JP3362602B2 - Active filter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active filter, and more particularly to an active filter incorporating a diamond type buffer circuit which is a buffer amplifier circuit having a two-stage buffer circuit configuration.

[0002]

2. Description of the Related Art FIG. 9 is a block diagram of an active filter having a buffer amplifier circuit which is well known in the art. The active filter 50 is composed of a CR network 1 and a buffer amplifier circuit 2 which are connected in series between an input terminal IN and an output terminal OUT. At this time, the buffer amplifier circuit 2 is composed of a series circuit of a front stage buffer circuit 3, a rear stage buffer circuit 4 and a resistor R _O ′. A feedback path 5 is provided from the output side of the rear stage buffer circuit 4 to the CR network 1.

FIG. 10 shows a conventional active filter 50.
The concrete circuit diagram of is shown. In this concrete circuit diagram, a diamond type buffer circuit is used as the buffer amplifier circuit 2. At this time, the front stage buffer circuit 3
Is composed of an NPN transistor Q1 and a PNP transistor Q2 whose bases are commonly connected, and the rear stage buffer circuit 4 is composed of an NPN transistor Q3 and a PNP transistor Q4 which are connected so as to form a bushable circuit. To be done.

The output of the CR network 1 is the input side of the pre-stage buffer amplifier circuit 3, that is, the base and PN of the NPN transistor Q1 forming the pre-stage buffer circuit 3.
It is connected to the base of the P-transistor Q2 and is on the output side of the pre-stage buffer amplifier circuit 3, that is, the emitter of the NPN transistor Q1 forming the pre-stage buffer circuit 3 and the PNP.
The emitter of the transistor Q2 is the latter stage buffer circuit 4
Input side, that is, the NPN forming the rear stage buffer circuit 4
Base of transistor Q3 and PNP transistor Q4
Connected to the base of. Further, the output side of the rear stage buffer circuit 4, that is, the emitter of the NPN transistor Q3 forming the rear stage buffer circuit 4 and the PNP transistor Q4.
The emitter of is connected to one end of the resistor R _O 'and is also connected to the CR network 1 via the feedback path 5.

Next, the operation of the conventional active filter 50 will be described. Now, assuming that the output impedance seen from the output terminal OUT is Z _O ′, Z _O ′ is expressed by the following equation (1). (However, in order to simplify the description, it is assumed that the current amplification factors of the transistors Q1 to Q4 forming the front stage buffer circuit 3 and the rear stage buffer circuit 4 are the same, and β is assumed.) Z _O ′ = R _O '+ R _O ' // Z _RC (1) At this time, r _O '= Z _RC ' / β ₂ : Active filter 5
Internal resistance of 0, Z _RC : Impedance seen from the output side of the rear stage buffer circuit 4 Z _RC ': Impedance seen from the input side of the front stage buffer circuit 3 Then, when this equation (1) is expanded, Z _O ' = R _O '+ 1 / (1 / Z _RC + β ₂ / Z _RC ') (2)

[0006]

However, when feedback is applied from the output side of the rear stage buffer circuit to the CR network as in the above-mentioned conventional active filter, it is apparent from the above equation (2). And output impedance Z
_{Since O'depends} largely on _ZRC and _ZRC 'which change depending on the frequency, there is a problem in that it greatly changes depending on the frequency.

The present invention has been made in order to solve such a problem, and it is an object of the present invention to provide an active filter capable of reducing the change in output impedance due to frequency and obtaining good filter characteristics. To aim.

[0008]

In order to solve the above-mentioned problems, the present invention provides at least a CR network and a subsequent stage.
A buffer amplifier circuit provided, in the configuration, the
The buffer amplifier circuit is composed of a front stage buffer circuit and a rear stage unit.
It is a diamond type buffer circuit consisting of a buffer circuit.
In addition , a feedback path is provided between the output side of the front stage buffer circuit and the CR network.

Also, an emitter follower circuit is provided on the output side of the preceding stage buffer circuit, and a feedback path is provided between the output side of the emitter follower circuit and the CR circuit network.

Further, an NPN transistor or a PN which constitutes the preceding stage buffer circuit of the buffer amplifier circuit.
One of the P transistors is an inverted Darlington connection.

Further, an NPN transistor or a PN that constitutes the latter stage buffer circuit of the buffer amplifier circuit.
A feature is that a capacitor is connected between the base of either one of the P transistors and the ground.

Further, a direct current blocking means is provided in a feedback path between the output side of the preceding stage buffer circuit of the buffer amplifier circuit and the CR network.

According to the active filter of the present invention, since the feedback path is provided between the output side of the pre-stage buffer circuit and the CR network, Z _RC and Z _RC 'which change according to the frequency are provided.
Can reduce the influence on the output impedance Z _O.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. In each of the embodiments, the same or equivalent parts as those of the conventional example are designated by the same reference numerals, and detailed description thereof will be omitted.

FIG. 1 shows a block diagram of an active filter of the present invention. The active filter 10 is similar to the conventional active filter 50 in that it has an input terminal IN and an output terminal O.
It is composed of a CR network 1 and a buffer amplifier circuit 2 which are connected in series with the UT. At this time, the buffer amplifier circuit 2 is composed of a series circuit of a front stage buffer circuit 3, a rear stage buffer circuit 4, and a resistor R _O. A feedback path 5 is provided from the output side of the front stage buffer circuit 3 to the CR network 1.

FIG. 2 shows a circuit diagram of a first embodiment of the active filter according to the present invention. Active filter 1
0 has almost the same configuration as the conventional active filter 50, but the output side of the front stage buffer circuit 3, for example, the emitter of the NPN transistor Q1 forming the front stage buffer circuit 3 is CR through the feedback path 5. They differ in that they are connected to the network 1.

Next, the operation of the active filter 10 of the first embodiment will be described. Now, assuming that the output impedance seen from the output terminal OUT is Z _O , Z _O is the following (3)
It is expressed as an expression. (However, in order to simplify the explanation, it is assumed that the current amplification factors of the transistors Q1 to Q4 forming the front-stage buffer circuit 3 and the rear-stage buffer circuit 4 are the same, and is β.) Z _O = R _O + r _O (3) At this time, r _O = (Z _RC // Z _RC '/ β) / β: Internal resistance of the active filter 10 Z _RC : Impedance viewed from the output side of the pre-stage buffer circuit 3 Z _RC ': Pre-stage the impedance viewed from the input side of the buffer circuit 3 and then expanding this equation _{(3), Z O = R O + 1} / (β / Z RC + β 2 / Z RC ') become (4).

In the above equation (2) (conventional example) and equation (4) (this embodiment), comparing the second term of each equation, β has a value of 50 to 200. Since 1 / Z _RC <β / Z _RC , 1 / Z _RC + β ₂ / Z _RC '<β / Z _RC + β ₂ / Z _RC ' and 1 / (1 / Z _RC + β ₂ / Z _RC ') > 1 / (β / Z _RC + β
₂ / Z _RC ').

As described above, according to the active filter 10 of the first embodiment, the output impedances Z _O and Z 0.
_{Since O} ′ is set to the same predetermined value (for example, 75Ω), the value of the second term of the equations (4) and (2), that is, the value of the term depending on the frequency, is output impedance Z _O ,
The ratio occupied by Z _O 'is smaller in the formula (4) than in the formula (2). Thus, the frequency dependence of the output impedance Z _O, can be made smaller than the conventional output impedance Z _O '.

FIG. 3 shows a circuit diagram of a second embodiment of the active filter according to the present invention. Active filter 1
5 is different from the active filter 10 of the first embodiment in that the output side of the front stage buffer circuit 3, for example, the emitter of the NPN transistor Q1 forming the front stage buffer circuit 3 is connected to the emitter follower circuit 16, for example, the PNP transistor Q5. Emitter-follower circuit 1 connected to the base of
The difference is that the output side of 6, ie the emitter of the PNP transistor Q5, is connected to the CR network 1 via the feedback path 5.

As described above, according to the active filter 15 of the second embodiment, the emitter follower circuit 16 is provided on the output side of the front stage buffer circuit 3, and the output side of the emitter follower circuit 16 and the CR network 1 are connected. Return path 5 between
Since it has, it is possible to obtain better filter characteristics.

FIG. 4 shows a circuit diagram of a third embodiment of the active filter according to the present invention. Active filter 2
Compared with the active filter 10 of the first embodiment, 0 is an inverted Darlington connection of the PNP transistor Q2 forming the pre-stage buffer circuit 3, and its output side is CR network via the feedback path 5. They differ in that they are connected to 1.

That is, the base of the NPN transistor Q6 is connected to the collector of the PNP transistor Q2 which constitutes the front stage buffer circuit 3, and the NPN transistor Q6 is connected.
PNP which forms the former stage buffer circuit 3 in the collector of 6
The emitter of the transistor Q2 is connected, and the emitter of the PNP transistor Q2 and the collector of the NPN transistor Q6 are connected to the CR network 1 via the feedback path 5.

Next, Table 1 shows the results of the frequency dependence of the output impedance in the active filter 20 of the third embodiment and the conventional active filter 50.

[0025]

[Table 1]

From this result, the frequency is changed from 1 MHz to 20 MHz.
The increase rate of the output impedance when changing to MHz is 58.3% in the conventional active filter 50 and 10.6% in the active filter 20 of the third embodiment, and the active filter 20 of the third embodiment is increased. It can be understood that the change in the output impedance with respect to the frequency is smaller than that of the conventional active filter 50.

As described above, according to the active filter 20 of the third embodiment, the PNP transistor Q2 forming the front stage buffer circuit 3 is in the inverted Darlington connection, and its output side is connected via the feedback path 5. Since the output impedance is connected to the CR circuit network 1, the frequency dependence of the output impedance is further reduced, and a better filter characteristic can be obtained.

FIG. 5 shows a circuit diagram of a fourth embodiment of the active filter according to the present invention. Active filter 2
5 is, compared with the active filter 20 of the third embodiment, an input side of the rear stage buffer circuit 4, that is, between the base of the NPN transistor Q3 forming the rear stage buffer circuit 4 and the base of the PNP transistor Q4 and the ground. The difference is that the capacitors C1 and C2 are connected to.

FIG. 6 shows the filter characteristics of the active filter 20 of the third embodiment and the active filter 25 of the fourth embodiment. In FIG. 6, the solid line shows the case of the active filter 20 of the third embodiment, and the broken line shows the case of the active filter 25 of the fourth embodiment. From this figure, it can be understood that the active filter 25 of the fourth embodiment has the same characteristics as the active filter 20 of the third embodiment to which a first-order LPF (low-pass filter) is added. .

As described above, according to the active filter 25 of the fourth embodiment, the capacitors C1 and C are provided between the base of the NPN transistor Q3 and the base of the PNP transistor Q4 forming the rear buffer circuit 4 and the ground.
Since 2 is connected, the same operation as in the case where the first-order LPF is added to the active filter 20 of the third embodiment can be performed.

FIG. 7 shows a circuit diagram of a fifth embodiment of the active filter according to the present invention. Active filter 3
0 is different from the active filter 20 of the third embodiment in that a DC blocking means, for example, a capacitor C3, is connected to the feedback path 5 between the pre-stage buffer circuit 3 and the CR network 1. .

As described above, according to the active filter 30 of the fifth embodiment, the capacitor C3 is provided in the feedback path 5.
Since it is possible to reduce unnecessary DC current flowing to the CR network 1, the DC operation of the buffer amplifier circuit 2 can be set independently of the resistance of the CR network 1. Therefore, the operations of the CR circuit network 1 and the buffer amplifier circuit 2 can be further improved, the frequency dependence of the output impedance can be further reduced, and a further better filter characteristic can be obtained.

The CR in the first to fifth embodiments
As the circuit network, for example, four resistors R as shown in FIG. 8 are used.
TW consisting of 1 to R4 and four capacitors C1 to C4
There is an IN-T type BRF circuit.

Further, in the buffer amplifier circuits of the third to fifth embodiments, the NPN transistor is added to the PNP transistor forming the pre-stage buffer circuit to make the inverted Darlington connection. A PNP transistor may be added to the constituent NPN transistor to make an inverted Darlington connection, and a feedback path may be provided from the output side of this buffer amplifier circuit to the CR network.

Further, P which constitutes the preceding stage buffer circuit
An NPN transistor may be added to the NP transistor, and a PNP transistor may be added to the NPN transistor to form two inverted Darlington connections, which are complementary inverted Darlington connections.

Also, a resistor may be connected to the inverted Darlington connection to apply voltage feedback from the rear transistor to the front transistor. In this case, gain is gained by voltage feedback, and a filter characteristic with higher Q is obtained.

Further, in the second embodiment, the case where the emitter follower circuit is provided at the emitter of the PNP transistor which forms the front stage buffer circuit has been described.
An emitter follower circuit may be provided in the emitter of the transistor.

In the fourth embodiment, the case where a capacitor is connected between the base of the NPN transistor forming the latter stage buffer circuit and the base of the PNP transistor and the ground has been described. However, the base of the NPN transistor or the PNP is connected. A capacitor may be connected between either one of the bases of the transistor and the ground.

[0039]

According to the active filter of the first aspect, since a feedback path is provided between the output side of the pre-stage buffer circuit and the CR network, Z _RC , Z which changes depending on the frequency.
The influence of _RC 'on the output impedance Z _O can be made smaller than before. Thus, the frequency dependence of the output impedance Z _O, can be made smaller than the conventional output impedance Z _O '.

According to the active filter of claim 2,
Since the emitter follower circuit is provided on the output side of the front stage buffer circuit and the feedback path is provided between the output side of the emitter follower circuit and the CR network, better filter characteristics can be obtained.

According to the active filter of claim 3,
Either the NPN transistor or the PNP transistor forming the pre-stage buffer circuit is in the inverted Darlington connection, and the output side of the pre-stage buffer circuit is connected to the CR network through the feedback path. The frequency dependence of
And good filter characteristics can be obtained.

According to the active filter of claim 4,
Since a capacitor is connected between the base of either the NPN transistor or the PNP transistor that constitutes the latter stage buffer circuit and the ground, the primary LPF
The same operation as in the case of adding can be performed.

According to the active filter of claim 5,
Since a DC blocking means is provided in the feedback path between the output side of the front stage buffer circuit and the CR network, unnecessary DC current flowing to the CR network can be reduced, and
The DC operation of the buffer amplifier circuit can be set independently of the resistance of the CR network. Therefore, the operations of the CR circuit network and the buffer amplifier circuit can be further improved, the frequency dependence of the output impedance can be further reduced, and further better filter characteristics can be obtained.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an active filter of the present invention.

FIG. 2 is a circuit diagram of a first embodiment according to the active filter of the present invention.

FIG. 3 is a circuit diagram of a second embodiment according to the active filter of the present invention.

FIG. 4 is a circuit diagram of a third embodiment according to the active filter of the present invention.

FIG. 5 is a circuit diagram of a fourth embodiment according to the active filter of the present invention.

6 is a diagram showing filter characteristics of the active filter of FIG. 4 and the active filter of FIG.

FIG. 7 is a circuit diagram of a fifth embodiment according to the active filter of the present invention.

FIG. 8 is a connection diagram for a TWIN-T type BRF circuit which is an example of a CR network.

FIG. 9 is a configuration diagram of a conventional active filter.

FIG. 10 is a specific circuit diagram of the active filter of FIG.

[Explanation of symbols]

10, 15, 20, 25, 30 Active filter 1 CR circuit network 2 Buffer amplifier circuit 3 Front stage buffer circuit 4 Rear stage buffer circuit 5 Feedback path 16 Emitter follower circuit Q1, Q3 NPN transistor Q2, Q4 PNP transistor C1, C2 Capacitor C3 DC blocking means (capacitor)

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) H03H 11/12 H03H 11/04

Claims (5)

(57) [Claims] 1. At least a CR network and a subsequent stage
A buffer amplifier circuit provided, and the buffer amplifier circuit comprises a front stage buffer circuit and a rear stage buffer circuit.
Diamond type buffer circuit consisting of step buffer circuit
, And the active filter and having a feedback path between the output side and the CR network of the first part a buffer circuit. 2. The emitter follower circuit is provided on the output side of the front stage buffer circuit, and a feedback path is provided between the output side of the emitter follower circuit and the CR network. Active filter. 3. The inverted Darlington connection of one of an NPN transistor and a PNP transistor forming a front stage buffer circuit of the buffer amplifier circuit, according to claim 1 or claim 2. Active filter. 4. A capacitor is connected between the base of either one of an NPN transistor or a PNP transistor which forms the latter stage buffer circuit of the buffer amplifier circuit and the ground. 3. The active filter according to any one of 3 above. 5. A feedback path between the output side of the preceding stage buffer circuit of the buffer amplifier circuit and the CR network,
5. The active filter according to claim 1, further comprising a DC blocking means.