JPH06140873A - Filter circuit - Google Patents

Abstract

PURPOSE:To decrease the number of elements and to reduce cost by sharing the filter circuit. CONSTITUTION:In a second order biquad circuit, switches 14, 15 and 15 are respectively provided at a load capacitor C1, which is connected to the positive input terminal and output terminal of a first amplifier 11, and a load capacitor C2 connected to the output terminal of a second amplifier 12. All the filter characteristics are switched by controlling the 1st, 2nd and 3rd switches 14, 15 and 16.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording / reproducing apparatus for video signals such as VTRs and movies (camera integrated VTRs), and more particularly to an active filter circuit which shares a filter when integrated.

[0002]

2. Description of the Related Art As a prior art, Japanese Patent Laid-Open No. 55-452
As described in Japanese Patent No. 24, an active filter suitable for integration is known. An example thereof will be described below. FIG. 6 shows a conventional filter circuit. In the figure, the emitters of the transistors Q1 and Q2 are connected to the variable constant current source A1, the base of the transistor Q1 is connected to the input terminal T1, and the collector thereof is the power supply terminal T1.
3 is connected. A constant current source A2 is connected to the collector of the transistor Q2, and a capacitor C is connected as an AC load between the collector and ground.

Further, the collector of the transistor Q2 is connected to the base of the transistor Q3, the emitter of the transistor Q3 is connected to the terminal T2, and the constant current source A3 is connected to the emitter of the transistor Q3. Connected to the base of.

In such a structure, the input voltage of the terminal T1 is V1, the output voltage of the terminal T2 is Vo, the signal current flowing through the transistors Q1 and Q2 and the capacitor C by the input voltage is Is, and the transistors Q1 and Q2 are formed. If the transconductance of the differential amplifier is Gm,

[0005]

## EQU1 ## The relation of Is = Gm (V1-Vo) is established.

A signal current Is flows through the capacitor C to generate a signal voltage, and the signal voltage is applied to a terminal T2 by a transistor Q3 which constitutes an emitter follower.
The output voltage Vo is obtained by
The signal voltage of the capacitor C is Vo. Therefore, if the angular frequency of the signal is ω,

[0007]

## EQU00002 ## Is / (j.omega.C) = Vo holds.

Therefore, when the transfer function is obtained from the above equations (1) and (2),

[0009]

## EQU00003 ## H (.omega.) = Vo / V1 = 1 / (1+ (j.omega.C) / Gm), which means that the cutoff frequency .omega.c is

[0010]

## EQU00004 ## It shows that it is a low pass filter of .omega.c = Gm / C.

Then, in the filter, a constant current source A1
If the sink current of 2 is
The emitter current of 2 becomes I, and between the emitter current and the transconductance Gm,

[0012]

[Formula 5] Gm = (αI / Vt · exp (Vi / Vt)) / (1 + exp (Vi / Vt)) ²

[0013]

## EQU6 ## Vt = (kT) / q α: current amplification factor of transistors Q1 and Q2 k: Boltzmann coefficient q: electron charge Vi: differential power potential difference, and the magnitude of current 2I of constant current source A1 The cutoff frequency ωc can be changed by changing the height.

Therefore, when the filter having the structure shown in FIG. 6 is integrated as an IC, the variation of the cutoff frequency with respect to the variation of the capacitance of the capacitor is caused by the constant current source A.
It can be adjusted by changing the size of 1. Therefore, it can be said that the filter shown in FIG. 6 is a filter suitable for being integrated into an IC.

In the filter shown in FIG. 6, transistors Q1, Q2 and Q3 and constant current sources A1 and A2 are used.
When A3 is taken out and considered, these elements constitute a differential amplifier and can be rewritten as shown on the left side of FIG. Assuming that the positive input terminal is T +, the negative input terminal is T-, and the output terminal is To, it can be considered as a (VI) conversion amplifier 11 having an amplification degree Gm as shown on the right side of FIG. If the potential difference between the positive input voltage and the negative input voltage is Vi, the output current Io is expressed by the following equation.

[0016]

## EQU00007 ## Io = Vi.Gm When a second-order biquad filter is constructed using such a differential amplifier, as shown in FIG. 8, the transconductance Gm of the first-stage differential amplifier 11 is different from that of the first-stage differential amplifier 11. Amplifier 1
2, the mutual conductance Gm2, the load capacitance of the first stage is C
A secondary low-pass filter is configured with the load capacitance of the first and second stages as C2. The input signal is input from the terminal T1 and the output signal is output from the terminal T2. Integration constant K
Are respectively expressed as follows.

[0017]

[Equation 8] K1 = Gm1 / C1

[0018]

## EQU9 ## K2 = Gm2 / C2 The transfer function H (ω) can be expressed by the following equation.

[0019]

H (ω) = (K1 · K2) / (ω ² + K2 · ω + K1 · K2) Similarly, if a high-pass filter is configured, the transfer function can be expressed by the following equation.

[0020]

H (ω) = ω ² / (ω ² + K2 · ω + K1 · K2) Similarly, if a bandpass filter is configured, the transfer function can be expressed by the following equation.

[0021]

[Equation 12] H (ω) = (K1 · ω) / (ω ² + K2 · ω + K1 · K2) Similarly, if a trap filter is configured, the transfer function can be expressed by the following equation.

[0022]

[Equation 13] H (ω) = (ω ² + K1 · K2) / (ω ² + K2 · ω + K1 · K2) If a delay filter is similarly configured, the transfer function can be expressed by the following equation.

[0023]

[Equation 14] H (ω) = (ω ² −K1 · ω + K1 · K2) / (ω ² + K2 · ω + K1 · K2) In the case of integrating a video filter required for a luminance and chroma video signal processing circuit, a conventional example The desired characteristics are created by connecting several stages of secondary biquad circuits as shown in FIG. In addition, in the recording / playback, NTSC / PAL, low band / high band modes, the characteristics of the filters are changed to improve the image quality. It is possible to control the bandwidth by using one block of the second-order biquad filter by operating the constant current source A1 as shown in FIG.

However, as shown in FIG. 9, when a low-pass filter is required in the recording mode and a trap filter is required in the PB mode, two sets of the low-pass filter and the trap filter are arranged, and these two sets of filter blocks are switched and used. Was.

[0025]

Since the conventional video signal processing circuit is configured as described above, there are filter blocks used only in the reproduction mode and filter blocks used only in the recording mode. .

When designing a circuit that realizes total filter characteristics corresponding to all modes such as recording / playback, NTSC / PAL, low band / high band, the number of filter blocks increases and the circuit configuration becomes complicated. The increase in the number of elements is
Increase IC cost.

The object of the present invention is to provide a filter device which can reduce the number of elements and can be manufactured at a low cost by using a filter block in order to solve the above problems. .

[0028]

The output terminal of the first amplifier is connected to the positive input terminal of the second amplifier, and the output terminal of the second amplifier is connected to the first and second amplifiers. In an active filter circuit connected to a negative input terminal and load capacitors connected to output terminals of the first and second amplifiers, respectively, an input signal is input to a positive input terminal of the first amplifier. Alternatively, a first switch for selecting a bias power supply is provided, and an input signal, a bias power supply, or an inverting amplifier that makes the input signal have an opposite phase to the load capacitance connected to the output terminal of the first amplifier. A second switch for selecting an output signal of the second amplifier, and a load capacitance connected to the output terminal of the second amplifier with a third switch for selecting an input signal or a bias power supply, The second , Second, and control the third switch,
A filter circuit characterized by switching all filter characteristics.

[0029]

The present invention has the above-mentioned configuration and is a secondary biquad.
Different filter blocks can be constructed by switching the input of the pattern filter block and switching the connection of the load capacitance.

[0030]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block circuit diagram showing an embodiment of the present invention, and shows a secondary biquad type filter circuit.

The output terminal of the first amplifier 11 is connected to the positive input terminal of the second amplifier 12, and the second amplifier 12 is connected.
Of the first and second amplifiers 1 and 2 are connected to the negative input terminals of the first and second amplifiers 11 and 12, respectively.
It is an active filter circuit in which load capacitors C1 and C2 are connected to output terminals 1 and 12, respectively.

The first of such active filter circuits
The switches are respectively provided to the positive input terminal of the amplifier 11, the load capacitance C1 connected to the output terminal of the first amplifier 11, and the load capacitance C2 connected to the output terminal of the second amplifier 12. The positive input terminal of the first amplifier 11 selects the input signal Vin or the bias power supply V1 for grounding the positive input terminal by the first switch 14. The load capacitance C1 connected to the output terminal of the first amplifier 11 causes the input signal Vin or the bias power supply V2 for grounding the load capacitance C1 or the input signal to have a reverse phase by the second switch 15. The output signal of the inverting amplifier 13 is selected. Second amplifier 12
The third switch 16 selects the input signal Vin or the bias power supply V3 for grounding the load capacitance C2, which is connected to the output terminal of the load capacitance C2.

The first, second and third switches 14, 15,
By controlling 16, all filter characteristics can be switched. The filter characteristics and switch operation are summarized in Table 1.

[0034]

[Table 1]

As described above, by adding the switches 14, 15 and 16 to the conventional circuit, all filter circuits can be selected.

Next, FIG. 2 shows a specific example in which the low-pass filter and the trap filter and the delay filter and the high-pass filter are commonly used. FIG. 2 is a system block diagram of a chroma low pass filter used for video signal processing. In the case of recording, it is configured by using four filters of a low pass filter, a trap filter, a low pass filter and a delay filter. In the case of reproduction, it is configured by using four filters of two trap filters, a low pass filter and a high pass filter. By sharing the recording low-pass filter and the reproduction trap filter and the recording delay filter and the reproduction high-pass filter, the number of elements can be reduced and the circuit configuration can be simplified. At this time, switching of the low-pass filter and the trap filter 21 is shown in Table 1.
Therefore, only the switch 16 is used for selection. A specific example is shown in FIG. Switching of the delay filter and the high pass filter 24 is selected by the switches 14 and 15 from Table 1. A specific example is shown in FIG.

FIG. 5 is a circuit diagram showing a specific example of sharing the trap filter and the low-pass filter in FIG.

In the figure, here, a biquad type secondary filter is formed, 53 is a power input terminal,
54 is a ground terminal, 55 is a bias source input terminal, 56 is a current source input terminal, 57 is a signal output input terminal, 58 is a signal input terminal, and 59 is a bias input terminal. When a voltage of 0.7 V or higher is applied to the base of the transistor 66,
The collector of the transistor 66 is grounded to 0V. Therefore, by grounding one side of the load capacitance 65,
It becomes a second-order low-pass filter. On the other hand, the transistor 6
When a voltage of 0V is applied to the base of 6, the collector of transistor 66 is open. Therefore, a signal of the same phase as the input signal is input from one side of the load capacitance 65 via the buffer, whereby a secondary trap filter can be obtained.

The sharpness Q, the trap frequency, or the cutoff frequency of the low-pass filter depends on the transistor 60,
It is determined by the transconductance Gm of the differential amplifier composed of 61, the differential amplifier composed of transistors 62 and 63, and the load capacitance of the capacitors 64 and 65. Therefore, by changing the current input from the input terminal 56, the transconductance Gm of these differential amplifiers can be changed, and the trap frequency and the cutoff frequency of the low-pass filter can be changed. The sharpness Q can be changed by connecting the bases of the transistors 67 and 68 to different current sources and changing the current ratio.

[0040]

As described above, according to the present invention, a plurality of different filter circuits can be constructed and the filter block can be used in common. Therefore, the number of elements can be reduced and the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a filter circuit according to the present invention.

2 is a block diagram showing a specific example of sharing a recording low-pass filter and a reproducing trap filter and sharing a recording delay filter and a reproducing high-pass filter according to FIG.

FIG. 3 is a block diagram showing a specific example of sharing a recording low-pass filter and a reproduction trap filter.

FIG. 4 is a block diagram showing a specific example of sharing a recording delay filter and a reproduction high-pass filter.

5 is a block diagram showing a specific example of sharing a recording low-pass filter and a reproduction trap filter in FIG.

FIG. 6 is a circuit diagram of a differential amplifier used in a conventional filter circuit.

FIG. 7 is a circuit diagram of a differential amplifier used in a conventional filter circuit.

FIG. 8 is a diagram showing an example of a conventional low-pass filter.

FIG. 9 is a diagram showing an example of conventional filter switching.

[Explanation of symbols]

11 ... First amplifier, 12 ... Second amplifier, 13 ... Inverting amplifier, 14, 15, 16 ... Switch, 21 ... Shared block of recording low-pass filter and reproduction trap filter, 24 ... Recording delay filter and reproduction high-pass filter Shared block with.

Front page continuation (72) Kazuo Kondo Inventor Kazuo Kondo 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Ltd.

Claims (1)

[Claims] 1. An output terminal of a first amplifier is connected to a positive input terminal of a second amplifier, and an output terminal of a second amplifier is negative input terminals of the first and second amplifiers. In the active filter circuit in which load capacitances are connected to the output terminals of the first and second amplifiers, respectively, in the positive input terminal of the first amplifier,
A first switch for selecting a bias power supply is provided, and an input signal, a bias power supply, or an output of an inverting amplifier that makes the input signal have an opposite phase to the load capacitance connected to the output terminal of the first amplifier. A second switch for selecting a signal is provided, and a load switch connected to the output terminal of the second amplifier is provided with a third switch for selecting an input signal or a bias power supply; A filter circuit characterized by controlling the first, second and third switches to switch all filter characteristics.