JP2000214190A - Amplification circuit with input capacitance adjustment circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectuate precise adjustment with good temperature characteristic and linearity and with high resolution by changing the voltage ratio of a voltage divider circuit, thereby increasing/decreasing charges stored in a capacitor and adjusting the input capacitance of an amplifier circuit. SOLUTION: An input capacitance C6 of an operational amplifier 1 is adjusted with utilization of a capacitor C7 and a voltage divider circuit 10. An input capacitance adjusting circuit is set which includes the voltage divider circuit 10, whose voltage dividing ratio set between an output terminal 52 and a ground potential point can be changed digitally and the capacitor set between a voltage dividing point of the voltage divider circuit and an input terminal of the operational amplifier 1. The input capacitance adjusting circuit adjusts the input capacitance C6 by changing the voltage dividing ratio of the voltage divider circuit 10. Accordingly, the input capacitance can be adjusted digitally. The circuit is adaptable to an automatically calibrating measuring apparatus by controlling the voltage divider circuit 10 with using a CPU.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplifier circuit using an operational amplifier, and more particularly to an improvement in an input capacitance adjusting circuit of the operational amplifier.

[0002]

2. Description of the Related Art The configuration of a conventional amplifier circuit with an input capacitance adjusting circuit will be described with reference to FIG. In the figure, an input terminal 51 is connected to one end of a resistor R3, and the other end of the resistor R3 is connected to one end of a resistor R4, one end of a variable capacitor C5, and an input terminal of the operational amplifier 1. The output of the operational amplifier 1 is connected to an output terminal 52,
It is connected to the inverting input of the operational amplifier 1 via a feedback path.

The other end of the resistor R4 and the other end of the variable capacitor C5 are connected to a ground potential point. Further, the high-frequency correction capacitor C2 is connected to the resistor R
3 are connected in parallel.

In the amplifier circuit having such a configuration, an input voltage ein is divided by a resistor voltage dividing circuit 31 composed of a resistor R3 and a resistor R4, and the divided voltage is supplied to an input terminal of the operational amplifier 1. Will be entered. This signal is amplified by the operational amplifier 1 and the output voltage eout
Is output to the output terminal 52.

[0005] The voltage dividing ratio A of the resistor voltage dividing circuit can be expressed by the following equation: A = R4 / (R3 + R4) (1) However, the above symbols are the resistance value and the capacitance value of each element. Hereinafter, symbols used in each equation are the resistance value and the capacitance value of each element.

Also, the input voltage ein and the output voltage eout
Can be expressed by eout = A × G × ein (2), where G is the amplification factor of the operational amplifier 1.

However, generally, an operational amplifier has several pF.
Input capacitance C6, and the value of the same type of parts varies due to factors caused by the manufacturing process, etc., so that the capacitance of the circuit 32 on the operational amplifier 1 side from the point f is set to a constant value. If you want to use it, inconvenience arises.

Therefore, in order to adjust the capacitance of the circuit 32 on the operational amplifier 1 side from the point f, a variable capacitor C5 is connected between the input terminal of the operational amplifier 1 and the ground potential point. By making such a connection, the variable capacitor C5 and the input capacitor C6 are equivalent to being connected in parallel. Therefore, by adjusting the variable capacitor C5, the total capacitance of the input capacitor C6 and the variable capacitor C5 is adjusted. The capacitance can be adjusted to a desired constant value.

That is, the variable capacitor C5 functions as an input capacitance adjusting circuit of the operational amplifier 1, and by adjusting this, the input capacitance C6 of the operational amplifier 1 can be adjusted.

Another advantage of the circuit for adjusting the input capacitance of the operational amplifier 1 with the above configuration will be described below.

In the amplifier circuit having the above-described configuration, the input capacitance C6 and the resistance value from the input terminal of the operational amplifier 1 to the resistance voltage dividing circuit 31 side (when the signal source resistance of the input voltage ein is 0). , R3 and R4), a low-pass filter is formed, and the frequency characteristic of the input signal ein is a response characteristic limited by the band of the low-pass filter.

Therefore, in order to improve the above-mentioned narrowing of the band, a high-frequency correcting capacitor C2 is connected in parallel with a resistor R3, and the variable capacitor C5 is connected as follows: R3: R4 = (C5 + C6): C2 (3) Is established, the frequency characteristics of the amplifier circuit shown in FIG. 11 become flat, and a constant voltage division ratio can be maintained regardless of the frequency. The operational amplifier 1 used here
Can obtain the same effect regardless of whether it is used for calculation purposes such as multiplication or subtraction or when it is used as a shock absorber.

[0013]

However, in the conventional amplifier circuit with an input capacitance adjusting circuit described above, a trimmer capacitor having a rotating mechanism is generally used as the variable capacitance capacitor C5. Temperature characteristics are significantly worse than fixed-capacitance capacitors, causing frequency characteristics to change even after adjustment due to temperature fluctuations; manual adjustment of capacitance requires poor efficiency; and characteristic deterioration at high frequencies Therefore, there is a problem that the adjustment resolution is relatively coarse.

Japanese Patent Laid-Open No. 7-11 / 1999 proposes a proposal for compensating for the problem caused by the manual adjustment and the disadvantage of the adjustment accuracy.
No. 3857. In this method, a variable capacitance diode is used in place of the variable capacitance capacitor C5, and a reverse applied voltage for determining the inter-terminal capacitance is applied by a D / A converter. As a result, the D / A converter is connected to the CPU.
The automatic adjustment is realized by controlling the system in a manner similar to that described above, and a high-resolution and fine adjustment resolution is realized by using the D / A converter.

However, even in this case, the temperature characteristic of the variable capacitance diode is worse than that of the fixed capacitance capacitor, so that the frequency characteristic changes even after adjustment due to temperature fluctuation, and the variable capacitance diode varies depending on the magnitude of the input signal. In this case, there is a problem that the inter-terminal capacitance changes, and as a result, the linearity of the voltage dividing circuit at high frequencies deteriorates.

The present invention solves the above-mentioned problems, and has good temperature characteristics and linearity, and can perform fine adjustment with high resolution.
It is another object of the present invention to provide an amplifier circuit with an input capacitance adjusting circuit that can be used for a measuring device that performs automatic calibration.

[0017]

According to the first aspect of the present invention, a voltage dividing ratio provided between an output terminal and a ground potential point can be changed by a digital value. A voltage dividing circuit, and a capacitor provided between a voltage dividing point of the voltage dividing circuit and an input terminal of the operational amplifier, and storing the voltage in the capacitor by changing a dividing ratio of the voltage dividing circuit. And an input capacitance adjusting circuit for adjusting the input capacitance by adjusting the input electric charge.

As a result, the input capacitance can be adjusted based on the digital value.

According to a second aspect of the present invention, in the first aspect of the present invention, the voltage dividing circuit includes a fixed resistor and an F
An ET is connected in series, and a D / A converter for supplying a gate current to the gate of the FET is provided.

Thus, an input capacitance adjusting circuit that does not use a variable capacitor can be realized.

According to a third aspect of the present invention, in the first aspect of the invention, the voltage dividing circuit controls a gate current of the FET by controlling a gate current of the FET by a digital value set in the D / A converter. The voltage dividing ratio is adjusted by changing the current flowing between the sources.

This makes it possible to adjust the voltage dividing ratio of the voltage dividing circuit with the high resolution and fine adjustment resolution of the D / A converter.

According to a fourth aspect of the present invention, in the first aspect of the present invention, a resistor R3 and a resistor R4 having the other end connected to a ground potential point are connected in series, and the resistor R3
And an input voltage dividing circuit configured so that an input signal is applied to a resistor R3 and an output of a common connection point between the resistors R3 and R4 is supplied to the amplifier circuit. A capacitor C8 connected thereto, wherein the frequency characteristic of the amplifier circuit can be adjusted by adjusting the value of α by a voltage divider circuit so that R3: R4 = (C8 + α): C2 holds. Things.

This makes it possible to improve the frequency characteristics of the amplifier.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings. FIG. 1 is a configuration diagram of an amplifier circuit with an input capacitance adjusting circuit according to the present invention. In the same figure, those performing the same operations as in FIG. 3 described in the conventional example are denoted by the same reference numerals, and description thereof will be omitted.

FIG. 1 differs from FIG. 3 which is a conventional example in that the variable capacitor C5 is changed to a fixed-capacitance capacitor C8, and a voltage divider is provided between the output of the operational amplifier 1 and the ground potential point. The circuit 10 is connected, and a capacitor C7 is connected between the voltage dividing point a of the voltage dividing circuit 10 and the input terminal of the operational amplifier 1.

The voltage dividing circuit 10 includes a fixed resistor 11 and an FE
T12 is connected in series, and D / A converter 13 is connected to the gate of FET12. This is D
FE is determined by the digital value set in the A / A converter 13.
This circuit adjusts the current between the drain and source of T12 to divide the output signal eout.

Here, the operation of adjusting the input capacitance of the operational amplifier in the amplifier circuit of the present invention will be described.

In the prior art, the input capacitance C6 of the operational amplifier 1 is adjusted by adjusting the capacitance of the variable capacitor C5. However, the circuit of the present invention utilizes the capacitor C7 and the voltage dividing circuit 10. Adjust.

Input terminal of operational amplifier 1 and voltage dividing circuit 1
Assuming that the voltage dividing ratio of the voltage dividing circuit 10 is B, A × ein−B × eout = A × ein−B × (A × B × ein) = (1−B × G) × A × ein (4)

Here, the charge Q7 stored in the capacitor C7 is given by Q7 = C7 × (A × ein × (1-B × G)) = (1-B × G) × C7 × (A × ein) (5), and (A × ein) in the above equation (5) is the above (2)
As is clear from the equation, since the input signal is the input signal of the operational amplifier 1, it is understood that the capacitor C7 viewed from the voltage dividing circuit 10 is equivalent to being multiplied by (1−B × G).

Therefore, by adjusting the voltage dividing ratio of the voltage dividing circuit 10, it is possible to adjust the charge stored in the capacitor C7 equivalent to being connected in parallel with the input capacitor C6 of the operational amplifier 1. , F, it is possible to adjust the input capacitance of the circuit as viewed from the operational amplifier 1 side.

Also, in the amplifier circuit of the present invention, similarly to the conventional example, a low-pass filter is formed by the input capacitor C6 and the resistance value from the input terminal of the operational amplifier 1 to the resistance voltage dividing circuit 31 side. Therefore, the frequency characteristic of the input signal ein is a response characteristic limited by the band of the low-pass filter.

Therefore, in order to improve the above-mentioned narrowing of the band, a high-frequency correcting capacitor C2 is connected in parallel with the resistor R3, and the voltage dividing ratio of the voltage dividing circuit 10 is changed to store the voltage in the capacitor C7. If the charge is adjusted and the capacitance is adjusted to the value of α such that R3: R4 = (C8 + α): C2 (6), the frequency characteristic of the amplifier circuit in FIG. Regardless, a constant partial pressure ratio can be maintained.

Further, by mounting the above-described amplifier circuit with an input capacitance adjusting circuit according to the present invention on the measuring device having the configuration shown in FIG. 2, a measuring device capable of automatically adjusting the input capacitance is realized. Becomes possible.

In FIG. 2, a signal changeover switch 61 is connected between an input terminal 51 and a resistor R3.
Is connected to the adjustment signal generator 62.

The signal changeover switch 61, the adjustment signal generator 62 and the D / A converter 13 are connected to a CPU 63, the operation of which is controlled by the CPU 63. Also, CP
U63 has an output signal eo output from the output terminal 52.
ut has been entered.

In the measuring instrument having such a configuration, the fixed contact p of the signal changeover switch 61 is connected to the changeover contact n during normal measurement, and the input signal ein is inputted to the resistor R3. The fixed contact p of the signal changeover switch 61 is switched to the changeover contact m side, and the adjustment signal output from the adjustment signal generator 62 is input to the resistor R3 according to a command from the CPU 63.

The CPU 63 calculates an optimum voltage dividing ratio of the voltage dividing circuit from the output signal eout corresponding to the adjusting signal, and sets a set value corresponding to the voltage dividing ratio in the D / A converter 13. The CPU 63 repeats this operation, and executes D / A
When the set value to be set in the converter 13 is determined, the signal changeover switch 61 is switched to the changeover contact n side, and the automatic capacity adjustment ends.

It should be noted that the foregoing description has been directed to specific preferred embodiments for the purpose of illustration and illustration of the invention. Therefore, the present invention is not limited to the above-described embodiments, and includes many more modifications without departing from the spirit thereof.
This includes deformation.

Therefore, the voltage dividing circuit 10 is not limited to the one configured by combining the D / A converter 13 and the FET 12 described above, but any circuit capable of performing the same operation is described with reference to FIG. An effect similar to that of the circuit can be obtained. For example, even with a structure in which a D / A converter is directly inserted at the position of the FET 12 to adjust the voltage division ratio, the same effect as the circuit described with reference to FIG. 1 can be obtained.

[0042]

As is apparent from the above description,
According to the present invention, the following effects can be obtained. According to the first aspect of the present invention, a voltage dividing circuit provided between an output terminal and a ground potential point is capable of changing a voltage dividing ratio by a digital value, and a voltage dividing point of the voltage dividing circuit. A capacitor provided between the input terminals of the operational amplifier, and an input capacitance adjusting circuit for adjusting an input capacitance by changing a voltage dividing ratio of the voltage dividing circuit, thereby adjusting an input capacitance by a digital value. Therefore, it is possible to provide an amplifier circuit with an input capacitance adjustment circuit that can also be used for a measurement device that performs automatic calibration by controlling the voltage division circuit using a CPU.

According to the second and third aspects of the present invention, in the first aspect of the present invention, the input capacitance adjusting circuit is configured using a resistor, a FET, and a D / A converter without using a variable capacitor. As a result, it is possible to realize an input capacitance adjustment circuit that has good temperature characteristics and linearity, and that can perform fine adjustment with high resolution.

According to a fourth aspect of the present invention, in the first aspect of the present invention, it is possible to prevent deterioration in frequency characteristics caused by a resistance voltage dividing circuit for dividing an input signal and an input capacitance of an operational amplifier. .

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of an amplifier circuit with an input capacitance adjustment circuit according to the present invention.

FIG. 2 is a diagram illustrating an application example of an amplifier circuit with an input capacitance adjusting circuit according to the present invention.

FIG. 3 is a configuration diagram illustrating an example of a conventional amplifier circuit with an input capacitance adjustment circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Operational amplifier 10 Voltage divider 11 Fixed resistor 12 FET 13 D / A converter 31 Resistor divider 51 Input terminal 52 Output terminal 61 Input signal changeover switch 62 Adjustment signal generator 63 CPU C2, C7, C8 Fixed capacitor C5 Variable capacitance capacitor C6 Input capacitance R3, R4 Fixed resistance

Claims (4)

[Claims] A voltage divider provided between an output terminal and a ground potential point and capable of changing a voltage division ratio according to a digital value; A capacitor provided between a voltage dividing point of a voltage dividing circuit and an input terminal of the operational amplifier, and by changing a voltage dividing ratio of the voltage dividing circuit, a charge stored in the capacitor is adjusted to increase or decrease an input of the amplifier circuit. An amplifier circuit with an input capacitance adjusting circuit, comprising an input capacitance adjusting circuit for adjusting a capacitance. 2. The voltage dividing circuit according to claim 1, further comprising a D / A converter that connects a fixed resistor and an FET in series and supplies a gate current to a gate of the FET. Amplifier circuit with input capacitance adjustment circuit. 3. The voltage dividing circuit controls a gate current of the FET according to a digital value set in the D / A converter, and adjusts a voltage dividing ratio by changing a current flowing between a drain and a source. 2. The amplifying circuit with an input capacitance adjusting circuit according to claim 1, wherein the amplifying circuit is configured as follows. 4. A resistor R3 and a resistor R4 having the other end connected to a ground potential point are connected in series with a preceding stage of the amplifier circuit, and a capacitor C2 is connected in parallel to the resistor R3 and an input signal is connected to the resistor R3. An input voltage dividing circuit configured to apply an output of a common connection point of the resistors R3 and R4 to the amplifier circuit; and a capacitor C8 connected in parallel with the R4, wherein R3: R4 = ( C8 + α): C2 (however, the above symbols are the resistance value and the capacitance value of each element), and the frequency characteristic of the amplifier circuit can be adjusted by adjusting the value of α by using a voltage divider circuit. 2. The amplifying circuit with an input capacitance adjusting circuit according to claim 1, wherein: