JPH0628328B2 - Filter circuit - Google Patents

Description

TECHNICAL FIELD The present invention relates to a filter circuit, and is particularly suitable for a semiconductor integrated circuit.

[Background technology]

Electronic devices tend to be smaller and lighter, and therefore, electronic circuits are being made into semiconductor integrated circuits. At this time, what becomes a problem is a circuit component such as a capacitor or a coil which is difficult to be formed into a semiconductor integrated circuit, and in many cases, it is an external component.

However, regarding capacitors, small capacitors can be formed within a semiconductor substrate. Therefore, when various filter circuits or capacitors are required, "Electronic Device Encyclopedia" (issued on March 20, 1976, first edition, published by Radio Technology Co., Ltd., pp160-16)
Variable capacitance element and junction capacitance as shown in 1),
Further, a MOS capacitor or the like is used.

According to the study of the present inventor regarding the variable capacitance element, it was found that the linearity of the capacitance change with respect to the applied voltage is not good and it is difficult to control to obtain a desired capacitance.
Further, regarding the junction capacitance, it has been found that the PN concentration must be increased in order to obtain a large capacitance, and in this case the breakdown voltage becomes low. Furthermore, it has been clarified that the capacitance change due to the input signal can be reduced if the power supply voltage is high, but it has the conflicting problem that only a small capacitance can be obtained when the power supply voltage is increased. Was also clarified by the inventor.

Therefore, the present inventor has considered using a MOS capacitor to form a filter circuit. In this case, M
It has been revealed that the OS capacity has an error of ± 30%, and the cutoff frequency cannot be accurately obtained without correcting the error.

In view of the above problems, the present inventor has made further studies, detects the capacitance of the capacitor that determines the cutoff frequency, and corrects the resistance value of the resistor by a control signal corresponding to this, determines the product of the capacitor and the resistor. It has been noticed that the cutoff frequency determined by the time constant, in other words, the reciprocal of the time constant, can be corrected to a desired value regardless of variations in capacitors.

Further, according to the above configuration, the filter circuit can be configured as a semiconductor integrated circuit integrally with other electronic circuits, and the current technological trends of electronic devices can be satisfied.
The present invention has been made from the above technical point of view.

[Object of the Invention]

It is an object of the present invention to provide a filter circuit which can be formed in a semiconductor integrated circuit and can accurately correct the cutoff frequency.

The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.

[Outline of Invention]

The following is a brief description of the outline of the present invention disclosed in the present application.

That is, a plurality of capacitors are formed in the semiconductor substrate by the same device process, and a capacitor value detection filter circuit for detecting the error of the capacitor is constituted by one of the capacitors and an amplifier (voltage follower circuit). The output signal and the voltage obtained by dividing the reference signal supplied to the capacitance value detection circuit are compared in voltage by a comparator, and the comparison output is used as a control signal for an amplifier of a filter circuit composed of another capacitor and an amplifier. It is an object of the present invention to obtain a filter circuit capable of correcting the cut-off frequency in accordance with the variation of the capacitor and controlling the resistance of the capacitor to a semiconductor integrated circuit.

〔Example〕

An embodiment of the filter circuit to which the present invention is applied will be described below.

It is to be noted that FIG. 1 is a circuit diagram of a filter circuit composed of a semiconductor integrated circuit (hereinafter referred to as IC).
(A) and (B) are schematic diagrams for explaining the low-pass filter and the high-pass filter, and FIG. 3 is a characteristic diagram for explaining the circuit operation.

The feature of this embodiment is that a capacitance value detection filter circuit is provided as a dummy circuit for detecting the capacitance error of the capacitor.
The output signal is used to correct the time constant of another filter circuit, in other words, the cutoff frequency.

The input signal Vin is supplied to the first and second terminals,
This signal may be supplied in common or individually. Reference numeral 1 is a low-pass filter circuit, which corresponds to the filter circuit of the present invention. The voltage follower circuit composed of the transistors Q _{1 to} Q ₅ corresponds to the amplifier in the present invention, and the resistance from the input side to the output side is changed by the control signal Vc supplied to the transistor Q _5. . Then, the capacitor C ₁ is formed in the same semiconductor substrate by the same device process as in each capacitor described later. Therefore, the voltage follower circuit and the capacitor C ₁ form a filter circuit having an integral characteristic, in other words, a low-pass filter circuit 1. The low-pass circuit 1 can be simplified and illustrated as shown in FIG.

By the way, the resistance of the voltage follower circuit is determined as follows.

That is, each emitter resistance r of the transistors Q ₁ and Q ₂
e is as known to those skilled in the art, Is determined by. In the above equation, the current I changes according to the control signal Vc, so the emitter resistance re also changes according to the current I.

Since the emitter resistors re of the transistors Q ₁ and Q ₂ are interposed between the input side and the output side of the voltage follower circuit to which the current feedback is applied, the resistance 2re between them is obtained. The impedance of the capacitor C ₁ with respect to the input signal Vin is 1 / jωc, and the gain Gv of the low-pass filter circuit 1 is Gv = 1 / (1 + j2ωcr
e). Since the resistance re is changed by the control signal Vc as described above regarding the emitter resistance re,
The Gv is also controlled by the current I.

The cutoff frequency of the low-pass filter circuit 1 is the resistance r
Since it is determined by the product of e and the capacitor C ₁ , if the resistance re is changed by the control signal Vc as described above, the cutoff frequency is also determined by the control signal Vc.

Incidentally, the resistance re is inversely proportional to the current flowing through the transistors Q ₁ and Q _{2 due} to the control signal Vc. The control signal Vc is obtained by the capacitance value detection filter circuit 3, the comparator 4 and the like which will be described later.

Next, the high pass circuit 2 provided as another example of the filter circuit will be described.

Transistor Q ₁₁ to Q ₁₅ constitutes a bolt presentation Huo Roi circuit, the capacitor C ₂ is one formed similarly to the capacitor C _1. Also in this case, the resistance of the voltage follower circuit is determined in the same manner as described above, and the reference voltage Vref
If the AC resistance of 1 is ignored, one end of the capacitor C ₂ will be grounded via the resistance of re.

Therefore, the high pass circuit 2 constitutes a differentiating circuit as shown in FIG. Also in the high pass circuit 2, the gain Gv and the cutoff frequency are determined in the same manner as above.

Next, the capacitance value detection filter circuit 3 (hereinafter referred to as a detection circuit) will be described.

Transistor Q ₃₁ to Q ₃₅ are the same manner as described above constitute the Bol presentation Huo Roi circuit, the capacitor C ₃ is one formed similarly to the capacitor C _2, C _1. Also in the detection circuit 3, the gain Gv and the cutoff frequency are determined in the same manner as above.

What should be noted here is the circuit operation when the reference signal Vs is supplied.

That is, when the reference signal Vs is supplied, the gain Gv
An output signal Vo corresponding to is obtained. The output signal Vo is
Needless to say, this corresponds to the error in the capacitance value of the capacitor C ₃ , and the error is considered to be the same error because the device process is the same as that of the other capacitors C ₁ and C _2. Good. Therefore, the output signal Vo corresponds to an error amount substantially the same as the error amount of the low pass circuit 1 and the high pass circuit 2. The output signal Vo is applied to the rectifying diode D ₁ . The capacitor C _A
Is for smoothing, and the output signal Vo is converted into a direct current and supplied to the inverting input terminal of the comparator 4.

On the other hand, the reference signal Vs is divided by the resistors R ₁ and R ₂ . Therefore, the voltage corresponding to the voltage level of the reference signal Vs is applied to the diode D ₂ . The capacitor Cb is for smoothing, and the divided reference signal Vs
Is converted into a direct current and supplied to the non-inverting input terminal of the comparator 4.

The comparator 4 detects the voltage difference between the two and obtains an output voltage corresponding to the voltage difference, in other words, a control signal Vc. The control signal Vc controls the resistance of each voltage follower circuit, that is, the resistance value of the re through the transistors ₅ , Q ₁₅ and Q _{35 in accordance} with the error of the capacitor C ₃ . Since the error of the capacitor C ₃ is substantially equal to the error of the capacitors C ₁ and C ₂ as described above, the resistance value of the resistor that determines the cutoff frequency is adjusted according to the error of the capacitors C ₁ and C _2. Therefore, the time constant determined by C ₁ · re or C ₂ · re, that is, the cutoff frequency of the filter circuits 1 and 2 is determined.

The determination of the cutoff frequency of the filter circuit will be described in more detail as follows.

By controlling the transistor Q35 by the output Vc of the comparator 4 shown in the figure, the gain of the detection circuit 3 with respect to the reference signal Vs is reduced by the series resistances R1 and R2.
R2) is controlled to have a cutoff frequency characteristic that is substantially equal to R2). That is, terminal 3
To Vo, the absolute value of the gain 1 / (1 + j2ωSC ₃ re) is made equal to R1 / (R1 + R2). Here, ωS is the angular velocity of the reference signal Vs, C ₃ is the capacitor C
The capacitance value of 3 and re is the emitter resistance of the transistors Q31 and Q32.

As a result, the combined emitter resistance of the transistors Q31 and Q32 Or It is said that However, A = [(R1 + R2) / R1)-
1]. This relationship clearly shows that the product of re and C _{3 at} the reference angular velocity ωs has a constant value regardless of the capacitance variation of the capacitor C ₃ .

The capacitors C1 to C3 have a sufficiently small relative variation according to the general characteristic of the semiconductor integrated circuit that the characteristics of the elements formed at the same time have a small relative variation. Therefore, for example, if the capacitance value of the capacitor C1 increases or decreases by n%, the capacitance value of the capacitor C3 also increases or decreases by n%. The control voltage Vc output from the comparator 4 is also commonly supplied to the transistors Q5 and Q15, so that the transistors Q1, Q2, and Q1.
2, Q11 has emitter resistances of transistors Q31, Q
It is changed at the same rate of change as the rate of change of the emitter resistance of 32. As a result, the product of the emitter resistance of the transistors Q1 and Q2, which determines the cutoff frequency of the filter circuit 1, and the capacitor C1 is controlled to a predetermined value without being substantially affected by the capacitance variation of the capacitor C1. Become. Similarly, the cutoff frequency of the filter circuit 2 is also controlled to a predetermined value.

As is clear from the above description, the capacitor C1
It suffices that C3 to C3 be formed at the same time so that the relative variation becomes small, and the magnitude of each capacitance value does not affect the degree of correction of the cutoff frequency. From this, it is clear that the cutoff frequencies of the filter circuits 1 and 2 can be set to appropriate values.

As is apparent from the above circuit operation, the detection circuit 3 and the comparator 4 operate as a so-called dummy circuit for detecting the error of the capacitors C ₁ and C ₂ formed in the IC, and the output signal The cutoff frequency of each filter circuit is corrected by the control signal Vc obtained as follows.

Therefore, the cut-off frequency can be set accurately despite the fact that the filter circuit is formed using the capacitor having a large error formed in the IC.

〔effect〕

(1) Multiple capacitors are formed in the IC, and one of them is
By controlling the resistance value of the resistor that determines the time constant in cooperation with other capacitors by a control signal corresponding to the error of the capacitor of, the cutoff frequency of the filter circuit formed in the IC can be accurately determined. The effect of setting to is obtained.

(2) Since the control signal is obtained by comparing with the voltage obtained by dividing the reference signal for obtaining the control signal, the level of the control signal by setting the voltage division ratio, and by extension, the reference level of the filter circuit is also arbitrarily set. The effect is that it can be set.

(3) According to the above (1), there is an effect that various filter circuits can be easily integrated into an IC.

The present invention made by the present inventor has been specifically described above based on the embodiments. However, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the invention. Needless to say.

For example, the cutoff frequencies of the filter circuits 1 and 2 may be set to different frequencies, and the output signal Vout of the filter circuit 1 may be used as the input signal of the filter circuit 2 to form a so-called bandpass filter.

The cutoff frequency may be changed by changing the emitter areas of the transistors Q ₅ and Q ₁₅ , and a resistor or the like may be provided in the current path of the voltage follower circuit.

Further, the capacitors Ca and Cb are capacitors C _{1 to} C ₃
It may be formed by the device process described above, or may be a large-capacity capacitor. Further, as the variable resistance circuit, a circuit having no frequency feedback and good frequency characteristics may be used.

[Field of application]

In the above description, an example in which the invention made by the present inventor is mainly applied to a filter circuit which is a field of application which is the background of the invention has been described. However, the invention is not limited thereto and, for example, an audio circuit that requires a filter circuit is used. It can be widely used in devices and the like.

[Brief description of drawings]

FIG. 1 is an overall circuit diagram of a filter circuit showing an embodiment of the present invention, FIGS. 2 (A) and 2 (B) are circuit diagrams of essential parts of the filter circuit, and FIG. 3 is a circuit of the filter circuit. The frequency characteristic figure explaining an example of operation | movement is shown. 1 ...... lowpass circuit, 2 ...... highpass circuit, 3 ...... capacitance detection filter circuit, 4 ...... _comparators, Q 1 to Q ₃₅ ...... _transistor, C 1 -C ₃ ...... _capacitors, R 1, _{R 2} …
... resistance, Vs ... reference signal.

Claims (3)

[Claims] (1) A filter circuit comprising a first capacitor formed in a semiconductor substrate and a first amplifier which substantially functions as a resistor, wherein the resistance is varied to a desired resistance value by a control signal. , (2) Second capacitor formed under the same conditions as the first capacitor
A capacitance value detection filter including a capacitor and a second amplifier which is substantially a resistor, the resistance being varied by a control signal, and a reference signal being supplied to obtain an output signal in response to the capacitance value of the second capacitor. A circuit, (3) for varying the resistance value of the filter circuit and the capacitance detection filter circuit to a desired resistance value by detecting the level difference between the output signal of the capacitance value detection filter circuit and the reference signal. A filter circuit comprising a comparator for generating a control signal. 2. The filter circuit according to claim 1, wherein the first capacitor and the second capacitor are constituted by MOS capacitance elements. 3. The filter circuit, wherein an amplifier acting as a resistance in a plurality of filter circuits including a low-pass filter circuit and a high-pass filter circuit varies a desired resistance value by a common control signal generated by the comparator. The filter circuit according to claim 1 or 2, wherein the filter circuit is controlled as follows.