JPS6134689B2 - - Google Patents

Description

[Detailed Description of the Invention] "Field of Industrial Application" This invention further develops an active filter using a state variable method that takes advantage of the characteristics of integrated circuits and operational amplifiers, and continuously increases its cut-off frequency by several thousand times. This invention relates to a filter circuit that can be changed.

``Prior Art'' In general, a state variable filter circuit is configured as shown in FIG. 1 and has excellent performance as a high pass filter (HPF), a hand pass filter (BPF), and a low pass filter (LPF). That is, operational amplifiers U ₁ , U ₂ , and U ₃ are combined into three stages, and the second and third stage operational amplifiers U ₂ and U ₃ are connected to a resistor R ₁ and a capacitor C ₁ and a cut-off frequency determining circuit.
It is formed by combining R ₂ and C ₂ as a pair.

However, in the past, simply changing the frequency was troublesome and was done by mechanically switching resistors and capacitors.

Also, to change the cutoff frequency continuously,
I was doing this by changing the resistance value of the variable resistor.

``Problems to be solved by the invention'' However, the method of changing the variable resistor in order to continuously change the cutoff frequency requires matching the characteristics of the components, which is extremely expensive and causes deterioration over time. It had drawbacks such as being difficult to control, being unstable, and not being able to be controlled remotely.

Originally, state variable filter circuits belong to relatively complex circuits, and to continuously change them electrically while satisfying dynamic range characteristics, temperature characteristics, and other various characteristics, the overall configuration would be extremely complicated and practical. It was considered unsuitable.

"Means for Solving the Problems" The present invention was made to solve these problems, and includes two sets of cut-off frequency determining circuits each pairing a multi-stage operational amplifier and a capacitor and a resistor. In a state variable type filter comprising:
The resistor of at least one of the two sets of cut-off frequency determining circuits is constituted by a variable resistor which is a light receiving element of a photocoupler, and the light emitting element is connected to a light emitting element of the photocoupler which is a control side of the variable resistor. The output side of a comparison circuit for controlling the resistance value of the variable resistor by the voltage applied to the variable resistor is coupled to one input terminal of the comparison circuit, a DC reference voltage input terminal is coupled to one input terminal of the comparison circuit, and a DC reference voltage input terminal is coupled to the other input terminal of the comparison circuit. is connected to the output terminal of the operational amplifier in the previous stage, and a control current input terminal is connected to the connection point between the variable resistor and the operational amplifier in the next stage, so that the cutoff frequency of the filter can be variably controlled. .

"Function" If you want to set the cutoff frequency to ₁ , use a variable resistor.
By simply passing I ₁ and I ₂ as control currents through R ₁ and R ₂ , this state variable filter automatically has a cutoff frequency of ₁ .

In other words, in this circuit, the current I ₁ flows through the variable resistor R ₁ to try to move the potential on the inflow side (point a ₁ ), but in reality, as a state variable filter, the potential at point _a remains unchanged, but instead b ₁
The potential at a point attempts to move exactly by I ₁ × R ₁ .
However, the local loop of the control circuit, which includes a comparison circuit and a photocoupler, operates and the potential at _one point b
Eb ₁ becomes a voltage equal to the DC reference voltage E ₁ . In the end, it passes through a double negative feedback circuit consisting of the loop circuit of the main signal line of the filter and the added local loop circuit.
The effective resistance R ₁ of the variable resistor becomes R ₁ =E ₁ /I ₁ due to the current I ₁ .

Similarly on the variable resistor R ₂ side, if E ₁ = E ₂ and I ₁ = I ₂ , then E ₁ /I ₁ = E ₂ /I ₂ = R ₁ = R ₂ = R, and = 1/
It becomes 2πCR.

The cutoff frequency is thus determined by the control current.

What is important in the filter circuit of the present invention is that the potential at point _b1 is I ₁ ×R ₁ and the potential at output point b ₂ of operational amplifier U ₂ is I ₂ ×R ₂ .

The filter circuit of this invention focuses on this point,
It was completed using already systemized feedback technology and analog technology.

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

U ₁ , U ₂ , and U ₃ are operational amplifiers connected in three stages in sequence, and a capacitor is connected between the output terminal b ₂ of the second stage operational amplifier U ₂ and the negative input terminal.
C ₁ and a resistor R ₁ is inserted between the negative input terminal and the output terminal b ₁ of the operational amplifier U ₁ in the previous stage. A cutoff frequency determining circuit is configured by pairing this capacitor _C1 and resistor _R1 . A similar capacitor C ₂ and resistor R ₂ are inserted in the third stage operational amplifier. The resistors R ₁ and R ₂ are variable resistors whose resistance value substantially changes depending on the voltage on the control side. More specifically, variable resistor
For R ₁ and R ₂ , the light receiving elements of the photo couplers P ₁ and P ₂ are used, and on the control side of the variable resistors R ₁ and R ₂ , the light receiving elements L ₁ and L ₂ of the photo couplers P ₁ and P ₂ are used. . These light emitting elements L ₁ and L ₂ are coupled to comparison circuits A ₁ and A ₂ consisting of differential input DC component amplifiers, and one input terminal E ₁ and E ₂ of these comparison circuits A ₁ and A ₂ is connected to a DC reference. Voltages E ₁ and E ₂ are applied, and the other input terminal
The output voltages Eb ₁ and Eb ₂ of the operational amplifiers U ₁ and U ₂ in the previous stage are applied to Eb ₁ and Eb ₂ . In addition, the variable resistor
The input terminals I ₁ , I 2 of the control currents I ₁ , I ₂ are connected to the connection points of R ₁ , R ₂ and the negative input terminals of the next-stage operational amplifiers U ₂ , _{U 3 .}
are combined. Note that a feedback path is formed via resistors R ₃ , R ₄ , R ₅ , and R ₆ so that the input operating points of the second and third stage operational amplifiers U ₂ and U ₃ are always at zero level. There is.

In the above configuration, when the cutoff frequency is set to ₁ , for example, currents I ₁ , I ₂ from control terminals I ₁ , I ₂
flow. This current I ₁ flows into the capacitor C ₁ transiently, but does not flow thereafter. Furthermore, in the state variable type, the signal does not flow to the negative input of the second stage operational amplifier _U2 . Therefore, all of the control current I ₁ flows through the resistor R ₂ . In the state variable filter circuit, due to this I ₁ , the potential at _one point a does not change, and only the potential Eb ₁ at _one point b changes.
Try to move by I ₁ × R ₁ . However, this potential Eb ₁ at point b ₁ is compared with the DC reference E ₁ , and the output voltage of comparator circuit A ₁ increases by the difference in voltage.
A current flows through the light emitting element L ₁ and it emits light, so R ₁
gradually becomes lower resistance. And finally, Eb ₁ is
Consistent with E ₁ , it becomes a stable state. At this time, the effective resistance R ₁ of the variable resistor becomes R ₁ =E ₁ /I ₁ from the control current I ₁ and the reference voltage E ₁ .

Similarly, current I ₂ flows through variable resistor R ₂ .
And if E ₁ = E ₂ and I ₁ = I ₂ , then E ₁ /I ₁ = E ₂ /I ₂ =
R ₁ = R ₂ = R, cutoff frequency = 1/2πCR
becomes. In BPF, it is the passing center frequency.

In this way, by flowing the control currents I ₁ and I ₂ to both the variable resistors R ₁ and R ₂ , the frequency can be varied over a range of thousands of times.

In addition, when obtaining a minute frequency variable range, if a control current is applied to only one of the variable resistors R ₁ and R ₂ to vary the resistance, a frequency variable range several times larger can be obtained.

In FIG. 2, the path of operational amplifier U ₂ -R ₄ -U ₁ and the operational amplifier U ₂ -U ₃ - are arranged so that the input operating point of operational amplifier U ₂ is always at zero level.
100% negative feedback is applied in the DC region through the path of U ₁ , so when current I ₁ flows into point A ₁ , the input of operational amplifier U ₂ returns to zero level, regardless of other parts. As shown, I ₁ ×R ₁ appears as the output voltage of U ₁ .

Similarly, operational amplifiers U ₃ , U ₁ , U ₂ are arranged so that the input operating point of operational amplifier U ₃ is always at zero level.
Since 100% negative feedback is applied in the _{DC region via , when current I 2 flows} into point a,
A potential of I ₂ ×R ₂ is generated at the output of operational amplifier U ₂ .

"Effects of the Invention" Since the filter circuit of the present invention is constructed as described above, the cutoff frequency can be changed continuously over a range of thousands of times. Furthermore, even if the variable resistor fluctuates due to temperature, since this variable resistor is in the servo circuit, it will be automatically corrected, making it possible to keep the cutoff frequency setting error, fluctuation, etc. below 0.5%. be. Furthermore, since a photocoupler light-receiving element is used as a variable resistor, and the resistance value is substantially changed in response to the voltage applied to the light-emitting element, distortion occurs more easily than when using a semiconductor variable transconductance element. The signal voltage and S/N ratio can be increased. Furthermore, the circuit configuration is simple and can be manufactured at low cost.

[Brief explanation of the drawing]

FIG. 1 is a basic circuit diagram of a conventional state variable type filter, and FIG. 2 is a circuit diagram showing an embodiment of the filter circuit of the present invention. C ₁ , C ₂ : Integrating capacitor, R ₁ , R ₂ : Integrating variable resistance element, P ₁ , P ₂ : Photocoupler, A ₁ , A ₂ : Comparison circuit (differential input DC component amplifier), U ₁ , U ₂ ，
U ₃ : Operational amplifier, R ₃ , R ₄ , R ₅ , R ₆ : Resistor, I ₁ ,
I ₂ : Control current input terminal.

Claims (1)

[Claims] 1 Negative feedback is applied to operational amplifiers connected in three stages in series, and integration circuits each consisting of a capacitor and a resistor that determine the cut-off frequency are connected to the second and third stage operational amplifiers. In the state variable type filter, the resistors of the two sets of integration circuits for determining the cutoff frequency are each constituted by a variable resistor which is a light receiving element of a photocoupler, and the light emitting element of the photocoupler which is a control side of the variable resistor is configured. The output side of a comparator circuit that controls the resistance value of the variable resistor by the voltage applied to the light emitting element is coupled to the output side of the comparator circuit, one input terminal of the comparator circuit is coupled to a DC reference voltage input terminal, and the other The output terminal of the preceding stage operational amplifier is coupled to the input terminal of the filter, and the control current input terminal is coupled to the connection point between the variable resistor and the next stage operational amplifier, thereby variably controlling the cutoff frequency of the filter. filter circuit. 2. The filter circuit according to claim 1, wherein the comparison circuit comprises a differential input DC component amplifier.