JPH0865128A - Proximity switch - Google Patents

Abstract

PURPOSE: To obtain the small sized proximity switch suitable for carrying by decreasing a power supply voltage of the proximity switch than a conventional switch and eliminating the need for an externally mounted capacitor in the case of circuit integration. CONSTITUTION: A constant current from a constant current circuit 7A is supplied to a bias circuit 3, and a forward voltage drop of diodes 3A, 3B is impressed to a base of an amplifier transistor(TR) 8A as a bias voltage. A constant current from a constant current circuit 7B is supplied to the bias circuit 4 and a forward voltage drop of the diode 4A is impressed to a base of an amplifier TR 8B as a bias current. A differential amplifier circuit consists of the TRs 8A, 8B, and a constant current circuit 7C and an output voltage of the TR 8B is converted into a current by a feedback TR 16 and fed back positively to a resonance circuit 1 via a resonance circuit 2, which is oscillated. Thus, a collector current of the TR 8B is reduced and a voltage on an oscillating output line is reduced more than that of a conventional switch.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high frequency oscillation type proximity switch.

[0002]

2. Description of the Related Art FIG. 2 is a circuit diagram showing an example of a conventional proximity switch. In FIG. 2, a resonance circuit 1 including a detection coil 1A whose impedance decreases as an object to be detected approaches and a capacitor 1B connected in parallel to the detection coil 1A, and a base thereof has diodes 3A and 3B.
Via the bias circuit 3 consisting of the amplifier circuit 8A, the collector of which is connected to the positive power supply terminal P through the resistor 11 and the emitter of which is connected to the positive power supply terminal P. The base is the collector of the amplification transistor 8A, and the collector is the detection coil 1A.
In the same manner as above, connected to one terminal a of the resonance circuit 1 through a resonance circuit 2 including a detection coil 2A whose impedance decreases as the detection object approaches and a capacitor 2B connected in series to the detection coil 2A. The feedback transistor 12 that has been turned on, the constant current circuit 7A connected between the base of the amplifying transistor 8A and the positive power supply terminal P, and the collector of the feedback transistor 12 and the negative power supply terminal N in series. A voltage dividing circuit 9 composed of resistors 9A and 9B connected to the oscillation amplitude discriminating circuit 13 connected to the collector of the feedback transistor 12 and an output transistor 14 connected to the oscillation amplitude discriminating circuit 13. The emitter of the amplifying transistor 8A is connected to the connection point between the resistors 9A and 9B of the voltage dividing circuit 9, and the other terminal b of the resonance circuit 1 is connected.
Is connected to the negative power supply terminal N.

The operation of this proximity switch is as follows. An almost constant current flows into the bias circuit 3 from the constant current circuit 7, and the forward voltage drop of the diodes 3A and 3B is applied as a bias voltage to the base of the amplifying transistor 8A. The voltage of the resonance circuit 1 is amplified by the amplification transistor 8A, and the output voltage of the amplification transistor 8A is converted into a current by the feedback transistor 12 and positively fed back to the resonance circuit 1 via the resonance circuit 2 to cause oscillation.
At this time, to the base of the amplifying transistor 8A, a voltage in which the oscillation AC voltage is superimposed around the bias voltage provided by the bias circuit 3 is applied. The diode of the bias circuit 3 has two diodes 3A and 3B connected in series.
One or three or more are connected in series and used. When the object to be detected is not close to the detection coil 1A, the detection coil 1A
Has a high impedance and the voltage of the resonance circuit 1 is high. Therefore, since the voltage applied to the base of the amplifying transistor 8A is high, its output voltage is high, and the feedback transistor 1
The output voltage of 2 is also high. This output voltage is discriminated by the oscillation amplitude discrimination circuit 13 as being higher than the set voltage, and at this time, the detection signal is not output from the output transistor 14. When the object to be detected approaches the detection coil 1A, the impedance of the detection coil 1A decreases and the voltage of the resonance circuit 1 decreases.
Therefore, since the voltage applied to the base of the amplifying transistor 8A is low, its output voltage is low, and the output voltage of the feedback transistor 12 is also low. This output voltage is discriminated by the oscillation amplitude discrimination circuit 13 as being lower than the set voltage, and the output transistor 14 outputs the detection signal of the detected object.

The resonance circuit 2 in which the detection coil 2A and the capacitor 2B are connected in series is the detection coil 1 of the resonance circuit 1.
It is for temperature compensation of A and is provided as necessary. Resonant circuit 1
The detection coil 1A and the detection coil 2A of the resonance circuit 2 are configured such that conductive wires made of the same material, for example, copper are wound around the same bobbin so that the same impedance changes depending on temperature. Then, for example, when the resistance value of the detection coil 1A increases due to a temperature rise and the oscillation voltage of the resonance circuit 1 decreases, the resistance value of the detection coil 2A also increases and the oscillation voltage of the resonance circuit 2 also decreases, so that the feedback On the contrary, the amount of positive feedback from the transistor 12 is increased to compensate for the decrease in the oscillation voltage of the resonance circuit 1. When the resonance circuit 2 for temperature compensation is not provided, the collector of the feedback transistor 12 is directly connected to the base of the amplification transistor 8A.

Further, the voltage dividing circuit 9 divides the oscillation output and changes the resistance values of the resistors 9A and 9B connected in series, so that the emitter is located at the voltage dividing point, that is, the connecting point between the resistors 9A and 9B. It is possible to control the magnitude of the base current of the connected amplification transistor 8A. As a result, the base current of the amplifying transistor 8A can be controlled to adjust the detection distance.

[0006]

In the proximity switch described above, a bias voltage is applied to the base of the amplifying transistor 8A by a bias circuit 3 composed of diodes 3A and 3B as shown in FIG. The voltage is amplified, and the output voltage of the amplifying transistor 8A is converted into a current by the feedback transistor 12 and is positively fed back to the resonance circuit 1 to cause oscillation. Considering that the oscillation is stopped, the diode 3 of the bias circuit 3 is provided at the base of the amplifying transistor 8A.
A voltage 2V _{D, which} is the sum of the forward voltages V _D of A and 3B, is applied, and the voltage at the connection point between the resistors 9A and 9B of the voltage dividing circuit 9 is 2V.
_D- V _BE (where V _BE is the base-emitter voltage of the amplifying transistor 8A). Therefore, the current i ₂ flowing through the resistor 9B is given by the equation (1).

[0007]

I ₂ = V _BE / R _9B (Equation 1) where: R _9B is the resistance value V _D ≈V _BE of the resistor 9B. The collector current i ₁ of the amplifying transistor 8A is given by the equation (2) from the base-emitter voltage V _BE of the feedback transistor 12 and the resistance value R ₁₁ of the resistor ₁₁ .

[0008]

[Number 2] _{i 1 = V BE / R 11} ··· formula (2) Therefore, the voltage V _C of the oscillation output line C is as shown in equation (3).

[0009]

## EQU3 ## V _C = i ₂ · R _9B + (i ₂ −i ₁ ) · R _9A = V _BE {1 + R _9A (1 / R _9B −1 / R ₁₁ ) ... Formula (3) In order to continue the oscillation, the power supply voltage V _S applied between the positive and negative power supply terminals P and N is set to a voltage value which is at least equal to or higher than the voltage V _C of the oscillation output line C shown in Expression (3). Indicates that it is necessary.

The voltage V _C on the oscillation output line C includes the resistance value of the resistor 11 and the resistance values of the resistors 9A and 9B of the voltage dividing circuit 9. Of these, the resistance values of the resistors 9A and 9B of the voltage dividing circuit 9 are included. Is required to divide the oscillation output to set the operating distance as a proximity switch and to determine it according to the characteristics of the detection coil to be used. Therefore, in order to lower the voltage V _C of the oscillation output line C, Has a limit, and it is necessary to set the power supply voltage V _S high.

An object of the present invention is to solve the above problems and to provide a proximity switch capable of setting a low power supply voltage. Since this kind of proximity switch circuit is usually formed as a semiconductor IC, it is necessary to solve the above-mentioned problems without using a large-capacity capacitor that is externally attached when the semiconductor IC is formed. It

[0012]

In order to achieve the above-mentioned object, the proximity switch of the present invention comprises a detection coil whose impedance is lowered due to the approach of the object to be detected and a capacitor connected in parallel to this detection coil. And a base thereof connected to a differential amplifier circuit for amplifying the voltage of the resonance circuit, and a collector of which is connected to one terminal of the resonance circuit and a positive power supply terminal through a first bias circuit. A first amplification transistor and a second amplification transistor whose collector is connected to a positive power supply terminal through a resistor, a collector of which is the positive power supply terminal, and a base of which is the collector of the second amplification transistor. Has its emitter connected to one terminal of the resonance circuit, converts the output voltage of the second amplification transistor into a current, and returns to the resonance circuit. Of the feedback transistor, the other terminal of the resonant circuit and the negative power supply terminal are connected, and the first resistor and the second resistor connected in series between this connection point and the emitter of the feedback transistor. Consisting of resistance,
The voltage dividing circuit in which the base of the second amplifying transistor is connected to the connection point of the first resistor and the second resistor via the second bias circuit, and the first and second bias circuits The first and second constant current circuits for flowing a constant current and the oscillation amplitude discriminating circuit connected to the collector of the feedback transistor are used. Then, it is convenient to provide a third constant current circuit for supplying a constant current to the collector / emitter of the feedback transistor. Further, it is preferable to form the circuit of the proximity switch as a semiconductor IC.

[0013]

In the proximity switch of the present invention, the oscillation output line is
In C voltage V_CIs the power supply voltage V _S, Feedback transistor
16 base-emitter voltage V_BE, Resistance value of resistor 15
R₁₅And the current i flowing through this resistor 15₃Determined by
The resonance circuit 1 and the oscillation that determine the basic characteristics of the oscillation circuit
First and second resistors 9A of the voltage dividing circuit 9 for dividing the output
Since it has nothing to do with the resistance values of 9B and 9B, the oscillation output line
The voltage of C can be made lower than the conventional product, and the power supply voltage V
_SCan be set low.

Further, a third constant current circuit for supplying a constant current to the collector / emitter of the feedback transistor is provided to supply a constant current at which the base-emitter voltage V _{BE of the} feedback transistor is almost saturated. Since the base-emitter voltage V _BE is held at a low constant voltage value, it is convenient to set the power supply voltage V _S lower.

[0015]

1 is a circuit diagram showing an embodiment of a proximity switch according to the present invention. In FIG. 1, a resonance circuit 1 including a detection coil 1A whose impedance decreases as an object to be detected approaches and a capacitor 1B connected in parallel to the detection coil 1A, and a base of which is a resistor 5 and diodes 3A and 3B. The amplifying transistor 8A is connected to the one terminal a of the resonance circuit 1 through the bias circuit 3, the collector of which is connected to the positive power supply terminal P, and the emitter of which is connected to the negative power supply terminal N through the constant current circuit 7C. When,
The amplification transistor 8B, whose collector is connected to the positive power supply terminal P via the resistor 15, its emitter is connected to the emitter of the amplification transistor 8A, its collector is the positive power supply terminal P, and its base is the amplification transistor 8B. The emitter of the detector is connected to the resonance circuit 2 including the detection coil 2A whose impedance is lowered by the approach of the object to be detected like the detection coil 1A and the capacitor 2B connected in series to the detection coil 2A. And a resistor 9 connected in series between the collector of the feedback transistor 16 and the negative power supply terminal N.
A bias circuit 4 and a resistor including a voltage dividing circuit 9 including A and 9B, a diode 4A connected in series between a connection point of the resistors 9A and 9B of the voltage dividing circuit 9 and the base of the amplifying transistor 8B. 6, a constant current circuit 7A connected between the connection point between the resistor 5 and the bias circuit 3 and the positive power supply terminal P, and between the connection point between the resistor 6 and the bias circuit 4 and the positive power supply terminal P. Connected constant current circuit 7B, constant current circuit 7D connected between the emitter of feedback transistor 16 and negative power supply terminal N, and feedback transistor 16
Oscillation amplitude discriminating circuit 13 connected to the emitter of the output transistor 1 and the output transistor 1 connected to the oscillation amplitude discriminating circuit 13
4 and the other terminal b of the resonance circuit 1 is connected to the negative power source terminal N.

The operation of this proximity switch is as follows. A substantially constant current flows into the bias circuit 3 from the constant current circuit 7A, and the forward voltage drop of the diodes 3A and 3B is applied as a bias voltage to the base of the amplifying transistor 8A. Further, a substantially constant current made up of the constant current circuit 7B flows into the bias circuit 4, and the forward voltage drop of the diode 4A is applied as a bias current to the base of the amplifying transistor 8B. The resistors 5 and 6 are base current limiting resistors. Amplifying transistors 8A and 8B
The constant current circuit 7C constitutes a differential amplifier circuit, the voltage of the resonance circuit 1 is amplified by this differential amplifier circuit, and this output voltage, that is, the output voltage of the amplifying transistor 8B is converted into a current by the feedback transistor 16. Then, it is positively fed back to the resonance circuit 1 through the resonance circuit 2 to cause oscillation. Other operations are similar to those of the conventional proximity switch shown in FIG.

In this proximity switch, when the base voltage of the amplifying transistor 8A rises and the collector current of the amplifying transistor 8A increases, the collector current of the amplifying transistor 8A constituting the differential amplifying circuit and the amplifying transistor 8A. Since the sum of the collector currents of the transistor 8B is the constant current value i ₀ of the constant current circuit 7C, the collector current of the amplifying transistor 8B decreases. Here, the voltage V _C of the oscillation output line C is given by the equation (4) when the collector current of the amplifying transistor 8B is i ₃ .

[0018]

[Equation 4] V _C = V _S −V _BE −R ₁₅ · i ₃ (4) where V _S : power supply voltage V _BE : base-emitter voltage of the feedback transistor 16 R ₁₅ : resistor 15 The resistance value of the constant current circuit 7D is for supplying a constant current at which the base-emitter voltage of the feedback transistor 16 is almost saturated and for maintaining the base-emitter voltage at a low constant voltage value. .

In this proximity switch, the voltage V _C of the oscillation output line _C is the power supply voltage V _S , the base-emitter voltage V _{BE of} the feedback transistor 16 and the resistance value R of the resistor 15 as shown in equation (4). ₁₅ and the resistance i of the first and second resistors 9A and 9B of the voltage dividing circuit 9 that divides the oscillation output, which is determined by the current i ₃ flowing through the resistor 15 and determines the basic characteristics of the oscillation circuit. Therefore, the voltage of the oscillation output line C can be made lower than that of the conventional product, and the power supply voltage V _S can be set low. Further, by conducting a constant current base-emitter voltage V _BE of the feedback transistor 16 is nearly saturated, since the base-emitter voltage V _BE is held at a low constant voltage value, the power supply voltage V _S It is convenient to set lower.

Furthermore, this proximity switch has a circuit IC.
When it is made into a semiconductor device, the structure is suitable for a semiconductor IC because a capacitor having a relatively large capacity, for example, several tens of picofarads or more, which is externally attached, is not used except the capacitor of the resonance circuit.

[0021]

The proximity switch of the present invention is suitable for various applications including portable use because it can lower the power supply voltage than conventional products and can be miniaturized without an externally attached capacitor when made into a semiconductor IC. .

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a proximity switch of the present invention.

FIG. 2 is a circuit diagram showing an example of a conventional proximity switch.

[Explanation of symbols]

 1A Detection coil 1B Capacitor 1 Resonance circuit 3 1st bias circuit 4 2nd bias circuit 7A 1st constant current circuit 7B 2nd constant current circuit 7D 3rd constant current circuit 8A 1st amplification transistor 8B 1st 2 amplification transistor 9A 1st resistance 9B 2nd resistance 9 Voltage dividing circuit 13 Oscillation amplitude discrimination circuit 15 Resistance 16 Feedback transistor

Claims (3)

[Claims] Claim: What is claimed is: 1. A resonance circuit comprising a detection coil, the impedance of which decreases when an object to be detected approaches, and a capacitor connected in parallel with the detection coil; A first amplifying transistor whose base to be amplified is connected to one terminal of the resonance circuit via a first bias circuit, and its collector is connected to a positive power supply terminal, and its collector is connected to a positive power supply terminal via a resistor. And a collector of the second amplifying transistor connected to the positive power supply terminal, a base of the second amplifying transistor to the collector of the second amplifying transistor, and an emitter of the second amplifying transistor to one terminal of the resonant circuit. A feedback transistor that converts the output voltage of the amplification transistor into a current and feeds back positively to the resonance circuit; the other terminal of the resonance circuit and the negative power supply terminal Of a first resistor and a second resistor connected in series between this connection point and the emitter of the feedback transistor. The first resistor and the second resistor are connected at the connection point of the first resistor and the second resistor. A voltage dividing circuit in which the base of the second amplifying transistor is connected via a second bias circuit, and first and second constant current circuits for flowing a constant current into the first and second bias circuits. A proximity switch comprising an oscillation amplitude discrimination circuit connected to the collector of the feedback transistor. 2. The proximity switch according to claim 1, wherein
A proximity switch characterized in that a third constant current circuit for supplying a constant current to the collector / emitter of the feedback transistor is provided. 3. The proximity switch according to claim 1, wherein the circuit is formed as a semiconductor IC.