JPS6012814A - Active filter - Google Patents

Abstract

PURPOSE:To use a high-frequency active filter properly by using a diode as a bias circuit of an amplifier circuit and varying frequency transmitting characteristics in accordance with current flowing into the diode. CONSTITUTION:When a current I1 flowing into the diode D1 is varied, the charging/discharging conditions of a feedback capacitor C1 are changed and the frequency characteristics are also changed. However, the potential difference between both the ends of the diode D1 is almost kept constant by the forward characteristic of the diode D1 even if the current I1 flowing into the diode D1 is changed. Consequently, the bias voltage applied to the base input side of the succeeding TR Q2 is almost fixed in spite of the change of the current I1 flowing into the diode D1. Therefore, amplification characteristics or the like are not almost changed and only the frequency characteristics are changed. The change of the frequency characteristics can be obtained only by operating the constant current value I1 of a circuit 10.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a technology that is particularly effective when applied to analog technology and also to frequency filters, for example, to a technology that is effective when applied to high frequency active filters in semiconductor integrated circuits. It is something.

[Background technology]

The present inventor has developed the following technology regarding analog technology, particularly high frequency filter technology.

That is, an active filter having a specific frequency transfer characteristic is configured by the time constant of the amplifier circuit. This will be explained in more detail below.

FIG. 1 shows an example of a high frequency active filter. The filter shown in the figure constitutes a low-pass type, so-called low-pass filter. This filter consists of bipolar transistors Ql, Q2 . Resistor R1゜R2, capacitor C1
, constant current circuit 10.12, and the power supply Vcc
, Vee. Transistors Ql and Q2 each constitute an emitter follower type amplifier circuit. and.

It is mainly configured as an internal circuit of a semiconductor integrated circuit device.

Transistor Q1 constitutes a first stage j11 width circuit.

A resistor R1 and a constant current circuit 10 are connected in series to the emitter side. The constant current circuit lσ is.

The constant current value T1 is configured to be variable.

Transistor Q2 constitutes a second stage amplifier circuit. There is a resistor R2 as a load on the collector side.

Constant current circuits 12 are connected in series to each emitter side. Further, a feedback capacitor c1 is connected from the collector to the base.

The output of the first stage amplifier circuit is directly connected to the input of the second stage amplifier circuit. The resistors R1, R2, etc. act as part of the amplification constant, and also act as a time constant that determines the frequency transfer characteristic. Therefore, by changing the values of these constants, such as the resistances R1 and R2, the characteristics of the filter, especially the cutoff frequency, can be changed.

However, the inventors have found that this technique has a problem in that it is not possible to easily vary only one frequency characteristic independently. In other words, the resistor R1 that determines the 8 wave number characteristics (frequency transfer characteristics),
Time constants such as R2.

At the same time, it also acts as part of the amplification constant, so if you change the resistors R1 and R2 or the constant current value ■1 of the constant current circuit 10 in order to change the cutoff frequency, for example, the amplification characteristics will change as well as the /# wave number characteristics. It's put away. moreover,
In a two-stage direct-coupled amplifier circuit as shown in Figure 1, if the circuit constants are changed, the bias condition on the base input side of the second stage transistor Q2 will be deviated from the bias condition of 2%.
A problem arises in that it is no longer possible to operate normally as an amplifier circuit or an active filter.

[Purpose of the invention]

An object of the present invention is to provide a filter technique that allows one frequency transfer characteristic to be easily varied.

Furthermore, the present invention provides a filter technology that allows only the frequency characteristics to be independently varied without affecting the amplification characteristics.

Furthermore, the present invention provides a high frequency active filter technology that allows the cutoff frequency to be easily adjusted.

The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

[Summary of the invention]

A brief overview of typical inventions disclosed in this application is as follows.

That is, by configuring a part of the circuit constant of the active filter with a diode, the purpose of making it possible to easily vary one frequency characteristic is achieved.

〔Example〕

Hereinafter, typical embodiments of the present invention will be described with reference to the drawings.

In one drawing, the same or corresponding parts are indicated by the same reference numerals.

FIG. 2 shows an embodiment of an active filter according to the invention.

First, the active filter shown in the figure is configured as an internal circuit of a semiconductor integrated circuit device. Also, mainly several hundred M
It is configured as a high frequency filter that operates in the Hz frequency range. moreover.

It constitutes a low-pass type, so-called low-pass filter.

The above filter includes transistors Q1. Q2. Diode D1. Resistor R2, capacitor CI, constant current circuit 10.1
2, the power supply VCC.

Operated by Vee. (Ibora transistors Ql and Q2 each constitute an emitter follower type amplifier circuit.

Transistor Q1 constitutes a first stage amplifier circuit. On its emitter side, there is a diode D1 and a constant current circuit lO.
are connected in series. This constant current circuit 10 is configured so that its constant current value 11 can be varied.

Transistor Q2 constitutes a second stage amplifier circuit. A resistor R2 as a load is connected in series to the collector side, and a constant current circuit 12 is connected to the emitter side. Further, a feedback capacitor C1 is connected from the collector to the base K.

The output of the first stage amplifier circuit is directly connected to the input of the second stage amplifier circuit. The diode D1, resistor R2, etc. each act as a part of the amplification constant, and also act as a time constant that determines one frequency transfer characteristic. Therefore,
If you change those constants, ie, the values of diode DI, resistor R2, etc.

Characteristics as a filter, especially the cutoff frequency, can be changed.

Here, the diode D1 inserted in series on the emitter side of the first stage transistor Q1 is connected to the power supply Vcc,
It is connected in the forward direction to ee.

The current 11 flowing through this diode D1 is limited by the constant current circuit 10. As described above, the constant current value 11 of the constant current circuit 10 can be varied.

Therefore, the current 11 flowing through the diode DI is also variably adjusted by the constant current circuit IO. When the current II flowing through the diode D1 is changed, the charging and discharging conditions of the feedback capacitor CI change, thereby changing the frequency characteristics. However, it should be noted that even if the current 1 flowing through the diode D1 is changed, the potential difference across the diode D1 remains almost constant due to the forward characteristics of the diode D1. As a result, the bias voltage applied to the pace input side of the second stage transistor Q2 remains approximately constant despite changes in the current 11 flowing through the diode DIK. As a result, only the frequency characteristics of the filter can be changed without changing the amplification characteristics or other operating conditions.

This can be obtained by simply manipulating the constant current value 11 of the constant current circuit 10.

In this manner, the low-pass characteristics of the high-frequency active filter, in particular its cut-off frequency, can be variably adjusted independently and easily, with little influence on other operating conditions.

Here, the operation of the active filter shown in FIG. 2 is analyzed as follows.

First, the cutoff frequency fc is expressed by the following equation.

fcocl/Re (1) Here, R is the forward resistance of the diode D1 and is of first order.

R=KT/qI [2 where I is the constant current value 11 flowing through the diode D1, is the Boltzmann constant, T is the absolute temperature, and q is the elementary charge.

Further, C is the capacitance due to the Miller effect of the capacitor CI as seen from the first stage transistor Q1 side, and is of first order.

C=(1+Av)C113 However, AM is the gain (Miller effect gain) of the second stage amplifier circuit.

From the above (lc2c3), the cutoff frequency fc is as follows.

fcoeIQ/KT(1+Av)C As described above, the cutoff frequency fc can be determined simply by varying the constant current value 11 of the variable current type constant current circuit 10, regardless of other operating conditions of one circuit. They can be easily changed independently and linearly. And its variable operation.

By controlling the constant current value (1), this can be easily performed from the outside.

〔effect〕

(11 By using a diode as a part of the circuit constant, 1
The effect is that the frequency transfer characteristics can be easily varied.

(21) By replacing some of the circuit constants, especially the resistors, with diodes, it is possible to obtain the effect that only the frequency characteristics can be varied independently without affecting the amplification characteristics.

(3) The cutoff frequency can be easily adjusted by replacing some of the circuit constants, especially the resistors, with diodes and by making it possible to vary the constant current flowing through the diodes.
This has the effect of being able to make a knot.

Due to the above (1) to (31), the frequency characteristics can be arbitrarily and easily varied, and furthermore, by controlling the current, it is possible to construct an active filter that can be easily performed from the outside. Effects can be obtained.

Although the invention made by the present invention number has been specifically explained above based on the examples, it goes without saying that this invention is not limited to the above-mentioned examples and can be modified in various ways without departing from the gist thereof. Nor. For example, the diode described above may be a constant voltage element such as a Zener diode, in addition to one having normal rectifying characteristics.

Furthermore, these elements are not limited to diodes in any way, but may be any device or circuit that does not substantially change the pace bias condition of transistor Q2 even if II is changed in FIG.

[Application field]

In the above explanation, the invention made by the present inventor was mainly explained in terms of high frequency analog technology, which is the field of application behind the invention, but it is not limited to this.
For example, it can be applied to low frequency analogue technology. Rina (also applicable to active filters where part of the circuit constants are composed of diodes).

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an example of an active filter developed by the present inventor prior to the present invention, and FIG. 2 is a circuit diagram showing an example of an active filter according to the present invention. Ql, C2...Transistors forming the amplifier circuit. R1, R2... time constant (resistance), C1... time constant (
capacitor), VCC2■ee ... power supply, 10...
・Current variable type constant current circuit, 12...constant current circuit, Vi
n...Input. vO...output, II...constant current value.

Claims (1)

[Claims] 1. An active filter that has a specific frequency transfer characteristic depending on the time constant of an amplifier circuit. A diode is used in the bias circuit of the above amplifier circuit,
An active filter whose frequency transfer characteristics are varied by the current flowing through the diode. 2. The active filter according to claim 1, wherein the diode is used as a gate that applies a bias potential to the input side of the amplifier circuit through the diode.