JP2001257576A - Piezoelectric sensor circuit - Google Patents

Abstract

(57) [Abstract] [Problem] To suppress unstable operation such as oscillation caused by downsizing. And A piezoelectric sensor 1, a first resistor R ₁ connected to ground in parallel with the piezoelectric sensor 1, the FET2 to the gate terminal S is connected in parallel a first and a resistor R _1, FET2 source The input capacitance of the piezoelectric sensor circuit having the second resistor R ₂ connected between the terminal S and the ground is within 0.2 times the element capacitance of the piezoelectric sensor 1 and the resistance value of the second resistor. R ₂
With the above impedance capacity, the operation is stabilized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a piezoelectric sensor circuit.

[0002]

2. Description of the Related Art A piezoelectric sensor circuit using a piezoelectric sensor as a sensor for detecting acceleration and impact is relatively simple in circuit configuration, so that it is easy to reduce the size. It is widely used for impact detection and acceleration detection in hard disks and meters.

[0003] A piezoelectric sensor circuit is composed of a piezoelectric sensor and an impedance conversion circuit comprising a source follower circuit or a grounded source circuit. When an electric charge is generated in the piezoelectric sensor by an inverse piezoelectric effect due to the application of acceleration, impact, or the like, the generated electric charge is extracted as a voltage signal by an impedance conversion circuit. By connecting an amplifier circuit and a filter circuit at the subsequent stage of the impedance conversion circuit, necessary signal magnitudes and frequency components can be obtained.

[0004] Examples of the piezoelectric sensor include those using piezoelectric ceramics and a piezoelectric single crystal. The element capacitance of a piezoelectric sensor varies depending on its size and element structure. In general, the element capacitance using piezoelectric ceramics is several tens to several hundreds of p.
F, those using a piezoelectric single crystal have several to several tens of pF, and the capacitance of a piezoelectric sensor using a piezoelectric single crystal is smaller.

[0005]

As the size of the piezoelectric sensor circuit is reduced, the dimensions of the circuit components are reduced and the distance between the lines is reduced. However, when the distance between the constituent elements is reduced and the distance between the electrodes of each element is reduced, the line capacitance between the elements, the line capacitance between the substrate and the element, or the electrode and the substrate is reduced. And the stray capacitance between them becomes as large as possible.

On the other hand, when the size and shape of the piezoelectric sensor itself are reduced in size, the distance between the electrodes is reduced, and the element capacitance of the piezoelectric sensor itself is reduced as much as possible. And
If the line capacitance and stray capacitance on the circuit side become too large to be ignored, compared to the element capacitance of the piezoelectric sensor itself, which becomes as small as possible, these capacitances become negative feedback and cause abnormal oscillation. May cause stable operation.

An object of the present invention is to provide a piezoelectric sensor circuit that operates stably.

[0008]

In order to achieve the above object, the present invention provides a piezoelectric sensor, a first resistor connected to ground in parallel with the piezoelectric sensor, and a gate in parallel with the first resistor. A piezoelectric sensor circuit comprising: a field effect transistor having a terminal connected thereto; and a second resistor connected between a source terminal of the field effect transistor and ground. Is an impedance capacity within 0.2 times the element capacity of the piezoelectric sensor and not less than the resistance value of the second resistor.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention according to claim 1 of the present invention is directed to a piezoelectric sensor, a first resistor connected to ground in parallel with the piezoelectric sensor, and a gate terminal in parallel with the first resistor. And a second resistor connected between the source terminal of the field-effect transistor and the ground, wherein the input capacitance of the piezoelectric sensor circuit is The impedance capacity is within 0.2 times the element capacity of the piezoelectric sensor and is equal to or larger than the resistance value of the second resistor, thereby enabling stable operation without oscillation and accurate measurement of acceleration. Can be implemented.

In the present invention, the input capacitance may be a capacitance between a gate terminal and a source terminal of the field-effect transistor, and the input capacitance may be a capacitance between the gate terminal and the source terminal. The output signal of the piezoelectric sensor circuit is preferably taken out from the source terminal.

Further, as described in claim 3, a third resistor is connected between a power supply of the piezoelectric sensor circuit and a drain of the field effect transistor, and the input capacitance is the field effect type transistor. It is the sum of the capacitance between the gate terminal and the source terminal of the transistor and the stray capacitance generated between the gate terminal and the drain terminal. The output signal of the piezoelectric sensor circuit is extracted from the drain terminal. Is preferred. In this case, the amplification factor is determined by the resistance ratio between the second resistor and the third resistor, and the amplification function can be added to the impedance conversion function.

Hereinafter, the present invention will be described in detail based on an embodiment. (Embodiment 1) As shown in FIG.
A piezoelectric sensor 1, the piezoelectric sensor 1 and the first resistor R ₁ connected to ground in parallel, a first resistor R ₁ and a field effect transistor whose gate terminal G is connected in parallel (hereinafter, F
ET) 2 and a second resistor R ₂ connected between the source terminal S of the FET 2 and the ground so that the acceleration signal Vs can be obtained from the source terminal S of the FET 2. Has become.

As the piezoelectric sensor 1, it is desirable to use a piezoelectric single crystal having a good temperature characteristic, a large coupling coefficient, and a high resonance sharpness. However, in this case, the capacity is larger than that using a piezoelectric ceramic. Become smaller.

A high-pass filter is formed by the piezoelectric sensor 1 and the first resistor R _1, and the cut-off frequency flc of the high-pass filter is obtained by the following equation (1) when the element capacitance of the piezoelectric sensor 1 is C _0. Can be

Flc = 1 / 2πC ₀ R ₁ (1) where the element capacitance C ₀ of the piezoelectric sensor 1 is
Is determined by the material and shape of the piezoelectric element that constitutes the above, and if the upper limit frequency of the necessary signal band is fh,
The first resistor R ₁ is determined by the following equation (2).

R ₁ ≧ 1 / 2πC ₀ fh (2) As apparent from the equations (1) and (2), the piezoelectric sensor 1
When the element capacitance C ₀ is small, it is necessary to increase the resistance value of the first resistor R _1. Conversely, when the element capacitance C ₀ is large, the resistance value of the first resistor R ₁ is small. Good.

In order to reduce the size of the piezoelectric sensor circuit, the size of each of the piezoelectric sensor 1, the FET 2, and the first and second resistors R ₁ and R ₂ is promoted. It is necessary to reduce the mounting area by reducing the connection distance and the area of the electrodes of each element. However, since the electrical connection between the elements is usually made via an electrode pattern formed on the circuit board on which the elements are mounted, if the mounting area is reduced, the stray capacitance generated between the substrate and the electrodes and the distance between the electrodes are reduced. Of the order of several hundred fF,
The input capacitance C _{i of the} FET 2 is not negligible.

Assuming that the capacitance between the gate and the source of the FET 2 is C _gs and the floating capacitance C _{GS of the} substrate generated between these terminals between the gate and the source, the input capacitance C of the FET 2
The approximate value of _i is obtained by the following equation (3).

C _i = C _gs + C _GS (3) When the capacitance ratio C _i / C ₀ of the input capacitance C _i for which the approximate value is obtained and the element capacitance C ₀ of the piezoelectric sensor 1 is obtained,
FIG. 2 shows a relationship between the capacitance ratio C _i / C ₀ and the oscillation level. As is clear from FIG. 2, the capacitance ratio C _i /
If C ₀ is 0.2 or more, the input capacitance C _i is negative feedback, the piezoelectric sensor circuit would oscillate. On the other hand, it can be seen that when the capacitance ratio C _i / C ₀ is 0.2 or less, no oscillation occurs and the device operates stably.

FET constituting an impedance conversion circuit
2 is always positive, the loop gain Aβ formed in the impedance conversion circuit must satisfy the following equation (4) in order for the piezoelectric sensor circuit to perform positive feedback and operate stably. is there.

1-Aβ> 0 (4) A: Gain of piezoelectric sensor circuit β: Feedback rate of piezoelectric sensor circuit In order for the piezoelectric sensor circuit to operate stably without oscillating, the magnitude relationship of impedance is The first resistor R ₁
And the input capacitance C _i , the following equation (5) is required.

R ₁ <1 / ωC _i (5) That is, if the input capacitance C _i is an impedance capacitance equal to or more than the _first resistance value R ₁ , positive feedback is provided and the device operates stably without oscillation. Hereinafter, this will be described in more detail.

In the source follower circuit of the FET as shown in FIG. 3, the gain A is obtained by the following equation (6).

A = μR ₂ / (r _d + (μ + 1) R ₂ ) (6) μ = g _m rd μ: FET amplification factor R ₂ : source resistance r _d : drain-source resistance g _m : FET Usually, since the amplification factor μ of the FET is much larger than 1, the gain A is A = 1 according to the above equation (6).

On the other hand, the feedback ratio β in this FET source follower circuit is obtained by the following equation (7).

Β = V ₂ / V ₁ = Z ₁ / ｛1 / (jωC ₂ ) + Z ₁ ＺZ ₁ = R ₁ // {1 / (jωC ₁ )} = R ₁ / ｛1 + jωC ₁ R ₁ ｝ β = R ₁ / {R ₁ (1 + C ₁ / C ₂ ) −j / ωC ₂ } (7) j: Imaginary number In the equation (7), the input capacitance C ₂ is larger than the gate-ground capacitance C ₁ . That is, C ₁ <C ₂ (C ₁ / C ₂ <
In the case of 1), the following equation (8) holds.

Β = R ₁ / {R ₁ −j / ωC ₂ } (8) The loop gain Aβ in this case is obtained by the following equation (9).

Aβ = 1 · R ₁ / {R ₁ −j / ωC ₂ } (9) The magnitude of the absolute value of the loop gain is given by the following (10)
It is obtained by the formula. _{| Aβ | = R 1 / {} R 1 2 + (1 / ωC 2) 2} 1/2 ... (10) Further, the phase theta, is determined by the following equation (11).

Θ = tan ⁻¹ (1 / ωC ₂ R ₁ ) (11) Here, since the loop gain Aβ is a complex number, FIG. 4 shows the vector locus. According to the Nyquist stability determination method, the range in which the possibility of oscillation is small and the operation is stable is as follows (12).

Aβ <1 (12) Comparing the magnitude relationship between the impedance R ₁ and the impedance capacity 1 / ωC ₂ , when R ₁ > 1 / ωC ₂ , (10)
The expression is | Aβ | = R ₁ / {R ₁ ² } ^1/2 = 1, and the piezoelectric sensor circuit does not satisfy the stability condition and performs unstable operation.

On the other hand, when R ₁ <1 / ωC ₂ , (1
Equation (0) is as follows: | Aβ | = R ₁ / ｛(1 / ωC ₂ ) ² ｝ ^1/2 = ωC ₂ R ₁ <1, and the piezoelectric sensor circuit satisfies the stable condition and maintains the stable operation.

As described above, the above expression (5) is satisfied.
That is, if the input capacitance C ₂ (= C _i ) is an impedance capacitance equal to or greater than the resistance value R ₁ (first resistance R ₁ ), the piezoelectric sensor circuit operates stably without oscillation.

On the other hand, in the above equation (7), C ₁ ≫
For C _2, the following (13) is established.

Β = R ₁ / {R ₁ (1 + C ₁ / C ₂ ) + 1 / jωC ₂ } = R ₁ / {R ₁ (C ₁ / C ₂ ) + 1 / jωC ₂ } = (C ₂ / C ₁ ) R ₁ / {R ₁ + 1 / jωC ₁ } (13) In this case, the vector locus of the loop gain Aβ is as shown in FIG. 5, the capacitance ratio _{C 2 / C 1, C 2} /
If C ₁ <1, the vector trajectory is (1, j0)
The piezoelectric sensor circuit is located closer to the origin, and the piezoelectric sensor circuit operates stably without oscillation.

As a configuration for suppressing the above-mentioned oscillation, FE
A structure is conceivable in which a capacitance is connected in parallel between the gate terminal G and the drain terminal D of T2, and positive feedback is newly provided. Owing to the occurrence of the positive feedback, the oscillation condition is not satisfied and the device operates stably without oscillation. Although the number of elements increases by one, the trouble of redesigning the electrode pattern and the like is not required, and oscillation can be easily suppressed.

As described above, in the piezoelectric sensor circuit, the input capacitance C _{i is set} to be equal to or less than 0.2 times the element capacitance C ₀ of the piezoelectric sensor 1 and equal to or greater than the impedance value of the first resistor R ₁ (gate resistance). By doing so, stable operation can be achieved without abnormal oscillation.

(Embodiment 2) In this embodiment, FIG.
The present invention is implemented in the piezoelectric sensor circuit shown in FIG. That is, this piezoelectric sensor circuit is
A first resistor R ₁ connected in parallel between the piezoelectric sensor 1 and the ground, an FET 2 having a gate terminal G connected in parallel with the first resistor R _1, and a source R between the source terminal S of the FET 2 and the ground. a second resistor R ₂ connected, and a third resistor R ₃ connected between the drain terminal D of the power supply Vcc and FET2, is amplified from the drain terminal D by the resistance ratio R ₃ / R ₂ The acceleration signal Vs is obtained.

The basic structure is the same as that of the first embodiment, except that the third resistor R ₃ is connected to the power supply Vcc side and the F
It is connected between the drain terminal D of ET2 and the output terminal is the drain terminal.

The gate terminal G and the drain terminal D of the FET 2
The electrostatic capacitance C _gd between, when the stray capacitance C _GD substrate generated between the gate terminal G and drain terminal D, the input capacitance C _i of FET2, the approximate value is obtained by the following equation (14).

C _i = C _gd + C _GD (14) The relationship between the capacitance ratio C _i / C ₀ of the input capacitance C _{i of the} FET 2 and the element capacitance C ₀ of the piezoelectric sensor 1 and the oscillation level is described in the embodiment. 1 is the same as that described with reference to FIG. That is, when the capacitance ratio C _i / C ₀ is 0.2 or more, the input capacitance C _i oscillates as negative feedback, and the capacitance ratio C _i
When / C ₀ is 0.2 or less, it does not oscillate and operates stably.

In order to obtain a positive feedback, the conditional expression (1-Aβ>
0), conditional expressions mentioned above for the first resistor R ₁ and the input capacitance C _i (5) is needed also in this embodiment.

Therefore, the input capacitance C _i must be within 0.2 times the element capacitance C ₀ of the piezoelectric sensor 1 and the first resistance R
₁ By setting the impedance capacity equal to or larger than the impedance value of (gate resistance),
It works stably. Hereinafter, this will be described in more detail.

In a common FET source circuit as shown in FIG. 7, the gain A is obtained by the following equation (15).

[0044] _{A = μR 3 / (r d} + (μ + 1) R 2) ... (15) μ = g m r d μ: amplification factor of FET R _3: drain resistance R _2: source resistance r _d: between the drain and the source Resistance g _m : Mutual conductance of FET Similar to the source follower circuit of the FET described in the first embodiment, since the amplification factor μ of the FET is extremely large with respect to 1, the gain A is obtained by the above equation (15). Becomes

On the other hand, the feedback ratio β in the common FET source circuit is obtained by the following equation (16).

Β = V_Two/ V₁= Z₁/ ｛1 / (jωC_Three) + Z₁｝… (16) Z₁= R₁// ｛1 / (jωC₁)｝ = R₁/ ｛1 + jωC₁R₁Ｊ j: imaginary number This equation (16) is equivalent to the equation (7) in the first embodiment.
Hit. Here, the expressions (16) and (7) are compared.
And the capacity C_TwoAnd capacity C_ThreeIs the only difference
The calculation formula is the same. Therefore, the same as in the first embodiment
For reasons, R ₁<1 / ωC_ThreeIn the case of (17)
If this formula is satisfied, this circuit will operate stably without oscillation
be able to. | Aβ | = R₁/ ｛(1 / ωC_Three)^Two｝^1/2= ΩC_ThreeR₁<1 ... (17)

[0047]

As described above, according to the present invention,
Oscillation conditions are not satisfied, and abnormal oscillation does not occur. As a result, a stable operation can be obtained, and an accurate acceleration signal can be output.

[Brief description of the drawings]

FIG. 1 is a circuit diagram illustrating a configuration of a piezoelectric sensor circuit according to Embodiment 1 of the present invention.

FIG. 2 is a diagram showing a relationship between a capacitance ratio and an oscillation level.

FIG. 3 is a circuit diagram for explaining the operation of the first embodiment;

FIG. 4 is a diagram provided to explain an operation of the first embodiment;

FIG. 5 is a diagram provided for describing an operation of the first embodiment;

FIG. 6 is a circuit diagram showing a configuration of a piezoelectric sensor circuit according to Embodiment 2 of the present invention.

FIG. 7 is a circuit diagram for explaining the operation of the second embodiment;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Piezoelectric sensor 2 FET (field effect transistor) R1 _1st resistance R2 _2nd resistance R3 _3rd resistance

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Hirofumi Taga 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. Co., Ltd. F term (reference) 5J050 AA11 BB23 CC09 DD06 EE14 EE22 FF32

Claims (3)

[Claims] 1. A piezoelectric sensor, a first resistor connected to ground in parallel with the piezoelectric sensor, a field effect transistor having a gate terminal connected in parallel with the first resistor,
A piezoelectric sensor circuit comprising a second resistor connected between a source terminal of the field-effect transistor and ground, wherein an input capacitance of the piezoelectric sensor circuit is set to 0. A piezoelectric sensor circuit having an impedance capacity within two times and a resistance value of the second resistor or more. 2. The piezoelectric sensor circuit according to claim 1, wherein the input capacitance is a capacitance between a capacitance between a gate terminal and a source terminal of the field-effect transistor and a capacitance between the gate terminal and the source terminal. The output signal of the piezoelectric sensor circuit is taken out from the source terminal. 3. The piezoelectric sensor circuit according to claim 1, further comprising a third resistor connected between a power supply of the piezoelectric sensor circuit and a drain terminal of the field effect transistor, wherein the input capacitance is Is the sum of the capacitance between the gate terminal and the source terminal of the field-effect transistor and the stray capacitance generated between the gate terminal and the drain terminal. The output signal of the piezoelectric sensor circuit is A piezoelectric sensor circuit which is taken out from a terminal.