JP2015082718A - Low-frequency amplifier circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-frequency amplifier circuit that is adaptable to a frequency of a reflected wave from an object and has dynamic range adaptable to a short distance to a long distance.SOLUTION: A low-frequency amplifier circuit includes: an amplifier element 3 amplifying a detection signal Vdet; an attenuation circuit 20 attenuating an output signal from the amplifier element 3; a control circuit 30 controlling an attenuation amount of the attenuation circuit 20; a first negative feedback circuit 40 connected between input and output terminals of the amplifier circuit 3; and a second negative feedback circuit 50 connected between an output terminal of the attenuation circuit 20 and the input terminal of the amplifier circuit 3. A capacitor is connected to one of the first negative feedback circuit 40 and the second negative feedback circuit 50, and a resistor is connected to the other of them. The attenuation circuit 20 has a first field-effect transistor 1. The control circuit 30 generates a control signal Vcnt having a control voltage value changed by a voltage value of the detection signal Vdet, and controls the attenuation amount of the attenuation circuit 20 by applying the control signal Vcnt to a gate G1 of the first field-effect transistor 1.

Description

  The present invention relates to a low-frequency amplifier circuit, and more particularly to a low-frequency amplifier circuit used in a wireless sensor device for detecting a movement of an object or a distance to the object using radio waves.

  Conventionally, a high-frequency signal output from an oscillation circuit is supplied to an antenna to radiate a radio wave, a reflected wave from an object is received, and the received signal is analyzed to detect the movement of the object or the distance to the object. There is a wireless sensor device. This is based on the principle that the frequency of the reflected wave slightly shifts from the frequency of the radiated radio wave when the object is irradiated with the radio wave, and the movement or speed of the object can be measured from the magnitude of the deviation. Further, the distance from the delay amount (phase change) of the reflected wave to the object can be measured. Such a wireless sensor device is provided with a detection circuit for outputting a voltage corresponding to the level of the received signal. In addition, a low-frequency amplifier circuit is provided after the detection circuit, and a reception signal amplified by the low-frequency amplifier circuit is analyzed by a signal processing circuit provided at the subsequent stage.

  As such a wireless sensor device, the invention described in Patent Document 1 is disclosed. A wireless sensor device 900 described in Patent Document 1 is shown in FIG.

  The wireless sensor device 900 generates a high-frequency transmission signal that is frequency-spread so that the transmission frequency continuously increases and decreases in a predetermined cycle and radiates it from the antenna 915, and also reflects the reflected wave from the object with the antenna 915. Receive. When the high frequency transmission signal and the reflected wave are input to the mixer circuit 916, the mixer circuit 916 operates as a phase detector when both frequencies coincide with each other, outputs a DC beat signal, and the DC beat signal is extracted by the low-pass filter 917. To do. Thereafter, the DC beat signal is amplified by the low-frequency amplifier circuit 918, and the signal processing circuit 919 analyzes the fluctuation of the DC beat signal to detect the movement of the object.

JP 2011-058836 A

  However, the low-frequency amplifier circuit 918 in the wireless sensor device 900 described in Patent Document 1 has the following problems. The distance from the object (mainly the human body) is assumed from a short distance to a long distance. A change occurs in the detection signal obtained by the mixer circuit 916 when the distance to the object changes. However, at a short distance, a signal having a voltage value exceeding the allowable input voltage range of the signal processing circuit 919 is input to the signal processing circuit 919 due to the high output voltage from the mixer circuit 916 and the amplification by the low frequency amplification circuit 918. There is a possibility. Therefore, there is a problem that signal processing with high accuracy becomes difficult. Further, at a long distance, the low frequency amplifier circuit 918 needs to have a high degree of amplification in order to increase the detection sensitivity for the object. That is, a dynamic range is required to cope with a short distance to a long distance. However, since the amplification degree of the low-frequency amplifier circuit 918 is fixed, it is difficult to obtain such a dynamic range. In addition, since the reflected wave from the object has different frequencies when the object to be detected is different, the frequency characteristic corresponding to the amplification characteristic is required, but the low frequency amplifier circuit 918 can cope with it. There wasn't.

  The present invention has been made in view of such a state of the art, and the object thereof is to cope with the frequency of a reflected wave from an object, and has a dynamic range capable of dealing from a short distance to a long distance. The object is to provide a low-frequency amplifier circuit.

  In order to solve this problem, a low-frequency amplifier circuit according to the present invention includes an amplification element that amplifies a detection signal output from a detection circuit, an attenuation circuit that attenuates an output signal from the amplification element, and the attenuation circuit. A control circuit for controlling the amount of attenuation, and a first negative feedback circuit is connected between the output terminal of the amplification element and the input terminal of the amplification element, A second negative feedback circuit is connected between the output terminal of the attenuation circuit and the input terminal of the amplifying element, and a capacitor is connected to one of the first negative feedback circuit or the second negative feedback circuit. And the other end is connected to a resistor, and the attenuation circuit includes a first field effect transistor having a drain connected to the output terminal of the amplifying element, and is connected to the source of the first field effect transistor. Frequency amplification circuit output The control circuit receives the detection signal, generates a control signal whose control voltage value changes according to the voltage value of the detection signal, and outputs the control signal to the first signal. The attenuation of the attenuation circuit is controlled by applying to the gate of the field effect transistor.

  The low-frequency amplifier circuit configured as described above has a first negative feedback circuit and a second negative feedback circuit, and a capacitor is connected to one of the first negative feedback circuit and the second negative feedback circuit. At the same time, since a resistor is connected to the other, it is possible to provide frequency characteristics corresponding to the frequency of the reflected wave from the object by combining the first negative feedback circuit and the second negative feedback circuit. . In addition, since the attenuation circuit includes the first field effect transistor, the attenuation amount of the amplified detection signal can be varied using the variable resistance action of the first field effect transistor. It is possible to easily provide a low-frequency amplifier circuit having a dynamic range that can handle distances.

  In the low frequency amplifier circuit of the present invention having the above-described configuration, a first resistor is connected between a drain and a source of the first field effect transistor in the attenuation circuit, and the first field effect transistor is provided. The second resistor is connected between the source and the ground.

  In the low-frequency amplifier circuit configured as described above, since the first resistor and the second resistor are connected to the first field effect transistor, the attenuation amount due to the variable resistance action of the first field effect transistor is set to the first resistor and the second resistor. It becomes possible to adjust by 2 resistance.

  In the low frequency amplifier circuit of the present invention having the above-described configuration, the control circuit includes a second field effect transistor having a gate connected to an output terminal of the detection circuit, and the drain of the second field effect transistor is A third resistor is connected between the power supply terminal and a connection point between the gate of the first field effect transistor and the drain of the second field effect transistor. The source of the second field effect transistor is connected to the ground.

  In the low-frequency amplifier circuit configured as described above, the control circuit includes the second field effect transistor, and the voltage value of the control signal can be varied using the variable resistance action of the second field effect transistor. Therefore, the attenuation circuit can be easily controlled.

  Further, the low-frequency amplifier circuit of the present invention having the above-described configuration is characterized in that the first negative feedback circuit is configured by only a resistor, and the second negative feedback circuit is configured by only a capacitor.

  In the low-frequency amplifier circuit configured in this way, since the second negative feedback circuit connected between the output terminal of the attenuation circuit and the input terminal of the amplifier element is configured only by the capacitor, the circuit from the amplifier element to the attenuation circuit is Of the frequencies handled by the entire low-frequency amplifier circuit, the gain in the high frequency region is particularly increased. Accordingly, it is possible to improve the frequency characteristics on the high frequency side of the low frequency amplifier circuit.

  Further, the low-frequency amplifier circuit of the present invention having the above-described configuration is characterized in that the first negative feedback circuit is composed of only a capacitor, and the second negative feedback circuit is composed of only a resistor.

  In the low-frequency amplifier circuit configured as described above, the second negative feedback circuit connected between the output terminal of the attenuation circuit and the input terminal of the amplifier element is configured only by a resistor. Of the frequencies handled by the entire low-frequency amplifier circuit, the gain in the low-frequency region is particularly increased. Therefore, it is possible to improve the frequency characteristics of the low frequency side of the low frequency amplifier circuit.

  The low-frequency amplifier circuit of the present invention includes a first negative feedback circuit and a second negative feedback circuit, and a capacitor is connected to one of the first negative feedback circuit and the second negative feedback circuit, and the other Since a resistor is connected to the, a combination of the first negative feedback circuit and the second negative feedback circuit can provide frequency characteristics corresponding to the frequency of the reflected wave from the object. In addition, since the attenuation circuit includes the first field effect transistor, the attenuation amount of the amplified detection signal can be varied using the variable resistance action of the first field effect transistor. It is possible to easily provide a low-frequency amplifier circuit having a dynamic range that can handle distances.

It is a block diagram which shows the structure of the radio | wireless sensor apparatus which has the low frequency amplifier circuit which concerns on embodiment of this invention. It is a circuit diagram which shows the structure of the low frequency amplifier circuit which concerns on embodiment. It is an equivalent circuit diagram which shows the principle of the low frequency amplifier circuit which concerns on embodiment. It is an equivalent circuit diagram which shows the principle of the low frequency amplifier circuit which concerns on embodiment. It is an equivalent circuit diagram which shows the principle of the low frequency amplifier circuit which concerns on embodiment. It is a graph which shows the relationship between a detection signal and a control signal in the low frequency amplifier circuit which concerns on embodiment. It is a circuit diagram which shows the structure of the 1st modification of the low frequency amplifier circuit which concerns on embodiment. It is a graph which shows the frequency characteristic of the low frequency amplifier circuit shown in FIG. It is a circuit diagram which shows the structure of the 2nd modification of the low frequency amplifier circuit which concerns on embodiment. 10 is a graph showing frequency characteristics of the low-frequency amplifier circuit shown in FIG. 9. It is a circuit diagram which shows the structure of the 3rd modification of the low frequency amplifier circuit which concerns on embodiment. It is a graph which shows the frequency characteristic of the low frequency amplifier circuit shown in FIG. It is a block diagram which shows the structure of the wireless sensor apparatus which concerns on a prior art example.

[Embodiment]
Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a block diagram showing a configuration of a wireless sensor device 150 having a low-frequency amplifier circuit 100 according to an embodiment of the present invention. The wireless sensor device 150 includes a transmission signal generation circuit 60, a transmission / reception antenna 90, a detection circuit 70, a low-frequency amplifier circuit 100 according to an embodiment of the present invention, and a signal processing circuit 80.

  The wireless sensor device 150 is a safety monitoring device (not shown) that detects, for example, an abnormality in a daily life state or a health state of a person who lives alone, such as an elderly person living alone, and monitors a comprehensive physical state. ) And the like. Referring to FIG. 1, a high-frequency signal output from the transmission signal generation circuit 60 is supplied to the transmission / reception antenna 90 to radiate a radio wave, and then a reflected wave from the object 91 is received, and the received signal is analyzed. The movement of the object 91 or the distance to the object 91 is detected. The movement of the object 91 includes not only an operation when a person such as an elderly person moves in the house but also biological information such as a person's breathing and heartbeat. The frequency of motion due to the heartbeat is a very low frequency of about 1 to 5 Hz, and the frequency of motion due to breathing is a lower frequency of about 0.2 to 0.5 Hz. The present invention provides a low-frequency amplifier circuit 100 that handles a low frequency of about 0.2 to 5 Hz.

  The transmission signal generation circuit 60 generates a high-frequency transmission signal that is frequency-spread by continuously changing the frequency at a predetermined period (for example, 2 μs) within a frequency range centered on a high frequency (for example, 2.45 GHz). . As shown in FIG. 1, a transmission / reception antenna 90 is connected to the transmission signal generation circuit 60, and the high-frequency transmission signal transmitted from the transmission signal generation circuit 60 is radiated from the transmission / reception antenna 90 to the atmosphere. In the present embodiment, the transmission / reception antenna 90 shares the transmission antenna and the reception antenna, but the transmission antenna and the reception antenna may be provided separately. The radiated high-frequency transmission signal reaches the object 91 and is reflected there, and then the reflected wave from the object 91 is received by the transmission / reception antenna 90. A high-frequency transmission signal transmitted from the transmission signal generation circuit 60 and a reflected wave from the object 91 received by the transmission / reception antenna 90 are input to the detection circuit 70.

  A low pass filter (not shown) is connected to the output stage of the detection circuit 70. The low-pass filter is made to function as a DC extraction circuit by setting passband characteristics so as to extract a low-frequency component (for example, 100 Hz or less). While the frequency of the high-frequency transmission signal and the reflected wave is different, the detection circuit 70 converts the frequency and outputs signals of various frequencies (high frequency and intermediate frequency), but these signals are suppressed by a low-pass filter. . When the frequency of the high-frequency transmission signal and the reflected wave match, the DC beat output from the detection circuit 70 is extracted by a low-pass filter.

  The low-frequency amplifier circuit 100 according to the embodiment of the present invention is connected to the subsequent stage of the detection circuit 70. The low-frequency amplifier circuit 100 amplifies the DC beat extracted by the low-pass filter to a voltage value that can be processed by a subsequent circuit. The signal processing circuit 80 in the subsequent stage includes a CPU, a memory, a program executed by the CPU, and the like. The low-frequency component (including the DC component) output from the detection circuit 70 is analyzed, and the movement of the target 91 and the target A function for detecting the distance to the object 91 is provided. The present invention relates to the low-frequency amplifier circuit 100 in the wireless sensor device 150.

  Next, the configuration of the low-frequency amplifier circuit 100 will be described with reference to FIG.

  FIG. 2 is a circuit diagram showing a configuration of the low-frequency amplifier circuit 100 according to the embodiment of the present invention. The low-frequency amplifier circuit 100 includes an input terminal 51, an amplifier 10, an attenuation circuit 20, a control circuit 30, a first negative feedback circuit 40, a second negative feedback circuit 50, and an output terminal 52. Has been. The input terminal 51 is connected to the output terminal of the detection circuit 70, and the input terminal 51 is connected to the input terminal of the amplifying unit 10. The output terminal of the amplifying unit 10 is connected to the input terminal of the attenuation circuit 20. The ends are connected. The input terminal 51 is also connected to the input terminal of the control circuit 30, and the control circuit 30 is also connected to the attenuation circuit 20. A first negative feedback circuit 40 is connected between the input and output terminals of the amplifying unit 10, and a second negative feedback circuit 50 is connected between the output terminal of the attenuating circuit 20 and the input terminal of the amplifying unit 10. Is connected.

  The amplifying unit 10 includes a series connection circuit of a coupling resistor 17 and a capacitor 18 connected to an input terminal 51, and an amplification element 3 connected in series to the series connection circuit. The capacitor 18 also has a DC cut function. An operational amplifier is used as the amplifying element 3 and amplifies the detection signal Vdet output from the detection circuit 70.

  The attenuation circuit 20 is configured to attenuate the output signal from the amplification element 3. The attenuation circuit 20 includes a first field effect transistor 1 (hereinafter referred to as FET 1 (FET: Field Effect Transistor)) whose drain D1 is connected to the output terminal of the amplifying element 3, and is connected from the source S1 of the FET 1 to the output terminal 52. The low frequency amplifier circuit output signal Vout is output. In the attenuation circuit 20, a first resistor 11 is connected between the drain D1 and the source S1 of the FET1, and a second resistor 12 is connected between the source S1 of the FET1 and the ground GND. The first resistor 11 has a resistance value R11, and the second resistor 12 has a resistance value R12. Further, the FET 1 has a drain-source on-resistance having a variable resistance value R1 when the FET 1 is turned on.

  The control circuit 30 includes a fourth resistor 14 having one end connected to the input terminal 51, a fifth resistor 15 connected between the other end of the fourth resistor 14 and the ground GND, a fourth resistor 14 and a fifth resistor. And a second field effect transistor 2 (hereinafter abbreviated as FET2) having a gate G2 connected to a connection point with the resistor 15. The drain D2 of the FET2 is connected to the gate G1 of the FET1, and the source S2 of the FET2 is connected to the ground GND. The third resistor 13 is connected between the connection point between the gate G 1 of the FET 1 and the drain D 2 of the FET 2 and the power supply terminal 53. The third resistor 13 has a resistance value R13, and the FET 2 has a drain-source on-resistance having a variable resistance value R2 when the FET 2 is turned on.

  The first negative feedback circuit 40 connected between the output terminal of the amplifying element 3 and the input terminal of the amplifying element 3, and is connected between the output terminal of the attenuating circuit 20 and the input terminal of the amplifying element 3. A capacitor is connected to one of the second negative feedback circuits 50, and a resistor is connected to the other.

  In the low frequency amplifier circuit 100 according to the present invention, the detection signal Vdet from the detection circuit 70 is input to the amplification element 3 in the amplification unit 10, and the detection signal Vdet from the detection circuit 70 is also input to the control circuit 30. The The control circuit 30 generates a control signal Vcnt whose control voltage value changes depending on the voltage value of the detection signal Vdet, and applies the control signal Vcnt to the gate G1 of the FET 1 of the attenuation circuit 20, thereby reducing the attenuation amount of the attenuation circuit 20. Is configured to control.

  Next, the operation and principle of signal attenuation by the attenuation circuit 20 and the control circuit 30 in the low-frequency amplifier circuit 100 will be described with reference to FIGS. 3, 4, and 5 are equivalent circuit diagrams illustrating the principle of the low-frequency amplifier circuit 100 according to the embodiment. FIG. 6 is a graph showing the relationship between the detection signal Vdet and the control signal Vcnt in the low-frequency amplifier circuit 100 according to the embodiment.

  As shown in FIG. 3, the FET 1 of the attenuation circuit 20 and the FET 2 of the control circuit 30 can be expressed as a first variable resistor VR1 (hereinafter abbreviated as VR1) and a second variable resistor VR2 (hereinafter abbreviated as VR2), respectively. As shown in FIG. 2 and FIG. 3, the FET 1 as VR1 has a drain-source-to-source voltage depending on the voltage difference between the control signal Vcnt applied to the gate G1 and the source S1, that is, the gate-source voltage Vgs1. The resistance value R1 of the on-resistance is varied. When the gate-source voltage Vgs1 is large, a large current flows through the FET1, so the resistance value R1 is small. When the gate-source voltage Vgs1 is small, a small current flows through the FET1, and the resistance value R1 is large. Similarly, the resistance value R2 of the drain-source on-resistance of the FET2 as VR2 is varied by the voltage between the gate G2 and the source S2, that is, the voltage value of the gate-source voltage Vgs2. When the gate-source voltage Vgs2 is large, a large current flows through the FET2, so the resistance value R2 is small. When the gate-source voltage Vgs2 is small, a small current flows through the FET2, and the resistance value R2 is large. The output voltage from the drain D2 of the FET2 is a control signal Vcnt output from the control circuit 30, and is used as a voltage for controlling the FET1. The control voltage value of the control signal Vcnt is obtained by dividing the power supply voltage Vcc supplied to the power supply terminal 53 by the resistance value R13 of the third resistor 13 and the resistance value R2 of the drain-source on-resistance of the FET2. Voltage.

  FIG. 3 shows a case where the detection signal Vdet from the detection circuit 70 has a minimum value as the first mode. Since the detection signal Vdet is small, the voltage value obtained by dividing the detection signal Vdet by the fourth resistor 14 and the fifth resistor 15 is also small, and the gate-source voltage Vgs2 is small. As a result, the resistance value R2 becomes the largest and becomes the resistance value R2max, and the control signal Vcnt from the drain D2 of the FET2 becomes the largest. Accordingly, the gate-source voltage Vgs1 increases and a large current flows through the FET1, so the resistance value R1 decreases and becomes the resistance value R1min. The resistance value R1min is approximately 0Ω.

  As described above, the first resistor 11 is connected between the drain D1 and the source S1 of the FET 1. However, since the resistance value R1 of the drain-source on-resistance is approximately 0Ω, Regardless of the resistance value, the resistance value between the amplifying element 3 and the output terminal 52 is approximately 0Ω. Accordingly, the attenuation amount in the attenuation circuit 20 is the minimum 0 dB. Therefore, the output voltage of the amplifying element 3 appears as it is as the low-frequency amplifier circuit output signal Vout.

  Next, FIG. 4 shows a case where the detection signal Vdet from the detection circuit 70 is an intermediate value as the second mode. Since the detection signal Vdet is an intermediate value, the voltage value obtained by dividing the detection signal Vdet by the fourth resistor 14 and the fifth resistor 15 is also an intermediate value, and the gate-source voltage Vgs2 of the FET 2 is also an intermediate value. As a result, the resistance value R2 of the drain-source on-resistance becomes a resistance value R2mid having a certain intermediate value, and the control signal Vcnt also takes an intermediate value. Since the control signal Vcnt is an intermediate value, the resistance value R1 also has a resistance value R1mid having a certain intermediate value. As described above, the first resistor 11 having the resistance value R11 is connected between the drain D1 and the source S1 of the FET1. Accordingly, the resistance value between the output terminal of the amplifying element 3 and the output terminal 52 is a parallel connection resistance value of the resistance value R1mid and the resistance value R11, that is, the resistance value (R1mid // R11). Therefore, a voltage value obtained by dividing the output voltage of the amplifying element 3 by the resistance value (R1mid // R11) and the resistance value R12 of the second resistor 12 appears as the low-frequency amplifier circuit output signal Vout. Accordingly, the attenuation amount is an intermediate value. As the low-frequency amplifier output signal Vout, the output voltage of the amplifier 3 is attenuated by this attenuation amount and output to the output terminal 52.

  Next, FIG. 5 shows a case where the detection signal Vdet from the detection circuit 70 has the maximum value as the third mode. Since the detection signal Vdet is the maximum value, the voltage value obtained by dividing the detection signal Vdet by the fourth resistor 14 and the fifth resistor 15 is also the maximum value, and the gate-source voltage Vgs2 is also the maximum value. As a result, since a large current flows through the FET 2, the resistance value R2 becomes small and becomes the resistance value R2min. The resistance value R2min is approximately 0Ω. Since the resistance value R2 is approximately 0Ω and the control signal Vcnt is also 0V, the resistance value R1 is the maximum resistance value R1max. As described above, the first resistor 11 having the resistance value R11 is connected between the drain D1 and the source S1 of the FET1. Therefore, the resistance value between the amplifying element 3 and the output terminal 52 is a parallel connection resistance value of the resistance value R1max and the resistance value R11, that is, the resistance value (R1max // R11). (Actually, R1max is very large and thus becomes almost R11.) Therefore, as the low-frequency amplifier circuit output signal Vout, the output voltage of the amplifying element 3 is set to the resistance value (R1max // R11) and the resistance of the second resistor 12. A voltage value divided by the value R12 appears. Therefore, the amount of attenuation is maximized.

  FIG. 6 is a graph showing the relationship between the detection signal Vdet and the control signal Vcnt from the first mode to the third mode described above. Here, when the detection signal Vdet is small, the level of the reflected wave from the object 91 received by the transmission / reception antenna 90 shown in FIG. 1 is small, and when the detection signal Vdet is large, the level of the reflected wave from the object 91 is Means big. Further, when the control signal Vcnt is small, the attenuation amount in the attenuation circuit 20 shown in FIG. 2 is large, and when the detection signal Vdet is large, it means that the attenuation amount in the attenuation circuit 20 is small. In the first mode, it can be seen that the control signal Vcnt is the maximum value (2.5 V) when the detection signal Vdet is the minimum value (0.7 V). At this time, the attenuation amount in the attenuation circuit 20 is minimized. Therefore, the gain of the low frequency amplifier circuit 100 is maximized. In the second mode, the detection signal Vdet is a value from the minimum value to the maximum value (0.7 V to 1.0 V), and the control signal Vcnt is a value from the maximum value to the minimum value (2.5 V to 0 V). I understand that. At this time, the attenuation amount in the attenuation circuit 20 becomes an intermediate value. Accordingly, the gain of the low-frequency amplifier circuit 100 is an intermediate value. In the third mode, it can be seen that the control signal Vcnt is the minimum value (0V) when the detection signal Vdet is the maximum value (1.0V). At this time, the attenuation amount in the attenuation circuit 20 is maximized. Therefore, the gain of the low frequency amplifier circuit 100 is minimized.

  Next, a change in the frequency characteristic of the gain in the low-frequency amplifier circuit 100 due to the capacitor or resistor in the first negative feedback circuit 40 or the second negative feedback circuit 50 will be described with reference to FIGS.

  FIG. 7 is a circuit diagram showing a configuration of a low-frequency amplifier circuit 200 as a first modification, and FIG. 8 is a graph showing frequency characteristics of gain of the low-frequency amplifier circuit 200. FIG. 9 is a circuit diagram showing a configuration of a low-frequency amplifier circuit 300 that is a second modification, and FIG. 10 is a graph showing frequency characteristics of gain of the low-frequency amplifier circuit 300. FIG. 11 is a circuit diagram illustrating a configuration of a low-frequency amplifier circuit 400 according to a third modification, and FIG. 12 is a graph illustrating a frequency characteristic of a gain of the low-frequency amplifier circuit 400.

  8, 10, and 12, the control signal Vcnt of each low frequency amplifier circuit in the second mode (intermediate region) shown in FIG. 6 is changed from 1.6 V to 2.0 V. The frequency characteristics in the case are shown. (In the region where the control signal Vcnt is 1.6V or less, or 2.0V or more, the difference is small compared to when the control signal Vcnt is 1.6V or 2.0V. In addition, in the first to third modifications of the low-frequency amplifier circuit according to the embodiment, the maximum gain when the control signal Vcnt is 1.6 V is substantially the same (about 84 dB). Thus, the resistance value R11 of the first resistor 11 and the resistance value R12 of the second resistor 12 are appropriately selected.

  In the first modification, as shown in FIG. 7, the first negative feedback circuit 40 is configured by a parallel connection circuit of a negative feedback resistor 21 and a negative feedback capacitor 22. Further, the second negative feedback circuit 50 is configured by only the negative feedback resistor 23. The negative feedback resistor 21 is generally used when negative feedback is applied to the entire frequency to be handled. The negative feedback capacitor 22 is used to improve the frequency characteristic of the amplification element 3 alone, and has an effect of flattening the characteristic at a particularly high frequency among the frequencies amplified by the amplification element 3. However, since the negative feedback by the negative feedback capacitor 22 does not work for the attenuation circuit 20, the low frequency amplifier circuit 100 as a whole may not improve the frequency characteristics at a high frequency. Further, since the resistance value of the negative feedback resistor 23 is set to be very large compared to the resistance value of the negative feedback resistor 21, the negative feedback by the resistor is mainly performed by the negative feedback resistor 21. The negative feedback by the negative feedback resistor 23 is used when fine adjustment is performed, but may be deleted when fine adjustment is not necessary.

  As shown in FIG. 8, in the first modification, for example, at a frequency of 2 Hz, the difference between the gain at Vcnt = 1.6V and the gain at Vcnt = 2.0V is about 14 dB, and the gain of the control signal Vcnt is It can be seen that the control range is wide. Therefore, the first modification is suitable for use in a scene where a wide dynamic range from a low detection signal voltage value to a high detection signal voltage value is required.

  However, in the first modification, it can be seen that the gain decreases at a high frequency of 2 Hz or higher. This frequency characteristic is improved in the second modification.

  In the second modification, as shown in FIG. 9, the first negative feedback circuit 40 is configured by only the negative feedback resistor 25, and the second negative feedback circuit 50 is configured by only the negative feedback capacitor 24. . As described above, the negative feedback resistor 25 applies negative feedback to the entire frequency to be handled, but does not significantly affect the frequency characteristics. Since the negative feedback capacitor 22 is connected between the output terminal of the attenuation circuit 20 and the input terminal of the amplification element 3, negative feedback is provided to the entire low-frequency amplification circuit 100 including the amplification element 3 to the attenuation circuit 20. Has the effect of multiplying.

  In addition, a resistance by parallel connection of the first resistor 11 and the ON-resistance between the drain and source of the FET 1 is provided between the output terminal of the amplifying element 3 and the output terminal of the attenuation circuit 20 (hereinafter referred to as a parallel connection resistance). Will be connected. Accordingly, a series connection circuit of the parallel connection resistor and the negative feedback capacitor 24 is connected between the output terminal of the amplification element 3 and the input terminal of the amplification element 3. Here, by setting the resonance frequency generated in the series connection circuit to an appropriate frequency, it is possible to improve the frequency characteristics of the low-frequency amplifier circuit 100, particularly in the high frequency region.

  FIG. 10 shows a result of improving the frequency characteristics of the low frequency amplifier circuit 100 by setting the resonance frequency by the series connection circuit to an appropriate frequency. As shown in FIG. 10, among the frequencies handled by the low-frequency amplifier circuit 100, the gain in the high frequency region of 2 to 5 Hz can be raised compared to the frequency characteristics shown in FIG.

  For example, when Vcnt = 2.0 V and a frequency of 1 Hz to 5 Hz, the gain difference is within about 2 dB. That is, a high gain is maintained at a frequency of 1 Hz to 5 Hz. Therefore, the second modification is suitable for use at a frequency of 1 Hz to 5 Hz. For example, it is desirable to use biometric information related to a heartbeat of a person who uses a frequency of 1 Hz to 5 Hz in a scene where it is required.

  However, in the second modification, it can be seen that the gain decreases at a low frequency of 0.5 Hz or less. For example, when Vcnt = 2.0 V and a frequency of 0.3 Hz, the magnitude of the gain is about 62.5 dB. Accordingly, it is desired to increase the gain in a low frequency region having a frequency of 0.5 Hz or less. The third modification is an improvement in the characteristics in the frequency domain.

  In the third modified example, as shown in FIG. 11, the first negative feedback circuit 40 is configured by only the negative feedback capacitor 26, and the second negative feedback circuit 50 is configured by only the negative feedback resistor 27. . Since the negative feedback capacitor 26 is connected between the output terminal of the amplifying element 3 and the input terminal of the amplifying element 3, negative feedback is applied to the frequency band amplified by the amplifying element 3. Negative feedback is not applied to the entire low-frequency amplifier circuit 100 including up to 20. Further, series resonance due to the resistor and the capacitor does not occur. Therefore, the gain in the high frequency region is not so flat. On the other hand, in the low frequency region, the entire low frequency amplifier circuit 100 is not subjected to negative feedback by the negative feedback capacitor, but is negatively fed only by the negative feedback resistor 27, so that the gain can be increased in the relatively low frequency region. Can be high.

  As shown in FIG. 12, in the third modification, for example, when Vcnt = 2.0 V and frequency 0.3 Hz, the magnitude of the gain is about 65 dB. This frequency is improved by about 2.5 dB compared to the second modification. Therefore, the third modification is suitable for use at a frequency of 0.2 Hz to 0.5 Hz. For example, it is desirable to use biological information related to breathing of a person who uses a frequency of 0.2 Hz to 0.5 Hz in a scene where it is required.

  As described above, the first modification is suitable for use in a scene where a wide dynamic range from a low detection signal voltage value to a high detection signal voltage value is required. The second modification is suitable for use at a high frequency of 1 Hz to 5 Hz. For example, it is desirable to use biometric information related to a heartbeat of a person who uses a frequency of 1 Hz to 5 Hz in a scene where it is required. Furthermore, the third modification is suitable for use at a low frequency of 0.2 Hz to 0.5 Hz. For example, it is desirable to use biological information related to breathing of a person who uses a frequency of 0.2 Hz to 0.5 Hz in a scene where it is required. Thus, in the embodiment of the present invention, the effects of the invention can be exhibited more clearly by properly using the first modification, the second modification, or the third modification according to the purpose of use. .

  As described above, the low-frequency amplifier circuit of the present invention has the first negative feedback circuit and the second negative feedback circuit, and the capacitor is connected to one of the first negative feedback circuit and the second negative feedback circuit. In addition, since a resistor is connected to the other, a combination of the first negative feedback circuit and the second negative feedback circuit can provide a frequency characteristic corresponding to the frequency of the reflected wave from the object. Is possible. In addition, since the attenuation circuit includes the first field effect transistor, the attenuation amount of the amplified detection signal can be varied using the variable resistance action of the first field effect transistor. It is possible to easily provide a low-frequency amplifier circuit having a dynamic range that can handle distances.

  The present invention is not limited to the description of the above embodiment, and can be implemented with appropriate modifications in a mode in which the effect is exhibited. For example, the low frequency amplifier circuit of the present invention may include components equivalent to the components shown in FIG.

DESCRIPTION OF SYMBOLS 1 1st field effect transistor 2 2nd field effect transistor 3 Amplifying element 10 Amplifying part 11 1st resistance 12 2nd resistance 13 3rd resistance 14 4th resistance 15 5th resistance 17 Resistance 18 Capacitor 20 Attenuation circuit 21 Negative feedback resistance 22 Negative feedback capacitor 23 Negative feedback resistor 24 Negative feedback capacitor 25 Negative feedback resistor 26 Negative feedback capacitor 27 Negative feedback resistor 30 Control circuit 40 First negative feedback circuit 50 Second negative feedback circuit 51 Input terminal 52 Output terminal 53 Power supply terminal 60 Transmission signal Generation circuit 70 Detection circuit 80 Signal processing circuit 90 Transmission / reception antenna 91 Object 100 Low frequency amplification circuit 150 Wireless sensor device 200 Low frequency amplification circuit 300 Low frequency amplification circuit 400 Low frequency amplification circuit D1 Drain D2 Drain S1 Source S2 Source G1 Gate G2 Gate R1 Anti value R2 resistance R11 the resistance value R12 resistance VR1 variable resistor VR2 variable resistor Vgs1 gate-source voltage Vgs2 between the gate and the source voltage Vcc supply voltage Vout low frequency amplifying circuit output signal Vcnt control signal Vdet detection signal

Claims (5)

A low frequency amplifier circuit comprising: an amplifying element for amplifying a detection signal output from the detection circuit; an attenuating circuit for attenuating the output signal from the amplifying element; and a control circuit for controlling the attenuation amount of the attenuating circuit. There,
A first negative feedback circuit is connected between the output terminal of the amplification element and the input terminal of the amplification element, and a second negative feedback circuit is connected between the output terminal of the attenuation circuit and the input terminal of the amplification element. A circuit is connected, a capacitor is connected to one of the first negative feedback circuit or the second negative feedback circuit, and a resistor is connected to the other,
The attenuation circuit includes a first field effect transistor having a drain connected to an output terminal of the amplification element, and is configured to output a low frequency amplification circuit output signal from a source of the first field effect transistor. And
The control circuit receives the detection signal, generates a control signal whose control voltage value changes according to the voltage value of the detection signal, and applies the control signal to the gate of the first field effect transistor to thereby attenuate the attenuation. A low-frequency amplifier circuit that controls the attenuation of the circuit.   The attenuation circuit has a first resistor connected between a drain and a source of the first field effect transistor and a second resistor connected between a source of the first field effect transistor and a ground. The low-frequency amplifier circuit according to claim 1, wherein The control circuit includes a second field effect transistor having a gate connected to an output terminal of the detection circuit,
A drain of the second field effect transistor is connected to a gate of the first field effect transistor, and a connection point between a gate of the first field effect transistor and a drain of the second field effect transistor and a power supply terminal The low-frequency amplifier circuit according to claim 1, wherein a third resistor is connected therebetween, and a source of the second field effect transistor is connected to the ground.   The low-frequency amplification according to any one of claims 1 to 3, wherein the first negative feedback circuit is configured by only a resistor, and the second negative feedback circuit is configured by only a capacitor. circuit. 4. The low-frequency amplification according to claim 1, wherein the first negative feedback circuit is configured by only a capacitor, and the second negative feedback circuit is configured by only a resistor. 5. circuit.