JP3229569B2 - Filter circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a filter circuit for removing noise superimposed on a power supply voltage.

[0002]

2. Description of the Related Art As a power source for electronic equipment, a direct current obtained from a commercial power source or a battery via a converter is used. However, various noises are superimposed on such a DC power supply depending on usage conditions. For example, when one DC power supply is used as a common power supply for a plurality of electronic devices, if there is a motor or the like in which noise is likely to occur as one of the electronic devices, a DC input which is a power supply for other electronic devices is provided. Is likely to have its noise superimposed. Conventionally, a method of removing such noise by connecting an RC filter or an LC filter to the power supply circuit of each electronic device has been used. Had the disadvantage of becoming larger.

FIG. 2 is a circuit diagram of a conventional filter circuit combining a transistor, a resistor, and a capacitor. A transistor Q2 is connected in series between the input terminal 1 and the output terminal 2, and a connection point between the resistor R2 and the capacitor C2 is connected to the base of the transistor Q2. One end of the resistor R2 is connected to the input terminal 1, and one end of the capacitor C2 is grounded. In such a filter circuit, a DC voltage from which noise has been removed by the circuit of the resistor R2 and the capacitor C2 is obtained at the emitter of the transistor Q2. However, even if the DC voltage at the output terminal 2 is the smallest compared with the DC voltage applied to the input terminal 1, the base voltage of the transistor Q2
There is a disadvantage that the emitter-to-emitter voltage is reduced by about 0.7 V. Despite the large number of filters that use passive elements or combine active and passive elements, conventional filters have been unsatisfactory due to such various disadvantages.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a filter circuit in which the voltage difference between input and output is small and which can be easily integrated.

[0005]

A filter circuit according to the present invention comprises a transistor connected in series between an input terminal and an output terminal, one end connected to a resistor connected to the input terminal and the other end.
A series circuit consisting of a constant current source , a capacitor connected at one end to a connection point between the resistor and the constant current source, and an input side connected to the connection point.
A voltage follower circuit connected to the transistor and having an output side connected to the transistor .

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A resistor having one end connected to an input terminal
And a series circuit consisting of a constant current source connected to the other end generates a voltage lower than the voltage of the input terminal. Noise superimposed on the low voltage is removed by a resistor and a capacitor connected to the connection point of the series circuit. Then, the output voltage of the voltage follower circuit as a reference voltage a voltage removed of noise, series-connected transistors is controlled between the input and output terminals.

[0007]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram showing an embodiment of a filter circuit according to the present invention. Note that FIG.
The same reference numerals are given to the same parts. In FIG.
A PNP transistor Q1 is connected in series between the input terminal 1 and the output terminal 2. One end of a resistor R1 is connected to the input terminal 1, and one end of a current source S1 is connected to the other end. One end of a capacitor C1 is connected to a connection point P1 between the resistor R1 and the current source S1. Capacitor C1
And the other end of the current source S1 is grounded. Operational amplifier A1
Forms a voltage follower circuit, the inverting input terminal of which is connected to the connection point P1, and the output side of which is connected to the base of the transistor Q1. The non-inverting input terminal is connected to the collector of the transistor Q1.

In the filter circuit configured as described above,
A connection point P is formed by a series circuit including a resistor R1 and a current source S1.
1 generates a voltage lower than the DC voltage applied to the input terminal 1. The voltage at the connection point P1 is lower than the voltage at the input terminal 1 by (R · I), where I is the current of the current source S1 and R is the resistance value of the resistor R1. When noise is superimposed on the voltage of the input terminal 1, noise that should be superimposed on the voltage of the connection point P1 is removed by the resistor R1 and the capacitor C1. The voltage follower circuit uses the voltage at the connection point P1 from which noise has been removed as a reference voltage to set the transistor Q
Control 1 At the output terminal 2, a DC voltage free of noise which is substantially lower by a voltage drop due to the resistor R 1 is obtained.

When a ripple noise having an amplitude of, for example, ± 0.1 V is superimposed on the DC voltage of the input terminal 1, the voltage at the connection point P 1 may generate a voltage lower by about 0.1 V than the DC voltage of the input terminal 1. desirable. Thereby, the noise on the minus side is removed by the voltage drop of the resistor R1, and the noise on the plus side is removed by the resistor R1 and the capacitor C1. By increasing the time constant of the resistor R1 and the capacitor C1, even low-frequency noise can be eliminated. Even if the value of the resistor R1 is increased, the voltage drop can be reduced by reducing the current of the current source S1.

For example, a voltage drop of 0.1 V causes the resistance R1 to reach 100K.
Ω and the current of the current source S1 is 1 μA. The value of the capacitor C1 can be about 10 μF. Such a value is a value easily obtained in a semiconductor integrated circuit, and can greatly increase the time constant. The resistor R1 has a role of generating a voltage lower than the voltage of the input terminal 1 at the connection point P1 and also has a role of forming a circuit for directly removing noise together with the capacitor C1. Since the resistor R1 is not connected in series between the input terminal 1 and the output terminal 2 through which a current flows, it goes without saying that loss is small.

The filter circuit of the present invention can be used alone as a series regulator. that time,
Obviously, the output voltage can be set by the input voltage and the resistor R1. Further, in the embodiment, the output side of the operational amplifier A1 is directly connected to the base of the transistor Q1, but a control circuit for the transistor Q1 may be provided and connected to the transistor Q1 via the control circuit. In short, it is only necessary that the transistor Q1 can be controlled by the output of the operational amplifier A1. The input side is directly connected to the connection point P1, but may be connected via an inverter circuit.

[0012]

As described above, the filter circuit of the present invention generates a voltage which is lower than the voltage of the input terminal and is free from noise, and uses the voltage as a reference voltage. The circuit controls a transistor connected in series between the input terminal and the output terminal, so that the difference between the input voltage and the output voltage can be reduced. Since there is no resistor connected in series between the input terminal and the output terminal through which a large current flows, loss can be reduced. Needless to say, since no coil is used, integration into an integrated circuit is easy. A resistor for generating a voltage lower than the voltage of the input terminal,
A resistor that directly forms a circuit for removing noise together with a capacitor is also used, and there is an advantage that a plane area when formed as an integrated circuit can be reduced. Another practical advantage is that the filter circuit can be directly converted to a series regulator.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram of a conventional filter circuit.

[Explanation of symbols]

 1 input terminal 2 output terminal Q1 transistor S1 constant current source

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03H 11/04 G05F 1/56

Claims (1)

(57) [Claims] 1. A serial connection between an input terminal and an output terminal.
Transistor, resistor with one end connected to input terminal and its
A series circuit consisting of a constant current source connected to the other end, a resistor and a constant current
A capacitor having one end connected to a connection point of the flow source, the connection point
Is connected to the input side and the output side is connected to the transistor.
Characterized by having a voltage follower circuit
Filter circuit.