JP2515814B2 - Time constant circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a time constant circuit, for example, to a technique effectively used for a CR filter (low-pass filter) incorporated in a semiconductor integrated circuit.

[Conventional technology]

As the most basic circuit of the low pass filter, a constant K type filter including a resistor and a capacitor arranged in series is well known. Further, there is known a Miller integrating circuit in which the capacitance value of a capacitor is increased by using an operational amplifier circuit. An example of such a Miller integrator circuit is:
Radio Technology Co., Ltd., December 20, 1979, "Linear IC Practical Circuit Manual" by Yojiro Yokoi, pages 113-116.

[Problems to be solved by the invention]

By the low-pass filter consisting of the above-mentioned resistor and capacitor, a low-pass filter in the voice band having a relatively low frequency, for example, when the cutoff frequency constitutes 1 KHz, and the resistance value of the resistance element is 100 KΩ, the capacitor is 1590 pF. It is necessary to set such a large capacity value. For this reason,
It is impossible to incorporate the above low-pass filter in an audio semiconductor integrated circuit. Further, since a large-sized capacitor is required, the cost becomes high, and it becomes a cause of hindering the miniaturization of the audio product which requires the above low-pass filter.

Although it is conceivable to use the Miller integrator circuit, the Miller integrator circuit requires a large signal amplitude, so that it is necessary to increase the operating voltage of the circuit.
It is not suitable for the above audio products that require battery operation.

An object of the present invention is to provide a time constant circuit capable of equivalently reducing the capacitance value of a capacitor under a relatively low operating voltage.

Another object of the present invention is to provide a time constant circuit suitable for a semiconductor integrated circuit.

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.

[Means for solving problems]

The outline of a typical one of the inventions disclosed in the present application will be briefly described as follows. That is,
A current source circuit that detects a current flowing through the resistance element forming the time constant circuit and forms a current whose absolute value is smaller than the current flowing through the resistance element is provided, and the current flowing through the resistance element and the current source circuit The current of the difference between the output currents is made to flow through the capacitor.

[Action]

According to the above-mentioned means, the charging / discharging current of the capacitor is reduced, so that the capacitance value of the capacitor viewed from the input signal can be increased.

[Embodiment 1] FIG. 1 shows a circuit diagram of an embodiment of a low-pass filter to which the present invention is applied. Although not particularly limited, each circuit element in the same drawing is formed on one semiconductor substrate such as single crystal silicon by a known semiconductor integrated circuit manufacturing technique.

A constant current source Io is provided at the common emitter of the NPN type differential transistors Q1 and Q2. The collectors of these differential transistors Q1 and Q2 are provided with current mirror type PNP transistors Q3 and Q4 forming an active load circuit. The following output circuit is provided using such a differential amplifier circuit as an input stage circuit. The NPN transistor Q5, which receives the collector output of the transistor Q4, operates as an emitter follower amplification transistor and performs a current / voltage conversion operation. The emitter of this transistor Q5 is provided with a constant current source Io via a diode-shaped transistor Q6 for level shifting (biasing the output transistor). This constant current source Io is shown with the same circuit symbol as the constant current source Io provided in the emitters of the differential transistors Q1 and Q2, but it does not necessarily mean that the same constant current flows (the same current It does not matter if there is).

The emitter output voltage of the transistor Q5 is supplied to the base of the NPN output transistor Q7, and forms a complementary push-pull output circuit together with the PNP output transistor Q8 which receives the output voltage level-shifted by the diode type transistor Q6.

Each of the above circuit elements is an amplifier circuit AM equivalent to an operational amplifier circuit with high input impedance and low output impedance.
It constitutes P. In this embodiment, the following circuit elements are added to the amplifier circuit AMP to form a current source circuit that forms a current corresponding to the input signal current.

That is, the collector side of the complementary push-pull output transistor Q7 is not directly connected to the power supply voltage Vcc as in the conventional operational amplifier circuit, but the operating voltage is changed via the diode-shaped PNP transistor Q9. Supplied. Further, the collector side of the output transistor Q8 is not directly connected to the circuit ground potential (or the negative power supply voltage in the dual power supply system) like a conventional operational amplifier circuit, but is a diode type NPN. The ground potential of the circuit (or a negative power supply voltage in the dual power supply system) is supplied via the transistor Q10. The diode-shaped PNP transistor Q9 and NPN transistor Q10 respectively include a current mirror-shaped PNP transistor Q11 and NPN.
A transistor Q12 is provided. With the transistor Q11 above
The collectors of Q12 are commonly connected to serve as the output current terminal. The output signal as this current source is
It is fed back to the non-inverting input (+) of.

The operational amplifier circuit (hereinafter also simply referred to as amplifier circuit) AM
The output terminal as P (the commonly connected emitter of transistors Q7 and Q8) is the transistor as the inverting input (-).
Connected to the base of Q2. As a result, the amplifier circuit
The AMP performs an operation as a voltage follower circuit, that is, an impedance conversion operation, by 100% negative feedback of its output signal.

At the base of the transistor Q1 serving as the non-inverting input (+) of the operational amplifier circuit AMP, a resistor forming a time constant circuit is provided.
The input signal Vin is supplied via R2. A capacitor C is provided between the non-inverting terminal (+) and the ground potential point of the circuit (which may be any AC ground point). And
The output signal Vout is obtained from the non-inverting terminal (+).

In this embodiment, since a current according to the current flowing through the resistor R2 is formed, the resistor R2 is provided between the input terminal to which the input signal Vin is supplied and the inverting input (−) of the amplifier circuit AMP.
1 is provided. The resistance value of the resistor R1 is set to a large value having a desired ratio as compared with the resistance value of the resistor R2, which will be understood from the description given later.

 Next, the operation of this embodiment circuit will be described.

The amplifier circuit AMP operates so that the voltage of its inverting input (-) becomes equal to the voltage of its non-inverting input (+). That is, since the amplifier circuit AMP has the voltage follower circuit configuration, the output voltage signal becomes equal to the voltage supplied to the non-inverting input (+), and the output signal is fed back 100%. Therefore, the voltage of the inverting input (-) is also equal. The resistor R1 has a current (Vin−Vin) according to a voltage difference (Vin−Vout) between the input signal Vin and the output signal Vout.
Vout) / R1 will flow. Since the output of the amplifier circuit AMP has low impedance, the current flowing through the resistor R1 flows through the output transistor Q7 or Q8. For example, when the input signal Vin is AC positive, the current (Vin-Vout) / R1 flowing through the resistor R1 flows through the output transistor Q8. This current is fed back to the non-inverting input (+) of the amplifier circuit AMP as a sink current through the transistors Q10 and Q12 in the current mirror form. This feedback current acts as a discharge current of the capacitor C. Conversely, when the input signal Vin has a negative polarity in terms of AC, the current (Vin-Vout) / R1 flowing through the resistor R1 flows through the output transistor Q7. This current is a non-inverting input (+) of the amplifier circuit AMP as a pushing current through the transistors Q9 and Q11 in the current mirror form.
Be returned to. This feedback current acts as a charging current for the capacitor C.

On the other hand, the resistor R2 is connected to the input signal Vin and the output signal Vout.
Current (Vin−Vout) / R2 according to the difference voltage (Vin−Vout) of
Flows. For example, when the input signal Vin is AC positive, the current (Vin−Vout) / R2 flowing through the resistor R2 acts as a charging current for the capacitor C. Conversely, when the input signal Vin has a negative polarity in terms of AC, the current (Vin−Vou
t) / R2 acts as a discharge current of the capacitor C. Therefore, the difference current between the currents flowing through the resistors R1 and R2 flows through the capacitor C. Therefore, since the charging / discharging current of the capacitor C viewed from the input signal Vin is reduced to the above difference current, it is considered that the capacitance value of the capacitor C is equivalently increased.

 This will be quantitatively explained as follows.

The current flowing through the resistor R2 is divided into the current flowing through the resistor R1 and the current flowing through the capacitor C, because the current flowing through the resistor R1 is supplied to the non-inverting input (+) in reverse phase.
Therefore, the following equation (1) is required.

(Vin−Vout) / R2 = (Vin−Vout) / R1 + jωC · Vout (1) The above equation (1) is transformed into the following equation (2).

Vout / Vin = 1 / (1 + jωCR) (2) Here, R is R1 · R2 / (R1−R2). Resistance R above
Assuming that the resistance ratio of 1 and R2 is n, in other words, the resistance R of the above equation (2) is nR2 / nR2.
(N-1). For example, if n is 1.1, R is 11
× It becomes like R2. That is, the time constant composed of the resistor R2 and the capacitor C is equivalently increased 11 times.

In the circuit of this embodiment, when a low-pass filter having a cutoff frequency of 1 KHz is formed as in the above example, assuming that the resistance value of the resistor R2 is 100 KΩ and the resistance value of the resistor R1 is 110 KΩ, it is equivalent to a capacitor. The capacitance value of C can be reduced to 160 pF. As a result, the capacitor C can be built in the semiconductor integrated circuit. Further, as described above, the equivalent capacitance value of the capacitor C, in other words, the cutoff frequency is set by the resistance ratio. Since the resistance ratio of the resistors R2 and R1 configured in the semiconductor integrated circuit can be configured accurately, a low pass filter suitable for the semiconductor integrated circuit can be obtained.

Further, in this embodiment circuit, a voltage follower circuit constituted by an amplifier circuit and a current source circuit for outputting a current according to the current flowing through the amplifier circuit are provided, and a time constant circuit provided at the input side of the amplifier circuit. By negatively feeding back the current to the capacitor C, the current flowing through the capacitor C is reduced at a constant rate. In this configuration, the output signal of the amplifier circuit has a voltage follower configuration, and thus has a voltage according to the input signal Vin. Therefore, unlike the Miller integrating circuit, the operating voltage can be lowered because the output signal is not amplified.

[Embodiment 2] FIG. 2 shows a circuit diagram of another embodiment of the low-pass filter to which the present invention is applied. In the figure, a circuit AMP indicated by a dotted line is an amplifier circuit similar to the transistors Q1 to Q8 and the constant current source Io shown in FIG. For the output transistors Q7 and Q8 of the amplifier circuit AMP, a diode-type PNP transistor Q9 and NPN transistor Q10 that form a current mirror circuit similar to the above are provided.

In this embodiment, in the amplifier circuit AMP, the inverting input (-) and the output terminal are commonly connected in the same manner as described above to form a voltage follower circuit. In this embodiment, a resistor R1 that forms a current according to the resistor R2 that constitutes the time constant circuit is used.
Is provided between the inverting input (−) and a predetermined reference voltage (DC bias) Vref.

Further, the current mirror circuit constituting the current source circuit is configured as follows in order to cause the current according to the resistor R1 to flow in the non-inverting input (+) of the amplifier circuit AMP in the opposite phase.
The collector output currents of the PNP transistor Q9 and the PNP transistor Q11 in the current mirror form are input to the NPN transistor Q14 in the diode form. Also,
The collector output currents of the NPN transistor Q10 and the NPN transistor Q12 in the current mirror form are input to the PNP transistor Q13 in the diode form. Then, a PNP type output transistor Q15 and an NPN type output transistor Q16 in a current mirror form are provided to these PNP transistor Q13 and NPN transistor Q14. The collectors of these output transistors Q15 and Q16 are commonly connected to form a current that is fed back to the non-inverting input (+) of the amplifier circuit AMP.

 Next, the operation of this embodiment circuit will be described.

Similarly to the above, the amplifier circuit AMP has its inverting input (-).
Operates so that the voltage of is equal to the non-inverting input (+). That is, since the amplifier circuit AMP has the voltage follower circuit configuration, the output voltage signal becomes equal to the voltage supplied to the non-inverting input (+), and the output signal is fed back 100%. Therefore, the voltage of the inverting input (-) is also equal. Therefore, the current Vout / R1 according to the output signal Vout flows through the resistor R1. Since the output of the amplifier circuit AMP has low impedance, the current flowing through the resistor R1 flows through the output transistor Q7 or Q8. For example, input signal Vin
Current is Vout / R1 that flows through the resistor R1
Contrary to the case of FIG. 1, the output transistor Q7
It will flow through. This current is applied to PNP transistors Q9, Q11 and NPN transistor Q1 in the current mirror form.
The phase is inverted through 4, Q16 and is fed back to the non-inverting input (+) of the amplifier circuit AMP as a sink current. This feedback current acts as a discharge current of the capacitor C. Conversely, when the input signal Vin has a negative polarity in AC, the current Vout / R1 flowing through the resistor R1 flows through the output transistor Q8. This current is the NP in the current mirror form.
The phase is inverted through the N-transistors Q10 and Q12 and the PNP transistors Q13 and Q15, and the above-mentioned amplifier circuit AM is used as the pushing current
It is fed back to the non-inverting input (+) of P. This feedback current acts as a charging current for the capacitor C.

On the other hand, the resistor R2 is connected to the input signal Vin and the output signal Vout.
Current (Vin−Vout) / R2 according to the difference voltage (Vin−Vout) of
Flows. For example, when the input signal Vin is AC positive, the current (Vin−Vout) / R2 flowing through the resistor R2 acts as a charging current for the capacitor C. Conversely, when the input signal Vin has a negative polarity in terms of AC, the current (Vin−Vou
t) / R2 acts as a discharge current of the capacitor C. Therefore, the difference current between the currents flowing through the resistors R1 and R2 flows through the capacitor C. Therefore, since the charging / discharging current of the capacitor C viewed from the input signal Vin is reduced to the above difference current, it is considered that the capacitance value of the capacitor C is equivalently increased.

 This will be quantitatively explained as follows.

The current flowing through the resistor R2 is divided into the current flowing through the resistor R1 and the current flowing through the capacitor C, because the current flowing through the resistor R1 is supplied to the non-inverting input (+) in reverse phase.
Therefore, the following equation (3) is required.

(Vin−Vout) / R2 = jωC · Vout−Vout / R1 (3) The above equation (3) is transformed into the following equation (4).

Vout / Vin = R1 / (R1-R2) ÷ (1 + jωCR) ··· (4) Here, R is R1 · R2 / (R1-R2). Above resistance R1
If the resistance ratio between R2 and R2 is n, in other words, the resistance
When setting as R1 = nR2, the above equation (4) becomes the following equation (5).

Vout / Vin = 11 / (1 + jωCR11) (5) That is, the time constant of the low pass filter of this embodiment can be expanded to 11 times the time constant CR1.

In the circuit of this embodiment, when a low-pass filter having a cutoff frequency of 1 KHz is formed as in the above example, the resistance value of the resistor R2 is 100 KΩ, and the resistance value of the resistor R1 is 110 KΩ. The constant can be increased 11 times and the capacitor C can be built in the semiconductor integrated circuit. Also, as described above, the capacitor C
The equivalent capacitance value of, in other words, the cutoff frequency is set. Since the resistance ratio of the resistors R2 and R1 configured in the semiconductor integrated circuit can be configured accurately, a low pass filter suitable for the semiconductor integrated circuit can be obtained.

Further, in this embodiment circuit, a voltage follower circuit constituted by an amplifier circuit and a current source circuit for outputting a current according to the current flowing through the amplifier circuit are provided, and a time constant circuit provided at the input side of the amplifier circuit. By negatively feeding back the current to the capacitor C, the current flowing through the capacitor C is reduced at a constant rate. In this configuration, the output signal of the amplifier circuit has a voltage follower configuration, and thus has a voltage according to the input signal Vin. Therefore, unlike the Miller integrating circuit, the operating voltage can be lowered because the output signal is not amplified.

[Third Embodiment] FIG. 3 shows a circuit diagram of another embodiment of the low-pass filter to which the present invention is applied. In the figure, a circuit AMP indicated by a dotted line is an amplifier circuit similar to the transistors Q1 to Q8 and the constant current source Io shown in FIG. For the output transistors Q7 and Q8 of the amplifier circuit AMP, a diode-type PNP transistor Q9 and NPN transistor Q10 that form a current mirror circuit similar to the above are provided.

In this embodiment, in the amplifier circuit AMP, the inverting input (-) and the output terminal are commonly connected in the same manner as described above to form a voltage follower circuit. In this embodiment, although not particularly limited, the capacitor C forming the time constant circuit is provided between the inverting input (-) and the reference potential point of the circuit. A resistor R2 is connected to the non-inverting input (+) of the amplifier circuit AMP.
The input signal Vin is supplied via. In this example,
The resistor R1 is omitted.

The current mirror circuit forming the current source circuit is configured as follows in order to allow the current according to the resistor R2 to flow in the non-inverting input (+) of the amplifier circuit AMP in reverse phase. Above PN
The collector output currents of the P-transistor Q9 and the PNP transistor Q11 in the current mirror form are input to the diode-shaped NPN transistor Q14. The collector output current of the NPN transistor Q10 and the NPN transistor Q12 in the current mirror form is
Input to NP transistor Q13. Then, a PNP type output transistor Q15 and an NPN type output transistor Q16 in a current mirror form are provided to these PNP transistor Q13 and NPN transistor Q14. The collectors of these output transistors Q15 and Q16 are connected together,
Form the current that is fed back to the non-inverting input (+) of the AMP.
Transistors Q11 and Q12 corresponding to the transistors Q9 and Q10 that form the current mirror circuit perform n times the current amplification operation by increasing the emitter area by n times. Further, although not particularly limited, the transistor Q1 corresponding to the transistors Q13 and Q14 forming the current mirror circuit.
The emitter areas of 5 and Q16 are respectively multiplied by m, so that the current amplification operation of m times is performed. As a result, the currents flowing through the output transistors Q7 and Q8 of the amplifier circuit AMP are multiplied by n × m and fed back to the non-inverting input (+) in antiphase. That is, the charge / discharge current flowing through the capacitor C is n ×
It is multiplied by m and fed back to the non-inverting input (+).

Similarly to the above, the amplifier circuit AMP has its inverting input (-).
Operates so that the voltage of is equal to the non-inverting input (+). When this is viewed in terms of current, the current flowing through the resistor R2 and the feedback current have opposite phases and are equal.

 This is expressed by the following equation (6).

(Vin−Vout) / R2 = jωC · Vout × n × m (6) The above equation (6) can be transformed into the following equation (7).

Vout / Vin = 1 / (1 + jωCR2 ・ n ・ m) (7) That is, the above formula (7) is obtained by using the resistor R2 and the capacitor C
It means that the time constant consisting of can be equivalently increased by n × m times.

In the circuit of this embodiment, when a low-pass filter having a cut-off frequency of 1 KHz is constructed as in the above example, if the resistance value of the resistor R2 is 100 KΩ and n is set to 2 and m is set to 5, it is equivalent. Moreover, the time constant can be expanded ten times, and the capacitor C can be built in the semiconductor integrated circuit.
Further, as described above, the equivalent capacitance value of the capacitor C, in other words, the cutoff frequency is set by the emitter area ratio (size ratio) of the transistor. Since the area ratio of the transistors included in the semiconductor integrated circuit can be accurately configured, a low pass filter suitable for the semiconductor integrated circuit can be obtained. Further, the resistor R1 can be omitted.

Further, in this embodiment circuit, a voltage follower circuit constituted by an amplifier circuit and a current source circuit for outputting a current according to the current flowing through the amplifier circuit are provided, and a time constant circuit provided at the input side of the amplifier circuit. By negatively feeding back the current to the capacitor C, the current flowing through the capacitor C is reduced at a constant rate. In this configuration, the output signal of the amplifier circuit has a voltage follower configuration, and thus has a voltage according to the input signal Vin. Therefore, unlike the Miller integrating circuit, the operating voltage can be lowered because the output signal is not amplified.

The following is a brief description of the function and effect obtained from the above-described embodiment. That is, (1) a current source circuit that detects a current flowing through a resistance element forming a time constant circuit and forms a current whose absolute value is smaller than the current flowing through the resistance element is provided, and By causing a current having a difference between the output currents of the current source circuit to flow through the capacitor, it is possible to obtain an effect that the capacitance value of the capacitor viewed from the input signal can be increased.

(2) Due to the above (1), it is possible to obtain an effect that a relatively large time constant can be configured by the resistor and the capacitor configured by the semiconductor integrated circuit.

(3) According to the above (1), even when the capacitor is composed of external parts, a small size such as a ceramic capacitor having a relatively small capacitance value can be used. As a result, it is possible to reduce the size of the audio device that requires the relatively large time constant circuit.

(3) As a means to increase the capacitance value of the capacitor equivalently, through a pair of complementary push-pull type output transistors of the operational amplifier circuit of the voltage follower configuration, a diode type PNP transistor and an NPN transistor are used. Are supplied with respective operating voltages, and the PNP transistor and the NPN transistor are respectively provided with an output transistor in the form of a current mirror or an output transistor having a phase inversion function, and the non-inverting input of the operational amplifier circuit transmits an input signal. The resistor element and one end of the capacitor are connected to each other, and the input signal or the reference voltage is used as an output terminal of the time constant circuit, and the inverting input of the resistor element has a resistance value larger than that of the resistor element. It is configured to convey. In this configuration, the time constant is equivalently increased by the resistance ratio, and since the resistance ratio can be accurately formed in the semiconductor integrated circuit, it is possible to obtain a time constant circuit suitable for the semiconductor integrated circuit. The effect of being able to do is obtained.

(4) As a means to equivalently increase the capacitance value of the capacitor, a pair of complementary push-pull output transistors of the operational amplifier circuit of the voltage follower configuration are connected via a diode-type PNP transistor and an NPN transistor. And supply an operating voltage to each of them, and the PNP transistor and the NPN transistor are provided with an output circuit that forms an output current that is current-amplified and phase-inverted from the PNP transistor and the NPN transistor by the transistor of the current mirror configuration.
The resistance element for transmitting an input signal and one end of the capacitor are connected to the non-inverting input of the operational amplifier circuit, and the capacitor is provided between the inverting input and a predetermined AC bias point and the time constant is constant. Use as the output terminal of the circuit. In this structure, the time constant is equivalently increased by the emitter area ratio of the transistors forming the current mirror circuit. Since the emitter area ratio can be accurately formed in the semiconductor integrated circuit, An effect that a suitable time constant circuit can be obtained is obtained.

(5) Since the amplifier circuit that equivalently increases the time constant has a voltage follower configuration, its output signal is a voltage according to the input signal. Therefore, unlike the Miller integrating circuit, the operating voltage can be lowered because the output signal is not amplified. As a result, it is possible to obtain the effect of being applicable to an audio device or the like that is driven by a battery and has a relatively low operating voltage.

Although the invention made by the inventor of the present application has been specifically described based on the embodiment, the invention of the present application is not limited to the embodiment and various modifications can be made without departing from the scope of the invention. Nor. For example, a configuration in which a current according to a current flowing through a resistor forming a time constant is detected and a current having a difference in recurrent is caused to flow through a capacitor can employ various embodiments. The specific configuration of the amplifier circuit may be any specific configuration of the operational amplifier circuit as long as it has a high input impedance and a low output impedance. Therefore, the amplifier element should be replaced with the MOSFET instead of the bipolar transistor as described above.
It may include (insulated gate type field effect transistor) or junction type FET.

The present invention can be widely used for low pass filters and various time constant circuits.

〔The invention's effect〕

The effect obtained by the representative one of the inventions disclosed in the present application will be briefly described as follows. That is, a current source circuit that detects a current flowing through a resistance element forming a time constant circuit and forms a current whose absolute value is smaller than the current flowing through the resistance element is provided, and the current flowing through the resistance element and the current source are provided. The equivalent time constant can be increased by causing a current corresponding to the difference between the output currents of the circuit to flow in the capacitor.

[Brief description of drawings]

1 is a circuit diagram showing an embodiment of a low pass filter to which the present invention is applied, FIG. 2 is a circuit diagram showing another embodiment of a low pass filter to which the present invention is applied, and FIG. FIG. 9 is a circuit diagram showing still another embodiment of the low pass filter to which the present invention is applied. AMP ... Amplification circuit

Claims (6)

(57) [Claims] 1. An operational amplifier having a differential amplifier circuit having an inverting input terminal and a non-inverting input terminal as an input stage circuit, and a complementary push-pull output circuit responsive to an output of the input stage circuit as an output circuit, The collector current of the PNP output transistor of the complementary push-pull output circuit is used as the input current, and the collector current of the PNP output transistor of the complementary push-pull output circuit is used as the input current. And a second current mirror circuit composed of an NPN transistor that forms a sink current. By connecting the output terminal of the complementary push-pull output circuit of the operational amplifier to the inverting input terminal, the operation described above is performed. Operates the amplifier as a 100% negative feedback voltage follower First, the first resistor is connected to the inverting input terminal of the operational amplifier, and the input signal is applied to the non-inverting input terminal of the operational amplifier via the second resistor. A capacitor is connected between the terminal and the AC ground point, and the capacitor connects the pushing current of the first current mirror circuit and the second current mirror circuit.
Of the capacitor by connecting between the capacitor and the outputs of the first current mirror circuit and the second current mirror circuit so that the capacitor is charged and discharged in response to the sink current of the current mirror circuit. A time constant circuit, wherein a capacitance value is equivalently increased to a multiplication factor related to the first resistance and the second resistance. 2. The time constant circuit according to claim 1, wherein the circuit element and the circuit other than the capacitor are incorporated in a semiconductor integrated circuit. 3. The time constant circuit is built in a semiconductor integrated circuit.
Time constant circuit described in section. 4. An operational amplifier having a differential amplifier circuit having an inverting input terminal and a non-inverting input terminal as an input stage circuit, and a complementary push-pull output circuit responsive to the output of the input stage circuit as an output circuit, PN that uses the collector current of the NPN output transistor of the complementary push-pull output circuit as the input current and forms a push current that is current-amplified by a prescribed multiple of the input current
A first current mirror circuit composed of a P-transistor, and an NP that forms a sink current, which is current-amplified by a predetermined multiple of the input current, using the collector current of the PNP output transistor of the complementary push-pull output circuit as an input current.
A second current mirror circuit composed of an N-transistor, and by connecting the output terminal of the complementary push-pull output circuit of the operational amplifier to the inverting input terminal, the operational amplifier is provided with 100% negative feedback. It is operated as a voltage follower, a capacitor is connected between the inverting input terminal of the operational amplifier and an AC grounding point, and an input signal is applied to the non-inverting input terminal of the operational amplifier via a resistor. The resistor, the first current mirror circuit and the second current so that the resistor flows a current in response to the pushing current of the first current mirror circuit and the sink current of the second current mirror circuit. By connecting between the output of the mirror circuit and the output of the mirror circuit, the capacitance value of the capacitor can be adjusted to the current amplification factor of the first current mirror circuit and the 2. A time constant circuit characterized in that it is equivalently increased to a multiplication factor related to the current amplification factor of the current mirror circuit of 2. 5. The time constant circuit according to claim 4, wherein the circuit element and the circuit other than the capacitor are incorporated in a semiconductor integrated circuit. 6. A time constant circuit according to claim 4, wherein the time constant circuit is built in a semiconductor integrated circuit.
Time constant circuit described in section.