JPH0279610A - Active filter - Google Patents

Abstract

PURPOSE:To realize a stable operation up to a high frequency by controlling the gain of the amplifier of an active filter in which a positive feedback circuit with frequency selectivity is connected between the output and input of the amplifier by applying DC negative feedback. CONSTITUTION:The title device is equipped with a high frequency amplifier A and the positive feedback circuit F with frequency selectivity connected between the output and input of the amplifier A, and the amplifier of DC control type controllable an amplification gain by a DC control signal supplied to a control terminal C is used as the amplifier A. Resistance division circuits R1 and R2 are provided at the output circuit of the amplifier. A first rectifier circuit D1 to rectify the output voltage of the resistance division circuit, a second rectifier circuit D2 to rectify the input voltage of the amplifier A, and a circuit B to supply a DC signal corresponding to the difference of the output of the rectifier circuits D1 and D2 to the control terminal C as a negative feedback control signal are provided. In such a way, it is possible to stabilize the gain of the amplifier A with high accuracy almost equal to the division ratio of the resistance division circuit, and to realize the stable operation even at high frequency.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is used as an electronic circuit element. The present invention relates to an active filter that can be used in a high frequency band (VHF band or UHF band).

The present invention provides an active filter in which a frequency-selective positive feedback circuit is connected between the output and input of an amplifier, and the active filter operates stably up to high frequencies by controlling the gain of the amplifier by direct current negative feedback. This is what you get.

[Conventional technology]

Filters that have been known for a long time are combinations of inductance and capacitance, but it is difficult to integrate inductance into an integrated circuit. For this reason, in a relatively low frequency band (IMHz or less), active filters that are composed of transistors, capacitors, resistors, etc. and do not use inductance have been put into practical use and have come to be used very widely.

Active filters can be roughly divided into (1) positive feedback type, (2)
) negative feedback type, and (3) calculation type. A positive feedback type is a circuit in which a feedback loop is provided in the amplifier, and the feedback loop is set to provide positive feedback at a specific frequency, thereby selectively obtaining gain at that frequency. Similarly, the negative feedback type obtains frequency selectivity by setting the feedback loop to provide negative feedback at a specific frequency. In the arithmetic type, the transfer function as a filter is expressed by a differential equation, and an arithmetic circuit is constructed based on the principle of an analog computer, and is constructed by a combination of a large number of integrating circuits and differentiating circuits.

In order to obtain sufficient frequency selectivity in the negative feedback type, an amplifier with sufficiently high gain is required. Generally, as the gain increases, the phase shift of the amplifier increases outside the high frequency band, making it difficult to construct a stable negative feedback loop. This is particularly noticeable when the frequency used is high. There are also limits to the operation of the arithmetic type up to high frequency bands. In this way, the negative feedback type and calculation type are not suitable for high frequency bands.

On the other hand, for the positive feedback type, a gain of ten or more dB is sufficient for the amplifier, and current monolithic technology makes it easy to obtain this level of gain, making it the most practical of small filters. expensive.

[Problem that the invention seeks to solve]

However, positive feedback active filters have the following problems regarding operational stability, and have not been put to practical use at high frequencies. That is, the second
The figure shows the configuration of a conventional secondary bandpass filter implemented as a positive feedback type.

The quality Q of this positive feedback active filter is expressed by the following equation.

However, R,=R2=R3=R, C2=C,=C.

and K is the gain of the amplifier.

According to the above equation (1), it is necessary to bring the gain K of the amplifier close to 5 in order to obtain a high Q value. Furthermore, in this case, the Q value is greatly affected by slight variations in gain. In extreme cases, if the amplifier K becomes larger than 5, the circuit may oscillate or become unstable.

In order to improve this, a method has been adopted to apply a sufficiently large negative feedback to the amplifier itself to reduce fluctuations in its gain, but in this case, the problem is similar to that of the negative feedback type filter mentioned earlier. As a result, it has been practically difficult to obtain a stable amplifier in a high frequency band of 100 MHz or higher.

The present invention solves the above-mentioned problems, and aims to realize an amplifier with extremely high gain stability and provide a monolithic active filter with a sufficiently high Q at high frequencies.

[Effect]

A resistor divider circuit is provided at the output end of the amplifier constituting the active filter, a level detection circuit is provided at the output end and the input end of this resistor divider circuit, and the gain of the amplifier is controlled by the detection output of this detection circuit. Gain can be stabilized.

[Means for solving problems]

The present invention comprises a high frequency amplifier (A) and a frequency selective positive feedback circuit (F) connected between the output and input of this amplifier.
), the amplifier is a DC control type amplifier whose amplification gain can be controlled by a DC control signal applied to the control terminal (C), and the output circuit of the amplifier is equipped with a resistance divider circuit (R
1, R2), a first rectifier circuit (DI) that rectifies the output voltage of this resistance divider circuit, a second rectifier circuit (D2) that rectifies the input voltage of the amplifier, and the first rectifier The present invention is characterized by comprising a circuit (B) that supplies a DC signal corresponding to the difference between the output of the circuit and the output of the second rectifier circuit to the control terminal (C) as a negative feedback DC control signal.

When the set gain of the amplifier is K, the division ratio of the resistor divider circuit is preferably approximately K.

〔Example〕

FIG. 1 is a block diagram of a circuit according to an embodiment of the present invention. This circuit is an active filter comprising a high frequency amplifier A and a frequency selective positive feedback circuit F connected between the output and input of the amplifier A.

Amplifier A is a DC control type amplifier including means for controlling its amplification gain by a DC control signal applied to control terminal C.

A resistor divider circuit composed of two resistors R1 and R2 is connected to the output circuit of amplifier A, and a first rectifier circuit D1 rectifies the output voltage of this resistor divider circuit, and a first rectifier circuit D1 rectifies the input voltage of amplifier A. a second rectifier circuit D2; a buffer circuit B that supplies a DC signal corresponding to the difference between the output of the first rectifier circuit and the output of the second rectifier circuit to the control terminal C at a phase that becomes a negative feedback DC control signal; Equipped with.

In a circuit with this configuration, if the division ratio of the resistor divider circuit formed by resistors R1 and R2 is K, then the gain of amplifier A is approximately K.
controlled so that

The division ratio 1/ is expressed by the following formula.

When two resistors are manufactured in the same process as in a monolithic IC, the ratio of the two resistors is maintained with high accuracy. That is, since (R2/R1) is kept constant, the value of 1/ is not affected by process variations or environmental changes during use. In addition, there are two rectifier circuits,
D2 can be composed of, for example, a diode, but if these two diodes are manufactured using a monolithic IC, their characteristics will be almost the same. Furthermore, since almost the same level is input, linearity as a rectifier circuit is not a problem. Therefore, the input levels of the two rectifier circuits are controlled to be equal with high precision.

As a result of the above, the gain of the amplifier A can be stabilized with high precision, approximately equal to the division ratio K of the resistance divider circuit. Therefore, even when obtaining a high Q that causes the circuit to operate on the verge of oscillation as described above, stable operation can be achieved. Furthermore, even if the gain to the amplifier becomes abnormally high for some reason, this circuit operates to reduce the gain of amplifier A, and as a result, oscillation is suppressed.

In the circuit shown in FIG. 1, a buffer circuit provides a DC signal corresponding to the difference between the output of the first rectifier circuit and the output of the second rectifier circuit to the control terminal C at a phase that becomes a negative feedback DC control signal. However, this circuit is a circuit that forms a DC control loop, and can realize desired control characteristics as a configuration including an amplifier, a filter, an integrator, a level converter, and the like.

In addition, in the circuit shown in Fig. 1, the positive feedback circuit F is a CR circuit corresponding to the conventional example shown in Fig. 2 as an example, but this positive feedback circuit F can be arbitrarily designed according to the characteristics of the active filter to be realized. It is of the nature of being

Also, in the circuit of Fig. 1, there are two rectifier circuits, and
2 are each shown as a single diode, but each can be realized as a peak detection circuit or other advanced circuit. In this case, the rectifier circuit can also be set to have a DC gain.

〔Effect of the invention〕

As explained above, in the present invention, there is no need to apply large negative feedback to the main amplifier of the active filter in the frequency band used, so there is no need to consider phase shifts in frequency bands higher than the frequency band used. Therefore, compared to conventional circuits, an active filter with a higher usable frequency band can be realized using an amplifier with the same high frequency characteristics.

Furthermore, since the amplifier is operated stably by direct current negative feedback, the margin for oscillation can be reduced, and a circuit with a high Q value can be realized. Since the circuit of the present invention includes a DC negative feedback circuit, it is suitable for implementation in a monolithic integrated circuit.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a circuit according to an embodiment of the present invention. FIG. 2 is a configuration diagram of a conventional circuit. A...Amplifier, F...Positive feedback circuit, B...Buffer circuit, C...Amplifier gain control signal input terminal, D
l, D2... Rectifier circuit. Patent Applicant Nippon Telegraph and Telephone Corporation Agent Patent Attorney Naotaka Ide Figure 2n

Claims (1)

[Claims] 1. An active filter comprising a high-frequency amplifier (A) and a frequency-selective positive feedback circuit (F) connected between the output and input of this amplifier, wherein the amplifier is controlled A resistor divider circuit (R_
1, R_2), a first rectifier circuit (D_1) that rectifies the output voltage of this resistance divider circuit, and a second rectifier circuit (D_1) that rectifies the input voltage of the amplifier.
2); and a circuit (B) that supplies a DC signal corresponding to the difference between the output of the first rectifier circuit and the output of the second rectifier circuit to the control terminal (C) as a negative feedback DC control signal. An active filter characterized by: