GB2205702A - Oscillator circuit with temperature-independent frequency, amplitude - Google Patents

Description

7 OR 53, AN OSCILLATOR CIRCUIT This invention relates to an oscillator

circuit having a piezoceramic sound-transmitter component which lies between the base emitter section of a first switching transistor and the collector emitter section of a second switching transistor, the base of which is coupled with the collector of the first switching transistor, in which respect the first switching transistor and the second switching transistor alternately block or conduct and the component is alternately flowed through by a positive or negative current pulse.

Such an oscillator circuit is described in the literary text Schwingquarze ein unverzichtbares Bauelement in der Elektronik" (Oscillator crystals an indispensable component in electronics), ZVEI, Vistas Verlag, Berlin 1985, pages 61 to 66 as a "Heegner Circuit". It has been shown that a piezoceramic sound- transmitter component (buzzer), if it is operated in such a known circuit, in a temperature range from O'C to 125C has neither a constant frequency nor a constant (sound) volume. This can be attributed to the fact that the reasonance frequency occurring in said circuit depends predominantly upon the electrical capacitance of the pie=ceramic sound-transmitter component. The said Heegner circuit also oscillates when the component is replaced by an electrical capacitor. The capacitance of the piezoceramic sound- transmitter component increases with rising temperature. The frequency changes accordingly.

It has furthermore been shown that piezoceramic sound-transmitter components having a comparatively large capacitance, for example 20 F and a comparatively low frequency, for example 2 kHz, cannot be operated reliably, since then the oscillating of the Heegner circuit is problematical and sudden changes of frequency may occur.

An aim of the present invention is to provide an oscillator circuit in which the sound frequency and the sound amplitude of the component, more especially in the temperature range between OOC and 1250C, may be largely or effectively independent of the ambient 2 temperature.

According to the present invention there is provided an oscillator circuit having a piezoceramic sound- transmitter component s which lies between the base-emitter section of a first switching transistor and the collector-emitter section of a second switching transistor, the base of which is coupled with the collector of the first switching transistor, and in which the first switching transistor and the second switching transistor alternately block or conduct and said component is alternately flowed through by a positive or negative current pulse, characterised in that a third switching transistor (e.g. T3) lies with its emitter-collector section in series with the collector-emitter section of the second switching transistor (e.g. T2) and with its base coupled to the collector of the first switching transistor (e.g. T1) the arrangement being such that, with the first switching transistor conductive or respectively blocking, the third switching transistor likewise conducts or respectively blocks, and such that, with the third switching transistor conductive, by way of the emitter-collector section thereof an excitation current pulse flows to said component which is such that the component mechanically overswings or overshoots, whereby at the base of the first switching transistor a potential arises such that the first switching transistor blocks, and then by way of the collector-emitter section of the second switching transistor a discharge current pulse flows which is such that the component mechanically overswings or overshoots in the opposite sense or direction, whereby at the base of the first switching transistor a potential arises which switches the first switching transistor conductive again.

The switching frequency of the circuit, which corresponds to the sound frequency of the piezoceramic sound- transmitter component, is here largely independent of the electrical capacitance of the component. This is because the switching of the first switching transistor and thus also the switching of the other switching transistors is dependent upon the mechanical overswing or overshoot of 3 the component and thus its piezoelectrical properties. These properties have little temperature-dependence, so that also the frequency is largely independent of the ambient temperature. Since the piezoceramic of the sound-transmitter component is excited comparatively severely, namely as f ar as overswinging or overshoot, also a high and largely temperatureindependent sound amplitude is achieved.

The conductive, third switching transistor makes possible the high excitation current pulse. The third switching transistor reduces in its blocked state the current consumption of the circuit.

The oscillator circuit is suitable for example for cooker timers which are exposed in operation to high temperatures, for example 110,C. As a result of the low current requirement it is possible to connect the oscillator circuit directly to a control circuit.

Advantageous developments of the invention will become apparent from the sub-claims and the following description of an exemplified embodiment. The single figure shows an oscillator circuit having a piezoceramic sound-transmitter component.

Referring to the drawing, a piezoceramic sound-transmitter component (1) lies between the base of a first switching transistor (T1) and the collector of a second switching transistdr (T2). The emitters of the switching transistors (T1, T2) are connected to a connection pole or terminal (2). The base of the second switching transistor (T2) is connected by way of a resistor (R1) to the collector of the first switching transistor (T1).

A second connection pole or terminal (3) of the circuit is connected by way of a low-ohmic barrier resistance (R2) to the emitter of a third switching transistor (T3), the collector of which is connected to the collector of the second switching transistor (T2) and to the component (1). The base of the third switching transistor (T3) is connected by way of a resistor (R3) to the collector of the first 4 switching transistor (T1). A resistor (R4) lies in parallel with the emitter-base section of the third switching transistor (T3).

A further resistor (R5) is provided between the emitter of the third switching transistor (T3) and the base of the first switching transistor (T1). A diode (D) poled in the blocking direction is connected in parallel with the base-emitter section of the first switching transistor (T1).

The described circuit consisting of the transistors (T1 to T3) and the resistors (Rl to R5) is so rated that, when the component (1) is not connected and voltage is applied to the connection poles or terminals (3, 2), a stable switching state occurs. Then, by way of the resistors (R2, R5) a base current flows to the first switching transistor (T1), which switches this conductive. The second switching transistor (T2) is in this respect blocked by way of the resistor (R1), whereas the third switching transistor (T3) is switched conductive by way of the resistor (R3). A capacitor provided instead of the component (1) could not or might not bring the circuit out of this stable state.

The mode of operation of the described circuit with connected component (1) is for instance as follows:

The connection pole or terminal is connected to a control circuit, for example a microprocessor, which, for switching-on the piezoceramic soundtransmitter component (1) (buzzer) applies voltage to the connection poles (3, 2). This voltage is at the same time the supply voltage for the circuit.

After the voltage is swit-ched on, the switching transistors (T1, T3) are conductive. The switching transistor (T2) is blocked. By way of the barrier resistor (R2) and the emitter-collector section of the transistor (T3) a strong excitation current pulse flows to the component (1). The time constant R2C0 based on the capacitance CO of the component (1) is negligible compared with the acoustic period of oscillation dictated by the mechanical acoustic structure of the component (1).

As a result of the strong excitation current pulse, the component (1) overswings or overshoots, at the end of the excitation current pulse, that stable position which corresponds to the applied voltage.

Thus the component may exceed a permitted stable value. Upon the return movement following on from the overswing, the component. (1) generates a negative potential at the base of the first switching transistor (T1), so that this blocks. In this way by way of the resistor (R1) the second switching transistor (T2) becomes conductive and the third switching transistor (T3) blocks.

Thereupon, a discharge current pulse flows from the component by way of the collector-emitter section of the second switching transistor (T2) and the diode (D). Also in this respect there occurs an overswing or overshoot, based on the mechanical structure of the component (1), which is followed by a corresponding swing back. Upon this swing back a negative potential arises at the base of the first switching transistor (T1), so that this becomes conductive again. The swinging or oscillating procedure is thus maintained until the supply voltage is switched off.

Since the third switching transistor (T3) blocks when the second switching transistor (T2) conducts, the current consumption of the oscillator circuit is considerably reduced when compared with a case where, instead of the emitter-collector section of the third switching transistor (T3), a resistor is provided.

The resistor (R5) ensures that the oscillator circuit reliably responds upon switching-on the control or supply voltage. It is, however, also possible to operate the oscillator circuit without the resistor (R5). In such a case, in the above-mentioned stable state of the circuit in which the component (1) is not connected, the first switching transistor (T1) will block and the second switching transistor (T2) will conduct. With the component (1) connected and 6 upon switching-on the control or supply voltage, then the component (1) receives by way of the third switching transistor (T3) a pulse which causes the first switching transistor (T1) to become conductive, so that the second switching transistor (T2), although it would 'per se' become conductive, is blocked.

It is to be understood that the scope of the present invention is not to be unduly limited by the particular choice of terminology and that a specific term may be replaced or supplemented by any equivalent or generic term where sensible. Further it is to be understood that individual features, method or functions related to the oscillator circuit or parts thereof, or circuits including same and/or combinations thereof might be individually patentably inventive. In particular, any disclosure in this specification of a range for a variable or parameter (e.g. ambient temperature) shall be taken to include a disclosure of any selectable or derivable sub-range within that range and shall be taken to include a disclosure of any value for the variable or parameter lying within or at an end of the range. The scope of the present invention may possibly extend to cover the

Claims (1)

1. characterising portion of Claim 1 (or functional equivalent thereto) - or

   of the following statement - in isolation to the pre-charactering portion.

   Further according to the present invention there is provided an oscillator circuit having a piezoceramic sound- transmitter component which lies between the base-emitter section of a first switching transistor and the collector-emitter section of a second switching transistor, the base of which is coupled with the collector of the first switching transistor, and in which the first switching transistor and the second switching transistor alternately block or conduct and the component is alternately flowed through by a positive or negative current pulse, characterised in that the sound frequency and sound amplitude of the component, in the temperature range O'C to 125"C, are rendered effectively independent of ambient temperature, and/or in which the switching of the first and second transistors is rendered dependent on a mechanical overswing or overshoot of the 7 component (and thus on its piezoelectrical properties rather than its capacitance), preferably, by the provision of a third switching transistor, preferably connected such that it is conductive when the first transistor is conductive and blocks when the first transistor blocks and such that when the first and third transistors conduct said component mechanically overswings or overshoots, and such that then the first transistor blocks thereby causing overshoot oi? overswing of said component in the opposite sense, and the first switching transistor is once again rendered conductive, and/or characterised in that the resonance frequency occurring in said circuit depends predominantly on the piezo-electric effect of said component.

   8 CLAIMS 1 An oscillator circuit having a piezoceramic sound- transmitter component which lies between the base-emitter section of a first switching transistor and the collector-emitter section of a second switching transistor, the base of which is coupled with the collector of the first switching transistor, and in which the first switching transistor and the second switching transistor alternately block or conduct and said component is alternately flowed,through by a positive or negative current pulse, characterised in that a third switching transistor lies with its emitter-collector section in series with the collector-emitter section of the second switching transistor and with its base coupled to the collector of the first switching transistor the arrangement being such that, with the first switching transistor conductive or respectively blocking, the third switching transistor likewise conducts or respectively blocks, and such that, with the third switching transistor conductive, by way of the emitter-collector section thereof an excitation current pulse flows to the component which is such that the component mechanically overswings or overshoots, whereby at the base of the first switching transistor a potential arises such that the first switching transistor blocks, and then by way of the collector-emitter section of the second switching transistor a discharge current pulse flows which is such that the component mechanically overswings or overshoots in the opposite sense or direction, whereby at the base of the first switching transistor a potential arises which switches the first switching transistor conductive again.

   2. A circuit according to Claim 1, in which connected in parallel with the baseemitter section of the first switching transistor is a diode which conducts upon the discharge current pulse.

   A circuit according to Claim 1 or 2, in which lying between the base of the first switching transistor and a connection pole or terminal is a resistor which switches the first switching transistor conductive upon switching-on the oscillator circuit.

   9 4. A circuit according to any one of the preceding claims, in which a resistor is provided between the collector of the first switching transistor and the base of the third switching transistor.

   5. A circuit according to any one of the preceding claims, in which a resistor lies between the collector of the first switching transistor and the base of the second switching transistor.

   6. An oscillator circuit substantially as herein described with reference to the accompanying drawings.

   7. An electrical circuit including an oscillator circuit as claimed in any one of the preceding claims and a control circuit, e. g. a microprocessor.

   8. A cooker timer including an electrical circuit as claimed in Claim 7.

   Published 1988 at The Patent Office, State House, 66171 I-hgh Holborn, London WClR 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BRS 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1/87.