JPH06283965A - Active low-pass filter - Google Patents

Abstract

(57) [Summary] [Object] An object of the present invention is to provide an active low-pass filter having a trap with a simplified configuration. According to the present invention, one end of the resistor R1 is connected to the input terminal I
N, the one end of the capacitor C1 is connected to the other end of the resistor R1, the one end of the resistor R2 is connected directly or via the buffer B3 to the other end of the resistor R1, and the one end of the capacitor C2 is connected to the resistor R2 Of the buffer amplifier B1 which is connected to the other end of the resistor R2 and which amplifies the drive current according to the signal from the input end.
The drive current of 1 is passed to generate a voltage, the voltage generated in the resistor R3 is taken out by the buffer amplifier B2 and applied to the other end of the capacitor C2, and the output end of the buffer amplifier B1 is connected to the other end of the capacitor C1. The input terminal or output terminal of the buffer amplifier B1 is connected to the signal output terminal OUT.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active low pass filter used by being incorporated in an IC in an electronic circuit.

[0002]

2. Description of the Related Art Recently, in an electronic circuit, an active type is often used when a low pass filter (hereinafter referred to as an LPF) is incorporated in an IC.
Since this active type LPF initially has a large variation in the element precision of the resistance and the capacitance that compose it, a method of correcting this variation using a variable voltage current converter (gm amplifier) has been adopted. . An active type LPF using a gm amplifier is typically a biquad type LPF. FIG. 3 shows the circuit configuration.

This LPF is composed of a first gm amplifier A and a second gm amplifier A.
1, A2 and voltage divider D are provided. The (+) input end of the first gm amplifier A1 is connected to the input terminal IN, and the output end thereof is grounded via the capacitor C11. The (+) input terminal of the second gm amplifier A2 is connected to the output terminal of the first gm amplifier A1, and its output terminal is connected to the output terminal OUT, and
The gm amplifiers A1 and A2 are feedback-connected to the (-) input terminals. The input terminal of the voltage divider D is connected to the input terminal IN, and the output terminal is connected to the output terminal OUT via the capacitor C12.

In the above structure, the input signal is Vin, the output signal is Vout, the voltage division ratio of the voltage divider D is k11, and the equivalent conductances of the gm amplifiers A1 and A2 are 1 / R1 respectively.
Assuming 1, 1 / R12, the transfer characteristic of this LPF is

[0005]

[Equation 1] Becomes

This characteristic is shown in FIG. That is, when the voltage division ratio k11 is 0, the low-pass characteristic is gradually lowered, and k
When 11 is not 0, a low-pass characteristic with a trap having a bottom at the angular frequency ωt is obtained. The angular frequency ωt is

[0007]

[Equation 2] Is expressed as

From this, it can be seen that this low-pass characteristic with a trap is effective when it is desired to sharply drop the high frequency range. The drawback of this LPF is that the number of circuit elements is very large because the gm amplifier is indispensable.

By the way, recently, the element accuracy in the IC has been dramatically improved, and a fixed constant can be used without adjustment in combination with the trimming technique. For this reason, LPFs with a simple structure called the Sallen-Key type have been widely used. FIG. 5 shows the circuit configuration of a Sallen-Key type LPF.

In this LPF, the gm amplifier is not used, and the input terminal IN is connected to the buffer amplifier B via the resistor R1.
3 is connected to the input terminal of the buffer amplifier B3, and the output terminal of the buffer amplifier B3 is connected to the input terminal of the buffer amplifier B1 via the resistor R2. The output terminal of the buffer amplifier B1 is connected to the output terminal OUT, is also feedback-connected to the input terminal of the buffer amplifier B3 via the capacitor C1, and the input terminal is grounded via the capacitor C2. The transfer characteristic in this circuit configuration is

[0011]

[Equation 3]

The same characteristic as k11 = 0 can be obtained in the bike quad type. In this case, the gm amplifier is unnecessary,
Since the number of elements is small, the circuit scale can be made extremely small.

However, in the Sallen-Key type LPF,
As is clear from the above equation, the bottom cannot be formed and can be used only when k11 = 0 in the bike quad type. The fact that the LPF with a trap is not configured in this way is very difficult to use in filter design.
There was a method of providing some traps, but all of them added a complicated time constant circuit.

[0014]

As described above, it has been impossible to realize an LPF with a trap with a simple structure in the past. The present invention has been made to solve the above problems, and an object thereof is to provide an active low-pass filter having a trap with a simple structure.

[0015]

In order to achieve the above object, an active low-pass filter according to the present invention has one end of a first resistor connected to a signal input terminal and one end of a first capacitor connected to a signal input terminal. 1 is connected to the other end of the first resistor, and one end of the second resistor is connected directly or via the third buffer amplifier to the first end.
Is connected to the other end of the resistor, one end of the second capacitor is connected to the other end of the second resistor, and the input end of the first buffer amplifier that amplifies the drive current according to the signal from the input end is connected to The second resistor is connected to the other end of the second resistor, and the drive current of the first buffer amplifier is passed through the third resistor to generate a voltage.
The voltage generated in the third resistor by the buffer amplifier is applied to the other end of the second capacitor, the output end of the first buffer amplifier is connected to the other end of the first capacitor, and the first buffer is connected. The input terminal or output terminal of the amplifier is connected to the signal output terminal.

[0016]

In the active low-pass filter having the above structure, the first and second resistors, the first and second capacitors, and the first and second capacitors are provided.
Based on the Sallen-Key type filter composed of the third buffer amplifier, the drive current to the first capacitor is taken out and passed through the third resistor, and the voltage generated in the third resistor is passed through the second buffer amplifier. The signal is fed back to the input terminal of the first buffer amplifier via the.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to FIG. In FIG. 1, the same parts as those in FIG. 5 are designated by the same reference numerals, and different parts will be mainly described here.

FIG. 1 shows the structure of the LPF according to the present invention. The input terminal IN is connected to the input terminal of the third buffer amplifier B3 via the resistor R1, and the output terminal of the third buffer amplifier B3 is connected to the first output terminal OUT1 via the resistor R2. It is connected to the base of the transistor Q which constitutes the first buffer amplifier B1.

The first buffer amplifier B1 is constructed by connecting the collector of the transistor Q to the VCC power source via the resistor R3, grounding the emitter to the ground via the constant current source I0, and connecting it to the second output terminal OUT2. To be done.

The emitter of the transistor Q has a capacitance C
It is connected to the input terminal of the second buffer amplifier B2 via 1. The collector of the transistor Q is connected to the input terminal of the second buffer amplifier B2, and the output terminal of the buffer amplifier B2 is connected to the input terminal (base of the transistor Q) of the first buffer amplifier B1 via the capacitor C2. To be done.

That is, in the LPF having the above structure, the first buffer amplifier B1 of the Sallen-Key type LPF shown in FIG. 5 is realized by the emitter follower of the transistor Q, and a resistor R3 and a second buffer amplifier B2 are slightly added. Only. The transfer characteristic of this circuit will be described. Now, when the transistor Q is idealized and only the AC component is considered, the collector current Ic of Q is

[0022]

[Equation 4] And the collector terminal voltage is V3, the buffer amplifier B3
Let V2 be the input of

[0023]

[Equation 5] From the relational expression of, the transfer characteristic is

[0024]

[Equation 6] Becomes

Here, comparing with the transfer characteristics of the conventional Sallen-Key type LPF, it can be seen that an LPF with a trap can be realized. The resonance frequencies of R1, C1, R2, and C2, which are the same as those of the conventional Sallen-Key type, differ, but they are the same if R1 + R3 is regarded as the conventional R1.

In FIG. 2, the third buffer amplifier B3 of FIG. 1 is eliminated, and the connection point of the resistor R1 and the capacitor C1 and the resistor R2 are directly connected. The result is that Q1 in the above equation is Q '= Q1 / 2. When the Q value may be low, the number of elements is reduced, which is effective.

Therefore, an LPF with a trap effective for filter design can be realized with a very simple structure, and a characteristic equivalent to a complicated circuit using a conventional gm amplifier can be obtained. The present invention is not limited to the above-described embodiments, and the present invention can be similarly implemented even if various modifications are made without departing from the scope of the present invention.

[0028]

As described above, according to the present invention, it is possible to provide an active low-pass filter having a trap with a simple structure.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of an embodiment of an active type low pass filter according to the present invention.

FIG. 2 is a circuit diagram showing the configuration of another embodiment according to the present invention.

FIG. 3 is a circuit diagram showing a typical biquad configuration as a conventional active LPF.

FIG. 4 is a characteristic diagram showing a transfer characteristic of the LPF of FIG.

FIG. 5 is a circuit diagram showing a configuration of a conventional Sallen-Key type LPF.

[Explanation of symbols]

A1, A2 ... First and second gm amplifiers, D ... Voltage divider, Q
... Transistor, I0 ... Constant current source, B1, B2, B3 ...
Buffer amplifier.

Claims (4)

[Claims] 1. A first resistor having one end connected to a signal input terminal, a first capacitor having one end connected to the other end of the first resistor, and one end having the first resistor. A second resistor connected to the other end of the second resistor, a second capacitor having one end connected to the other end of the second resistor, and an input end connected to the other end of the second resistor, A first buffer amplifier for amplifying a drive current according to a signal from an input end;
A third resistor for generating a voltage when a drive current of the buffer amplifier is supplied, and a second buffer amplifier for extracting the voltage generated in the third resistor and applying it to the other end of the second capacitor. An active end, wherein an output end of the first buffer amplifier is connected to the other end of the first capacitor, and an input end of the first buffer amplifier is connected to a signal output terminal. Type low-pass filter. 2. An active low-pass filter according to claim 1, wherein a third buffer amplifier is interposed between the other end of the first resistor and the other end of the second resistor. filter. 3. A first resistor having one end connected to a signal input terminal, a first capacitor having one end connected to the other end of the first resistor, and one end having the first resistor. A second resistor connected to the other end of the second resistor, a second capacitor having one end connected to the other end of the second resistor, and an input end connected to the other end of the second resistor, A first buffer amplifier for amplifying a drive current according to a signal from an input end;
A third resistor for generating a voltage when a drive current of the buffer amplifier is supplied, and a second buffer amplifier for extracting the voltage generated in the third resistor and applying it to the other end of the second capacitor. An active low-pass filter, comprising: an output end of the first buffer amplifier connected to the other end of the first capacitor and a signal output terminal. 4. An active type low-pass filter according to claim 3, wherein a third buffer amplifier is interposed between the other end of the first resistor and the other end of the second resistor. filter.