JPH0429247B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a switched capacitor integrator used, for example, in electronic filters, speech recognition circuits, speech synthesis circuits, etc.

FIG. 1 shows the basic circuit of a switched capacitor circuit, and FIG. 2 shows its equivalent circuit. In FIG. 1, the first contact a of the changeover switch S is the input terminal 11.
The second contact point b is connected to the output terminal 12, and the common connection point c is connected to the ground terminal via the capacitor _CS . Ground potentials Vi _{and Vp} are applied to the input terminal 11 and output terminal 12, and the switch S is switched _S times per second. now,
As shown in FIG. 1a, switch S is connected to input terminal 11.
When connected to the side, the charge Q ₁ charged in the capacitor C _S becomes "Q ₁ = C _S ·V _i ". Next, Figure 1b
When the switch S is connected to the output terminal 12 side as shown in the figure, the charge Q ₂ of the capacitor C _S becomes ``Q ₂ = C _S・
V _p ”. Therefore, switch S is input terminal 11
It is considered that the charge of ΔQ was moved from the input terminal 11 to the output terminal 12 by a series of operations of switching from the side to the output terminal 12 side.

ΔQ=Q ₁ −Q ₂ =C _S (V _i −V _p ) ...(1) Since the switch S switches _S times per second, the average current i from the input terminal 11 to the output terminal 12 is expressed as i=ΔQ・_S = C _S (V _i −V _p ) _S ...(2) will flow.

If the switching frequency _S of the switch S is sufficiently larger than the frequency of the voltages V _i and V _p , the current i becomes equal to the current determined by the instantaneous values of V _i and V _p , and the circuit of FIG. 1 becomes as shown in FIG. Input terminal 11, output terminal 12
This is equivalent to a circuit with a resistor R connected between them. Here, R=V _i −V _p /i=1/ _CS · _S (3).

In other words, a switched capacitor circuit is equivalent to obtaining a resistance R by switching the capacitor C _S as described above, and a switched capacitor integrator is an integrator constructed using this equivalent resistance. It is.

FIG. 3 shows a Miller integrator using an operational amplifier 31, and it is well known that its input/output characteristics are given by the following equation.

V _p /V _i =-1/S・R _S・C _f ...(a) _Vi : Input voltage V _p : Output voltage R _S : Between the input terminal 11 and the inverting input terminal (-) of the operational amplifier 31 Input resistance S _S connected between: Capacitor connected between the output terminal of the operational amplifier 31 and the inverting input terminal (-) Note that V _DD and V _SS in FIG. 3 are power supplies, and the operational amplifier 31 The non-inverting input terminal (+) of is grounded.

FIG. 4 shows a Miller integrator configured using a switched capacitor circuit 41 instead of _the resistor R _S in FIG.
This is obtained by substituting R in the previous equation (3) into .

V _p /V _i = - _S /S (C _f /C _S ) ...(5) In other words, the input/output characteristics of the Miller integrator shown in Fig. 4 are determined by the capacitance ratio of capacitors C _S and C _f and the switching of switch S. It is a function of frequency _S , especially a linear expression of frequency _S. This shows that the integration time constant can be changed in proportion to the frequency _S , and if the mirror integrator shown in Figure 4 is used as a filter unit, the filtering frequency can be changed in proportion to the switching frequency _S. becomes possible.

On the other hand, FIG. 5 shows a Miller integrator equivalent to that shown in FIG. 4, in which a switched capacitor circuit 50 is used as a negative resistance having an equivalent negative resistance value. This switched capacitor circuit 50
The capacitor is controlled by two changeover switches S ₁ and S ₂ .
It is configured to switch both ends of C _S simultaneously. That is, the first contact of the first changeover switch _S1
a ₁ to input terminal 11, and the second changeover switch
The first contact a ₂ of S ₂ is connected to the inverting input terminal (-) of the operational amplifier 31, and the second contacts b ₁ and b ₂ of the switches S ₁ and S ₂ are connected together and connected to the reference power supply V _ref (in this example is connected to ground potential).

Now, as shown in figure a, selector switch _S1 is set to
When the contact _A1 side is connected to the second contact B2 side, and the switch _S2 is connected to the second contact _B2 side, a potential difference "V _i −V _ref " is applied to both ends of the switched capacitor C _S , so the following equation shows: A charge Q _a is charged.

Q _a = C _S (V _i - V _ref ) Next, as shown in figure b, the changeover switch S ₁ switches to the second
When the switch S ₂ is connected to _the contact b ₁ side and the first contact a ₂ side, a potential difference “V _ref −
V _i ” is applied, the amount of charge Q _b is expressed by the following equation.

Q _b = C _S (V _ref −V _i ′) Here, V _i ′ is the inverting input terminal (−) of the operational amplifier 31
voltage.

Therefore, the amount of charge movement ΔQ at this time is ΔQ=−(Q _a −Q _b )=−C _S (V _i +V _i ′−2V _ref ) (6). The reason for the negative sign in the above equation is that in the switching state shown in figure a, it is assumed that the current flows from the potential V _i side to the potential V _ref side, whereas in the switching state shown in figure b, it is assumed that the current flows from the potential V′ _i side. This is because a current flows toward the potential V _ref side. Since the operational amplifier 31 operates so that the inverting input terminal (-) of the operational amplifier 31 is virtually grounded to the potential V _ref ,
If we set “V′ _i =V _ref ”, the previous equation (6) becomes ΔQ=−C _S (V _i −V _i ′) ……(7), and the switches S ₁ and S ₂ increase _S in one second. When the switching state shown in figures a and b is repeated, the current I becomes I=ΔQ· _S =−C _S (V _i −V′ _i ) _S (8). Therefore, the equivalent resistance R due to this switched capacitor circuit 50 is R=V _i −V′ _i /I=−1/C _S・_S (9), which is compared with the previous equation (3). For example, it can be understood that this switched capacitor circuit 50 functions as a negative resistance.

By the way, as shown in FIGS. 4 and 5, the switched capacitor integrator used as a mirror integrator has two terminals for the operational amplifier power supplies V _DD and V _SS and a reference power supply V _ref ( Requires one terminal for grounding. Therefore, such a switched capacitor integrator can be used with two power supplies (V _DD , V _SS ).
In order to mix it with the normal random logic used, it becomes necessary to add one more power supply terminal.

However, increasing the number of power supply terminals is fatal, especially in integrated circuits. In other words, in integrated circuit design, this leads to a longer design period, an increase in the chip area of the integrated circuit, and difficulty in pattern design for the three power supply terminals.In addition, power amplification during printed board mounting makes printed board design difficult. This would make it more difficult and lead to a significant increase in costs.

This invention was made in view of the above-mentioned circumstances, and its purpose is to reduce the number of power supplies used and to create a switch that can be easily integrated into integrated circuits since the number of power supply terminals is small. The present invention provides a capacitor integrator.

An embodiment of the present invention will be described below with reference to the drawings.

Figure 6 shows its configuration, and in the figure,
The switched capacitor circuit 60 connects the capacitor C _S to the first contact by switching switches S ₁ and S ₂ that operate simultaneously.
It is connected to the a ₁ and a ₂ side or the second contact b ₁ and b ₂ side, and the switching frequency is _S. The first contact _a 1 of the changeover switch S ₁ is connected to the input terminal 61 to which the input voltage V _i is applied, and the second contact b ₁ is connected to the power supply
Connected to _VDD . In addition, the _second switch of changeover switch S2
Contact _a2 is connected to the inverting input terminal (-) of the operational amplifier 31,
The second contact _b2 is connected to the power supply _VSS .

On the other hand, the operational amplifier 31 is supplied with power supplies V _DD and V _SS , and its output terminal is connected to the output terminal 62 as well as to the inverting input terminal (-) via the capacitor C _f , and the non-inverting input terminal ( +) is the above power supply V _DD
A voltage intermediate between the voltage and the power supply V _SS voltage is applied.
This intermediate voltage is generated by the power supplies V _DD and V _SS , and its magnitude is appropriately selected depending on the characteristics of the operational amplifier 31. If you want to obtain, for example, "1/2 (V _DD - V _SS )" as the above intermediate voltage, the drain of the N-channel transistor T ₁ whose gate and drain are connected is connected to the power supply V as shown in FIG. _DD , the source of an N-channel transistor _T2 whose gate and drain are also connected is connected to the power supply _VSS , and the source of the transistor _T1 and the drain of the transistor _T2 are connected, and this connection point It is sufficient to connect E to the non-inverting input terminal (+).

Next, the operation of the switched capacitor circuit configured as described above will be explained. Now, Figure 6a
As shown in the figure, the changeover switch _S1 is placed on the first contact _A1 side,
When the switch S ₂ is connected to the second contact b ₂ side, a potential difference “V _i −V _SS ” is applied to both ends of the switched capacitor C _S , so if “V _SS = 0” is set, A charge Q _a as shown in the following equation is charged.

Q _a = C _S・V _i Next, as shown in figure b, selector switch S ₁ is set to
When the switch S ₂ is connected to _the contact b ₁ side and the first contact a ₂ side, a potential difference “V _DD −
V′ _i ” is applied, the amount of charge Q _b is expressed by the following equation.

Q _b = C _S (V _DD −V′ _i ) Therefore, the amount of charge movement ΔQ at this time is ΔQ=−(Q _a −Q _b ) = −C _S (V _i +V′ _i −V _DD ) ...(10) becomes. The non-inverting input terminal (+) of the operational amplifier 31 is supplied with "V _DD /2" by a bias circuit.
As in the case of _{FIG .}
2”, so “V′ _i =
V _DD /2'', the previous equation (10) becomes ΔQ=-C _S (V _i -V' _i ), and the average current I flowing between contacts a ₁ and a ₂ and the equivalent resistance R are given by the following equation. become that way.

I=ΔQ・_S =−C _S (V _i −V′ _i )・_S ……(11) R=V _i −V′ _i /I=−1/C _S・_S ……(12) The above formula ( 11) and (12) are the same as the previous equations (8) and (9), and it can be seen that this switched capacitor circuit acts as a negative resistance. Therefore, the circuit in Figure 6 is as described above. It functions in the same way as the circuit shown in FIG. 5, and the input/output characteristics of the integrator shown in FIG. 6 are expressed by the following equation.

V _p /V _i =C _S /S·C _{f S} (13) FIG. 7 shows an example in which the switched capacitor circuit 60 portion of FIG. 6 is integrated into an integrated circuit. In the switched capacitor circuit 70, T ₃ to _{T 6} are each, for example, an N-channel field effect transistor, and a transistor T ₃ functioning as a first switch circuit and a transistor T ₄ functioning as a second switch circuit are connected to each other. A transistor T 5 corresponding to one of the changeover switches S ₁ in FIG _. 6 and functioning as a third switch circuit, and a transistor functioning as a fourth switch circuit.
T ₆ corresponds to the other changeover switch S ₂ . Then, the transistors T ₃ and T ₆ corresponding to the first and fourth switch circuits are in the same switch state,
Switching is controlled so that transistors T ₄ and T ₅ corresponding to the second and third switch circuits are in the same switch state. That is, the drain of the transistor _T3 is connected to the input terminal 61, and the drain of the transistor T3 is connected to the input terminal 61.
The source of T ₄ is connected to the power supply V _DD , and one end of the capacitor _CS is connected to the connection point between the source of the transistor T ₃ and the drain of the transistor T ₄ . On the other hand, the drain of the transistor _T5 is connected to the inverting input terminal (-) of the operational amplifier 31, the source of the transistor _T6 is connected to the power _{supply VSS} , and the source of the transistor _T5 is connected to the drain of the transistor _T6 . The other end of the capacitor C _S is connected to the contact. The gates of the transistors T ₃ and T ₆ are connected together to the clock input terminal 71, and the gates of the transistors T ₄ and T ₅ are collectively connected to the clock input terminal 72. 72, as shown in Figure 8 a or b, each has a period of 1/ _S and is simultaneously "1".
Clock pulses φ ₁ and φ ₂ that do not reach the level are introduced. Therefore, when φ ₁ = “1” and φ ₂ = “0”, transistors T ₃ and T ₆ are in the on state, and transistors T ₄ and T ₅ are in the off state, which is the same as the circuit state in Fig. 6a. Become. On the other hand, φ ₁ = “0”, φ ₂ =
When the signal is "1", transistors T ₃ and T ₆ are turned off, and transistors T ₄ and T ₅ are turned on, resulting in the same circuit state as in FIG. 6b.

In the circuit shown in FIG. 7, one transistor T3 to _T3 is used as each of the first to fourth switch circuits.
Although _T6 is used, other analog switches may be used instead, such as transistor switches such as transmission gates.

FIG. 9 shows another embodiment of the present invention, in which a switched capacitor circuit 90 is provided in which the power supply V _DD and V _SS terminals of the switched capacitor circuit 60 in the circuit shown in FIG. 6 are replaced. It is something. Even in such a configuration, the switched capacitor circuit can be operated as a negative resistance similarly to the above embodiment.

In each of the embodiments described above, a potential (for example,
The bias circuit for applying V _DD -V _SS /2) can be modified in various ways, and it goes without saying that a circuit with low current consumption, such as a step-down circuit, may be used. In addition, in the case of the operational amplifier 31 using MOS transistors in the input stage, the input impedance of the non-inverting input terminal (+) is almost infinite, so the bias circuit may have a high input impedance, and such a bias The circuit can sufficiently reduce current consumption.

As explained above, according to the present invention, the operational amplifier power supply is used for the discharge path of the switched capacitor circuit, and the operational amplifier power supply is used to bias the non-inverting input terminal of the operational amplifier. Since a bias circuit is provided for applying voltage, the number of power supplies used can be reduced and a single power supply can be used, and the number of power supply terminals can be reduced when integrating circuits, making it easy to integrate switched capacitor integrators. can get.

[Brief explanation of drawings]

Figures 1 a and b are circuit diagrams showing different operating states of the basic circuit of a switched capacitor circuit, Figure 2 is an equivalent circuit of Figure 1, and Figures 3 and 4 are circuit diagrams of a conventional Miller integrator. The circuit diagram shown in FIG. 5a,
6b is a circuit diagram showing different operating states of a conventional switched capacitor integrator; FIGS. 6a and 6b are circuit diagrams showing different operating states of a switched capacitor integrator according to an embodiment of the present invention; Figure 7 is the above 6th
A circuit diagram showing a specific example of the configuration of the switched capacitor circuit in the circuit shown in the figure; FIGS. 8a and 8b are timing diagrams shown to explain the operation of FIG. 7;
The figure is a circuit diagram showing another embodiment of the invention. 31... Operational amplifier, 60, 70, 90... Switched capacitor circuit, 61... Signal input terminal, 62
…Output terminal, C _S …Switching capacitor, C _f …
Capacitor, T ₁ to T ₆ ...transistor, V _DD , V _SS
…power supply.

Claims (1)

[Claims] 1. An operational amplifier, a capacitor connected between an inverting input terminal and an output terminal of the operational amplifier, and a voltage dividing circuit between one power supply and the other power supply for the operational amplifier. a bias circuit that generates a bias voltage and supplies it to the non-inverting input terminal of the operational amplifier; and a switching circuit that is provided between the signal input terminal to which the input signal voltage is applied and the inverting input terminal of the operational amplifier. A switching capacitor is provided at both ends of the switching capacitor and the switching capacitor, and the switching capacitor is connected between the signal input terminal and one power supply for the operational amplifier in the first operation period, and and switching means for alternately forming circuits connecting switching capacitors between the other power supply for the operational amplifier and the inverting input terminal of the operational amplifier. vessel. 2 The bias circuit has a first circuit connected in series between one power source and the other power source for the operational amplifier.
2. A switched capacitor integrator according to claim 1, characterized in that said bias voltage is obtained from a connection point of said transistor, and said bias voltage is obtained from a connection point of said transistor. 3. The switching means includes a first transistor switch connected between the signal input terminal and one end of the switching capacitor and whose conduction is controlled by a first signal, and one end of the switching capacitor and the operational amplifier. a second transistor switch connected between the other power supply of the operational amplifier and whose conduction is controlled by a second signal; and a second transistor switch connected between the other end of the switching capacitor and the inverting input end of the operational amplifier; a third transistor switch whose conduction is controlled by the signal; and a fourth transistor which is connected between the other end of the switching capacitor and one power supply for the operational amplifier and whose conduction is controlled by the first signal. A switched capacitor integrator as claimed in claim 1, characterized in that it comprises a switch.