JPH09167941A - Low pass filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To configure an integrated LPF without the need for an external component for an audio band by changing part of a current of a capacitor being a component of the LPF into a signal being an increased output signal from a differential amplifier circuit so as to increase the capacitance apparently. SOLUTION: An input signal and a reference voltage Vref are fed to an inverting input terminal and a non-inverting input terminal of a differential amplifier circuit 2 via an input resistor 1 and a capacitor 3 is connected between the inverting input terminal and an output terminal 6. Furthermore, a collector current (i) of transistors(TRs) 2a, 2b of an output stage of the circuit 2 is an input current to current mirror circuits 4, 5 whose mirror ratio is set to be 1:N. Thus, the inverted output current from the circuits 4, 5 is a multiple of N of the current (i) and part of the current fed to the capacitor 3 is supplied or absorbed depending on an output difference current. Thus, the time constant of the capacitor 3 is reduced to increase the capacitance apparently, then the integrated LPF is formed at the output terminal of the circuit 2 without the need for an externally mounted component for an audio frequency band.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-pass filter that can be used in the frequency band of audio signals and is suitable for IC implementation.

[0002]

2. Description of the Related Art Conventionally, as a low-pass filter, a CR type low-pass filter composed of a resistor R and a capacitor C, or an operational amplifier in place of the resistor R is used for LP.
An active low-pass filter that obtains an F characteristic is known. Since these LPF circuits are well known, description of their operation will be omitted.

In the CR type low pass filter,
The cutoff frequency fc is fc = 1 / 2π · C · R. Further, in the case of an active type low pass filter, the conductance gm of the operational amplifier is used as a resistor,
The cutoff frequency fc ′ is fc ′ = gm / 2π · C.

[0004]

However, the low-pass filter is used in the frequency band (20 Hz to 2) of the audio signal.
When used at 0 KHz), either the resistor R or the capacitor C becomes a value that cannot be integrated into an IC,
The resistor R or the capacitor C was an external element. Therefore, there is a problem in that the number of external pins of the IC is increased when the IC is formed. Especially,

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned points, and an input terminal to which an input signal is applied, an output terminal from which an output signal having a low-pass filter characteristic is obtained, and an input resistor are provided. A negative input terminal to which an input signal is applied via, a positive input terminal to which a reference voltage is applied, and an amplifier circuit having an output terminal connected to the output terminal, a negative input terminal of the amplifier circuit, and the output terminal And a current increasing circuit for increasing the output current of the amplifier circuit and mixing the increased current with the input signal.

Further, the amplifier circuit generates two output signals, and the current increasing circuit sets the mirror ratio to 1: N and inverts and inverts the two output signals of the amplifier circuit. First, the mixed current is mixed to obtain the output current.
And a second current mirror circuit.

[0007]

FIG. 1 is a diagram showing an embodiment of the present invention, in which (1) is an input resistance to which an input signal is applied,
(2) is an amplifier circuit including the output stage transistors (2a) and (2b), (3) is a capacitor connected between the negative input terminal and the output terminal of the amplifier circuit (2), and (4) is a mirror ratio. Is set to 1: N, the first current mirror circuit that inverts the collector current of the output stage transistor (2a), and (5) has the mirror ratio set to 1: N.
a second current mirror circuit for inverting the collector current of b),
(6) and (7) are the first and second output terminals from which the low-pass filter characteristic is obtained.

In FIG. 1, an input signal is applied to the negative input terminal of the amplifier circuit (2) via the input terminal IN and the input resistor (1). Since the reference voltage Vref is applied to the positive input terminal of the amplifier circuit (2), the voltage of its negative input terminal becomes equal to the voltage Vref. However, the positive and negative input terminals of the amplifier circuit (2) are virtually short-circuited. Since the bias voltage of the input signal is the voltage Vref, a current corresponding to the voltage difference between the input signal voltage and the voltage Vref and the resistance value of the input resistance (1) flows through the input resistance (1). Ignoring the input current of the negative input terminal of the differential amplifier circuit (2), the current flowing through the input resistor (1) is common to the capacitor (3) and the first and second current mirror circuits (4) and (5). It is shunted to the output end.

By the way, the collector currents of the output stage transistors (2a) and (2b) become the input currents of the first and second current mirror circuits (4) and (5), respectively, and the input signal is inverted at a mirror ratio of 1: N. Therefore, the output currents of the first and second current mirror circuits (4) and (5) are N times the collector current. The output differential currents of the first and second current mirror circuits (4) and (5) are applied to the negative input terminal of the amplifier circuit (2). Then, the output stage transistor (2
Since the magnitudes of the collector currents of a) and (2b) change according to the positive or negative amplitude of the input signal, the first and second
The output differential current of the current mirror circuits (4) and (5) changes, and a part of the current flowing through the capacitor (3) is supplied or drawn according to the differential current.

When the amplitude of the input signal is positive, the current flowing through the output stage transistor (2b) becomes large, so that the current flowing through the capacitor (3) flows from the input resistor (1) to the output terminal of the amplifier circuit (2). Flow in the direction. At this time, the output current of the second current mirror circuit (5) is the first current mirror circuit (4).
Of the current flowing through the capacitor (3) is supplied to the second current mirror circuit (5). On the contrary, when the amplitude of the input signal is negative, the current flowing through the output stage transistor (2a) becomes large, so that the current flowing through the capacitor (3) from the output end of the differential amplifier circuit (2) to the input resistance (1). Flowing in the direction. At this time, since the output current of the first current mirror circuit (4) becomes larger than the output current of the second current mirror circuit (5), the difference current between the first and second current mirror circuits (4) and (5) is It is supplied to the input resistor (1). By the above operation, the time constant can be reduced by reducing the current flowing through the capacitor (3), so that the capacitance value of the capacitor (3) can be apparently increased. Therefore, the actual capacity of the capacitor (3) can be reduced in order to obtain the same cutoff frequency.

Next, the operation of FIG. 1 will be described using equations.
When the AC voltage of the input signal is Vin and the resistance value of the input resistance (1) is R1, the current i1 flowing through the input resistance (1) is
Is

[0012]

[Equation 1]

## EQU1 ## Further, the current i1 flowing in the input resistor (1) can ignore the input current of the negative input terminal of the amplifier circuit (2), and the current i2 flowing in the capacitor (3) and the first and second current mirror circuits (4) and Since it is shunted to the current i3 flowing in the connection output end direction of (5),

[0014]

(Equation 2)

## EQU1 ## Further, the current i2 flowing through the capacitor (3) is the output stage transistors (2a) and (2
b) becomes the difference current of the output current, and the output currents of the current mirror circuits (4) and (5) become the output stage transistors (2
Since the input currents of a) and (2b) are N times, the current i3 is

[0016]

(Equation 3)

## EQU1 ## Then, regarding the output voltage Vout of the first output terminal (6), when the capacitance value of the capacitor (3) is C, the current i2 flowing through the capacitor (3) is

[0018]

(Equation 4)

## EQU1 ## However, ω is an angular frequency. Then, substituting equations (1), (3), and (4) into equation (2),

[0020]

(Equation 5)

The transfer characteristic of the first output terminal (6) is

[0022]

(Equation 6)

It becomes Further, the transfer characteristic of the second output terminal (7) is that the output signal of the amplifier circuit (2) is inverted by the inverting circuit (8).

[0024]

(Equation 7)

## EQU1 ## Therefore, formula (6) or formula (7)
Therefore, the cutoff frequency fc of the low pass filter of FIG.

[0026]

(Equation 8)

## EQU1 ## The time constant of the cutoff frequency fc becomes a predetermined value, and the time constant is a time constant determined by the resistance value R1 of the input resistance (1) and the capacitance value of the capacitor (7),
It is a value obtained by multiplying a constant determined by the mirror ratio (N).
That is, the cutoff frequency can be changed according to the mirror ratio of the first and second current mirror circuits (4) and (5),
When trying to obtain the same cutoff frequency, the capacitance of the capacitor (3) can be reduced by increasing the mirror ratio, and the input resistor (1) and the capacitor (3) can be integrated.

When a rectangular wave signal as shown in FIG. 1 is applied as an input signal, an output signal as shown in FIG. 1 is obtained at the first output terminal (6) and an output signal as shown in FIG. Such an output signal is obtained. Therefore, since the current mirror circuit is used to set the cutoff frequency fc, the circuit configuration can be simple and the capacitance value of the capacitor (3) can be easily set.

[0029]

As described above, according to the present invention, the capacitance of the capacitor is changed by changing a part of the current flowing through the capacitor forming the low-pass filter into a signal obtained by increasing the output signal of the differential amplifier circuit. Since the value can be apparently increased, the value of the capacitor can be reduced, and an integrated LPF without an external element can be configured in the frequency band of the audio signal.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

[Explanation of symbols]

 1 Input Resistance 2 Differential Amplifier Circuit 2a, 2b Output Stage Transistor 3 Capacitor 4 First Current Mirror Circuit 5 Second Current Mirror Circuit 6 First Output Terminal 7 Second Output Terminal 8 Inversion Circuit

Claims (2)

[Claims] 1. An input terminal to which an input signal is applied, an output terminal from which an output signal having a low-pass filter characteristic is obtained, a negative input terminal to which the input signal is applied via an input resistor, and a positive voltage to which a reference voltage is applied. An amplifier circuit having an input terminal and an output terminal connected to the output terminal, a capacitor connected between the negative input terminal of the amplifier circuit and the output terminal, and an output current of the amplifier circuit increasing. A low pass filter comprising: a current increasing circuit for mixing the increased current with an input signal. 2. The amplifying circuit generates two output signals, and the current increasing circuit sets a mirror ratio to 1: N and inverts and inverts the two output signals of the amplifying circuit. 2. A first and a second current mirror circuit for mixing different currents to obtain an output current.
The low-pass filter described.