JPH09167938A - Low-pass filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the IC circuit of an LPF without an external element. SOLUTION: This filter is provided with a first differential amplifier 1 on which an input signal is impressed, a second differential amplifier 2 whose one input terminal is connected with an output terminal, a resistance 3 to which the output terminals of the first and second differential amplifiers 1 and 2 are supplied, first and second diodes 4 and 5 performing voltage conversions for the output signals of the first and second differential amplifiers 1 and 2, a third differential amplifier 6 on which the output voltage of diodes 4 and 5 is applied and the capacitor 7 connected with the output terminal. The output terminal of the third differential amplifier 6 is connected with the output terminal and an LPF characteristic is obtained at the output terminal.

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, an LPF (low-pass filter) circuit has a C composed of a resistor R and a capacitor C.
L-type LPF or L instead of R
An active filter that obtains a PF characteristic is known. In such a CR type LPF circuit, the cutoff frequency fc
Is fc = 1 / 2π · C · R. Further, in the case of the active filter, since the conductance gm of the operational amplifier is used as the resistance, the cutoff frequency fc ′ is fc ′.
= Gm / 2π · C.

[0003]

However, the conventional LPF is used in the frequency band (20 to 20 KH) of the audio signal.
z), either the resistor R or the capacitor C becomes a value that cannot be integrated into an IC, and the resistor R
And 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. Also, in the audio signal frequency band, LP
When F was used, it was difficult to make all the devices IC.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and includes a first differential amplifier to which an input signal is applied and a second differential amplifier having one input end connected to an output terminal. A differential amplifier, a resistor to which the output signals of the first and second differential amplifiers are supplied, first and second variable loads for converting the output signals of the first and second differential amplifiers into a voltage, and a voltage A third differential amplifier to which the converted output signals of the first and second differential amplifiers are applied, and a capacitor connected to the output terminal are provided, and the output end of the third differential amplifier is output to the output. It is characterized by connecting to a terminal and obtaining a filter characteristic.

The operating current of the third differential amplifier is variable.

[0006]

1 is a diagram showing an embodiment of the present invention, in which (1) is a transistor (1a) and (1b).
A first differential amplifier circuit, and (2) is a transistor (2
a) and a second differential amplifier circuit (2b), (3) a resistor connected between the bases of the transistors (1b) and (2a), (4) and (5) a first and a second difference. The first and second diodes for converting the output currents of the dynamic amplifiers (1) and (2) into a voltage, and the output voltages of the first and second diodes (4) and (5) are applied to their bases, respectively. A third differential amplifier circuit comprising transistors (6a) and (6b), and (7) is a capacitor connected to the collector of the transistor (6a).

In FIG. 1, the input signal is applied to the base of the transistor (1a), and the first differential amplifier circuit (1).
Is differentially amplified. The output signal of the first differential amplifier circuit (1) is generated from the emitter of the transistor (8), is fed back to the base of the transistor (1b), and is voltage-converted by the first diode (4) to be the transistor (6a). Applied to the base of. On the other hand, the output signal generated from the output terminal is the transistor (2b) of the second differential amplifier circuit (2).
And is differentially amplified. Second differential amplifier circuit (2)
Is generated from the emitter of the transistor (9), fed back to the base of the transistor (2a), converted into a voltage by the second diode (5), and applied to the base of the transistor (6b). Transistors (6a) and (6b) that are differentially connected are transistors (6a)
And (6b) the base input signal is differentially amplified, and an output current corresponding to the base-to-base voltage of the transistors (6a) and (6b) is generated from the collector of the transistor (6a),
The capacitor (7) is charged and discharged according to the output current. Therefore, an output signal having a low-pass filter characteristic at the output terminal can be obtained.

Next, the LPF characteristic obtained at the output terminal will be specifically described. If the AC voltage of the input signal is Vin, and the base voltage of the transistor (2a) is Vx, the current i1 flowing through the resistor (3) is

[0009]

[Equation 1]

[0010] However, R is the resistance value of the resistor (3). Further, the base-to-base voltage Va of the transistors (6a) and (6b) is equal to the on-resistance of the first diode (3) r.
If we say e1,

[0011]

(Equation 2)

## EQU1 ## Further, regarding the voltage Va, the output current difference between the transistors (6a) and (6b) is i2,
The emitter resistance of the transistors (6a) and (6b) is r
If we say e2,

[0013]

(Equation 3)

## EQU1 ## From equations (1) to (3),

[0015]

(Equation 4)

[0016] Since the current i2 is supplied to the capacitor (7), the terminal voltage of the capacitor (7), that is, the output voltage Vout, is C, where the capacitance of the capacitor (7) is

[0017]

(Equation 5)

## EQU1 ## The voltage Vx is Vx = Vout from the transistors (2a) and (2b). Therefore, from the expressions (4) and (5) and Vx = Vout, the relationship between the input voltage Vin and the output voltage Vout is

[0019]

(Equation 6)

## EQU1 ## And its transfer characteristic is

[0021]

(Equation 7)

## EQU1 ## Therefore, the cutoff frequency fc is

[0023]

(Equation 8)

## EQU1 ## The time constant of the cutoff frequency fc is a predetermined value, and the time constant is the time constant (CR) determined by the resistor (3) and the capacitor (7) and the emitter resistance of the transistors (6a) and (6b) and the It is a value multiplied by a constant (re2 / re1) determined by the ON resistance of one diode (4). Therefore, the LPF of FIG. 1 can reduce the values of the resistor and the capacitor as compared with the LPF that is simply composed of the resistor and the capacitor. However, r
The condition of e2> re1 is required.

More specifically, for example, when setting the cutoff frequency of the LPF of FIG. 1 to 3.9 KHz, re
If 1 = 1.3 KΩ and re2 = 26 KΩ, the capacitance of the capacitor (7) is 100 pF and the value of the resistor (3) is 2
It can be 0 KΩ. With such a value,
Even if the LPF of the audio signal frequency band, resistance (3)
Also, since the capacitor (7) can be integrated into an IC, an LPF without external elements can be formed.

Further, when the variable current source (10) for supplying the operating current to the third differential amplifier circuit (6) is changed, the emitter resistance re2 of the transistors (6a) and (6b) is changed. The cutoff frequency fc can be changed from (8). When the output current of the variable current source (10) is increased, the emitter resistance re2 is decreased, so that the cutoff frequency fc is increased from the equation (8). Conversely, if the output current of the variable power supply (10) is reduced, the emitter resistance re
Since 2 becomes larger, the cutoff frequency fc becomes lower than that of the equation (8). Therefore, if the configuration is such that the magnitude of the output current of the variable current source (10) is controlled, the cutoff frequency of the LPF is variable.
Can be configured.

[0027]

As described above, according to the present invention, the LP
The cutoff frequency of F is set by multiplying the time constant of the resistor and the capacitor by the ratio of the on resistance of the diode and the emitter resistance of the transistor, so that the value of the resistor and the capacitor can be reduced, and the audio signal It is possible to construct an IC-type LPF having no external element in the frequency band of.

Further, it is possible to provide an LPF in which the cutoff frequency can be easily changed only by changing the operating current of the differential amplifier circuit.

[Brief description of the drawings]

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

[Explanation of symbols]

 1 1st differential amplifier circuit 2 2nd differential amplifier circuit 3 resistance 4, 5 diode 7 capacitor 10 variable current source

Claims (2)

[Claims] 1. A first differential amplifier to which an input signal is applied, a second differential amplifier whose one input terminal is connected to an output terminal, and output signals of the first and second differential amplifiers are supplied. A resistance, a first and second variable load for converting the output signals of the first and second differential amplifiers into a voltage, and a voltage-converted output signal of the first and second differential amplifiers A low-pass filter comprising a third differential amplifier and a capacitor connected to the output terminal, wherein an output end of the third differential amplifier is connected to the output terminal to obtain a filter characteristic. 2. The low pass filter according to claim 1, wherein the operating current of the third differential amplifier is variable.