JPH04416B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application This invention relates to an oscillation circuit for a high frequency oscillation/loss detection type proximity switch.

(b) Prior art The oscillation circuit of a high-frequency oscillation/loss detection type proximity switch utilizes the fact that the high-frequency loss of the detection coil increases when a metal object approaches the detection coil. An oscillation circuit is constructed and the detection operation is performed by attenuating the amplitude of the oscillation circuit or stopping the oscillation due to the approach of a nearby object. The oscillation circuit of this high-frequency oscillation/loss detection type proximity switch must be able to (a) oscillate over a wide frequency range, (b) have a steep change in oscillation amplitude in response to a change in loss in the detection coil, and (c) have the detection coil have the simplest possible configuration. (d) The oscillation frequency can be easily determined; (e) The loss value of the detection coil at which the oscillation amplitude begins to attenuate or oscillation stops can also be easily varied; (f) The oscillation amplitude (Both ends of the detection coil) are required to be as large as possible.

From the viewpoint of satisfying each of these points to some extent and having a simpler circuit configuration, the circuit shown in FIG. 1 has been proposed as a circuit configuration suitable for IC implementation. To explain this circuit, the transistor
Tr ₁ is connected as an emitter follower, and its output current Ic is fed back as a feedback current _IF through a current mirror circuit made up of transistors Tr ₂ and Tr ₃ .
This feedback current I _F is supplied to a resonant circuit consisting of a detection coil L and a capacitor C. Detection coil L is 2
It is a terminal, and one end is connected to the 0V side of the power supply.
If a voltage V _T is generated in this parallel resonant circuit (tank circuit), the emitter follower circuit consisting of transistor _Tr1
A current Ic of approximately V _T /Re (Re is the value of the resistor Re connected to the emitter of the transistor Tr ₁ ) flows through the collector of Tr ₁ . A current I _F that is approximately equal to this current Ic
is the transistor Tr ₃ due to the action of the current mirror circuit.
The current flows into the collector of , and is supplied to the parallel resonant circuit.
The current I _F fed back to the parallel resonant circuit via the current mirror circuit can be adjusted by the value of the resistor Re, so the magnitude of the feedback current can be varied by the resistor Re. That is, the strength of oscillation can be varied by changing the resistance Re. Further, the oscillation frequency is determined by the resonant frequency of the parallel resonant circuit, and is determined by the inductance of the detection coil L and the capacitance of the capacitor C, if phase delays in the emitter follower circuit and the current mirror circuit are ignored.

The circuit shown in FIG. 1 is an extremely simple circuit and is particularly suitable for IC implementation as an oscillation circuit for a proximity switch. However, there are drawbacks as described below. The base linear bias of transistor Tr ₁ is low, so the relationship between base input voltage V _T and collector current Ic is not linear.Even if V _T is a sine wave, Ic has a distorted waveform, and as a result, the feedback current I _F also becomes , which naturally results in a distorted waveform. When the feedback current I _F has a distorted waveform in this way, the only component that can excite the parallel resonant circuit is the fundamental wave component, so the ability of the oscillation circuit is determined by the magnitude of the fundamental wave component I _F1 of I _F with respect to V _T. must be evaluated accordingly.
In other words, the negative conductance G _OSC of the active circuit viewed from both ends of the parallel resonant circuit is G _OSC = I _F1 /V _T = Ic ₁ /V _T (However, Ic ₁ is the collector current Ic of transistor Tr ₁ .
fundamental wave component). If the value of this negative conductance G _OSC remains constant with respect to the magnitude of the amplitude V _T , the change in the oscillation amplitude will be
It becomes steep as the loss changes. If the base DC bias of transistor Tr ₁ is inappropriate, the distortion component will increase according to the amplitude V _T and the fundamental wave component will decrease, so G _OSC will depend on the magnitude of the amplitude V _T and a steep oscillation amplitude will occur. No change can be obtained.

FIG. 2 shows data showing the results of investigating how the characteristics of G _OSC change with respect to the amplitude V _T when a direct current bias voltage Eb is applied to the amplitude V _T . When the bias voltage Eb is 0, G _OSC is extremely nonlinear, and even if the bias voltage Eb is applied to compensate for the base-emitter voltage V _BE of the transistor Tr ₁ , it is still insufficient.
It has been shown that nonlinear characteristics appear. From this figure 2, it is clear that in order to eliminate the dependence of G _OSC on the amplitude V _T and obtain a linear characteristic, at least the oscillation amplitude is
It can be seen that a bias voltage Eb equal to the total amplitude of V _T is required.

(c) Purpose This invention improves the oscillation amplitude by applying an optimal bias voltage so that the change in oscillation amplitude with respect to a change in loss in a detection coil becomes steeper, and also stabilizes the oscillation amplitude by stabilizing this bias voltage. It is an object of the present invention to provide an oscillation circuit for a proximity switch that is improved so as to be able to accurately discriminate changes in oscillation amplitude.

(D) Structure The oscillation circuit of the proximity switch according to the present invention includes: a first transistor connected as an emitter follower; a first current mirror circuit for feeding back the output current of the first transistor; and a first current mirror circuit for feeding back the output current of the first transistor. a parallel resonant circuit consisting of a detection coil and a capacitor to which a current fed back by a mirror circuit is supplied; a second current mirror circuit whose output current side is connected to the base of the first transistor; A constant current circuit connected to the reference current side of the current mirror circuit No. 2, a series circuit of a plurality of resistors, a base connected in the middle of this series circuit, and an emitter connected to one end of the series circuit. The transistor includes a second transistor, and a third transistor whose base is connected to the collector of the second transistor, whose emitter is connected to the other end of the series circuit, and whose collector is connected to the power supply. , one end is the connection point between the base of the third transistor and the collector of the second transistor, and the other end is the connection point between the emitter of the second transistor and one end of the series circuit, and both ends thereof are connected to each other. A level shift circuit for base bias of the first transistor is connected between the base of the first transistor and the first current mirror circuit connection side of the resonant circuit. It is becoming.

(E) Embodiment In Fig. 3, the transistor Tr ₁ is connected as an emitter follower, and its output current passes through a current mirror circuit consisting of transistors Tr ₂ and Tr ₃ to a parallel resonant circuit consisting of a detection coil L and a capacitor C. He has been returned. A level shift circuit for base bias is inserted between this parallel resonant circuit and the base of transistor _Tr1 . This base bias level shift circuit consists of a transistor Tr ₇ whose base is connected to the base of the transistor Tr ₁ and whose collector is connected to the power supply, and two resistors R ₁ whose one end is connected to the emitter of the transistor Tr ₇ . , _R2 , and a transistor whose collector is connected to the base of the transistor Tr ₇ and whose base is connected to the middle of the series circuit.
It is composed of a constant voltage circuit with Tr ₆ . This level shift circuit includes transistors Tr ₄ ,
Current I _B of the constant current circuit flows through a current mirror circuit consisting of Tr ₅ .

Here, the voltage generated across the resistor R ₂ is the base-emitter voltage V _BE of the transistor Tr ₆ (approximately
When the voltage exceeds 0.6V, the transistor _Tr6 becomes conductive and works to reduce the voltage between its collector and emitter, exhibiting constant voltage characteristics. This means that the DC base voltage of the transistor Tr ₁ is level-shifted by the collector-emitter voltage of the transistor Tr ₆ .

This base bias voltage can be easily adjusted using resistors R ₁ and R ₂ . If the current flowing through resistor R ₁ is I ₁ and the current flowing through resistor R ₂ is I ₂ , then the transistor
The collector-emitter voltage Vout of Tr ₆ is Vout=I ₁・R ₁ +2V _BE I ₂ =V _BE /R ₂ , and since I ₁ ≒ I ₂ can be established, Vout={2+( R ₁ /R ₂ )}V _BE , and the bias voltage can be adjusted with (R ₁ /R ₂ ). Since it can be adjusted by the resistance ratio in this way, the bias voltage can be adjusted optimally compared to bias circuits that use constant voltage elements such as diodes, where the bias voltage is fixed at a value specific to that element. This is extremely advantageous.

The reason why the current I _B is supplied by the current mirror circuit composed of transistors Tr ₄ and Tr ₅ in FIG. 3 is as follows. In order to increase the amplitude V _T of the parallel resonant circuit, the transistor Tr ₁
It is necessary to keep a wide range of dynamite cleanses. For this purpose, it is necessary to increase the base bias voltage of transistor _Tr1 . and the amplitude V _T
If there is a large change in the constant current circuit, this will have an adverse effect on the constant current circuit and cause a current drift, so a current mirror circuit was provided to eliminate this.

(F) Effect According to the present invention, a second transistor whose base is connected to the base of the first transistor and whose collector is connected to a power supply, and a plurality of transistors whose one end is connected to the emitter of the second transistor are connected. The base bias level shift circuit is configured by a constant voltage circuit having a series circuit of resistors and a third transistor whose collector is connected to the base of the second transistor and whose base is connected in the middle of the series circuit. , easy to adjust bias voltage,
It is possible to apply the optimal bias voltage,
As a result, it is possible to improve the oscillation amplitude so that the change in oscillation amplitude with respect to the loss change in the detection coil becomes steep.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a conventional example, FIG. 2 is a graph showing the relationship between negative conductance G _OSC and amplitude V _T , and FIG. 3 is a circuit diagram of an embodiment of the present invention. L...detection coil, C...capacitor that forms a parallel resonant circuit with detection coil L, Tr ₁ ...
Emitter follower connected transistor, Tr ₂ ,
Tr ₃ ... Transistors forming a current mirror circuit constituting the feedback circuit, Tr ₄ , Tr ₅ ... Transistors forming a current mirror circuit for supplying current to the level shift circuit, R ₁ , R ₂ ... Resistors, Tr ₆ , _Tr7 ...
A transistor that forms a constant voltage circuit that functions as a level shift circuit together with resistors R ₁ and R ₂ .

Claims (1)

[Claims] 1. A first transistor connected as an emitter follower; a first current mirror circuit for feeding back the output current of the first transistor; and a current mirror circuit for feeding back the output current of the first transistor; a parallel resonant circuit consisting of a detection coil and a capacitor to which a current is supplied; a second current mirror circuit whose output current side is connected to the base of the first transistor; and a reference for the second current mirror circuit. A constant current circuit connected to the current side, a series circuit of a plurality of resistors, a second transistor whose base is connected in the middle of this series circuit and whose emitter is connected to one end of the series circuit, and this transistor. a third transistor whose base is connected to the collector of the second transistor, whose emitter is connected to the other end of the series circuit, and whose collector is connected to the power supply; One end is the connection point between the base and the collector of the second transistor, the other end is the connection point between the emitter of the second transistor and one end of the series circuit, and both ends are the base of the first transistor. and a level shift circuit for base bias of the first transistor, which is connected between the resonant circuit and the first current mirror circuit connection side of the resonant circuit. .