Transistor blocking oscillator stabilized against changes in bias voltage and temperature

#The invention relates to a blocking oscillator for use in a buzzer, and more particularly, to a transistor oscillator circuit which is stabilized against changes in ambient temperature and fluctuations of a supply voltage.

#A blocking oscillator for use in a buzzer and comprising npn or pnp transistor is well known, and is described in German Laid-open Application No. 2,110,918 and U.S. Pat. No. 3,564,542, for example. One form of known blocking oscillator comprising an npn transistor is illustrated in FIG. 1 where terminals 1 and 2 are adapted to be connected with a power supply. A current flow through resistor and control coil L.sub.2 establishes a bias voltage across the base and emitter of a transistor T, which therefore conducts to permit a current flow through its collector-emitter path including a drive coil L.sub.1. The both coils L.sub.1 and L.sub.2 are disposed on a common core to provide an inductive coupling between them. The current flow through the drive coil L.sub.1 induces a voltage across the control coil L.sub.2 which is of a polarity to reduce the base-emitter voltage V.sub.BE of the transistor T below the bias voltage, thus rendering the transistor cut-off. Thereupon a reverse e.m.f. is induced across the control coil L.sub.2 to maintain the transistor cut-off. As the reverse e.m.f. is discharged through a loop including the emitter and base of the transistor and the voltage V.sub.BE across the base and emitter path of the transistor increases, it is again rendered conductive. The process is periodically repeated at a rapid rate, which determines the frequency with which an armature is magnetically driven by the core.

#One of the requirements for a proper operation of the blocking oscillator is that the transistor T must be rendered conductive when the power is initially turned on. Another requirement relates to the voltage induced across the control coil L.sub.2 which must be sufficient to reduce the voltage across the base and emitter of the transistor below the bias voltage so that the latter is made cut-off. This requires that the transistor maintains a proper value of the bias voltage. However, when the oscillator is used in an environment having a varying ambient temperature, a variation in the operational characteristic of the transistor prevents a proper operation from being achieved if the bias voltage across the base and emitter is maintained constant.

#FIG. 1 is a circuit diagram of a standard blocking oscillator;

#FIG. 2 is a circuit diagram of a known blocking oscillator including a temperature compensation diode;

#FIG. 3 is a circuit diagram of a known blocking oscillator including a voltage compensation Zener diode;

#Referring to FIG. 6, the blocking oscillator of the invention also includes a pair of terminals 1, 2 for connection with a power supply. A drive coil L.sub.1 is connected in series with the collector-emitter path of a transistor T across the terminals 1, 2. The transistor is shown as npn type, and hence the terminal 1 is connected with a positive terminal and the terminal 2 with a negative terminal of the power supply. A series circuit including a resistor R, which supplies a base bias to the transistor T, and a control coil L.sub.2 is also connected across the terminals. In accordance with the invention, this series circuit also includes a diode D.sub.1 which is connected between the control coil L.sub.2 and the terminal 2 and is forwardly poled. The junction between the resistor R and the control coil L.sub.2 is connected with the base of the transistor. The drive coil L.sub.1 and the control coil L.sub.2 are disposed on a common core 3 as a bifilar winding and have a close inductive coupling therebetween. Also, in accordance with the invention, another diode D.sub.2 is connected in shunt with the control coil L and is forwardly poled.

#The transistor T and the diodes D.sub.1 and D.sub.2 are formed of the same kind of semiconductor material. For example, if a silicon transistor is used, the diodes are formed of silicon. During the wiring of the circuit connection, they are preferably disposed so as to be placed under the same thermal condition. As a consequence of these considerations, it is assured that the I.sub.C - V.sub.BE characteristic of the transistor and the I.sub.F - V.sub.F characteristic of the diodes D.sub.1, D.sub.2 are similar in configuration as is the amount of changes which occur in these characteristics as the ambient temperature changes. Thus it is possible to provide a bias voltage which compensates for a variation in the threshold voltage of the transistor. In the arrangement of the invention, when the I.sub.C - V.sub.BE characteristic of the transistor remains unchanged but the supply voltage fluctuates, it will be understood that since the voltage across the control coil L.sub.2 is maintained substantially constant by the diode D.sub.2, a bias voltage across the base and emitter of the transistor can also be maintained substantially constant by a series circuit formed by the diodes D.sub.1 and D.sub.2. Thus a normal oscillating condition of the transistor T can be maintained for all fluctuations of the supply voltage.

#FIG. 7 shows another embodiment of the invention in which the blocking oscillator comprises a pair of transistors T.sub.1 and T.sub.2 connected in Darlington configuration. In this case, the drive coil L.sub.1 is connected between the terminal 1 and the collector of transistor T.sub.2 in the final stage. As before, diode D.sub.2 is connected in shunt with the control coil L.sub.2. A pair of diodes D.sub.1 and D.sub.3 are connected in series with the parallel combination of the diode D.sub.2 and the control coil L.sub.2 so as to correspond to the use of the pair of transistors T.sub.1 and T.sub.2. The operation remains unchanged.