 Condenser 33 microfarad 0.01 Condenser 38 do 0. 001 +B volts 200 Fig. 2 is a circuit diagram representing a modified form of the invention which is essentially similar to the arrangement of Fig. 1, similar circuit elements being designated by similar reference characters, except; that there is provided a different form of means responsive to a characteristic of the oscillations generated by generator I9 for deriving the control-bias potential which is utilized to control the audio-frequency amplifier of unit I5. The output circuit of generator I9 is coupled through a condenser 40, the valve of which is not critical but may be of the order of micro-microfarads, to one of the diode rectifier anodes of vacuum tube 35. A diode load resistor 4I, the value of which may be of the order of 1 megohm, is connected between this diode anode and ground. There is thus developed across the resistor M, by peak rectification of the oscillations generated by generator I9, a negative unidirectional potential which is applied, as in the Fig. 1 arrangement, through the filter network 3|, 32, and 33 to the input circuit of the audio-frequency amplifier stage of unit I5 to control the amplification thereof. The operation of this modified form of the invention is otherwise essentially similar to that of the Fig. 1 arrangement and will, therefore, not be repeated.

From the foregoing description of the invention, it will be evident that a carrier-signal receiver embodying the invention has the advantages that the high negative potential output of the oscillation generator is substantially constant for all such values of input control-bias potential as permit the generator to oscillate, and is more than sufiicient when produced to block the controlled audio-frequency amplifier, whereby the output of the receiver is either completely on or completely off and there is no intermediate range within which a distorted output can occur. Additionally, all tubes including the first audiofrequency tube operate with their cathodes substantially at ground potential. The automatic amplification control system of the invention is of simple circuit arrangement, is relatively inexpensive, and is of suihciently sturdy construction that its operation is not impaired by mechanical vibration or shock of the type usually encountered in mobile equipment.

While there has been described what is at present considered to be the preferred embodiinent 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. A signal amplification control system comprising, an input circuit adapted to have signals applied thereto, a vacuum-tube amplifier coupled to said input circuit, a relaxation oscillator having a bias voltage-output amplitude characteristic which has an abrupt discontinuity at one value of bias voltage upon variation thereof in one sense and a second abrupt discontinuity at another value of bias voltage upon variation thereof in opposite sense, means responsive to a predetermined characteristic of applied signals for deriving a bias voltage varying with said signal characteristic and for applying said voltage to said oscillator, and means responsive to the output amplitude of said oscillator for controlling the amplification of said amplifier.

2. A signal amplification control system comprising, an input circuit adapted to have signals applied thereto, a vacuum-tube amplifier coupled to said input circuit, a relaxation oscillator having a bias voltage-output amplitude characteristic which has an abrupt discontinuity at one value of bias voltage upon variation thereof in one sense and a second abrupt discontinuity at another value of bias voltage upon variation thereof in opposite sense, means responsive to a predetermined characteristic of applied signals for deriving a bias Voltage varying with said signal characteristic and for applying said voltage to said oscillator, said oscillator characteristic having substantially zero value for values of bias voltage greater than the smaller of said two values upon variation of said bias voltage in the sense corresponding to decreasing values of said signal characteristic, and means responsive to the output amplitude of said oscillator for controlling the amplification of said amplifier.

3. A signal amplification control system comprising, an input circuit adapted to have signals applied thereto, a vacuum-tube amplifier coupled to said input circuit, a relaxation oscillator having a bias voltage-output amplitude characteristic which has an abrupt discontinuity at one value of bias voltage upon variation of one sense and a second abrupt discontinuity at another value of bias voltage upon variation thereof in opposite sense, means responsive to a predetermined characteristic of applied signals in either of two ranges of values for developing bias voltages in excess of the higher of said values or less than the other of said values and for applying said voltages to said oscillator, and means responsive to the output amplitude characteristic of said oscillator for controlling in accordance with the amplitude of generated oscillations the amplification of said amplifier.

4. A signal amplification control system comprising, an input circuit adapted to have signals applied thereto, a vacuum-tube amplifier coupled to said input circuit, a relaxation oscillator having a bias voltage-output amplitude characteristic which has an abrupt discontinuity at one value of bias voltage upon variation thereof in one sense and a second abrupt discontinuity at another value of bias voltage upon variation thereof in opposite sense, each of said abrupt discon- Cal tinuities dividing said characteristic into a relatively large output amplitude portion and a relativel'y small outputamplitude" portion, means responsive to a predetermined characteristic of applied signals for deriving a bias voltage varying with said signal characteristic and for applying said voltage 'to said oscillator, and means "respon sive to the large output portion of said oscillator characteristic for reducing the amplification of said amplifier.

5. A signal amplification control system comprising, an input circuit adapted to have signals applied thereto, a vacuum-tube amplifier coupled to said input circuit, a relaxation oscillator having a bias voltage-output amplitude characteristic which has an abrupt discontinuity at one value of bias voltage upon variation thereof in one sense and a second abrupt discontinuity at another value of bias voltage upon variation thereof in opposite sense, each of said abrupt discontinuities dividing said characteristic into a relatively large output amplitude portion and a relatively small output amplitude portion, means responsive to the amplitude characteristic of applied signals for deriving a bias voltage varying with said signal characteristic and for applying said voltage to said oscillator, and means responsive tothe large output portion of said oscillator characteristic for blocking said amplifier.

6. A signal amplification control system comprising, an input circuit adapted to have signals applied thereto, a vacuum-tube amplifier coupled to said input circuit, a relaxation oscillator having a bias voltage-output amplitude characteristic which has an abrupt discontinuity at one value of bias voltage upon variation thereof in one sense and a second abrupt discontinuity at another value of bias voltage upon variation thereof in opposite sense, each of said abrupt discontinuities dividing said characteristic into a relatively large output amplitude portion and a relatively small output amplitude portion, means responsive to the amplitude characteristic of applied signals for deriving a bias voltage varying with said signal characteristic and for applying said voltage to said oscillator, and means responsive to the small output and large output portions of said oscillator characteristic for respectively permitting normal amplification and reducing the amplification of said amplifier.

7. A signal amplification control system comprising, an input circuit adapted to have signals applied thereto, a vacuum-tube amplifier coupled to said input circuit, a relaxation oscillator having a bias voltage-output amplitude characteristic which has an abrupt discontinuity at one value of bias voltage upon variation thereof in one sense and a second abrupt discontinuity at another value of bias voltage upon variation thereof in opposite sense, means responsive to a predetermined characteristic of applied signals for deriving a bias voltage varying with said signal characteristic and for applying said voltage to said oscillator, means for rectifying oscillations generated by said oscillator to derive a unidirectional control-bias potential, and means for utilizing said control-bias potential to control the amplification of said amplifier.

8. A signal amplification control system comprising, an input circuit. adapted to have signals applied thereto, a vacuum-tube amplifier coupled to said input circuit, a relaxation oscillator having a bias voltage-output amplitude characteristic which has an abrupt discontinuityat one value of bias voltage upon variation thereof in one sense and a second abrupt discontinuity at another value of bias voltage upon variation thereof in opposite sense, said oscillator having a circuit for developing a relatively high negative-bias potential varying with said oscillator characteristic, means responsive to a predetermined characteristic of applied signals for deriving a bias voltage varying with said signal characteristic and for applying said Voltage to said oscillator, and means responsive to said bias potential for controlling the amplification of said amplifier.

NELSON P. CASE.