US 3727112 A
A generator for producing energy in the ultrasonic range for driving a transducer having variable load characteristics in which the generator includes an oscillator circuit having the capability of accepting and operating with transducers whose natural resonant frequencies vary over a wide range; which has good frequency stability under low or no-load conditions of operation; and good frequency stability under loaded conditions of the transducer. The generator circuit also includes a self-monitoring circuit for keeping a relatively constant output power over wide ranges of loading of the transducer.
Description (OCR text may contain errors)
Unite. States atent [191 Popescu GENERATOR FOR PRODUCING ULTRASONIC ENERGY  Inventor: Gabriel Popescu, Astoria, NY.
 Assignee: Surgical Design Corp., Long Island City, NY.
 Filed: June 14, 1971  App1.No.: 152,947
Related US. Application Data  Continuation-impart of Ser. No. 854,240, Aug. 29,
1969, Pat. No. 3,629,726.
 US. Cl. ..317/146, 310/81, 310/26,
331/109, 331/116 M, 331/157, 331/186  Int. Cl. ..H03b 5/40  Field of Search ..3 lO/8. l 26;
[5 6] References Cited UNITED STATES PATENTS 3,199,052 8/1965 Verstraelen ..331/116 51 Apr. 10, 1973 Randall ..331/186 Pepescu ..310/8.l
Primary Examiner-L. T. Hix
. Attorney-Darby & Darby [57 ABSTRACT A generator for producing energy in the ultrasonic range for driving a transducer having variable load characteristics in which the generator includes an oscillator circuit having the capability of accepting and operating with transducers whose natural resonant frequencies vary over a wide range; which has good frequency stability under low or no-load conditions of operation; and good frequency stability under loaded conditions of the transducer. The generator circuit also includes a self-monitoring circuit for keeping a relatively constant output power over wide ranges of loading of the transducer.
17 Claims, 4 Drawing Figures a FIG. 1
PATENTEU 1 01975 3, 727, 1 l2 sum 1 OF 2 1 N 9 00 Ll. 1
GABRIEL POPESCU PATENTEBAPRl 0197s 7 7, 1 12 SHEET 2 BF 2 uiiuz 3 c s e Q TEMPORARY INVENTOR. GABRIEL POPESCU PRODUCING ULTRASONIC ENERGY RELATED APPLICATION This application is a continuation-in-part of my copending application Ser. No. 854,240, filed Aug. 29, 1969, now US. Pat. No. 3,629,726, entitled Oscillator and Oscillator Control Circuit, which is assigned to the same assignee.
DESCRIPTION OF PRIOR ART In my aforesaid application, a generator (oscillator) and oscillator control circuit is provided for the production of electric energy which is used to supply an ultrasonic transducer. The transducer is of the magnetostrictive or piezoelectric type which converts the electrical energy into mechanical energy. The energy is in the frequency range of l khz to 100 khz, which is broadly called the ultrasonic frequency range where the transducer is to perform some work function. In a typical medical, dental or commercial application, the transducer drives a workpiece, such as the tip'of a dental handpiece or a horn in a commercial application, to produce a useful work function at ultrasonic frequency.
As is known, in such applications, the transducer is subjected to varying loads. There loads are of the type, for example, wherein a tip of a handpiece, which the transducer drives when used by a dentist, is pressed down against the teeth of the patient; a horn in a commercial application is immersed into a fluid for agitating the fluid; the horn is loaded against an object to generate heat; the medium in which the transducer is located is changed, for example, from air to water, etc.
In the most common form of ultrasonic transducer, a stack of magnetostrictive elements of a suitable material, such as nickel or a nickel alloy are used. The elements of the stack are selected to have a predetermined resonant frequency and are connected together in a manner so that their outputs will be additive in phase to produce a desired power output at or near the resonant frequency. The magnetostrictive elements are supplied with the necessary electrical driving current through a single wound inductance coil, or several. such coils located around the stack. The current causes the stack elements to undergo a stress or strain and the mechanical output is produced. Usually, an acoustic transformer is connected between the output'end of the stack. and the workpiece to transmit the vibrational energy of the stack to the workpiece.
Each stack of magnetostrictive elements has a natural resonant frequency. If the electrical current supplied to the stack is of this resonant frequency, then the stack will produce maximum mechanical output power. Whenthe transducer is operated with a load, the net effect is to change the input impedance of the transducer from that at the natural resonant frequency. This effectively changes the resonant frequency. Thus, in order to provide maximum power transfer of driving current to the transducer, the output characteristics of the oscillator must be shifted in a corresponding manner to compensate for this change in the transducer input impedance and resonant frequency.
It has also been a problem that while transducers of the magnetostrictive type are designed to have a particular resonant frequency, due to variations in materi- GENERATOR FOR als and tolerances during production, the natural resonant frequency of two transducers designed to be the same will not necessarily be so. Therefore, it is desirable to have an oscillator circuit that will accept transducers having natural resonant frequencies within a relatively wide range so that different oscillator circuits will not have to be designed to work with each transducer. The latter would be undesirable from a manufacturing point of view.
The present invention is an improvement over the oscillator and oscillator control circuit described in my aforesaid application. The improvements achieved by the invention of the subject application include the 5 ability to accept and operate with transducers whose natural resonant frequencies vary over a wider bandwidth; a better frequency stability at lower output power levels, that is, when the transducer operates with little or no load, the frequency of the oscillator will not shift; an increased stability or oscillation frequency under a loaded condition; and the ability to self-monitor the amplitude of the output power to produce a fairly constant output power when the load varies over a relatively wide range without external control circuitry.
it is therefore an object of the invention to provide a generator for producing energy for driving an ultrasonic transducer and having a high efficiency of power transfer.
Another object is to provide a generator for driving an ultrasonic transducer which will operate with transducers whose natural resonant frequencies vary over a relatively wide range while maintaining good power transfer efficiency.
A further object is to provide a generator for producing energy to drive an ultrasonic transducer'which is relatively simple, having one transistor for an oscillator and a second transistor for manual power setting.
An additional object is to provide a generator for an ultrasonic transducer which has good stability at lowload conditions of the transducer.
Other objects and advantages of the present invention will become more apparent upon reference to the' following specification and annexed drawings, in which:
FIG. 1 is a schematic diagram of the generator of the subject invention;
FIG. 2 is a schematic diagram simplified for purposes of analysis;
FIG. 3 is a diagram illustrating certain operating principles of the generator; and
FIG. 4 is a diagram illustrating other operating principles of the generator.
Referring to FIG. 1, the circuit of the subject invention includes a first transistor 10, which is the amplitude monitoring transistor for setting the desired power level for the generator, and a second transistor 12 connected as an oscillator for producing the electrical current to drive a transducer 60. The power for the circuit is supplied from a suitable'transformer 20 which has a pair of isolated secondary windings 20a and 20b. The primary of the transformer is to be connected to a suitable source of A.C. voltage. A ground terminal at the secondary side is shown by reference numeral 21. An isolated transformer is used since the circuit, when used for dental and medical applications, is preferably isolated from ground for safety purposes. Of course, in some cases this may not be necessary.
The first secondary 20a of transformer 20 is connected across the upper and lower terminals of a full wave bridge, rectifier circuit 24. The upper end'of the secondary 20a is connected to the top terminal of the bridge through a resistor 26.
The secondary winding 20b of the transformer 20 supplies'voltage to the upper'and lower terminals of a second bridge rectifier 30. As is explained below, the output of bridge 30 is used to supply a polarizing current to he transducer. A pair of filter capacitors 32 and 34 are connected across the output'terminals of the respective bridge rectifier circuits and connected together at a common neutral potential point 36.
The unregulated D.C. output voltage fro the upper end ofcapacitor 32 is applied to the collector of the first transistor .l0. The base of transistor is biased by a voltage divider formed by series connected resistors 40 and 42, connected between the point of unregulated potential at the upper end of capacitor 32 and the neutral line 36. A capacitor 44 is connected between the emitter of transistor 10 and the neutral point 36. The reactance of capacitor 44 is low at the natural resonant frequency of theoscillator and transducer. Therefore, the A.C. component of the transducer is not reflected back at the power supply.
Transistor 10 functions as a series voltage regulator.
, The conducting level of'the transistor is set by the amount of 'voltageatits base as selected from the slider of potentiometer 42 which is part of a voltage divider The transducer 60 is connected between the emitter of transistor 12 and neutralpoint 36. FIG. 2 shows schematically the transducer and the effect of loading. A stack of magnetostrictive elements can be represented for purposes of analysis 'as capacitance, inductance and resistance elements connected in series. This, of course, is only a rough approximation which in the present case is sufficient'The three elements are designated 60C, 60L and. 60R .and they are the mechanical impedance components at the resonant frequency of the transducer. The transducer load is designated as Z When the load is of the damping variety, such as when a tip attached to the transducer is pressed against an object, the load is primarily resistive. In this case it can be designated R The driving coil 60d of the transducer which is actually connected to the emitter of transistor 12, is not shown in FIG. 2. In
40, 42. The voltage at thefemitter of transistor 10,
which is supplied to the collector of oscillator transistor 12, is maintained substantially constant for variations in power supply voltage. The ratio of the two resistors of the voltage divider is selected preferably so that the minimum voltage output will be approximately onetenth of the maximum I The second bridge rectifier circuit 30 provides 'a D.C. polariz ing current to the transducer 60 which is connected between the emitter of the oscillator transistor 12 and the neutral line 36. This current is supplied through an inductor 46 and a resistor 48 from he positive output terminal of bridge 30 to the top end of a single coil 60a which drives the transducer stack. A capacitor 49 is connected from the top end of coil 60a to ground. The inductor 46 and the capacitor 49 prevent the transducer oscillations from shorting to ground. The positive current from bridge 30 is supplied to the transducer coil 60a to pre-bias the elements of the stack, i.e., to shift the point of operation of the magnetostrictive elements to a desired point on the hysteresis curve wherethe stack elements will respond to signals of relativelylow level to produce a magnetostrictive effect. If the polarizing current were not used, the-output of the oscillator would have. to befof a greater amplitude to produce magnetostrictive action of a magnitude sufficient to operate the transducer with a reasonable quantity of output power.
The collector of transistor 12 receives operating potential from the regulated output at the emitter of transistor 10 which is applied to the collector through the primary winding of a feedback transformer 57. A series circuit of a resistor 58 and a diode 59 is connected across the prirnary'of transformer 57. The purpose of this is described below;
the circuit of FIG. 1, the effect of the driving coil inductance is neutralized by the capacitors 61 and 49 connected in parallel with coil 60a. (The capacitors 49 and 61 are part of a bridge containing the ends of the transducer coil. By grounding the junction of the capacitors the voltages across the coil ends are transmitted to the stack in phase opposition. This brings the stack voltage to ground potential.)
The base of oscillator transistor 12 is biased through a network comprising the series resistors 50, 51, 52 and 53.'The resistor 50 is connected to the collector of the regulator transistor 10. A positive feedback path is provided from the collector to the base of the transistor 12 through the secondary of transformer 57. A trimming capacitor is connected in parallel with the secondary of transformer 57 to adjust the phase of the feedback voltage-to secure oscillation. A pair of series, connected capacitors 65 and 66 are connected across the secondary winding and capacitor 64 to improve the power transfer, as is explained below. A-feedback connection is provided from the junction of capacitors 65, 66 over line 69 to the junction of resistors 50 andSl. A diode 54 is connected between the collector of transistor 12 and resistor 53 connected to the base. The purpose of this device is also explained below.
As in the circuit of my aforesaid patent application, the oscillator transistor 12 has the transducer 60 connected thereto so that thevoltage appearing across the transducer 60 appears at the emitter of the transistor '12 and the current flowing through the transducer flows through the transistor'and therefore through the primary winding of feedback transformer 57. Thus, the feedback voltage produced on line 69. and applied back to the base of transistor 12 through the resistors 51, 52 and 53 correspondstothe current through the transducer. As explained before, the transducer changes its characteristics, essentially its input impedance, in response to changes inload. Therefore, both the voltage across the transducer and the current through it will vary, depending upon the loading conditions.
The oscillator portion of the circuit of FIG. 1 operates by having a'collector to base feedback path through the transformer 57 which is phase inverted to produce a positive feedback signal and thereby sustain oscillations. The oscillator output voltage u appears across-the combination of the diode 59 and resistor 58 which is in the collector circuit of transistor 12. A portion of this collector voltage is transformed by the tuned transfomier 57 to provide a feedback voltage a,
appearing across capacitor 65 which is applied through the resistors 51, 52, 53 to the transistor base electrode. Voltage u, is proportional to the current through the transducer 60 since the current passing through the transducer is effectively the same current (less the base current), passing through the transistor 12. This current appears across diode 59 resistor 58 as the output voltage. The feedback voltage u, applied to the base is therefore proportional to'the current through transducer 60.
The voltage across transducer 60 appears as a voltage u between the emitter of transistor 12 and the neutral point 36. This voltage is in phase opposition with the feedback voltage it, applied to the transistor base so that the effect of the two voltages u, and u with respect to the transistor is vectorially additive.
FIG. 3 shows graphically the effect of the circuit of FIG. 1 on the transducer. The X axis of the graph is in the frequency scale while the Y axis is the amplitude scale for u, and a The series resonant frequency of the transducer is designated f, Curve 70 is the voltage u appearing at the emitter of the transistor 12. In the absence of the transducer stack, this voltage maintains a fairly level amplitude over a relatively wide range of frequencies due to the presence of the coil for transducer 60 of value L and the neutralizing capacitor 61 of value C in parallel with the coil. The voltage u has a sharp dip, shown by the portion 71, around the resonance frequency f,, which is caused by the resonance frequency of the stack of magnetostrictive elements.
Curve 75 represents the voltage u, The voltage U, is directly proportional to the transducer current as previously explained. The slowly changing amplitude portion on each side of the resonant frequency f is due to the presence of the tuned feedback circuit transformer 57 and capacitors 64, 65 and 66. This voltage peaks, as shown by the curve 751:, at the resonant frequency due to thepresence of the transducer stack in the emitter circuit. The solid portion of the curve 75 below the peak 75a indicates thevoltage which would appear without the transducer stack being in the emitter circuit. With the transducer stack there is a resonance of the transducer which causes an additional peaking of the voltage at and on either side of the resonant frequency 1,
Curve 74 is a combination of the two voltage u, u which is shown as the base to neutral voltage in FIG. 1. Here it can be seen that there is a peak voltage at the resonant frequency of the stack. In normal operation of the circuit, the resonant frequency of the stack will.
change due to loading of the transducer. Also, for a number of transducers designed for a certain resonant frequency, some of these will fall on each side of the design center by varying amounts. In the oscillator circuit of FIG. 1, the oscillator will accommodate for both types of variations within a relatively wide range. That is, the oscillator will deliver good power output as the transistor shifts on either side of f, due to loading, as shown by FIG. 3. Also, it will operate with transducers whose resonant frequencies are on either side of the design center j], The oscillator circuit of FIG. I is highly effective since two feedback signals are used, one to the base and one to the emitter, both of which have peak values at the transducer stack resonant frequency.
The oscillator 12 is biased so that it operates substantially in a Class B manner. Accordingly, selecting the proper values for the feedback components 57, 64, 65, 66 can be accomplished quite readily but certain precautions have to be taken for certain conditions. Because of the Class B type operation, power (current) is transmitted to the transducer substantially only on positive half cycles of the oscillator output, that is, when the oscillator 12 is conductive. FIG. 4 shows a cycle from time t to t with the middle at Collector current I, flows from t to )2 the positive half cycle, and there is no current from to t The voltage across the transducer is again shown as 14 and it follows I,
The power required to drive the transistor oscillator base electrode can put a large stress on the Q (quality factor) of the oscillator circuit and bring it down. Thus, precautions should be taken to see that the feedback path to the base electrode does not load the Q of the circuit unnecessarily.
Referring to FIG. 1, resistor 50 feeds the junction of capacitors 65, 66 from the output of the rectifier bridge 24. This provides an essentially constant current source to the junction of capacitors 65, 66 and sets up the bias at the base of transistor 12 through resistors 51, 52, 53. As indicated above, current flows through the transducer only on positive half cycles of input signal to the transistor base. Thus, during the negative half cycles of base input signal transistor'lz does not conduct and the secondary of transformer 57 is substantially unloaded due to the equilibrium condition established by the two capacitors 65, 66 whose junction is supplied with a voltage from resistor 50.
The Q of the secondary of transformer 57, called Qmmpmm in FIG. 4, is given by Qlen'ip 0)/( oL) where R the load on the secondary of transformer 57 presented by the load on the base of transistor 12 w, the center frequency of transformer 57 L the inductance of transformer 57 The load R, can be represented generally by an equivalent resistance in parallel with a single equivalent capacitor, which represents the combined capacitance of capacitors 64, 65, 66. Both the equivalent resistor and the capacitor parallel the transformer secondary. The Rload varies as a function of time, due to whether or not current is flowing in the oscillator circuit. Thus, referring to FIG. 4, R (t)=R fort t t fort t t 2) (since the transistor current is effectively zero) where R,,( t) is the varying value of the load.
R, is obtained by integration of the effect of R,,(t) for the whole period t, to and multiplied by a function of the capacitors 65, 66; It can be shown mathematically that R, remains relatively high over the period t, to i compared to the load resistor. Resistors 51 and 52 keep the circuit Q reasonably high during'the time the integrations are made. Thus, there is a good power transfer from the tank circuit 57, 65, 66 to the base without seriously lowering the Q of the tank circuit.
An essentially constant current is also supplied from the rectifier bridge 24 through the resistors 50, 51, 52, 53. However, the resistor 56 between the terminals of the primary of transformer 57 should be kept small for the positive values of current through the transistor 12 and transducer 60. If this is not done, then power is transferred to the collector of transistor 12 from the emitter, where it should be delivered. When the voltage at the upper terminal of the primary winding of 57 is more positive than the lower terminal (t to t diode 59 will conduct and resistor 58 will appear a the collector load. When the voltage at the upper terminal is less than that at the lower terminal (t to t,,), diode 59 is non-conductive and the resistor 58 is not in the circuit. This results in a better Q for the tank 57 for negative voltages (t to t across the transducer 60 and no collector current flowing, that is, the primary of transformer 57 is not loaded by resistor 58 since diode 59 is non-conductive. It also results in a poorer Q for tank 57 when the current flows through thetransistor (t, to t and the feedback is active, since resistor 58 loads the primary of transformer 57. Thelatter is desirable since for active periods of .the transducer the feedback is more decided primarily by the resonant characteristics of the transducer itself. During inactive periods of the transducer (t to t the oscillation of the circuit is maintained by the higher Q of the tank circuit 57.
Another way of viewing the circuit is that when transistor 12 conducts and diode 59 becomes conductive the resistance of resistor-58 placed across the primary oftransformer 57 lowers the impedance in the collector circuit. This results in a greater voltage (u being developed across transducer 60 and an increase in the power delivered to the transducer. When transistor 12 is non-conductive, the resistor 58 is not effective and the collector load impedance is higher. The larger portion of the current flowing through transistor 12 will now appear as a voltage drop across transformer 57 and be used on the feedback signal.
The feedback between collector and base of transistor vl2 should be sufficiently high to maintain oscillations even when the load on the transducer is small and there is high current flowing through transistor 12. Peak currents developed under little or no load conditioncould produce direct conduction between the collector and base electrode, that is, I
. would be high. This could either destroy the transistor or latch up the circuit in a conducting state with little or no effect or further base excitation.
I Diode 54 and resistor 53 prevent I from reaching dangerous levels. When the operating condition of the circuit is such that the collector voltage (u is less than the base voltage (u which would cause the latch-up condition, the diode conducts. The voltage from base to collector (a is given as:
u u, u When diode 54 conducts, u,, will exceed u only by the voltage dropacross the diode (a which can be, for ex- (3) arc tan Q=are tan to o: (n
where the phase angle of the feedback voltage (0,, the resonant frequency of the transducer to the operating frequency Q the quality factor of the transducer L and R the values of the inductance and resistance of the transducer at resonance R theimpedance of the load.
A resistive load is assumed for purposes of simplicity, the total reactive expression being somewhat complicated.
By keeping the current through the transducer constant, the power in the transducer will increase with increasing load. The power delivered to a load represented by a resistance R is equalto:
P= FR I 4 Thus, if the load increases causing an increase in R the total power P will increase if I is held constant.
The circuit of FIG. 1 maintains arelatively constant current I to the transducer and its load R Referring to FIG. 2, the generator which includes the oscillator circuit l2, behaves essentially as a constant current generator. In the equivalent circuit of FIG. 2, the cur rent] through the transducer end load is given as:
aen iv: I. .IXTR
where u the output voltage of the generator R the equivalent circuit output resistance of the generator R the resistance of the transducer stack which is R in FIG. 2 ,X the reactive component of the transducer stack impedance formed by 60L and 60C; and
R the load.
At resonance, the ,X component vanishes. Therefore, the current I at resonance is: o aen TR L As a practical matter R R and R R Therefore:
R R R,
This means that L, is substantially independent of the load resistance, or
n nen 1R Thus, for changes in load R the power P will follow as per equation (4) since the current I remains substantially constant.
As should be apparent, a relatively simple but highly efficient circuit has been provided for driving an ultrasonic transducer. The circuit comprises only two transistors but yet provides all of the advantages discussed above.
What is claimed is: g
1. In combination ultrasonic transducer means whose impedance-varies as a function of its loading, oscillator circuit means including amplifying means having at least a control electrode and first and second output electrodes, means for connecting said transducer means to one of said output electrodes to receive driving current from said amplifying means, frequency responsive circuit means including at least one reactive circuit element connected to the output electrode of said amplifying means to serve as the load impedance therefor, means coupling energy from said frequency responsive circuit means to said control electrode to provide a positive feedback signal to produce and sustain oscillations of said amplifying means, and means connected to said frequency responsive circuit means for changing the load impedance seen by said other output electrode during a given portion of each cycle of its oscillations to increase the power delivered to the transducer.
2 The combination of claim 1 wherein said means for changing the load impedance of the frequency responsive circuit means includes means responsive to the current through said amplifying means to lower the Q of the frequency responsive circuit means when current is flowing through said amplifying means.
3. The combination of claim 2 wherein said ultrasonic transducer means includes means forming a frequency responsive circuit.
4. The combination of claim I wherein said frequency responsive circuit means includes a tuned resonant circuit, said means for changing the load impedance seen by said other output electrode comprising a diode and a first means having a resistive component which are connected to said resonant circuit, said diode becoming conductive during said give portions of each cycle of oscillation of said amplifying means to place said first means in circuit with said resonant circuit means to lower the impedance of the latter, said diode being non-conductive during the remainder of each cycle of oscillation of said amplifying means.
5. The combination of claim 2 wherein said frequency responsive circuit means includes a tuned resonant circuit, said means for changing the load impedance of said circuit comprising a diode and a first means having a resistive component which are connected to said resonant circuit, said diode becoming conductive during said given portions of each cycle of oscillation of said amplifying means to place said first means in circuit'with said resonant circuit means to lower the Q of the latter, said diode being non-conductive during the remainder of each cycle of oscillation of said amplifying means.
6. The combination of claim 1 wherein said means for connecting said transducer to said amplifying means connects it in series with said one electrode and a point of reference potential so that the current through said amplifying means is proportional to the current through said transducer means and a first voltage is present between said one output electrode and said point of reference potential which is proportional to the voltage across said transducer, means for connecting said frequency responsive circuit means to said other electrode so that a second voltage is present thereon which is proportional to the current through said amplifying means, said coupling means applying said second voltage to said control electrode so that said first and second voltages are vectorially additive with respect to the operation of said amplifying means to produce said oscillations.
7. In the combination of claim 6 wherein said three electrode amplifying means is a semiconductor device having an emitter, a collector and a base electrode which are respectively said first and second output electrodes and said control electrode.
8. In the combination of claim 7 wherein said transducer is connected to said emitter electrode and said frequency responsive circuit means for applying the second voltage to said base electrode comprises tuned feedback circuitmeans connected between the collector and base electrodes.
9. In combination an ultrasonic transducer means whose impedance varies as a function of its loading, oscillator circuit means including amplifying means having at least a control electrode and first and second output electrodes, meansfor connecting said transducer means in series between a point of reference potential and said first output electrode so hat the current through said amplifying means is proportional to the current through said transducer means and a first voltage appears between said first output electrode and said point of reference potential which is proportional to the voltage across said transducer, and means connected to said second output electrode for producing a second voltage proportional to the current through said amplifying means and for applying said second voltage to said control electrode so that said first and second voltages are vectorially additive with respect to the operation of said amplifying means to produce and sustain oscillations, said last named means comprising a tune resonant circuit including a transformer having a primary and a secondary winding, a pair of series connected capacitors connected in parallel across said secondary winding and means connected between the junction of said pair of capacitors and said control electrode for supplying said second voltage to said control electrode.
10. The combination of claim 9 further comprising means connected to said frequency responsive circuit means for changing the impedance load that it presents to said second output electrode during a given portion of each cycle of its oscillations.
11. The combination of claim 1 wherein said tuned resonant circuit means comprises a transformer having primary and secondary windings, said means for changing the impedance load presented by said tuned resonant circuit means being connected to said primary winding, said means for coupling energy to said control electrode comprising a pair of series connected capacitors connected across said secondary winding, and means connected between the junction of said pair of capacitors and said control electrode to supply said positive feedback signal to said control electrode.
12. The combination of claim 7 further comprising diode means connected between said collector and base electrodes to prevent the potential at the base electrode from exceeding the potential at the collector electrode.
13. In combination an ultrasonic transducer means whose impedance varies as a function of its loading, said ultrasonic transducer means including a plurality of magnetostrictive elements and a single coil in proximity thereto for supplying driving current to said elements, oscillator circuit means including amplifying means having at least a control electrode and first and second output electrodes, means for connecting said single coil of said transducer means in series between a point of reference potential and said first output electrode so that the current through said amplifying means if proportional to the current through the coil of said transducer means and a first voltage appears between said first output electrode and said point of reference potential which is proportional to the voltage across pedance presented by said circuit means.
14. The combination of claim 13 wherein circuit last means comprises a tuned resonant circuit including a transformer having a primary and a secondary winding, a pair of series connected capacitors connected in parallel across said secondary winding and means connected between the junction of said pair of capacitors and said control electrode for supplying said second voltage to said control electrode.
15. The combination of claim 13 further comprising an additional impedance element, said'circuit means including a tuned resonant circuit, and said first means including second means for placing said additional impedance element in circuit with said tuned resonant circuit.
16. The combination of claim 15 wherein said tuned resonant circuit includes a transformer having a primary winding with one end connected to the said second output electrode, said second means and said additional impedance element connected in seriesacross at least a portion of said transformer.
17. The combination of claim 16' wherein said second means is a diode and said diode and said additional impedance element are connected in series across the primary winding of the transformer.