JPH04299607A - Charge amplifier - Google Patents

Abstract

PURPOSE:To attain stable charge amplification by connecting a T-shaped primary low pass filter composed of a negative feedback static capacitance element, a resistor and a capacitor in parallel between an output terminal and the inverting input terminal of a differential input operational amplifier. CONSTITUTION:A weight converter S is connected to the inverting input terminal of an operational amplifier A and a T-type primary low pass filter F is connected between the input terminal and an output terminal. The resistance of a resistor Rf connected in parallel with a capacitor Cf in a feedback circuit is divided into R1 and R2 to form a T-type filter F. The bias current of the operational amplifier A is supplied through the resistor Rf at a very low frequency region including DC and the function of the charge amplifier is enhanced by using the capacitor Cf for a frequency region in which the transfer admittance of the T-type circuit is negligible. Since the gain of a general-purpose operational amplifier is abnormally increased for the frequency of 100-400Hz, phase lead compensation is applied to the amplifier for a narrow frequency range in the vicinity of the frequencies. The frequency characteristic is made slow by the insertion of a phase compensation circuit J composed of the resistors R1, R2 and a capacitor C2 and a stable amplifier is realized and a flat characteristic is obtained over a wide frequency range.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charge amplifier that converts charge or voltage generated in a capacitive high impedance sensor into a stable analog voltage.

0002

2. Description of the Related Art Capacitive high impedance sensors include, specifically, piezoelectric transducers using crystal oscillators, ceramic piezoelectric elements, Rochelle salt, and the like, and capacitive sensors.

As is well known, the equivalent circuit of the piezoelectric transducer shown in FIG. 5(a) can be expressed as shown in FIG. 5(b), and the charge ΔQ generated by the load acting on the piezoelectric transducer is Capacity Co and generated voltage Δ
This is equivalent to a series circuit with a V power supply. Generally, Co does not have a very large capacitance, so if the output signal is routed through a shielded wire or the like, the output voltage ΔV will be divided by the stray capacitance Cs of the shielded wire, as shown in FIG. 6, resulting in a large error.

Therefore, as shown in FIG. 7, if the input impedance is received by a current amplifier close to zero, the influence of stray capacitance is eliminated.

[0005]

[Problems to be Solved by the Invention] An example of a charge amplifier is shown in FIG. 8, but the inverting input terminal (-) is a virtual ground, and the input impedance mentioned above becomes zero as long as the gain of operational amplifier A is large. However, if we focus on the resistance Rf and capacitance Cf of the feedback circuit connected between the output terminal OUT and the inverting input terminal (-), we can see that the feedback circuit of operational amplifier A is connected to the resistance Rf. When forming the transducer only, the output voltage is obtained by differentiating the transducer output, so by connecting it in parallel with the capacitance Cf, the input current ratio is integrated and taken out as an output proportional to ΔQ. Since the reactance of the capacitance Cf at the operating frequency is very large, the resistor R connected in parallel is
It is necessary that f has a very high resistance, and it must be an operational amplifier with special performance.

Nevertheless, the presence of resistor Rf precludes the fabrication of highly accurate charge amplifiers. The above-mentioned conventional technology had the following problems. (1) Since the conventional charge amplifier described above requires a very large resistance Rf, a general-purpose operational amplifier cannot be used.

(2) Since it includes a high impedance circuit, it is susceptible to commercial frequency induction and external noise. (3) It is impossible to minimize the capacitance Cf in order to increase the gain. (4) The accuracy of the charge amplifier is low.

The negative feedback circuit of the charge amplifier is mainly composed of a capacitance Cf, and the purpose of the resistor Rf is to stably supply a bias current to the operational amplifier A, and it is better not to have a high resistance unless there are restrictions on the characteristics. However, the frequency response of the charge amplifier is fL = 1/(2・π・Cf・Rf) ‥‥
(1) is set as the low cutoff frequency, and even if the transducer has a wide band, the charge amplifier imposes a limit on the low frequency characteristics. For example, when Cf=100pF, Rf=10MΩ, fL
≒159 Hz, and to improve this, the capacitance C
When f is increased, the amplification degree Av of the charge amplifier becomes as follows (2
) As in the formula Av=-Co/Cf
. . . (2), and the degree of amplification inevitably decreases in relation to Co.

That is, the problem to be solved is to maintain the necessary frequency characteristics and stable operation of the amplifier while keeping the product (Cf·Rf) at an appropriately small value. The present invention
While obtaining a DC path with Rf having an appropriate resistance value as a bias current supply circuit, the signal current is prevented from passing through the resistor Rf so that the low cut-off frequency fL does not rise thereby. Furthermore, it is an object of the present invention to provide a good charge amplifier by preventing disturbance and instability of frequency characteristics caused by changes in phase characteristics.

0010

[Means for Solving the Problems] The charge amplifier of the present invention includes:
A negative feedback capacitance element connected between the output terminal and the inverting input terminal of the differential input operational amplifier, and a T-type resistor and capacitance as a DC bias current supply circuit connected in parallel with the negative feedback capacitance element. It is characterized by connecting a first-order low-pass filter.

[0011]

[Operation] According to this configuration, a DC path is obtained with Rf having an appropriate resistance value as a bias current supply circuit, and the signal current is thereby prevented from passing through Rf so that the low cutoff frequency fL does not rise. Furthermore, a compensation circuit is provided to prevent disturbance and instability of frequency characteristics caused by changes in phase characteristics, thereby realizing a good charge amplifier.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1 to 4. It should be noted that the same reference numerals are given to the parts having the same functions as in FIGS. 5 to 8 showing the conventional example.

The charge amplifier for a load transducer according to the present invention is a piezoelectric transducer S for a load transducer as shown in FIG.
is connected to the inverting input terminal (-) of the operational amplifier A, and a T-type primary low-pass filter F is connected between the output terminal OUT of the operational amplifier A and the inverting input terminal (-).

This first-order low-pass filter F will be explained. First, after determining the resistance value of the resistor Rf, as shown in FIG. 2, the resistance value is divided into R1 and R1 to form a T-type first-order low-pass filter (first-order lag circuit). This T-type circuit is shown in Figure 6.
Therefore, its cutoff frequency fT is fT =
1/π・C1・R1
(3) In general, if fT is selected to be 1/100 to 1/1000 of the lower limit of the frequency used, the signal component appearing at the amplifier output will be greatly attenuated and will be returned to the inverting input terminal (-) through R1. The signal received is minute,
It fulfills the original role of a DC bus.

That is, in an extremely low frequency range including direct current, the bias current of the operational amplifier A is supplied through the resistor Rf, and as a charge amplifier, the function is shared by Cf in the frequency range where the transfer admittance of the T-type circuit can be ignored. ,
Demonstrate.

Generally, the gain of a general-purpose operational amplifier has a first-order pole at a low frequency, for example, about 10 Hz, and at frequencies above this, the gain monotonically decreases at -6 dB/oct. In other words, it has first-order lag characteristics and has a frequency of several hundred Hz.
In this case, a phase rotation of approximately -(π/2) rad is involved. This means that when considering the phase characteristics of the T-shaped circuit described above, a frequency occurs where the phase delay is almost close to -π [rad] and the loop gain is positive. That is, the loop gain vector passes through a point very close to (I.jo), and at that frequency the charge amplifier sees an abnormal increase in gain. Generally, this frequency is between 100 [Hz] and 400 [Hz]. Therefore, by performing phase compensation at this frequency, the frequency characteristics become gentle and a stable amplifier can be obtained. Phase compensation is performed in a narrow frequency range around the point where the abnormal gain occurs. Figure 4 shows the resistance R
1, such a compensation circuit J consisting of R2 and capacitance C2
This T-type first-order low-pass filter F is connected between the output terminal OUT and the inverting input terminal (-) of the operational amplifier A.

[0017]

[Effects of the Invention] As described above, according to the present invention, since the T-type primary low-pass filter is connected between the output terminal and the inverting input terminal of the differential input operational amplifier, the feedback resistance value can be prevented from becoming excessive. Stable operation can be ensured, and the following effects can be achieved.

(1) The effects of Cf and Rf are each achieved according to their original purpose, and interference with each other is extremely small. (2) Effective operation even with very small Cf.

(3) Since it has flat characteristics over a wide frequency range, it is also suitable for amplifying rectangular pulse waves and the like. (4) High accuracy.

(5) A general-purpose operational amplifier can be used. (6) Strong against induction and noise.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of a charge amplifier of the present invention.

FIG. 2 is an explanatory diagram of a circuit interposed between an inverting input terminal and an output terminal of an operational amplifier of the same device.

FIG. 3 is an explanatory diagram of the circuit of FIG. 2;

FIG. 4 is a circuit diagram interposed between an inverting input terminal and an output terminal of an operational amplifier in the same device.

FIG. 5 is a circuit diagram and an equivalent circuit diagram of a piezoelectric transducer.

FIG. 6 is an equivalent circuit diagram when the output signal of the piezoelectric transducer is connected to an amplifier.

FIG. 7 is a configuration diagram of a general amplifier.

FIG. 8 is a configuration diagram of a conventional charge amplifier.

[Explanation of symbols]

A Operational amplifier [differential input operational amplifier] F
Primary low-pass filter S Piezoelectric transducer J Phase compensation circuit

Claims (1)

[Claims] Claim 1: A negative feedback capacitance element connected between the output terminal and the inverting input terminal of a differential input operational amplifier, and a DC bias current supply circuit connected in parallel with the negative feedback capacitance element, including a resistor and a capacitance element. A charge amplifier connected to a T-type primary low-pass filter consisting of a capacitor.