JPH06204789A - Active filter circuit - Google Patents

Abstract

PURPOSE:To prevent malfunctions which deviation in a frequency characteristic due to dispersion in the capacitance of a capacitor of the active filter is automatically corrected. CONSTITUTION:The active filter circuit 10 is provided with a filter circuit main body composed of an active filter using a gm amplifier, a phase shifter 12 employing an active filter the same as the active filter used for the filter circuit main body and shifting the phase of a signal of a prescribed frequency, and a phase difference detector 13 detecting a phase difference with respect to the reference phase shift angle of a signal passing through the phase shifter 12. Then a gm control circuit 14 revises the gm of the gm amplifier of the active filter 10 in the filter circuit main body and the phase shifter so as to eliminate the phase difference based on the phase difference detected by the phase difference detector 13. Furthermore, the circuit 10 is provided with a limit circuit for regulating the adjustment range of the gm revised by the gm control circuit 14.

Description

Detailed Description of the Invention

[0001]

The present invention relates to an active filter circuit using the g _m amplifier an integrated circuit used in sound multiplex demodulator and the like.

[0002]

2. Description of the Related Art An audio multiplex demodulator is designed to support a stereo broadcast and an audio multiplex broadcast, and a carrier sub-audio signal of 2f _H and 3.5.
Various filter circuits including a bandpass filter for separating the carrier multiplexing control signal of f _H are provided. Since these filter circuits can be easily integrated into an IC and the frequency characteristics can be easily adjusted, an active filter using a g _m amplifier is often adopted.

FIG. 3 is a circuit diagram of an active filter using a g _m amplifier. The active filter 20 includes g _m amplifiers 21a and 21b, buffers 23a and 23b, a capacitor 22a,
It is composed of 22b and is designed to pass only a signal of a specific frequency. The buffers 23a and 23b
Since both g _m amplifiers 21a and 21b output current,
It is provided to convert its output into a voltage. However, the active filter as shown in FIG. 3 has a drawback that the frequency characteristic of the circuit is deviated because the capacitance of the capacitor varies.

Therefore, a circuit as shown in FIG. 1 has been proposed so that the frequency characteristic does not shift even if the capacitance of the capacitor varies (Japanese Patent Laid-Open No. 01-264009). That is, when the capacitance of the active filter capacitor deviates from the normal value, the signal that has passed through the BPF (bandpass filter) 11 and the phase shifter 12 has a difference with respect to the reference phase shift angle by an angle corresponding to the deviation. Occurs. This phase difference is detected by the phase difference detector 13 and guided to the g _m control circuit 14 as an error current. The g _m control circuit 14 includes g filters in the active filter used in the filter circuit main body, the BPF (band pass filter) 11 and the phase shifter 12 so that the error current output from the phase difference detector 13 becomes zero. Controls the g _m value of the _m amplifier. Then, the deviation of the frequency characteristic due to the variation in the capacitance of the capacitor of the active filter is eliminated.

[0005]

However, if an attempt is made to widen the frequency adjustment range beyond the variation in the capacitance of the capacitors of the active filter, the above-mentioned circuit is used as a reference signal when the carrier multiplexing control signal of 3.5 f _H is lost. There is a risk of locking to a signal other than that and causing a malfunction.

The present invention was created under the background described above, and an object of the present invention is to make a malfunction while automatically correcting the deviation of the frequency characteristic due to the variation in the capacitance of the capacitor of the active filter. An object of the present invention is to provide an active filter circuit in which the above problem does not occur.

[0007]

An active filter circuit according to the present invention uses a filter circuit body composed of an active filter using a g _m amplifier and an active filter which is the same as the active filter used in the filter circuit body. A phase shifter for shifting a signal of a predetermined frequency,
A phase difference detector that detects the phase difference of the signal that has passed through the phase shifter with respect to the reference phase shift angle, and the filter circuit body and phase shifter that eliminate the phase difference based on the phase difference detected by the phase difference detector. A g _m control circuit for changing the g _m value of the g _m amplifier of the active filter in the filter, and a limiting circuit for restricting the adjustment range of the g _m value changed by the g _m control circuit. There is.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an active filter circuit according to the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a part of an active filter circuit. FIG. 2 is a circuit diagram of the g _m control circuit and the limiting circuit. In addition,
The characteristic parts of the circuit of the present invention are the g _m control circuit and the limiting circuit shown in FIG. 2, and the parts of the other circuits shown in FIG. 1 are the same as the conventional circuit. However, for convenience of explanation, FIG.
The circuit of the present invention will be described with reference to FIG.

The active filter circuit described by way of example here is provided in the audio multiplex demodulator of the audio receiving circuit in the television receiver compatible with the audio multiplex broadcast, and g _m in the audio multiplex demodulator. As an active filter main body using an amplifier, it has a function of automatically correcting deviations in frequency characteristics due to variations in the capacitance of the capacitors.

In the figure, reference numeral 10 is a BPF (bandpass filter) for extracting a 3.5 f _H carrier multiplexing control signal from the voice multiplexing signal.
Is. The BPF 10 uses the same active filter as that used in the filter circuit body. The carrier multiplexing control signal of 3.5 f _H taken out by the BPF 11 is guided to the phase shifter 12.

The phase shifter 12 uses the same active filter as the active filter used in the filter circuit main body, and the phase of the carrier multiplex control signal of 3.5 f _H is 90 deg.
Shift at the standard phase shift angle of. The signal whose phase is shifted by the phase shifter 12 is guided to the phase difference detector 13 which is a balanced modulator.

The phase difference detector 13 detects the phase of the signal after passing through the phase shifter 12 and the phase of the signal before passing through the phase shifter 12.
The comparison is performed with 0 deg as a reference, and if the phase difference deviates from 90 deg, an error current occurs. The error current output from the phase difference detector 13 is guided to the g _m control circuit 14.

In the g _m control circuit 14, the above-mentioned error current is zero.
Filter circuit body, BPF 11 and phase shifter 12
The g _m value of the g _m amplifier in each of the active filters used in the above is controlled according to the error voltage.

Each g provided in the active filter circuit
_Although the g _m value of the _m amplifier is controlled by the g _m control circuit 14 described above, when the g _m value is out of the preset range,
g _m control circuit restriction circuit 15 added to the 14 working, g _m
The change of the value is regulated, and in any case, the g _m value is changed within a preset range.
That is, the limiting circuit 15 is configured to regulate the adjustment range of the g _m value. In this embodiment, the circuit configuration shown in FIG. 2 is adopted.

First, the g _m control circuit 14 will be described.
The g _m control circuit 14 includes a constant current source I, transistors Q8 and Q
The circuit configuration is centered on a differential amplifier circuit composed of 9 or the like. Phase difference detector on the base side of the transistor Q9
A resistor R _L for converting an error current as an error signal derived from 13 into a voltage is connected. On the other hand, a voltage source E that generates a reference voltage Vcc / 2 is connected to the base side of the transistor Q8. Note that Vcc is a power supply voltage.

That is, the difference between the error voltage generated across the resistor R _L and the reference voltage Vcc / 2 is determined by the differential amplifier circuit including the transistors Q8 and Q9, and the current corresponding to the difference is calculated as g _m value. (G _m control amount) is output. The collector current of the transistor Q9 is converted into a voltage by the resistor R2. This voltage signal is the control circuit 15
Has been introduced into the system to recognize the g _m value.

The limiting circuit 15 includes transistors Q3, Q4,
The circuit configuration is centered on a comparator circuit including Q5 and the like. Base side of the transistor Q3, while g _m component serving the current source I of the control circuit 14, the transistor Q9, a resistor R2 circuit is connected, the base of the transistor Q4, the current source I, transistors Q1, resistors A circuit composed of R1 is connected. However, the current source I
Part of the current of the transistor is drawn by the transistor Q2. A current generated by the constant current source I and the transistor Q7 also flows through the resistor R1.

That is, the same constant current I flows through the g _m control circuit 14 and the limiting circuit 15. Here, if the emitter area ratio of the transistor Q1 and the transistor Q2 is set to 1: N,
A current of 1 / (1 + N) is applied to the resistor R1 and the transistor Q
The sum of the currents of 7 flows. Therefore, when the current of the resistor R2 of the g _m control circuit 14 is about to exceed the current, the comparator circuit including the transistors Q3, Q4, and Q5 operates to flow the current into the resistor R _L. By applying feedback in this way, the output current of the g _m control circuit 14 can be limited to a certain value. The current that can be limited is transistor Q1,
Since it is determined by the number of Q2, it is possible to limit the adjustment range without error.

Next, the operation of the active filter circuit configured as described above will be described. If the capacitance of the active filter capacitor used in the phase shifter 12 deviates from the normal value, the phase difference between the signal after passing through the phase shifter 12 and the signal before passing through the phase shifter 12 is It does not reach 90 deg. Therefore, the phase difference detector 13 produces an error current,
By controlling the g _m value of the g _m amplifier in the active filter in the phase shifter 12 by the g _m control circuit 14, the phase difference becomes 90 deg. As a result, the phase shifter
In the active filter in 12, the g _m value of the g _m amplifier is changed to absorb the deviation of the capacitance of the capacitor from the normal value, and the frequency characteristic is corrected.

The active filter used in the filter circuit main body and the BPF 11 is the same as the active filter in the phase shifter 12, and like the active filter in the phase shifter 12, the g _m value of the g _m amplifier is g _m control circuit 14
Since they are controlled by, the deviation of their frequency characteristics is eliminated as in the active filter in the phase shifter 12.

Conventionally, if the 3.5 f _H carrier multiplex control signal disappears for some reason, the above circuit locks to a signal other than the reference signal, causing a malfunction. For example, it may be locked to the sub audio signal. However, in the case of the circuit of this embodiment, the limiting circuit 15 regulates the adjustment range of the g _m value, so that the problem as described above does not occur.

The active filter circuit according to the present invention is not limited to the above embodiment, and the limiting circuit is also
_It can be changed according to the circuit configuration of the _m control circuit.

[0023]

As described above, in the case of the active filter circuit according to the present invention, the value of g is changed by the g _m control circuit.
_Since it is configured with a limiting circuit that regulates the adjustment range of the _m value, there is no malfunction even while automatically correcting the deviation of the frequency characteristics due to the variation in the capacitance of the capacitors of the active filter. . Therefore, there is an advantage that the reliability of the circuit is increased.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an embodiment of an active filter circuit according to the present invention and is a block diagram showing a part of the active filter circuit.

FIG. 2 is a circuit diagram of a g _m control circuit and a limiting circuit.

FIG. 3 is a diagram for explaining a conventional active filter circuit.

[Explanation of symbols]

10 Active filter circuit 11 BPF 12 Phase shifter 13 Phase difference detector 14 g _m Control circuit 15 Limiting circuit

Claims (1)

[Claims] 1. A filter circuit main body comprising an active filter using a g _m amplifier, and a phase shifter for phase-shifting a signal of a predetermined frequency by using the same active filter as the active filter used in the filter circuit main body. A phase difference detector that detects the phase difference of the signal that has passed through the phase shifter with respect to the reference phase shift angle, and the filter circuit body and phase shifter that eliminate the phase difference based on the phase difference detected by the phase difference detector. and g _m control circuit for changing the g _m value of g _m amplifier of the active filter in the vessel, and characterized by further comprising a limiting circuit for regulating an adjustment range of g _m value is changed by g _m control circuit Active filter circuit.