JPH05226968A - Semiconductor integrated circuit - Google Patents

Abstract

PURPOSE:To reduce noise by providing a 2nd filter circuit used to adjust a characteristic of a 1st filter circuit in response to an input signal inputted to the 1st filter circuit used to decide a basic characteristic of a filter to the integrated circuit so as to obtain a high filtering effect in response to the input signal. CONSTITUTION:Diode series connection circuits 4 being components of a 2nd filter circuit 5 have a forward voltage VF and are in a high impedance condition when the forward voltage applied to the circuits 4 is the voltage VF or below. Thus, the circuits 4 act like diodes having a forward voltage nVF. When the amplitude of an input signal is the voltage nVF or below as to the 2nd filter circuit 5, the circuits 4 are nonconductive and the characteristic of the 1st filter circuit A is independent of the effect of the 2nd filter circuit 5. When the serial, signals are the voltage nVF or over, the circuits 4 are conductive and the characteristic of the circuit A is adjusted by the 2nd filter circuit 5. The filter characteristic in this case is closer to a characteristic caused by short-circuiting the circuits 4 as the amplitude of the input signal is higher.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor integrated circuit capable of adjusting filter characteristics.

[0002]

2. Description of the Related Art FIG. 5 is a circuit diagram showing a most basic high-pass filter circuit in a conventional semiconductor integrated circuit. In the figure, 1 is a capacitor, 2 is a resistor, and 6 is an input terminal connected to a capacitor 1. , 7 is an output terminal connected to the capacitor 1 and the resistor 2, and 8 is an input terminal (GND side) connected to the resistor 2.

Next, the operation will be described. When the capacitance of the capacitor 1 that constitutes the filter circuit is C [F] and the resistance value of the resistor 2 is R [Ω], the frequency of the input signal is f [Hz].
Then, the output signal can be expressed as follows. V ₀ = 1/1
-J (1 ÷ 2πfCR) Here, when 2πf <CR,
The filter circuit has an attenuation characteristic and the frequency f when 2πf = CR is called a cutoff frequency.

[0004]

Since the conventional filter circuit is constructed as described above, the cutoff frequency of the filter is fixed and determined by the circuit constant. If the circuit element is variable, the filter characteristic can be adjusted, but the characteristic cannot be changed according to the state of the input signal.

The present invention has been made to solve the above problems, and an object thereof is to obtain a semiconductor integrated circuit having a filter function capable of adjusting the characteristics of a filter according to the state of an input signal. ..

[0006]

According to another aspect of the present invention, there is provided a semiconductor integrated circuit which adjusts a characteristic of a first filter circuit according to an input signal input to the first filter circuit which determines a basic characteristic of the filter. Two filter circuits are provided.

[0007]

In the semiconductor integrated circuit according to the present invention, the circuit constant of the filter changes according to the state of the input signal, and the characteristics of the filter are adjusted.

[0008]

EXAMPLES Example 1. Embodiment 1 of the present invention will be described below with reference to the drawings. In FIG. 1, reference numerals 1, 2, 6 to 8 are the same as those of the conventional one, and a first filter circuit A connected between an input terminal 6, 8 and an output terminal 7 with a capacitor 1 and a resistor 2 is used. Constitute. 3 is a resistor whose one end is connected to the output terminal 7, 4 is a diode series body in which a plurality (n) of diodes are connected in series, two diode series bodies 4 are connected in anti-parallel, and one end is a resistor body 3 And the other end is connected to the input terminal 8. A second resistor connected to the latter stage of the first filter circuit A by the resistor 3 and the diode series body 4.
The filter circuit 5 is constructed.

Next, the operation will be described. The diode series body 4 forming the second filter circuit 5 has a forward voltage VF, and is in a high impedance state when the forward voltage applied to the diode series body 4 is VF or less.
Therefore, the diode series body 4 operates as a diode having a forward voltage of nVF. In the second filter circuit 5, when the amplitude of the input signal is nVF or less, the diode series body 4 becomes non-conductive, and the characteristics of the first filter circuit A are affected by the second filter circuit 5. Absent.
When the input signal is nVF or higher, the diode series body 4 becomes conductive, and the characteristics of the first filter circuit A are adjusted by the second filter circuit 5. The filter characteristic at this time becomes closer to the characteristic in which the diode series body 4 is short-circuited as the amplitude of the input signal increases.

Embodiment 2. As shown in FIG. 2, the second filter circuit may include a resistor 3, a diode series body 4, an NPN transistor 9, and a PNP transistor 10.
In the filter circuit of FIG. 2, when the voltage between the output terminals 7 and 8 is (n + 1) VF or more, a current flows through the base of the NPN transistor 9 and the NPN transistor 9 becomes conductive. At this time, the PNP transistor 10 becomes non-conductive. In addition, the voltage between the output terminals 7 and 8 is-(n +
1) In the case of VF or less, the PNP transistor 10 becomes conductive and the NPN transistor 9 becomes non-conductive. Output terminal 7
When the voltage between the terminal 8 and the terminal 8 is within the range of ± (n + 1) VF, the NPN transistor 9 and the PNP transistor 10
Becomes non-conductive.

Embodiment 3. As shown in FIG. 3, the second filter circuit includes an NPN transistor 9, a PNP transistor 10, a resistor 11, a capacitor 12, a peak value holding circuit 13, a comparator 14, a positive power supply 15, a negative power supply 16, and a block 17. You may comprise. The filter circuit of FIG. 3 controls the second filter circuit in block 17. Resistor 11 and capacitor 12
When the component passing through the high-pass filter constituted by (3) becomes large, the peak value is held by the peak value holding circuit 13 and becomes the input of the comparator 14. The comparator 14 is configured as shown in FIG. 4, and when the potential of 14c is lower than the potential of 14d, the value is within the potential of the terminal 8 plus about 0.2V, and the second filter circuit is turned off. Also,
When the potential of 14c is higher than the potential of 14d, the output of the comparator 14 takes only a value close to the positive power supply voltage or the negative power supply voltage, and turns on the second filter circuit.
The polarity of the output is positive when the potential of the input terminal 6 is higher than the potential of the terminal 8 and negative when the potential is low.

[0012]

As described above, according to the present invention, the second filter circuit which adjusts the characteristic of the first filter circuit according to the input signal inputted to the first filter circuit which determines the characteristic of the filter. Since the above is provided, a high filter effect can be obtained according to an input signal, and a semiconductor integrated circuit that reduces noise can be obtained.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a filter circuit according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing a filter circuit according to a second embodiment of the present invention.

FIG. 3 is a circuit diagram showing a filter circuit according to a third embodiment of the present invention.

FIG. 4 is a circuit diagram showing a comparator in a third embodiment of the present invention.

FIG. 5 is a circuit diagram showing a conventional filter circuit.

[Explanation of symbols]

 A first filter circuit 5 second filter circuit

Claims (2)

[Claims] 1. A semiconductor integrated circuit having a filter function, wherein the characteristic of the first filter circuit corresponds to the amplitude of the input signal according to the input signal input to the first filter circuit which determines the basic characteristic of the filter. And a second filter circuit for adjustment. 2. The semiconductor integrated circuit according to claim 1, wherein the characteristics of the filter circuit are adjusted according to the magnitude of a specific frequency component included in the input signal.