JP3389764B2 - Low frequency amplifier circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a low-frequency amplifier circuit which is suitable for very amplify the band of the low frequency at high gain, in particular, utilized in the circuit for amplifying the output signal of the pyroelectric sensor It is what is done.

[0002]

2. Description of the Related Art FIG. 8 shows a circuit configuration of a general human body detection sensor. This human body detection sensor includes a pyroelectric sensor 3 that detects infrared rays emitted from the human body and outputs a minute voltage signal, and a low-frequency amplifier circuit 4 that amplifies a minute voltage signal output from the pyroelectric sensor. , A window comparator 5 that outputs a human body detection signal when the amplified signal fluctuates beyond a certain level. The frequency components of the output voltage from the pyroelectric sensor 3 due to the movement of the human body are concentrated in the range of about 0.1 to 10 Hz. The frequency range lower than this range shows noise components due to, for example, changes in the temperature around the sensor and very slow temperature changes in the area optically monitored by the sensor (such as when heated by sunlight). Contains a lot. Further, in the frequency range higher than the above range, originally, the frequency characteristic of the high range with respect to the infrared incident power of the pyroelectric element is poor, the signal itself is gradually attenuated, and a lot of electrical external noise is included. Therefore, the S / N ratio is poor. From the above, in order to prevent malfunction, it is necessary for the low-frequency amplifier circuit to select (filter) a frequency in the range of about 0.1 to 10 Hz for amplification. Moreover, since the output voltage from the pyroelectric sensor is about several tens of microvolts, the threshold voltage of the comparator is several hundred mV.
A gain of about 80 dB is required to amplify up to.

A low-frequency amplifier circuit satisfying the above requirements has hitherto been achieved by a circuit as shown in FIG. In the figure, IC3
IC4 is an operational amplifier, Rf1, Rf2, Rs1, Rs2
Is a resistor, Cs1 and Cs2 are large-capacity (47 μF) capacitors, and Cf1 and Cf2 are small-capacity (10 nF) capacitors. The amplification factor of the operational amplifier IC3 is Rf1 / Rs.
The cutoff frequency is determined by the time constant of the resistor Rs1 and the capacitor Cs1. The amplification factor of the second operational amplifier IC4 is determined by Rf2 / Rs2, and the cutoff frequency is determined by the time constant of the resistor Rs2 and the capacitor Cs2. In this example, Rf1 and Rf2 are 2 MΩ, Rs1,
Rs2 is set to 20 KΩ, and each operational amplifier has about 1
By obtaining the amplification factor of 00 times, it becomes about 100 as a whole.
A gain of 00 times is obtained.

[0004]

However, in the circuit of FIG. 8, since a low frequency (about 0.1 Hz) is used as the cutoff frequency, a large capacity capacitor is required.
It is Rs and C that determine the cutoff frequency in the low frequency range.
Since the time constant τs is the product of s, the time constant changes depending on the gain of the amplifier. On the contrary, in order to set the gain high, it is necessary to reduce Rs, but in order to maintain a constant time constant, a large Cs is required. From the above, when trying to obtain a high gain in one step, a very large capacitor is required, which is not realistic. Therefore, in general, in most cases, two-stage amplification as shown in FIG.
Each amplifier is designed to amplify about 40 dB, and a low-frequency cut capacitor of several tens of μF is used. For the above reasons
The conventional circuit structure inevitably requires a large-capacity capacitor, and its size and cost are problems.

Conventionally, an aluminum or tantalum electrolytic capacitor has been used as a capacitor having such a capacity, but the electrolytic capacitor has a large leakage current as compared with a film capacitor or a ceramic capacitor and is not stable. Therefore, this causes a problem in the DC stability of the circuit and adversely affects the temperature characteristics, environment resistance characteristics, and long-term reliability of the sensor, which is a problem.

The present invention has been made in view of the above points, and can be realized by a ceramic or film capacitor without using an electrolytic capacitor whose size, cost and characteristics are problems. Even if the capacitance value is used, gain / frequency characteristics equivalent to the conventional one can be realized.
An object of the present invention is to provide a low frequency amplifier circuit for amplifying an output signal of a pyroelectric sensor .

[0007]

In the low frequency amplifier circuit of the present invention, in order to solve the above problems, as shown in FIG. 1, an input signal is applied to a first differential input terminal. , A first operational amplifier IC1 having a feedback resistor Rf connected between the second differential input terminal and the output terminal, and a second operational amplifier IC2 having the first differential input terminal connected to the output terminal. , A capacitor C1 connected between the second differential input terminal of the second operational amplifier IC2 and the constant potential point, the second differential input terminal of the first operational amplifier IC1 and the second operational amplifier IC2.
Is connected between the second differential input terminal of the first operational amplifier IC1 and the second differential input terminal of the second operational amplifier IC2. Second
Of a resistor R2 possess, first of the first operational amplifier IC1
The input signal applied to the differential input terminals is the output of the pyroelectric sensor.
It is characterized by being a force signal .

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the basic structure of a low frequency amplifier circuit according to the present invention. In the figure, IC1 and IC2 are operational amplifiers, and the input impedances of the inverting input terminal (− side input terminal) and the non-inverting input terminal (+ side input terminal) thereof are extremely high, and substantially no input current flows. Also,
A voltage obtained by differentially amplifying the inputs of the inverting input terminal and the non-inverting input terminal is output to the output terminal, and the amplification factor of the differential amplification is extremely high. Therefore, the potential difference between the inverting input terminal and the non-inverting input terminal is substantially zero (so-called imaginary short). The first operational amplifier IC1 has a single amplification stage of a conventional amplifier that low-frequency amplifies an input signal applied to an input terminal and outputs the amplified signal to an output terminal. The second operational amplifier IC2 is an operational amplifier which is a feature of the present invention, and the capacitor C connected to the non-inverting input terminal of the second operational amplifier IC2.
1 has the effect of apparently amplifying. Now, when looking at the output of the second operational amplifier IC2 from the inverting input terminal of the first operational amplifier IC1 (point "A"), the equivalent capacitance is C
When 0 is set, C0 = C1 × R2 / R1 ... This is because the operational amplifier IC2 is subjected to DC feedback, and the output terminal and the non-inverting input terminal of the operational amplifier IC2 are constantly at the same potential (imaginary short). Therefore, the current flowing from the point "A" into the resistors R1 and R2 is always the ratio of the reciprocal of the resistance value. By the way, as shown in FIG. 2, when the current i is injected at a constant current, the capacitance of the capacitor C1 is the voltage V at the time t.
Is as shown in FIG. 3, and is calculated by the following equation. C = i × t / V ...

Since the current Ic1 flowing into the capacitor C1 in FIG. 1 is expressed as Ic1 = Ia × R1 / R2 ... Using the current Ia flowing from the “A” point, the equivalent capacitance seen from the “A” point. C0 becomes C0 = (R2 × Ic1 × t) / (R1 × V) ... And (Ic1 × t) / V on the right side represents C1, so the formula is obtained. Therefore, the equivalent capacitance C0 viewed from the point "A" is R2 / R1 times the capacitance C1.

Further, the DC impedance seen from the point "A" is because the output impedance of the operational amplifier IC2 is very small (one of the open gain of the non-feedback output resistance).
It is equal to R1. Therefore, the circuit of FIG. 1 works equivalently like the circuit of FIG. Therefore, the basic gain Av of this circuit is given by Av = (Rf / R1) +1 ... And the time constant τh in the high range and the time constant τl in the low range are
.Tau.h = Rf.times.Cf ... .tau.l = R1.times.C0 ..

[0011]

FIG. 5 shows a first embodiment of the present invention.
The circuit shown in this figure is designed so as to achieve a gain of 10001 times that of the conventional two-stage amplifier circuit shown in FIG. First, the gain Av is given by the equation, and Rf = 2M
In the case of Ω, R1 = 2MΩ / (10001-1) = 200Ω. Also, the capacitance C1 that determines the time constant of the low frequency range
Is calculated by C1 = (20 KΩ × 47 μF) / (200 Ω × 10000) = 0.47 μF. Cf based on the time constant τh in the high frequency range is the same as the conventional one, and Cf = 10 nF. Also, R2 = R3 =
It is set to 2 MΩ. By adopting the above circuit configuration and circuit constants, the gain and cutoff frequency similar to the conventional one can be obtained by one-stage amplification. However, regarding the frequency characteristic, the cut-off frequency can be designed in the same manner as that of the conventional one step, but since the slope has a one-step configuration, 6d
It becomes B / oct.

A second embodiment of the present invention is shown in FIG. Each circuit constant is the same as that of the embodiment of FIG. 5, but in this embodiment, the resistance R1 of 200Ω is the variable resistance VR1. In the figure, consider the change in overall gain with respect to the change in R1 (= VR1). Now, the low time constant τ
l is calculated from the equation and the following equation: τl = R1 × (R2 / R1) × C1 = R2 × C1. That is, the resistance R1 that influences the overall gain
It turns out that is not affected by. This is because the equivalent capacitance C0 becomes 1/2 even if R1 is doubled. This means that as shown in the embodiment of FIG.
This means that even if the gain is changed by using 1, the entire frequency characteristic does not change, which is a great advantage that the conventional example does not have. This is because, in the case of the conventional circuit configuration as shown in FIG. 8, when Rs1 is changed to change the gain, the time constant τl in the low frequency band changes, so that the capacitor Cs1 is also kept to keep the time constant constant. Because I had to change it at the same time. In other words, even if the resistance Rs1 is a variable resistance in an attempt to adjust the gain, if the value of the capacitor is fixed, the frequency characteristic changes at the same time as the gain, making it difficult to perform the optimum adjustment. Compared with this, in the present invention, only the gain can be purely adjusted without changing the frequency characteristic.

FIG. 7 is a circuit diagram of the third embodiment of the present invention. In the present embodiment, an example in which the low frequency amplifier circuit of the present invention is used for the human body detection sensor is shown. As shown in FIG. 9, the pyroelectric sensor 3 receives infrared rays from the human body M through the infrared lens L and outputs an electric signal generated in the pyroelectric element 1 to the FET.
2 is output via. Rg is the input resistance, Rs
Is an output resistance, and R4 is a power supply resistance. The output of the pyroelectric sensor 3 is amplified by the low frequency amplifier circuit of the present invention, and the amplified signal is compared with the threshold value by the voltage comparators IC5, IC6 of the window comparator 5. Resistance R
5 to R7 are voltage dividing resistors for setting threshold values for voltage comparison. The resistor R8 is a voltage comparator IC
5, output resistor for pulling up the open collector output of IC6. When the output of the pyroelectric sensor 3 fluctuates in the range of 0.1 to 10 Hz due to the movement of the human body M, the frequency component is selectively amplified and the output of the window comparator 5 is inverted. To adjust the sensitivity of the human body detection sensor, the variable resistor VR1 may be adjusted, but at that time, since the frequency characteristic of the low frequency amplification circuit does not change,
It is very convenient.

[0014]

According to the present invention, the following effects can be obtained. (A) The capacitor that determines the low-frequency time constant of the low-frequency amplifier circuit can be realized by using the conventional capacity of 1 / 10,000 to 1 / 10,000. Therefore, the low-frequency amplifier circuit can be configured with ceramics, film capacitors, etc. without using an electrolytic capacitor. As a result, the output signal of the pyroelectric sensor
It is possible to reduce the size and cost of the low-frequency amplifier circuit for amplifying the . In addition, if ceramic or film capacitors are used, the leakage current, which is a problem with electrolytic capacitors, is much reduced, and the DC stability of the circuit, temperature characteristics and environmental resistance characteristics when used as sensors, long-term reliability Will be good. (B) Since it is possible to realize a capacitor having a size that is difficult to obtain with ordinary small-sized components, it is possible to realize a low-frequency amplifier circuit with a large gain in one stage. Therefore, when the whole circuit is integrated into an IC, a large number of external parts and a large number of pins pose problems, which is particularly effective. In addition, by reducing the number of peripheral parts, a low-frequency boost for amplifying the output signal of the pyroelectric sensor
The width circuit can be downsized and the cost can be reduced. (C) The gain of the amplifier can be controlled by changing only one resistance value without changing the frequency characteristic of the low frequency amplifier circuit at all. Therefore, in order to adjust the gain, as compared with the conventional example in which the combination of the resistor and the capacitor is changed, the output signal of the pyroelectric sensor is amplified.
The low-frequency amplifier circuit can be adjusted easily, accurately and promptly.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a basic configuration of the present invention.

FIG. 2 is a charging circuit diagram of a capacitor for explaining the operation of the present invention.

FIG. 3 is a charge characteristic diagram of a capacitor for explaining the operation of the present invention.

FIG. 4 is an equivalent circuit diagram for explaining the operation of the circuit shown in FIG.

FIG. 5 is a circuit diagram of a first embodiment of the present invention.

FIG. 6 is a circuit diagram of a second embodiment of the present invention.

FIG. 7 is a circuit diagram of a third embodiment of the present invention.

FIG. 8 is a circuit diagram of a human body detection sensor using a conventional low frequency amplifier circuit.

FIG. 9 is a diagram showing a usage state of a human body detection sensor using a conventional low frequency amplifier circuit.

[Explanation of symbols]

IC1 operational amplifier IC2 operational amplifier R1 first resistance R2 second resistance Rf feedback resistance C1 capacitor

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ identification code FI H03H 11/04 H03H 11/04 J (58) Fields investigated (Int.Cl. ⁷ , DB name) H03F 3/181 G01J 1 / 02 G01J 1/44 G01J 5/02 H03F 3/45 H03H 11/04

Claims (3)

(57) [Claims] 1. A first operational amplifier having an input signal applied to a first differential input terminal and a feedback resistor connected between a second differential input terminal and an output terminal, and a first differential amplifier. A second operational amplifier having an input terminal connected to an output terminal, a capacitor connected between the second differential input terminal of the second operational amplifier and the constant potential point, and a second operational amplifier of the first operational amplifier. A first resistor connected between the differential input terminal of the first operational amplifier and the output terminal of the second operational amplifier, and a second difference between the second differential input terminal of the first operational amplifier and the second operational amplifier. a second resistor connected between the dynamic input terminals possess, input applied to the first differential input terminal of the first operational amplifier
A low-frequency amplifier circuit characterized in that the force signal is the output signal of the pyroelectric sensor . 2. The low frequency amplifier circuit according to claim 1, wherein the first resistor is a variable resistor. 3. A first differential input terminal of a second operational amplifier.
The low frequency amplifier circuit according to claim 1 or 2, wherein the child and the output terminal are connected via a third resistor .