JPS60239114A - Filter circuit - Google Patents

Abstract

PURPOSE:To miniaturize an IC area and to reduce the cost by providing the 1st resistor connected to a signal input terminal, the 2nd resistor connected in series between the 1st resistor and the amplifier input terminal and a capacitor connected between the output terminal of the amplifier and the 1st, 2nd resistors in an active filter circuit. CONSTITUTION:The resistors R1, R2, a capacitor C1 and a base input capacitor of transistors (TRs) Q2, Q5 constitute a time constant circuit, the signal is fed back to a capacitor C2 by an amplifier 4 so as to constitute the secondary positive feedback type LPF. An output signal is extracted from an emitter of the TRQ5. The base input capacitance is utilized as the 2nd capacitor, and the secondary positive feedback type LPF is constituted of one capacitor by utilizing the base input capacitance of the TR as the 2nd capacitor like the capacitor C2, then the capacitance value of the 2nd capacitor is selected freely depending on the area of the TRQ5 and the number of TRs connected in parallel with the TRQ5.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a method for forming a silicon wafer on a silicon wafer or the like.
The present invention relates to a filter integrated circuit suitable for integrating a filter in a V-type IC.

[Background of the invention]

Integration of electrical circuits (monolithic IC, hereinafter simply IC)
As the technology (hereinafter referred to as ``electronic circuits'') progresses, the use of ICs for external block filters is becoming an important issue in realizing circuit miniaturization and cost reduction. A conventional filter is composed of an inductance L, a capacitance C1, and a resistor R. However, it is difficult to integrate the inductance L into an IC, and it is difficult to integrate a filter composed of L, C, and R into an IC. Therefore, an active filter that can be configured only with a capacitor C1 and a resistor R is an IC.
It is suitable for As a low-pass filter (hereinafter abbreviated as LPF), a positive feedback LPF shown in FIG. 1 is well known. In the figure, 1 is an operational amplifier with a gain of 5 times.

If the resistance R9 and the capacitance C are respectively resistance R1m, R1*, capacitance 'l, and mC1, then the resonant frequency fo is expressed as follows. Il+j is an input signal source, and 116 is an output terminal. Furthermore, the Q of the filter determines the peaking near the resonance frequency. The Q of the LPF in FIG. 1 is expressed as follows. By increasing Q, peaking can be increased. In general, block filters are composed of high-order filters, and many of those actually used are of fourth order or higher. However, when constructing a filter, combining filters with equal low-order frequency characteristics,
If a high-order filter is configured, phase rotation and group delay will be added around the cutoff frequency, which is not favorable for signal transmission. As for the characteristics of the filter, it is desirable that the phase characteristics and group delay characteristics in the passband be as flat as possible. Therefore, generally, a high-order LPF is constructed by combining low-order filters with anisotropic frequency characteristics. As an example, a 4th-order LP that combines 2nd and 3rd-order positive feedback LPFs
F is shown in FIG. In LPF 2, the Q is high and the third
With the amplitude characteristics shown in the figure, the LPF of No. 2 and Otsu are combined to have the amplitude characteristics shown in FIG. 4 by lowering the Q and the amplitude characteristics shown in FIG. 5 are obtained.

In order to obtain high Q characteristics as shown in Figure 3, formula (2) is used.
Therefore, the ratio of the capacitance C3 to the capacitance C2 can be increased, but if the capacitance C2 is made small, the influence of parasitic capacitance becomes a problem, so it is necessary to make the capacitance C1 large. But I
In C, the area occupied by a capacitor is larger than that of a resistor or a transistor, resulting in an increase in the chip area of the IC, resulting in a problem of increased cost due to a decrease in yield.

[Purpose of the invention]

It is an object of the present invention to provide a filter circuit which eliminates the above-mentioned drawbacks.

[Summary of the invention]

In positive feedback type LPP, by using the base input capacitance of the transistor instead of the second capacitance, the second
can be omitted and its capacitance value is small, so it is possible to create an LP with high Q characteristics without increasing the first capacitance value.
It becomes possible to obtain F.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIG. In the same figure, R,, R, is the resistance s, CI is the first capacitance, 4
are transistors (h, Qt, Qs, (h and resistor 8.
The amplifier constituted by 9.10.11.12, the Q transistor, and the resistor 16 constitute an emitter follower. 5, 6.7 are voltage sources. A time constant circuit is constructed with the resistor R1p R1, capacitor C8, and base input capacitance of transistor Q2 + QIl, and the signal is sent to amplifier 4 with a large capacitance C! The feeder is connected to the secondary positive feedback type L.
PF is configured.

Output signal C is output by transistor Q, 9-emitter. The base input capacitance of the transistor is used as the second capacitance, and this is mainly the junction capacitance that occurs because the P-N junction of the pace-collector is reverse biased in the active region of the transistor. In this embodiment, by using the base input capacitance of the transistor as the second capacitor such as capacitor C7 shown in FIG.
A second-order positive feedback LPF can be configured with one capacitor, and
The capacitance value of the second capacitor can be freely selected depending on the area of the transistor Q and the number of transistors connected in parallel with the transistor Q. As mentioned above 2 LPF
It is easy to understand that a second-order LPF can be constructed even if the input capacitance of only four amplifiers or only an emitter follower is used in the circuit.

〔Effect of the invention〕

As explained above, the filter circuit according to the present invention can configure a second-order LPF with one capacitor, and is therefore extremely effective in reducing the size and cost of an IC.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing an example of a positive feedback type LPF, Figure 2 is a circuit diagram of an example of a fourth-order LPF constructed by combining two positive feedback type LPFs, Figures 13 and 4. is positive feedback type L
FIG. 5 is an amplitude characteristic diagram of a PF, FIG. 5 is an amplitude characteristic diagram of a fourth-order LPF, and FIG. 6 is a block diagram of an embodiment of the present invention. R1+ RR... Resistor C1 that constitutes the second-order LPF
...secondary LPF's first capacity representative patent attorney Akio Gao Figure 1 CI Figure 2 Akio: Figure 3 Figure 4 Panasonic S diagram

Claims (1)

[Claims] 1. In an active filter circuit constructed on a semiconductor wafer and consisting of a folding capacitor and an amplifier, a first resistor connected to the signal input terminal and a resistor connected in series between the first resistor and the amplifier input terminal a second resistor connected to the output terminal of the amplifier and the first . A filter circuit characterized by being connected between a second resistor and having a friend capacitance.