JPH09167939A - High pass filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a high pass filter into an integrated circuit by multiplying a value depending on an emitter-resistance of a transistor(TR) and an on- resistance of a diode with a time constant consisting of a resistor and a capacitor so as to select a cut-off frequency thereby eliminating the need for an external element. SOLUTION: A cut-off frequency is obtained by multiplying a constant depending on resistance of 1st and 2nd resistors 2, 7, an on-resistance of 1st to 4th diodes 4, 5, 14, 15, and an emitter-resistance of transistors(TRs) 16a, 16b, 116a, 116b with a time constant depending on 3rd and 4th resistors 9, 12 and a capacitor 17. For example, in the case of setting the cut-off frequency of be 2.2kHz, the capacitance of the capacitor 17 is 100pF, the resistance of the 3rd, 4th resistors 14, 15 is 20KΩ let the resistance of the 1st resistor 2 be 40KΩ, the resistance of the 2nd resistor 7 be 20KΩ, the on-resistance of the 1st and 2nd didoes 4, 5 be 360Ω, the emitter-resistance of the TRs 6a, b be 26KΩ, the on-resistance of the 3rd and 4th didoes 4, 5 be 360Ω, and the emitter- resistance of the TRs 16a, b be 26KΩ, then the components of the 1st to 4th resistors 2, 7, 9, 12 and the capacitor 12 are integrated in an IC and no externally mounted component is required.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-pass filter which is used in an audio signal band and which is suitable for use as an IC in which an external element can be incorporated in the IC.

[0002]

2. Description of the Related Art Conventionally, an HPF (high-pass filter) circuit has a C composed of a resistor R and a capacitor C.
R type HPF or op amp instead of resistor R
An active filter that obtains a PF characteristic is known. In such a CR type HPF circuit, the cutoff frequency fc
Is fc = 1 / (2π · C · R). Also, in the case of an active filter, the conductance gm of the operational amplifier
Is used as a resistor, the cutoff frequency fc ′ is f
c ′ = gm / (2π · C).

[0003]

However, the HPF
Is used in the frequency band (20 to 20 KHz) of the audio signal, either the resistor R or the capacitor C has a value that cannot be integrated into an IC, and the resistor R and the capacitor C are external elements. Therefore, there is a problem in that the number of external pins of the IC is increased when the IC is formed.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and includes a first transistor to which an input signal is applied and an emitter of the first transistor via a first resistor. A connected second transistor, first and second variable loads to which collector currents of the first and second transistors are supplied respectively, and output voltages of the first and second variable loads are applied, and output terminals thereof A first differential amplifier circuit connected to the second resistor, a third transistor to which the output signal of the first differential amplifier circuit is applied to the base, and an emitter of the third transistor via a third resistor. A fourth transistor connected to the emitter, a fifth transistor having a collector and a base commonly connected to a collector and a base of the fourth transistor, and an emitter connected to the fifth transistor via a fourth resistor. A sixth transistor connected to the emitter of the transistor, third and fourth diodes to which collector currents of the third and sixth transistors are respectively supplied, and output voltages of the third and fourth diode are applied to output A second differential amplifier circuit to which a signal is applied to the base of the sixth transistor; and a capacitor connected between the bases of the first and sixth transistors, and an output terminal of the second differential amplifier circuit. It is characterized in that the filter characteristics are obtained.

Further, the operating current of the first differential amplifier is made variable to change the cutoff frequency. Further, the operating current of the second differential amplifier circuit is made variable and the amount of attenuation is changed.

[0006]

1 is a diagram showing an embodiment of the present invention, (1) is a first transistor to which an input signal is applied, and (2) is connected to an emitter of the first transistor (1). The first resistance is (3), and the emitter is the first resistance (2).
A second transistor (4) and (5) connected to the emitter of the first transistor (1) via
First and second diodes for converting the collector currents of the transistors (1) and (3), respectively, and (6) is the first diode.
And a first differential amplifier circuit comprising transistors (6a) and (6b) to which the output voltages of the second diodes (4) and (5) are respectively applied, and (7) is a first differential amplifier circuit (6). A second resistor connected between the output terminal and the terminal to which the reference voltage is applied; (8) a third transistor to which the output signal of the first differential amplifier circuit (6) is applied;
(9) is a third resistor connected to the emitter of the third transistor (8), and (10) is a fourth transistor whose emitter is connected to the emitter of the third transistor (8) through the third resistor (9). , (11) has the fourth collector and base
A fifth transistor commonly connected to the collector and base of the transistor (10), (12) a fourth resistor connected to the emitter of the fifth transistor (11), and (13) an emitter of the fourth resistor (12). A sixth transistor connected to the emitter of the fifth transistor (11) through
(14) and (15) are third and fourth diodes for converting the collector currents of the third and sixth transistors (8) and (13), respectively, and (16) is a third diode to the base.
And a second differential amplifier circuit comprising transistors (16a) and (16b) to which the output voltages of the fourth diodes (14) and (15) are applied, and (17) is the first and sixth transistors (1) and ( A capacitor connected between the bases of 13) and (18) is a buffer circuit to which the output signal of the second differential amplifier circuit (16) is applied.

In FIG. 1, an input signal is applied to the base of the transistor (1), and collector currents corresponding to the input signal are generated from the collectors of the first and second transistors (1) and (3) which are differentially connected. Occur. The collector current is voltage-converted by the first and second diodes (4) and (5), and the output voltages of the first and second diodes (4) and (5) are different by the first differential amplifier circuit (6). Dynamically amplified. First
The output signal of the differential amplifier circuit (6) is applied to the base of the third transistor (8) and also to the base of the second transistor (3) via the second resistor (7),
Negative feedback is given. Furthermore, since the third and fourth transistors (8) and (10) are differentially connected, and the fifth and sixth transistors (11) and (13) are differentially connected,
Collector currents of the third and sixth transistors (8) and (13) are generated according to the output signal of the first differential amplifier circuit (6). The collector current is voltage-converted by the third and fourth diodes (14) and (15) and then applied to the second differential amplifier circuit (16). Second differential amplifier circuit (1
The output signal of 6) is applied to the base of the first transistor (1) via the capacitor (17) and is generated at the output terminal via the buffer circuit (18).

Next, the HPF characteristic obtained at the output terminal will be specifically described. When the AC voltage of the input signal is Vin and the value of the first resistor (2) is R1, the current i1 flowing through the first resistor (2) is

[0009]

[Equation 1]

[0010] The difference V2 between the output voltages of the first and second diodes (4) and (5) is equal to the difference between the first and second diodes.
If the on resistance of the diodes (4) and (5) is re1,

[0011]

(Equation 2)

[0012] Further, a second differential amplifier circuit (6)
Of the output signal i2 of the transistors (6a) and (6b)
If the emitter resistance of is re2,

[0013]

(Equation 3)

## EQU1 ## In addition, i3 is a current flowing through the bases of the fourth and fifth transistors (10) and (11)
Let R3 be the value of the third and fourth resistors (9) and (12),
If the output voltage of the second differential amplifier circuit (6) is Va and the output voltage of the output terminal is Vout,

[0015]

(Equation 4)

## EQU1 ## Then, the difference V3 between the output voltages of the third and fourth diodes (14) and (15) is the on resistance of the third and fourth diodes (14) and (15).
If it is 3,

[0017]

(Equation 5)

## EQU1 ## Further, the second differential amplifier circuit (1
If the emitter resistances of the transistors (16a) and (16b) are re4, the output current i5 of 6) is

[0019]

(Equation 6)

And again,

[0021]

(Equation 7)

It also becomes Here, the output voltage Va of the first differential amplifier circuit (6) is set to R2 when the value of the second resistor (7) is R2.

[0023]

(Equation 8)

Therefore, when the equations (3) to (1) are sequentially substituted into the equation (8),

[0025]

(Equation 9)

## EQU1 ## On the other hand, when the expressions (6) to (4) are sequentially substituted into the expression (7),

[0027]

(Equation 10)

It becomes Then, by substituting the equation (9) into the equation (10) so as to eliminate the voltage Va, the transfer function of the input signal and the output signal becomes

[0029]

[Equation 11]

## EQU1 ## From the equation (11), the circuit of FIG.
It becomes the characteristic of the PF circuit. Further, from the equation (11), the frequency f1 at which the attenuation amount is −6 dB is

[0031]

(Equation 12)

Therefore, the cutoff frequency fc on the high frequency side is

[0033]

(Equation 13)

## EQU1 ## Therefore, the time constant that determines the frequency f1 or fc becomes a predetermined value, and the time constant determined by the third and fourth resistors (9) and (12) and the capacitor (17) becomes equal to the first and second resistors (2 ) And (7), and the first
And the on-resistance re1 of the second diodes (4) and (5)
An emitter resistance re2 of the transistors (16a) and (16b), an on resistance re3 of the third and fourth diodes (14) and (15), and a transistor (16a).
And the emitter resistance re4 of (16b).

Therefore, in the HPF of FIG. 1, the values of the resistor and the capacitor can be made smaller than those of the HPF which is simply composed of the resistor and the capacitor. However, in this case,

[0036]

[Equation 14]

It is necessary to satisfy the following conditions. As a more specific example, when setting the cutoff frequency of the HPF of FIG. 1 to 2.2 KHz, R1 = 40 KΩ and R2 = 20 K
Ω, re1 = 360Ω, re2 = 26KΩ, re3 = 3
Assuming 60Ω and re4 = 26KΩ, the condenser (1
The capacitance of 7) is 100 pF, and the third and fourth resistors (14)
The value R3 of (15) and (15) can be 20 KΩ.
With such values, the first to fourth resistors and the capacitor (12) of the HPF of FIG. 1 can be integrated into an IC, so that the HPF without external elements can be formed.

When the operating current of the first differential amplifier circuit (5) is made variable, the value of the emitter resistance re2 changes, and the frequency f1 changes according to the equation (12). Therefore, Figure 2
As shown in (a), when the operating current is increased, the frequency f1 is increased, and when the operating current is decreased, the frequency f1 is decreased. Further, the initial gain G that determines the attenuation amount of the filter circuit from the equation (11) is

[0039]

(Equation 15)

The initial gain G varies depending on the emitter resistance re2. Therefore, as shown in FIG. 2B, the amount of attenuation increases as the operating current increases, and the amount of attenuation decreases as the operating current decreases. The cutoff frequency fc can be changed not only by changing the operating current of the first differential amplifier circuit (16) but also by changing the operating current of the second differential amplifier circuit (16).

[0041]

As described above, according to the present invention, HP
Since the cutoff frequency of F is set by multiplying the time constant of the resistor and the capacitor by the value determined by the on resistance of the diode and the emitter resistance of the transistor, it is possible to reduce the values of the resistor and the capacitor. When it is realized, an HPF having no external element can be constructed.

Further, it is possible to provide an LPF in which the cutoff frequency and the maximum attenuation amount 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.

FIG. 2 is a characteristic diagram for explaining the present invention.

[Explanation of symbols]

 1 1st transistor 2 1st resistance 3 2nd transistor 4 1st diode 5 2nd diode 6 1st differential amplifier circuit 7 2nd resistance 8 3rd transistor 9 3rd resistance 10 4th transistor 11 5th transistor 12 4th Resistor 13 6th transistor 14 3rd diode 15 4th diode 16 2nd differential amplifier circuit 17 Capacitor

Claims (3)

[Claims] 1. A first transistor to which an input signal is applied, a second transistor whose emitter is connected to an emitter of the first transistor through a first resistor, and collector currents of the first and second transistors. The first and second variable loads respectively supplied, the first differential amplifier circuit to which the output voltages of the first and second variable loads are applied, and the output terminals thereof are connected to the second resistor, and the first differential amplifier circuit to the base. A third transistor to which the output signal of the first differential amplifier circuit is applied; a fourth transistor whose emitter is connected to the emitter of the third transistor through a third resistor; and a collector and a base of the fourth transistor. A fifth transistor commonly connected to the collector and the base, and a sixth transistor having an emitter connected to the emitter of the fifth transistor through a fourth resistor, Third and fourth diodes to which collector currents of the third and sixth transistors are supplied respectively, output voltages of the third and fourth diodes are applied, and an output signal is applied to a base of the sixth transistor. Second
A high-pass filter comprising a differential amplifier circuit and a capacitor connected between the bases of the first and sixth transistors, and a filter characteristic is obtained at an output terminal of the second differential amplifier circuit. . 2. An operating current of the first differential amplifier is made variable, and a cutoff frequency is changed.
High pass filter as described. 3. The high-pass filter according to claim 1, wherein the operating current of the second differential amplifier circuit is made variable and the amount of attenuation is changed.