JPH0221794Y2 - - Google Patents

Description

[Detailed Description of the Invention] Technical Field The present invention relates to a frequency characteristic compensation circuit for a wideband active filter for video signals that includes C, R and an amplifier as constituent elements. That is, it compensates for deterioration in frequency characteristics due to the GB product and the Q value of the active filter.

Prior Art This type of active filter, which is well known in the past, uses a frequency range that is mainly limited to a low frequency band such as an audio frequency band. There was no need to consider the effects of the finite GB product and the Q value of the active filter on the frequency characteristics, but if the frequency range used extends to the video frequency band, the wideband frequency characteristics necessary for the video active filter will be affected. The drawback is that it deteriorates due to the influence of the input capacity of the operational amplifier, finite GB product, and Q value of the filter, and that the conventionally known configuration does not have a limited wide-band frequency characteristic sufficient as an active filter for video. It was hot.

Main points of the invention The purpose of the invention is to eliminate the above-mentioned conventional drawbacks, and to reduce the input capacity of the component amplifier and the finite GB.
An object of the present invention is to provide an active filter frequency characteristic compensation circuit for a video signal which can obtain sufficiently wide band frequency characteristics by compensating for the influence of the product and the Q value of the active filter.

In other words, the active filter frequency characteristic compensation circuit of the present invention has a feedback circuit including a resistor and a capacitance element, and inputs an input signal of an amplifier constituting an active filter and an output signal via the feedback circuit to the amplifier. A resistor divider consisting of a first resistor _R1 and a second resistor _R2 connected between the non-inverting input terminal of the active filter and the signal input terminal of the active filter and between the non-inverting input terminal and the ground potential point, respectively. a Q of the active filter, which is proportional to the reciprocal of the gain-bandwidth product of the amplifier and in parallel to the first resistor R ₁ in the resistive divider circuit; Provide a capacitive element having a capacitance C* such that C ^* = R ₂ /R ₁ × C _io + C _pp with respect to a capacitance C _pp at least approximately proportional to the value and an input capacitance ^C in at the non-inverting input terminal of the amplifier. The present invention is characterized in that the frequency characteristics of the active filter are compensated for.

Embodiments The present invention will be described in detail below with reference to the drawings.

First, FIG. 1 shows an example of a circuit configuration in which the active filter frequency characteristic compensation circuit for video signals according to the present invention is applied to an active filter in the form of a so-called Wilson circuit to form an all-pass active filter. However, in this type of conventional circuit configuration, regarding the operational amplifier OPA used as an amplifier, the input capacitance C _io associated with the non-inverting input terminal (+) that supplies the input signal is
The frequency amplitude characteristics of the passband deteriorate due to the finite GB product of the operational amplifier OPA and the Q value of the active filter, making it difficult to obtain sufficient wideband frequency characteristics required for active filters for video. It became clear that there was no.

Therefore, in the active filter frequency characteristic compensation circuit for video signals according to the present invention, the deterioration of the frequency characteristic caused by the input capacitance and finite GB product of the operational amplifier is compensated for by the non-inverting operation of the operational amplifier OPA constituting the Wilson circuit. Input terminal (+)
A dividing circuit with a resistor R ₁ in series and a resistor R ₂ in parallel is provided, and a compensating capacitor C ^* is added and connected in parallel to the resistor R ₁ , so that the capacitance C ^* of the compensating capacitor is approximately R ₂ By making it equal to /R ₁ ×C _io +C _pp , a sufficiently flat passband characteristic can be obtained. Of this compensation capacitance C ^* = R ₂ / R ₁ × C _io + C _pp , the first term R ₂ /R ₁ × C _io cancels and eliminates the influence of the input capacitance C _io of the operational amplifier OPA. Furthermore, the second term C _pp is for compensating for the decrease in frequency characteristics caused by the finite GB product of the operational amplifier OPA.

In other words, the first thing to consider about the deterioration of the passband characteristics of an active filter configured in the form of a Wilson circuit is the series resistance R ₁ , the parallel resistance R ₂ , and the input capacitance C at the non-inverting input terminal (+) of the operational amplifier OPA. _This is a deterioration in frequency characteristics due to a decrease in the amplitude of the high-frequency component of the input signal due to the input signal being divided by the IO parallel circuit. Therefore, in order to prevent such deterioration of frequency characteristics, it is recommended to connect a capacitor in parallel to the series resistor R ₁ to make the division ratio of the input signal amplitude ^independent of the _frequency . /R ₁ ×C _io . However, according to the results of various experiments and analysis using a computer, so-called CAD analysis, for the circuit configuration shown in Figure 1, it has been found that only the addition of the compensation capacitance R ₂ /R ₁ ×C _io to the input capacitance C _io In the end, it was found that the flat characteristic required as the passband characteristic of an all-pass active filter could not be obtained.

In other words, in Figure 2, which plots the results of the experiments and CAD analysis described above, the input capacitance compensation component
As shown by the characteristic curve in the case of C _pp = 0, in which the compensation capacity C _pp for a finite GB product, which will be described later, is zero using only the compensation capacity R ₂ /R ₁ × C _io , the above-mentioned input capacitance C _io If only compensation is made for the resulting deterioration in frequency characteristics, the frequency amplitude characteristics will undulate around a specific frequency point, and the high-frequency characteristics will deteriorate, making it impossible to obtain the necessary flat characteristics. As a result of various studies on specific frequency points at which waviness occurs on the frequency-amplitude characteristic curve of the active filter, we found that the gain-bandwidth product of the operational amplifier OPA, that is,
It was found that when the GB product is finite, the rapid phase rotation within the band corresponds to the frequency occurring in the operational amplifier. In other words, for example, in an active filter configured to obtain an all-pass flat passband characteristic, the frequency changes around the angular frequency ω ₀ where a sudden phase rotation occurs due to the influence of the finite GB product of the operational amplifier. This results in waviness in the amplitude characteristics.

Therefore, in the active filter frequency characteristic compensation circuit of the present invention, in addition to the compensation capacitance R ₂ /R ₁ ×C _io for the input capacitance C _io of the operational amplifier OPA, the Q value required for the active filter and the GB of the operational amplifier are A compensation capacitor C _pp having an optimum capacitance value corresponding to the product is also added in parallel to the input series resistor R ₁ to eliminate the waviness of the frequency amplitude characteristic curve and the drop in the high frequency characteristic described above, and to reduce the active voltage for video signals. To obtain sufficiently flat frequency characteristics when using the filter as an all-pass filter, for example. The results of experiments and CAD analysis conducted with various values of the compensation capacitance _Cpp are presented in the second section.
As shown in the figure, in the illustrated example, the specifications of the active filter are Q=1.427, GB
= 210MHz, C _io = 1pF, R ₁ = 519.1Ω, R ₂ = 1kΩ, the optimal compensation capacitance C _pp is 3pF,
Phase rotation angular frequency ω ₀ = 24.21×10 ⁶ rad/s In other words, the waviness of the frequency amplitude characteristic curve and the decline in high-frequency characteristics that occurred around the phase rotation frequency of 3.85 MHz are reduced when the compensation capacitance C _pp = 3 pF. was removed, and a sufficiently flat passband characteristic was obtained.

In the actual design of the active filter for video signals according to the present invention, the input capacitance compensation capacitance R ₂ /R ₁ ×C _io is determined by the circuit design conditions and the input capacitance of the operational amplifier used, which is usually several pF. In addition, in a practical design area where the Q value of the active filter for video signals is 0.5 to 2.5, the compensation capacitance C _pp can be calculated from the Q value and GB product as shown in Figure 3. Based on the relationship, an appropriate value can be found from the GB product of the operational amplifier actually used and the Q value of the required active filter.

Also, from the configuration shown in Figure 1 and the relationship shown in Figure 3 with the frequency characteristics shown in Figure 2, the required compensation capacitance C _pp is a value that depends on the reciprocal of the GB product and the Q value of the active filter. can be approximated by the reciprocal of the GB product and the capacitance value proportional to the Q value of the active filter, and as another approximation, by the relational expression C _pp = k/GB・Q ^3/2 . You can also ask for it. In this relational expression, the constant k=350, and the compensation capacitance value C _pp is approximated by the Q value of the active filter: Q and the gain bandwidth product: GB (MHz) of the operational amplifier OPA.

Note that the approximation relational expression C _pp =k/GB·Q ^3/2 described above can be approximated fairly well under the conditions of the specifications shown in _FIG .
Even for video signal active filters with different _R2 constants, the number of CR feedback circuits to be connected, etc., a required approximate relational expression is set according to the technical idea of the present invention described above, and the calculation is performed using the approximate relational expression. By adding the compensation capacitor C _pp in parallel to the series resistor R ₁ of the decomposition circuit, the frequency characteristics of the active filter can be easily and effectively compensated.

Effects As is clear from the above explanation using an active filter with all-pass characteristics as an example, the present invention can be used for high-performance video signals used, for example, in pre-filters and interpolation filters required for digital processing of video signals. The filter can be made much smaller and lighter than conventional LC filters.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing a configuration example of the active filter frequency characteristic compensation circuit of the present invention, Fig. 2 is a characteristic curve diagram showing an example of the frequency characteristics of the filter, and Fig. 3 is the characteristic of the compensation capacitance of the filter. FIG. 3 is a characteristic curve diagram showing an example. OPA...Operation amplifier, V _io ...Input video signal,
V _put ...Output video signal.

Claims (1)

[Claims for Utility Model Registration] 1. An input signal of an amplifier comprising a feedback circuit including a resistor and a capacitance element to constitute an active filter and an output signal via the feedback circuit are connected to a non-inverting input terminal of the amplifier. The amplifier is connected to the amplifier via a resistance divider circuit consisting of a first resistor _R1 and a second resistor _R2 connected between the signal input terminal of the active filter and between the non-inverting input terminal and the ground potential point. and in parallel with the first resistor R 1 in the resistor divider circuit, the resistor R ₁ is proportional to the reciprocal of the gain-bandwidth product of the amplifier and at least approximately equal to the Q value of the active filter. Regarding the proportional capacitance C _pp and ^the input capacitance C in at the non-inverting input terminal of the amplifier, a capacitance element having a capacitance C ^* = R ₂ /R ₁ × C _io + C _pp is provided, and the frequency of the active filter is An active filter frequency characteristic compensation circuit characterized by compensating the characteristics. 2 Utility Model Registration In the compensation circuit according to claim 1, the capacitance C _pp is proportional to the 3/2 power of the Q value of the active filter and inversely proportional to the gain bandwidth product of the amplifier. An active filter frequency characteristic compensation circuit characterized by: