JPH03155212A - Capacitance changeover device for capacitor - Google Patents

Abstract

PURPOSE:To reduce a leakage current and an on-resistance by connecting an operational amplifier to the output and providing the operational amplifier between a capacitor and a semiconductor switch. CONSTITUTION:For example, when a semiconductor switch 1 is connected to the output of an operational amplifier 3 and a semiconductor switch 2 is grounded, a voltage at both electrodes of a capacitor C3 is kept the same which is equivalent to the circuit without the capacitor C3. Then the equivalent circuit is a time constant circuit comprising a resistor R1 and a capacitor C4. In such a case, each switch contact part of the semiconductor switches, 1, 2 is driven with a low impedance, then the leakage current of the semiconductor switch 1 is all absorbed by the operational amplifier 3 and the leakage current of the semiconductor switch 2 flows to ground.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application 1] The present invention relates to a capacitor capacity switching device that mainly uses an analog switch to switch the capacitance of a capacitor used in, for example, a time constant circuit.

[Prior art] Figure 3(a) shows a low-pass filter (hereinafter referred to as LPF)
(b) is a diagram showing an outline of the time constant circuit that constitutes the main part of a bypass filter (hereinafter referred to as HPF). , R, , R2 are resistors, and C, , C2 are capacitors.

The time constant of the circuit shown in Figure 3 is RI, CI or R2.
, C, so when switching this time constant, it is sufficient to change the value of either the resistor R1 (R2) or the capacitors C, (C2).

FIG. 4 is a diagram showing a circuit for switching the capacitor in the time constant circuit of FIG. C3°C, , CS, C, is a capacitor, 5W-1, 3W-2, 591-:l.

591-4 is a switch.

In the circuit shown in Figure 4, if a mechanical switch is used as the selector switch, the resistance when the switch is turned on is small and there is no need to worry about it, and the leakage current when it is turned off is minute, so a mechanical switch can be used. However, electronic control is preferable as a circuit control mechanism, so it is better to use semiconductor switches, which are analog switches, than to use relays, etc., consisting of mechanical switches, for this electronic control. Conventionally, semiconductor switches have been used because of their low power consumption.

[Problems to be Solved by the Invention] In the conventional time constant circuit as described above, if a mechanical switch is used as the changeover switch to change the capacitance of the capacitor, both the resistance and leakage current when the switch is connected are small. It may not be such a problem, but it is desirable to use electronic control as a circuit control mechanism, and for this purpose, semiconductor switches, which consume less power, are increasingly used instead of mechanical switches such as relays. However, when a semiconductor switch is used, there is some resistance when the switch is on, and leakage current becomes a problem when the switch is off. Particularly in the case of a portable type device with a small power supply capacity, it is difficult to use a relay due to power consumption, so there are major problems with resistance when the E-leg switch is turned on and leakage current when it is turned off.

The present invention has been made to solve such conventional problems, and provides a capacitor capacity switching device that can reduce the on-resistance when a semiconductor switch is turned on and the leakage current when the switch is turned off. The purpose is to

[Means for Solving the Problems] In order to achieve the above object, in the capacitor capacity switching device of the present invention, one electrode of each capacitor is connected to a positive phase input terminal of an operational amplifier, and Each of the other electrodes of each of the capacitors is connected to the output terminal of the operational amplifier through a separate semiconductor switch, or to the ground or signal side, and is further connected to the negative phase input terminal of the operational amplifier. and the output terminals are connected to each other, or each of one electrode of each capacitor is connected to the positive phase input terminal of the first operational amplifier, or each of the other electrode of each of the capacitors is connected to the positive phase input terminal of the first operational amplifier. each connected to the output side of a separate operational amplifier,
The positive-phase input terminals of each of these separate operational amplifiers are each connected via a separate semiconductor switch to the output terminal of said first operational amplifier or to the ground or signal side and further connected to said first operational amplifier. The operational amplifier has a configuration in which a negative phase input terminal and an output terminal are connected to each other.

[Function] One electrode of each of the capacitors is connected to the positive phase input terminal of the operational amplifier, and the other electrode of each of the capacitors is connected to the output of the operational amplifier via a separate semiconductor switch. A configuration in which the operational amplifier is connected to a terminal or connected to ground or a signal side, and further, the negative phase input terminal and output terminal of the operational amplifier are connected to each other reduces leakage current when the semiconductor switching element is turned on. Something will happen. Further, each of one electrode of each capacitor is connected to a positive phase input terminal of a first operational amplifier, or each of the other electrode of each of the capacitors is connected to an output side of a separate operational amplifier, The positive-phase input terminals of each of these separate operational amplifiers are each connected via a separate semiconductor switch to the output terminal of said first operational amplifier or to the ground or signal side and further connected to said first operational amplifier. According to the configuration in which the negative phase input terminal and the output terminal of the operational amplifier are connected to each other, it is possible to reduce the on-resistance and also solve the problem of leakage current.

[Embodiment] FIG. 1 is a circuit diagram schematically showing the main parts of a capacitor capacity switching device according to an embodiment of the present invention.

FIG. 5(a) is a diagram showing the case of use in an LPF, and FIG. 2(b) is a diagram showing the case of use in an HPF.

In Fig. 1, 1, 2, 4.5 are semiconductor switches called so-called analog switches, 3゜6 is an operational amplifier (hereinafter referred to as an operational amplifier), and Figs.
The same reference numerals as in the figures indicate the same or corresponding parts.

In a time constant circuit, etc., the time constant is determined by the product of resistance and capacitance, so especially when the time constant is large, leakage current becomes a big problem because impedance increases when the resistance value is high.

Normally, an active circuit is used on the output side of the time constant circuit for impedance conversion, but if this output is used and the configuration shown in Figure 1 is adopted, the capacitor Cz
, C4 or c s, ca and semiconductor switch 1.
Since the connection with 2 or 4.5 is always driven with low impedance, leakage current is not a problem.

In the circuit of Figure 1(a), if semiconductor switch 1 is connected to the output side of operational amplifier 3 and semiconductor switch 2 is connected to the ground (GND) side, both ends of capacitor C3 are kept at the same potential. This is equivalent to not continuing this capacitor C3 in the circuit, so
The circuit shown in FIG. 1(a) is equivalent to a time constant circuit including a resistor R and a capacitor C4. In this case, the semiconductor switch 1.2
Since each connection due to switching in is driven with low impedance, all leakage current related to semiconductor switch 1 is absorbed by operational amplifier 3, and leakage current of semiconductor switch 2 is absorbed by GND, so problem 1 is solved. It won't happen.

Additionally, although there is generally an offset voltage in an operational amplifier, this offset voltage remains constant regardless of the presence or absence of input voltage, so the potential difference across the capacitor Ci is kept constant in the case of Figure 1(a) as well. Therefore, regardless of the offset voltage of the operational amplifier 3, no current flows through the capacitor C3, so it can be considered that the capacitor C3 does not exist.

Furthermore, in the circuit used for the HPF shown in FIG. 1(b), for example, if the semiconductor switch 4 is connected to the output side of the operational amplifier 6, and the semiconductor switch 5 is connected to the input signal side, then the circuit shown in FIG. ), the capacitor C
Both ends of C5 are held at the same potential, making it equivalent to not having this capacitor C5 in the circuit. At the same time, each switching connection in the semiconductor switch 4 is driven with low impedance, so all leakage current is absorbed by the operational amplifier 6, and leakage current from the semiconductor switch 5 is absorbed by the signal side, so there is no problem. .

FIG. 2 is a circuit diagram schematically showing the main parts of a capacitor capacity switching device showing another embodiment of the present invention; FIG. ) is H
It is a figure showing the case where it is used for PF.

In Figure 2, 7.10 is a semiconductor switch.

8.11 is a first operational amplifier, 9.12 is a second operational amplifier, and the same reference numerals as in FIG. 1.3.4 indicate the same or equivalent parts.

In addition, in the circuit of FIG. 2, for convenience of explanation, a separate operational amplifier is used only for capacitor C4 or C6.
A second operational amplifier 9 or 12 and a semiconductor switch 7 or lO connected thereto are provided, but a third operational amplifier and a semiconductor switch connected thereto are also provided as a separate operational amplifier for the capacitor Cat or C5. Of course, the capacitance of the capacitor C or C can be changed by providing the capacitor C or C.

In the circuit of FIG. 2(a), if the semiconductor switch 7 is connected to the GND side, for example, the capacitor C4 is equivalent to being connected to the GND, and in addition, the semiconductor switch 7 is connected to the GND side. Since the input impedance of the second operational amplifier 9 has a high resistance compared to the on-resistance, the on-resistance of the semiconductor switch 7 can be ignored. That is, the on-resistance of a semiconductor switch is as high as several hundred ohms, and as low as about 30 to 500, which is sufficiently low compared to the input impedance of an operational amplifier.

On-resistance is not a problem.

Furthermore, since the output impedance of the operational amplifier is l/loop gain, it is sufficiently low compared to the on-resistance of an analog switch, and is therefore equivalent to using a switch with a low on-resistance similar to a mechanical switch.

Next, when the semiconductor switch 7 is connected to the output side of the first operational amplifier 8, both ends of the capacitor C4 are kept at the same potential as in the case of FIG. It is equivalent to not existing in the circuit. Furthermore, when the capacitor C4 is equivalent to not being present, one circuit is always driven at low impedance, and there is no problem with leakage current due to switching of the semiconductor switch 7 when changing the capacitance of the capacitor C4.

Also, in the circuit used for the HPF in FIG. 2(b),
When connected to the output side of the semiconductor switch 10 or the first operational amplifier 11, both ends of the capacitor C6 are kept at the same potential, and it is equivalent to the capacitor C6 not existing in the circuit.

At that time, the second
Since the input impedance of the operational amplifier 12 is high resistance, the on-resistance of the semiconductor switch IO can be ignored, as in the case of FIG. 2(a). Further, as described in the case of FIG. 2(a), problems regarding leakage current and on-resistance do not occur.

[Effects of the Invention] As explained above, according to the present invention, leakage current and on-resistance can be reduced with a simple circuit configuration by connecting an operational amplifier to the output side and by providing an operational amplifier between a capacitor and a semiconductor switch. can solve the problem.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram schematically showing the main parts of a capacitor capacity switching device that is an embodiment of the present invention, and (a)
FIG. 2 is a circuit schematically showing the main parts of a capacitor capacity switching device showing another embodiment of the present invention. In the figure, the figure (a) is a diagram showing the case where it is used for LPF,
Figure 3 (b) is a diagram showing the case where it is used in HPF, and Figure 3 (
(a) is a diagram showing an outline of the time constant circuit that constitutes the main part of the LPF, (b) is a diagram showing the outline of the time constant circuit that makes up the main part of the HPF, and Fig. 4 is a diagram showing the outline of the time constant circuit that makes up the main part of the HPF. The same figure (
a) is a capacitor switching circuit used in LPF, and (b) is the same figure.
is a capacitor switching circuit diagram used in the HPF. In the figure. R,, R2: resistance

Claims (2)

[Claims] (1) In a capacitor capacity switching device in a circuit having a plurality of capacitors each connected in parallel with each other, one electrode of each of the capacitors is connected to a positive phase input terminal of an operational amplifier, and Each of the other electrodes of the capacitors are each connected via a separate semiconductor switch to the output terminal of the operational amplifier, or to the ground or signal side, and are further connected to the anti-phase input and output terminals of the operational amplifier. A capacitor capacity switching device characterized in that the capacitors are connected to each other. (2) In a capacitor capacity switching device in a circuit having a plurality of capacitors each connected in parallel with each other, one electrode of each capacitor is connected to a positive phase input terminal of a first operational amplifier, and , each of the other electrodes of each of the capacitors is connected to the output side of a separate operational amplifier, and each positive-phase input terminal of each of these separate operational amplifiers is connected to the first operational amplifier via a separate semiconductor switch. Capacitance switching of a capacitor, characterized in that the capacitor is connected to an output terminal of an amplifier, or connected to a ground or signal side, and further, an anti-phase input terminal and an output terminal of the first operational amplifier are connected to each other. Device.