JPH0712141B2 - Proximity sensor oscillator circuit - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an oscillation circuit of a linear output type proximity sensor that provides an output proportional to a distance to a detection object, and particularly to an oscillation circuit at a level corresponding to the distance to the detection object. The present invention relates to a proximity sensor oscillator circuit.

[Outline of Invention]

The proximity sensor oscillation circuit according to the present invention is a negative resistance type oscillation circuit using a current mirror circuit, in which the mirror current of the current mirror circuit is a constant current type and is oscillated in proportion to the resonance resistance of the resonance circuit by the detection coil and the capacitor. In addition to obtaining the amplitude, the limiter circuit limits the base voltage of the transistor that switches the oscillating voltage and feeds back the constant current to a predetermined value to reduce the inrush current that flows when the transistor is switched. In this way, it is possible to prevent the oscillation amplitude due to the inrush current or the linearity or the fluctuation of the amplitude due to the temperature change.

[Background of the Invention]

FIG. 3 is a circuit diagram showing an example of an oscillating circuit as a premise of the proximity sensor oscillating circuit according to the present invention. This oscillation circuit has a resonance circuit composed of a detection coil 1 and a capacitor 2, and the hot mirror side of this resonance circuit has a current mirror circuit 3
It is connected to the collector terminal of one transistor 3a. The collector of the transistor 3b is connected to the collector of the transistor 4 which converts a voltage into a current, and the collector end of the transistor 3a is further connected to the base of the transistor 7 via the resistor R1, the base-emitter connection of the transistors 5 and 6. There is. The transistor 7 is an emitter follower type transistor, and the voltage change amount of the resistor R2 is transmitted to the base of the transistor 4 via the detection circuit 8 in the subsequent stage and the resistor R3.
Here, the emitter of the transistor 4 is the transistor 9a, 9b
It is connected to a current mirror circuit 9 configured as a constant current circuit. The collector of the one transistor 9b is connected to the common base and the power supply terminal via the resistor R9, and the collector of the other transistor 9a is connected to the emitter terminal of the transistor 4.

According to such an oscillating circuit, a current flows into the detection coil 1 and the capacitor 2 through the resistor R6, the transistors 5 and 6 and the resistor R1, and the voltage thereof is impedance-converted by the emitter follower type transistor 7 so that the transistor 4
Join in. At this time, since the current flowing through the transistor 4 is equal to or less than the current value determined by the current mirror circuit 9, the transistor 9a operates as a large conductance, and positive feedback is performed to the resonance circuit via the current mirror circuit 3. When the collector current of the transistor 3b becomes equal to the current value determined by the current mirror circuit 9, the current mirror circuit 9 becomes a constant current circuit and the collector current is limited to a constant value. Therefore, the current fed back from the collector of the transistor 3a to the resonance circuit is also made constant. Therefore, the oscillation circuit oscillates with an amplitude corresponding to the conductance of the resonance circuit, and the output thereof is converted into a DC voltage by the detection circuit 8 and transmitted to the outside.

When the oscillation circuit oscillates, the terminal voltage on the hot end side of the detection coil 1 has a sine waveform as shown in FIG. 4 (a), and the base voltage of the transistor 4 is positive as shown in FIG. 4 (b). It becomes a half-wave rectified waveform that flows only in the half cycle of. At this time, the emitter voltage of the transistor 4 is limited to a predetermined value determined by the current mirror circuit 9, but during switching of the transistor 4, the collector-base capacitance of the transistor 9a is rapidly charged. Since this charging current is multiplied by hfe in the transistor 9a, an inrush current Ip1 flows as shown in FIG. 4 (c) when the transistor 4 is turned on. The inrush current Ip1 is also fed back through the current mirror circuit 3, but the magnitude of the inrush current Ip1 largely depends on the oscillation amplitude and further fluctuates due to variations in the characteristics of the transistors 4, 9a and 9b and temperature changes. Therefore, there is a problem that the oscillation amplitude of the proximity sensor oscillation circuit may fluctuate.

[Object of the Invention]

The present invention has been made in view of the problem of such a proximity sensor oscillation circuit, and a proximity sensor oscillation circuit which does not cause variations or fluctuations in the oscillation amplitude by reducing the inrush current flowing at the time of switching of a transistor. The purpose is to provide.

[Constitution and effect of the invention]

The present invention is an oscillation circuit of a linear output type proximity sensor,
A resonance circuit including a detection coil, a current mirror circuit in which the current output end of one transistor is connected to the resonance circuit, and an emitter follower type first transistor in which the base is connected to the resonance circuit via a constant voltage element A second transistor which is intermittently connected based on the emitter voltage of the first transistor and which feeds back current to the other transistor of the current mirror circuit when conducting, and the second transistor which is connected to the output terminal of the second transistor. A constant current circuit that limits the feedback current of the second transistor to a constant value, and a limiter circuit that limits the base voltage of the second transistor to the constant value.

According to the present invention having such a feature, the terminal voltage of the second transistor is limited to a predetermined value by the limiter circuit, so that the rush current during switching of the second transistor is independent of the magnitude of the oscillation amplitude. It is possible to limit it to a small level. The current of the second transistor is fed back to the resonance circuit as it is, but the feedback current can be stabilized because the inrush current hardly changes. Therefore, it is possible to significantly reduce the effects of fluctuations in oscillation amplitude, variations in element characteristics, and temperature changes.

[Explanation of Examples]

FIG. 1 is a circuit diagram showing an embodiment of a proximity sensor oscillator circuit according to the present invention. In this figure, the same parts as those of the proximity sensor oscillation circuit in FIG. 3 are designated by the same reference numerals. In this embodiment, the detection coil 1 and the capacitor 2 are the same as in the oscillation circuit described above.
The hot end side of the resonant circuit consisting of transistors 3a, 3
It is connected to the collector terminal of one transistor 3a of the current mirror circuit 3 composed of b. Resistors R4 and R5 are connected to the emitters of the transistors 3a and 3b from the power source end as shown in the figure. And the hot end side of the detection coil 1 is a resistor
R1 is connected to the base of the first transistor 7 via the base-emitter connection pair of the transistors 5 and 6. The transistors 5 and 6 are used as constant voltage elements because the forward drop voltage between the base and the emitter is constant. The transistor 7 is an emitter follower type transistor whose collector is connected to the power supply. The transistor 7 supplies the voltage change at the emitter end to the transistor 4 via the resistor R3, as in the oscillation circuit described above.

In the present invention, a series connection body of diodes D1, D2, D3 is connected between the base and the ground as a limiter for limiting the base voltage of the second transistor 4 to a predetermined value or less. Other configurations are the same as those of the oscillation circuit described above,
The emitter terminal of the transistor 4 is connected to the collector terminal of the transistor 9a of the current mirror circuit 9.

Next, the operation of this embodiment will be described with reference to the waveform chart. FIG. 2 is a waveform diagram showing the waveform of each part of the oscillator circuit. Also in this embodiment, if an object is not close to the detection coil 1, a current flows into the resonance circuit through the resistor R6, the transistors 5 and 6, and the resistor R1, and the voltage is impedance-converted by the transistor 7 and added to the transistor 4. ,
Positive feedback is performed via the current mirror circuit 3 to start oscillation. FIG. 2A shows the oscillation voltage at the end of the detection coil 1. At this time, since the base voltage of the transistor 4 is clipped by the three diodes D1 to D3 as shown in FIG. 2 (b), it is limited to a predetermined value in a positive half cycle of the oscillation amplitude as shown in the figure. A wavy signal is obtained.
The emitter voltage of the transistor 4 is lower than this base voltage by the forward drop voltage of one diode, and its emitter current becomes the collector current of one transistor 9a of the current mirror circuit 9 and its current value is the second value.
It is limited as shown in FIG. At this time, since the emitter voltage of the transistor 4 is limited to a predetermined value, the charging current of the parasitic capacitance between the collector and the base of the transistor 9a flows for a short time as described above, but as shown in FIG. At this time, the peak current Ip2 is much smaller than the peak current Ip1 described above and can be ignored. The collector current of the transistor 9a, that is, the emitter current of the transistor 4 is positively fed back to the detection coil 1 as it is through the current mirror circuit 3. However, the peak voltage Ip2 is always kept constant regardless of the oscillation amplitude level. Therefore, the effective value of the feedback current can be kept constant, and it is possible to suppress variations in the inrush current due to variations in the oscillation amplitude, changes in the inrush current due to variations in elements, and variations in the inrush current due to temperature changes. Therefore, it is possible to facilitate the configuration of the linearization circuit for the detection output distance of the detection output, and it is possible to greatly reduce the output fluctuation, which is good for element variations and temperature changes.

Although three diodes are connected in series as a limiter in this embodiment, it goes without saying that another constant voltage element such as a constant voltage diode can be used.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a proximity sensor oscillator circuit according to the present invention, FIG. 2 is a waveform diagram showing waveforms of respective parts thereof, and FIG. 3 is an example of a proximity sensor oscillator circuit on which the present invention is based. FIG. 4 is a circuit diagram showing a waveform of each part. 1 ... Detection coil, 2 ... Capacitor, 3,9 ... Current mirror circuit, 3a, 3b, 9a, 9b, 4 to 7 ... Transistor, R1 to R
9 ... Resistance, 8 ... Detection circuit, D1-D3 ... Diode

Claims (2)

[Claims] 1. A resonance circuit including a detection coil, a current mirror circuit in which the current output end of one transistor is connected to the resonance circuit, and an emitter whose base is connected to the resonance circuit via a constant voltage element. A follower-type first transistor, a second transistor that is interrupted based on the emitter voltage of the first transistor and that feeds back current to the other transistor of the current mirror circuit when conducting, and a second transistor output terminal A proximity current circuit connected to the second transistor for limiting the feedback current of the second transistor to a constant value, and a limiter circuit for limiting the base voltage of the second transistor to a constant value. Sensor oscillator circuit. 2. The proximity sensor oscillation circuit according to claim 1, wherein the limiter circuit is three diodes connected in series to the base end of the second transistor.