JP2625283B2 - Low-pass filter - Google Patents

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, and more particularly to a low-pass filter for a current which is effective for removing unnecessary signals from various protection circuits in a semiconductor integrated circuit device.

[0002]

2. Description of the Related Art A conventional low-pass filter will be described with reference to the drawings. FIG. 5 is a general conventional circuit diagram. In the figure, 1 is a current signal source, 10 is a first resistor, 1
1 is a fifth transistor, 4 is a capacitor, and 12 is a second transistor.
, 13 is a sixth transistor, 5 is an output terminal, 6
Is a positive power supply terminal, and 7 is a ground.

Next, the operation of the low-pass filter will be described. When a current signal flows from the current signal source 1,
The fifth transistor 11 operates, whereby the sixth transistor 13 operates to transmit a signal to the output terminal 5.

However, when an unnecessary signal having a high frequency such as noise is generated from the current signal source 1, the unnecessary signal is absorbed by the capacitor 4 and is not transmitted to the output terminal 5. The frequency of the erroneous signal that can be absorbed depends on the capacitor 4 and the first resistor 10.
And the gain of the fifth transistor 11. Special
The normal signal from the current signal source 1 has a relatively wide pulse width.
Moreover, it is a low frequency pulse signal,
Explanation of the case where pulse noise with narrow pulse width is mixed
I do. When there is no regular signal, the base of transistor 11
Is the same potential as the ground 7, and the capacitor 4 is
And the base side is charged with a charge of-. That
When pulse noise occurs in the current signal source 1 in this state,
Part of the current flows through the first resistor 10 and the rest is
You can move toward the base of transistor 11, but
Then, the base of the transistor 11 is charged to the capacitor 4.
From the base because it has the same potential as the emitter
It cannot flow to the emitter and the initial current
The sensor 4 is charged (-discharges) and the base potential is increased.
Consumed by Ruko. Therefore, the pulse width of the pulse noise is
If it is very narrow and its current value is also small, the transistor
No current flows through 11 to block the passage of pulse noise
You. A legitimate signal is a relatively wide pulse signal.
Therefore, charge the capacitor 4 sufficiently (discharge-) and
The transistor 11 is energized.

[0005]

In the above-described conventional low-pass filter, since the output impedance of the fifth transistor 11 is low, the gain is low and the Miller effect of the capacitor 4 is small. Also, the regular pulse width is wide
The effect of removing narrow noise pulses mixed in the signal
I will explain the situation because it is not enough. Figure 5 Circuit smell
Therefore, when there is no regular pulse signal,
The potential of the base of the resistor 11 is the same as that of the ground 7,
If pulse noise occurs at that time, the initial current is
There is no resistance in between for the capacitor 4 and for the ground 7
Has the first resistor 10, so most of the capacitor 4
The current flowing toward the first resistor 10 is small.
Therefore, the capacitor 4 is quickly charged and the transistor 1
The time to keep 1 in the blocking state is short. Therefore a little pulse width
Broad noise and large current noise are allowed to pass.
In FIG. 5, the first resistor 10 is set to 7KΩ, and the second resistor 12 is set to 5K.
Ω, the fifth transistor 11 and the sixth transistor 13
Circuit with hFE of 100 and duty 50 at 250 μApeak
FIG. 7 shows the relationship between the frequency that can be absorbed when the current square wave is output from the current signal source 1 and the capacitance of the capacitor 4. FIG.
In order to remove an erroneous signal of, for example, 500 KHz, a capacitor of 100 pF is required. When this circuit is incorporated in a semiconductor integrated circuit device, a capacitor of 100 pF has a large area, and a larger capacitor is required in consideration of the variation between the capacitor and other components, which is not very practical.

When it is required that the relationship between the signal of the current signal source 1 and the output current signal (collector current of the sixth transistor 13) be constant (for example, the same), the difference in hFE between the two transistors is considered. It is necessary to design the first resistor 10 and the second resistor 12 in accordance with the above, and the design is not easy.

[0007]

SUMMARY OF THE INVENTION Accordingly, the present invention provides an input current signal that is injected into an emitter and based on an output current signal.
A first transistor of one conductivity type to be derived, a second transistor of another conductivity type which inputs an output current signal of the first transistor to a base, amplifies the current, and outputs the amplified signal to a collector; A low-pass filter having a capacitor that bypasses a high frequency component of an output current signal of one transistor from a base of the second transistor.

Furthermore, the input current signal is injected into the emitter, the output current signal and the first transistor made of one conductivity type to derive the base, based on the output current <br/> current signal of the first transistor Input, amplify current and output to collector
To a second transistor formed of other conductivity type, the second
The current signal output to the collector of the transistor
Other signals that are injected into the
A third transistor of a conductivity type;
Input the output current signal of the transistor to the base and amplify the current.
Fourth transistor of one conductivity type for outputting to collector
When is the low-pass filter and having a capacitor for bypassing from the base of the output current signal or (and) the third next transistor high-frequency component of the output current signal of the transistor of the first transistor .

[0009]

According to the above construction, the current signal is made 1 / (hFE + 1) by the first transistor, and the high frequency component is passed through the capacitor. Therefore, the high frequency absorption effect of the capacitor is enhanced, and even if the capacitor has a small capacity, it is high. A low-pass filter effect can be obtained. Also, the pulse width is relatively wide
Used to remove narrow pulse width noise mixed with regular signals
The current output to the base of the first transistor
Does not matter whether the noise signal is rising at the beginning.
1 / (hFE) of the noise current always injected into the emitter
+1) and it takes time to charge the capacitor
The second transistor is also used for a wide pulse noise.
Hold in blocking state. Since the signal multiplied by 1 / (hFE + 1) is multiplied by hFE by the second transistor,
If the hFE of both transistors is substantially equal, the low frequency component including the DC component of the input current and the output current are substantially the same, so that they can be easily mounted between signals of various circuits.

However, it is not so easy to make the hFE of both transistors of different conductivity types uniform in a semiconductor integrated circuit device. Therefore, the first and second transitions
Opposite the first circuit consisting of a star and each transistor
If a signal is cascaded through the second circuit of the conductivity type, it is easy to make the characteristics of the transistors of the same conductivity type the same, so that the input current and the output current generated in the first circuit are reduced. The difference is canceled by the second circuit, and it is easy to make the input current and the output current the same as a whole.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram of one embodiment of this circuit. 1 is a current signal source, 2 is a first transistor, 3 is a second transistor, 4 is a capacitor, 5 is an output terminal, 6 is a positive power supply terminal, and 7 is a ground.

The first transistor 2 is a PNP transistor and the second transistor 3 is an NPN transistor, and both have the same hFE.

In this low-pass filter, the power signal source 1 is connected to the emitter of the first transistor 2, the collector is connected to the ground 7 which is the other end of the power source, and the base is connected to the base of the second transistor 3 and the capacitor. 4 is connected. The emitter of the second transistor 3 is connected to the ground 7
And the collector is connected to the other terminal of the capacitor 4 and the output terminal 5.

Next, the operation of the low-pass filter will be described. The input signal IIN of the current signal source 1 flows into the emitter of the first transistor 2. At this time, a current of 1 / (hFE + 1) ・ IN flows through the base of the first transistor 2. This current is supplied by the second transistor 3 to hFE
Doubled. Assuming that the output current of the output terminal 5 is IOUT,

(Equation 1)

[0015]

The input current IIN and the output current IOU
T is almost the same.

In the case of a high-frequency signal, the current passing through the capacitor 4 is 1 / (hFE + 1) ・_IN , so that the effect of the capacitor on the low-pass filter is enormous. In addition,
The normal signal of the flow signal source 1 is pulse-like,
When mixed pulse noise with narrow pulse width
Of the second transistor 3 during the time when there is no regular signal
The potential of the base is the same as the ground 7, and the current signal
The noise pulse current having a narrower pulse width than the source 1
When injected into the emitter of transistor 2, its base
Regardless of whether the pulse current is initial or not.
A current as small as 1 / (hFE + 1) is output. Follow
The capacitor 4 is charged slowly and the second transistor
A current flows through the base of the star 3 and thus a current flows through the collector
It takes time to reach the base potential where
When using the same capacitor compared to the conventional circuit shown
Wide noise also blocks passage.

As shown in FIG. 2, the first transistor 2A is an NPN transistor and the second transistor 3A
Is a PNP transistor, and the collector of the first transistor and the emitter of the second transistor 3A are connected to the power supply terminal 7A, the same effect can be obtained.

That is, in the embodiment of FIG. 2, the ground 6A is one end of the power supply, and the positive power supply terminal 7A is the other end of the power supply.

[0020]

Second Embodiment FIG. 3 is a circuit diagram of a second embodiment of the present invention. This embodiment is the same as the first embodiment except that a third transistor 8 and a fourth transistor 9 are added to the first embodiment. The description is omitted.

The first transistor 2 and the fourth transistor 9 are PNP transistors, and their hFE is hFEp. The second transistor 3 and the third transistor 8 are N
In the PN transistor, hFE is hFEn. In the circuit of the second embodiment, the emitter of the third transistor 8 is connected to the connection point between the collector of the second transistor 3 and the capacitor 4, and the collector of the third transistor 8 and the emitter of the fourth transistor Are connected to the positive power supply terminal 6, the base of the third transistor 8 is connected to the base of the fourth transistor 9, and the collector of the fourth transistor 9 is connected to the output terminal 5.

The operation of this embodiment is based on a high-frequency noise signal.
In the case of pulse noise, the same effect as in the first embodiment can be obtained. For low-frequency signals and signals with a wide pulse width ,

[0023]

The following holds.

As can be seen from this equation, in the circuit of the second embodiment, h of the PNP transistor and the NPN transistor
Even if FE is different, the input current IIN and the output current IOUT are almost the same. PNP transistor in semiconductor integrated circuit and N
It is very difficult to make the PN transistors have the same hFE, and this second embodiment is effective.

Also, as shown in FIG.
A and the fourth transistor 9A are NPN transistors, and the second transistor 3A and the third transistor 8A
Is a PNP transistor, the collector of the first transistor 2A and the emitter of the second transistor 3A are connected to the power supply terminal 7A, and the collector of the third transistor 8A and the emitter of the fourth transistor 9A are connected to the ground 6A. Has the same effect.

Further, the capacitor 4 is connected between the base and the collector of the fourth transistor 9A and the second transistor 3A.
Between the base of the third transistor 8A and the base of the third transistor 8A.
Similar effects can be obtained by connecting between the base of the transistor 3A and the ground 6A, between the base of the third transistor 8A and the power supply terminal 7A, and the like.

[0028]

As described in the foregoing, the present invention because it has passed through the condenser in the 1 / (hFE + 1) by transistor current signal <br/> No. enhances the high frequency absorption effect of the capacitor, a small capacity high pass filter effect in the capacitor is Ru obtained. Positive if used in pulse signal circuits
Eliminates pulse noise that has a narrower pulse width than the normal signal
High effect is obtained. Also to ensure that be easily mounted between the signals of various circuits as almost the same input and output currents of the low-pass filter is a low-frequency signal
I can .

For example, in the circuit of FIG. 6 using the first embodiment, the second resistor 12 is set to 5 KΩ, the first transistor 2, the second transistor 3, and the sixth transistor 13 are set to 10 hFE.
0, 250 μA peak, duty 50 from current signal source 1
FIG. 7 shows the relationship between the frequency of the signal that can be absorbed and the capacitance of the capacitor 4 when the square wave is output. As can be seen from FIG. 7, the capacity of the capacitor 4 can absorb a square wave of 85 kHz at 10 pF, and has a function of increasing the capacitor capacity by about 54 times as compared with the conventional circuit.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram of a modification of the first embodiment of the present invention.

FIG. 3 is a circuit diagram of a second embodiment of the present invention.

FIG. 4 is a circuit diagram of a modification of the second embodiment of the present invention.

FIG. 5 is a circuit diagram of a conventional technology.

FIG. 6 is a circuit diagram for explaining the effect of the application circuit using the first embodiment of the present invention.

FIG. 7 is a graph showing the relationship between the frequency of a square wave that can be absorbed and the capacitance of a capacitor according to the first embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 current signal source 2, 2A first transistor 3, 3A second transistor 4 capacitor 5 output terminal 6 positive power supply terminal (one end of power supply) 6A ground (one end of power supply) 7 ground (other end of power supply) 7A positive power supply Terminal (the other end of the power supply) 8,8A Third transistor 9,9A Fourth transistor

Claims (17)

(57) [Claims] An input current signal is injected into an emitter, and an output
In the first transistor and the other conductivity type to output the current amplification to the collector receives the output current signal of said first transistor to <br/> base made in one conductivity type to derive a current signal to the base A low-pass filter comprising: a second transistor; and a capacitor that bypasses a high-frequency component of an output current signal of the first transistor from a base of the second transistor. 2. The method according to claim 1, wherein a current amplification factor of the first transistor is substantially equal to a current amplification factor of the second transistor, and an output current signal is substantially equal to a low frequency component including a DC component of the input current signal. Low-pass filter. 3. An emitter connected to one end of a power supply via a current signal source, a collector of which is connected to a first transistor of one conductivity type connected to the other end of the power supply and one end of the power supply via a load. A collector is connected to the output terminal, and an emitter is a second transistor of another conductivity type connected to the other end of the power supply. The base of the first transistor is connected to the base of the second transistor. 3. The low-pass filter according to claim 1, wherein one of the capacitors is connected to the connection point. 4. The low-pass filter according to claim 3, wherein the other of said capacitors is connected to a collector of said second transistor. 5. The low-pass filter according to claim 3, wherein the other end of said capacitor is connected to one end of said power supply. 6. The low-pass filter according to claim 3, wherein the other end of said capacitor is connected to the other end of said power supply. 7. An input current signal is injected into an emitter, and an output
A first transistor of one conductivity type that derives a current signal on the base, and a second transistor of another conductivity type that inputs the output current signal of the first transistor to the base, amplifies the current, and outputs the amplified signal to the collector . A transistor and the second transistor
The current signal output to the collector of the
With other conductivity types that are based on the output current signal
A third transistor, and the third transistor
Input the output current signal to the base and amplify the current to collect
A fourth transistor formed of one conductivity type to be output to said first transistor of the output current signal or (and) a capacitor for bypassing from the base of the third next transistor high-frequency component of the output current signal of the transistor When
Low-pass filter and having a. 8. A first transistor having one conductivity type and a fourth transistor having one conductivity type.
The current amplification factor of the second transistor is substantially equal to that of the third transistor, and the current amplification factor of the second transistor and the third transistor of other conductivity types are substantially equal to each other. 8. The low-pass filter according to claim 7, wherein the low-pass filter has substantially the same frequency. 9. A first transistor of one conductivity type having an emitter connected to one end of a power supply via a signal source and a collector connected to the other end of the power supply, and another transistor having an emitter connected to the other end of the power supply. A second transistor of a conductivity type, a third transistor of another conductivity type having an emitter connected to the collector of the second transistor and a collector connected to one end of the power supply, and an emitter connected to one end of the power supply The collector comprises a fourth transistor of one conductivity type connected to an output terminal connected to the other end of the power supply via a load, and
The base of the third transistor is connected to the base of the second transistor, the base of the third transistor is connected to the base of the fourth transistor, and the base of the first transistor and the second transistor are connected. 9. The low-pass filter according to claim 7, wherein one of the capacitors is connected to a connection point with the base. 10. The low-pass filter according to claim 9, wherein the other of said capacitors is connected to a collector of said second transistor. 11. The low-pass filter according to claim 9, wherein the other end of said capacitor is connected to one end of said power supply. 12. The low-pass filter according to claim 9, wherein the other end of said capacitor is connected to the other end of said power supply. 13. A first transistor of one conductivity type having an emitter connected to one end of a power supply via a signal source and a collector connected to the other end of the power supply, and another transistor having an emitter connected to the other end of the power supply. connecting a second transistor formed of a conductive type, an emitter connected to the collector of the second transistor, a third transistor made of other conductivity type having a collector connected to one end of the power, at one end of the emitter power The collector is a fourth transistor of one conductivity type connected to the output terminal connected to the other end of the power supply via the load, and the base of the first transistor is connected to the second base. , A base of the third transistor and a base of the fourth transistor are connected, and a connection point of the base of the third transistor and the base of the fourth transistor is connected to a connection point of a capacitor. Low pass filter according to claim 7 or claim 8, characterized in that it is connected. 14. The low-pass filter according to claim 13, wherein the other of said capacitors is connected to a collector of said fourth transistor. 15. The low-pass filter according to claim 13, wherein the other of said capacitors is connected to a base of said second transistor. 16. The low-pass filter according to claim 13, wherein the other end of said capacitor is connected to one end of said power supply. 17. The low-pass filter according to claim 13, wherein the other end of said capacitor is connected to the other end of said power supply.