LC-switched transistor oscillator for vibrator excitation

A self-oscillating vibrator exciting circuit utilizing a drive transistor that operates in a switching mode. The ON-period of the transistor is determined by base/emitter inductance and capacitance and base/collector capacitance, while the OFF-period of the transistor is determined by collector/emitter inductance and capacitance. The vibrator is connected between transistor collector and emitter, and the ON-period frequency of the transistor is set in the neighborhood of the resonance frequency of the vibrator so that the vibrator exciting circuit oscillates generally at the resonance frequency of the vibrator.

BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION 
This application claims the benefit of the priority under the International 
Convention and hereby specifically incorporates by reference the 
disclosure of Japanese Utility Model application No. 57-90509 filed 29 May 
1982. 
The present invention relates to a vibrator exciting circuit suited to 
drive a vibrator with horn and, particularly, to a vibrator exciting 
circuit which is adapted to drive a piezoelectric vibrator efficiently at 
the resonance frequency of the vibrator. 
A conventional vibrator of the Langevin type or the like, wherein the 
amplitude of oscillation is amplified by a horn connected with the 
vibrator, is driven at a point where the resonance frequency has a low 
impedance. This is because the amplitude becomes maximum at the resonant 
point. Therefore, most prior art oscillation systems have been arranged as 
shown in FIG. 1, such that a vibrator TD is provided with a pickup 
electrode 1, and the voltage on the electrode is fed back to an amplifier 
2, so that oscillation follows the maximum amplitude. This method, 
however, needs a third electrode on the vibrator, and further requires a 
phase compensation circuit and the like, causing the circuit 
disadvantageously to become complex and thus expensive. 
On the other hand, in applications where the accuracy of frequency and 
efficiency are not the primary concerns, such as a self-oscillator used in 
a cleaner, an arrangement has been made as shown in FIG. 2, such that a 
feedback transformer 3 or an LC tuning circuit provided in the feedback 
loop is combined with an amplifier 4 and a vibrator TD. However, this 
circuit arrangement disadvantageously requires the fine adjustment of the 
tuning circuit when each vibrator has a different resonance frequency. 
Vibrator exciting circuits without a resonant point for use in supersonic 
sprays are disclosed in Japanese Utility Model Publication No. 56-33659 
and Japanese Patent Publication No. 56-40640. However, these arrangements 
are modified Colpitts circuits by utilization of the inductive portion of 
the vibrator, and do not oscillate at the resonance frequency; thus they 
are not the best arrangements for sprays that operate efficiently at the 
maximum amplitude of oscillation. 
In view of the foregoing prior art deficiencies, the present invention 
contemplates providing a vibrator exciting circuit which has a simple 
circuit arrangement, drives the vibrator at its resonant point, and 
provides a satisfactory power transmission characteristic. Briefly, the 
circuit utilizes a drive transistor operating in a switching mode, in 
which the ON-period of the transistor is determined by base/emitter 
inductance and capacitance and base/collector capacitance, while the 
OFF-period of the transistor is determined by collector/emitter inductance 
and capacitance, and in which the vibrator is connected between the 
collector and emitter of the transistor, with the frequency of the 
ON-period of the transistor being set in the neighborhood of the resonance 
frequency of the vibrator so that the exciting circuit oscillates at the 
resonance frequency of the vibrator. 
The following describes presently preferred embodiments of the present 
invention, with reference to the drawings.

DETAILED DESCRIPTION 
In FIG. 3, a DC power source 10 has a positive terminal connected to the 
collector of a transistor Q and one terminal of a capacitor C1 through a 
diode D1 which protects the circuit when the power source is connected in 
the opposite polarity. A negative terminal of source 10 is connected to 
the emitter of the transistor Q through a coil L1 and also to another 
terminal of the capacitor C1. Connected between the collector and the base 
of the transistor Q are a capacitor C2 and a series connection of a 
vibrator TD and a coil L2. The vibrator TD is connected in parallel with a 
bias resistor R, and a capacitor C3 is connected between the emitter of 
the transistor Q and the junction of the vibrator TD and the coil L2. A 
diode D2 is connected between the base and the emitter of the transistor 
Q, and a capacitor C4 is connected between the collector and the emitter. 
Capacitors C5 and C6, shown with dashed connections, represent the 
capacitances existing in the transistor Q, i.e., inter-electrode 
capacitances. 
Representative values of the circuit components for providing an 
oscillation frequency of 100 kHz under a 12 volt supply voltage of the DC 
power source 10 are as follows: 
L1: 156 .mu.H 
L2: 82 .mu.H 
C2: 100 pF 
C3: 0.2 .mu.F 
C4: 6800 pF 
FIG. 4A depicts the collector current I.sub.C of the transistor Q in the 
vibrator exciting circuit shown in FIG. 3. FIG. 4B depicts the collector 
to emitter voltage V.sub.CE of the transistor Q. FIG. 4C depicts the base 
current I.sub.B of the transistor Q. FIG. 4D depicts the current I.sub.L 
in the coil L1. 
FIG. 5 shows an example of a vibrator having a horn excited by the vibrator 
exciting circuit of FIG. 3, wherein vibrator TD is fixed on the major end 
face of a horn 20, and a resonance plate 21 is provided integrally on the 
opposite end of the horn 20. 
The following is the operation of the circuit of FIG. 3. First, the 
principle of oscillation will be described on the assumption that the 
vibrator TD is absent from the circuit. When power is supplied to the 
circuit, the base current I.sub.B is supplied through the bias resistor R 
to the transistor Q, and consequently the collector current I.sub.C starts 
flowing as shown in FIG. 4A. At the same time, the collector to emitter 
voltage V.sub.CE of the transistor Q falls, and the emitter voltage 
V.sub.E relative to the voltage at the negative terminal of the DC power 
source 10 rises. The base current I.sub.B shown in FIG. 4C increases and 
decreases in a period (frequency) determined by the resonant circuit 
constituted by the coil L2 and capacitances C2, C3, C5 and C6, and 
maintains the transistor Q in an ON state during the positive cycle. The 
base current cuts off the transistor Q sharply when it enters the negative 
cycle. Once the transistor has been cut off, the collector voltage V.sub.C 
rises sharply, causing a current to flow in the loop including the coil L1 
and the capacitors C1 and C4; then the emitter voltage V.sub.E starts to 
rise. When the emitter voltage V.sub.E turns to fall, the base current 
I.sub.B starts to flow again. At this time, the current through the diode 
D2 makes a sharp increase in the base current I.sub.B. Thus the transistor 
Q turns on and off repetitively, with its ON-period determined by the coil 
L2 connected between the base and the emitter and a capacitor C2 and/or 
capacitance C5 connected and/or existing between the base and the 
collector, and with its OFF-period determined by the coil L1 and the 
capacitors C1 and C4 connected between the collector and the emitter. 
Consequently, the current I.sub.L of the coil L1 has a waveform with two 
periods as shown in FIG. 4D. 
When the vibrator T.sub.D, with its resonant Q being higher than that of 
the circuit, is connected in the circuit, the period of the base current 
I.sub.B is determined by the resonance frequency of the vibrator TD. For 
example, a circuit as in FIG. 3 has an oscillation frequency of 93 kHz for 
the circuit without involving the vibrator, and it operates in an 
oscillation frequency pulled in the resonance frequency of the vibrator TD 
of 100 kHz. In the pull-in oscillation state, the circuit current is 
determined by the minimum impedance of the vibrator. 
The effects achieved by the foregoing embodiment are as follows. 
(1) The circuit arrangement is simple and thus inexpensive. The circuit 
provides a satisfactory power transmission characteristic and high 
efficiency, since it operates at the resonance frequency of the vibrator 
TD. 
(2) Oscillation proceeds even if the connection for the vibrator TD is 
open-circuited, preventing the abnormal current caused by that event which 
would otherwise destroy the transistor Q and other components. 
(3) The transistor Q operates in a complete switching mode during 
oscillation, generating less heat by transient power loss. 
(4) The circuit can drive the vibrator TD with a current waveform 
approximating a sine wave. 
It will be appreciated that the capacitor C2 connected between the 
collector and the emitter of the transistor Q may be eliminated, and 
substituted by the internal capacitance C5 existing between the collector 
and the base when the transistor has a sufficiently high current gain. 
FIG. 6 shows another embodiment of the present invention. In this 
arrangement, a vibrator TD is connected through an impedance matching 
transformer 30 to the collector and the base of the transistor Q, with a 
DC blocking capacitor C7 provided in series with the primary winding of 
the transformer. The remaining portions of the circuit are the same as 
shown in the circuit of FIG. 3, except that the capacitor C2 of FIG. 3 has 
been omitted, and the inter-electrode capacitances C5 and C6 have not been 
shown. 
The arrangement of FIG. 6 is advantageous in maintaining a satisfactory 
impedance matching between the vibrator TD and the circuit when the supply 
voltage is varied. 
According to the present invention, as described above, a vibrator exciting 
circuit having a simple circuit arrangement and capable of driving a 
vibrator at its resonant point for satisfactory power transmission is 
achieved. 
It will be appreciated that the presently preferred embodiments of the 
invention may be modified. The invention, therefore, is to be defined by 
the following claims.