JPS632170B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a filter using a biquad circuit in which signal polarities at input and output terminals can be made in phase.

Conventionally, a filter using a biquad circuit composed of a plurality of switched capacitors and two stages of integrators has the configuration shown in FIG. Operational amplifier OP ₁ ,
OP ₂ and integrating capacitors C ₁ and C ₂ (C ₁ = C ₂ = C
), two stages of integrators are connected in cascade, and the switched capacitors are connected in four stages: K ₀ C, K ₁ C, K ₂ C, and K ₄ C.
In a filter using a capacitor K ₃ C, the input voltage V ₁ and the output voltage V ₂ have opposite polarities. If the integral function connected to the switched capacitor is expressed approximately as an analog integral function -1/S (S=jω), the equivalent circuit shown in FIG. 2 is obtained.

In Figure 2, α ₀ =K ₀ /T, α ₁ =K ₁ /T, α ₂ =K ₂ /T α ₃ =K ₃ , α ₄ =K ₄ /T (T is the switched capacitor ), and the relationship between V ₁ , V _n , and V ₂ is expressed by an equation: V _n = α ₀ (-1/S) V ₁ + α ₁ (-1/S) V ₂ ...(1) V ₂ =-α ₂ (-1/S) V _n +α ₄ (-1/S) V ₂ -α ₃ V ₁
...(2) Eliminate V _n from equations (1) and (2) to find V ₂ /V ₁ : V ₂ /V ₁ = - (α ₀ α ₂ + α ₃ S ² ) / α ₁ α ₂ + α ₄ S＋S
² ...(3) The negative sign in equation (3) is the output signal for input signal V ₁ .
V ₂ indicates that the polarity is reversed. Therefore, when obtaining an output signal V ₂ that is in phase with V ₁ , the first
It is necessary to additionally use an operational amplifier for phase inversion at the output section (or input section) shown in the figure. As a result, power consumption increases, and the filter becomes larger and more expensive.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a filter using a biquad circuit which improves the above-mentioned drawbacks and obtains an output signal in phase with the input with a simple configuration.

Embodiments of the present invention shown in the drawings will be described below. FIG. 3 is a circuit configuration diagram showing a first embodiment of the present invention, and the same reference numerals as in FIG. 1 indicate the same components. K ₃ C is a switched capacitor for operational amplifier OP ₁ , which constitutes the first stage integrator.
It is connected in parallel with K ₀ C and passes the input signal V ₁ . Operational amplifier OP ₂ that constitutes the second stage integrator
A capacitor K ₆ C connected in parallel to the switched capacitor K ₂ C and a switched capacitor K ₅ C for directly applying the input signal V ₁ are connected to the input of the switch. A capacitor K ₅ C is used to cancel unnecessary output signal components caused by connecting K ₃ C and K ₆ C. If the equivalent circuit is drawn in the same manner as in FIG. 2, it will become as shown in FIG. 4. The relational expression among V ₁ , V _n , and V ₂ is found as follows in the same manner as in Fig. 2.

α ₀ = K ₀ /T, α ₁ = K ₁ /T, α ₂ = K ₂ /T, α ₃ = K ₃ α ₄ = K ₄ /T, α ₅ = K ₅ /T, α ₆ = K ₆ V _n =-α ₀ (-1/S)V ₁ +α ₁ (-1/S)V ₂ -α ₃ V ₁
…(4) V ₂ = −α ₂ (−1/S) V _n +α ₄ (−1/S) V ₂ −α ₅ (
− 1/S) −α ₆ V _n ……(5) Eliminate V _n from equations (4) and ( ₅ ) to find V ₂ /V 1. V ₂ /V ₁ = α ₀ α ₂ − (α ₂ α ₃ − α ₅
+α ₀ α ₆ )S+α ₃ α ₆ S ² /α ₁ α ₂ +(α ₄ −α ₁ α ₆ )S
+S ² ...(6) Here, if each capacitance value is selected so that α ₅ = α ₂ α ₃ + α ₀ α ₆ ...(7), equation (6) becomes V ₂ /V ₁ = α ₀ α ₂ +α ₃ α ₆ S ² /α ₁ α ₂ +(α ₄ −α ₁ α ₆
)S+S ² ...(8) Equation (8), like equation (3), represents the function of the band elimination filter. There is no negative sign on the right side of equation (8).
This shows that the polarities of V ₁ and V ₂ are in phase. That is, the circuit shown in FIG. 4 can realize a band elimination filter in which the phases of input and output signals are not inverted.

If the capacitance value is selected to satisfy the following conditions, a low-pass filter with the same phase of input and output signals,
It is also possible to implement a high-pass filter or a band-pass filter.

A Low-pass filter α ₅ =α ₂ α ₃ +α ₀ α ₆ α ₃ =0 (or α ₆ =0).

V ₂ /V ₁ =α ₀ α ₂ /α ₁ α ₂ + (α ₄ −α ₁ α ₆ )S+S ² …
…(9) B High-pass filter α ₅ = α ₂ α ₃ + α ₀ α ₆ α ₀ = 0 V ₂ /V ₁ = α ₃ α ₆ S ² /α ₁ α ₂ + (α ₄ −α ₁ α ₆ )S+S ²
...(10) C Band pass filter α ₀ = 0, α ₃ = 0 V ₂ /V ₁ = α ₅ S/α ₁ α ₂ + (α ₄ −α ₁ α ₆ )S+S ² ……(
11) Next, FIG. 5 is a circuit diagram showing a second embodiment of the present invention in which two output signals having the same phase and opposite phase with respect to the input signal are obtained. The output of V 2 is in phase with respect to the input from the V ₁ terminal, and the output of _{V 2 is} in reverse phase with respect to the input from the V ₃ terminal. As for the input terminal of the operational amplifier OP ₁ , there is a path for the input from the V ₃ terminal to pass through the switched capacitor.
The operation using this path is the same as that shown in FIG. 1, and when the input is from the _V1 terminal, the operation shown in FIG. 3 is performed, so it can be used by switching as appropriate. They can also be used at the same time.

In this way, according to the present invention, in a filter using a biquad circuit, a configuration in which the output signal is in phase with the input signal can be easily obtained, so that the filter is small and simple.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a conventional filter, Fig. 2 is an equivalent circuit diagram of Fig. 1, Fig. 3 is a circuit diagram of the first embodiment of the present invention, and Fig. 4 is an equivalent circuit diagram of Fig. 3. 5 are circuit configuration diagrams of a second embodiment of the present invention. OP ₁ , OP ₂ ... operational amplifier, C ₁ , C ₂ ... capacitor, K ₀ C, K ₁ C, K ₂ C, K ₄ C, K ₅ C ... switched capacitor.

Claims (1)

[Claims] 1. In a filter using a biquad circuit composed of a plurality of switched capacitors and two stages of integrators, the input signal is directly coupled to the first stage integrator by connecting the switched capacitor in parallel. The input of the second stage integrator is provided with a capacitor to
A connection circuit that inputs the stage integrator output via a switched capacitor, a capacitor connected in parallel with the connection circuit, and a second stage integration that inputs the first stage integrator input via another switched capacitor. A filter using a biquad circuit, characterized by comprising a connection circuit for inputting an input to a filter. 2 The first stage integrator input is connected only through another switched capacitor, and the second stage integrator input is further provided with an input terminal that goes through a parallel connection circuit of a switched capacitor and a capacitor. A filter using a biquad circuit according to claim 1.