JPS60223317A - Filter integrated circuit - Google Patents

Abstract

PURPOSE:To attain miniaturization and light weight of an electronic circuit by combining a Twin-T circuit where the product between integrated circuit capacitance and integrated circuit resistance decides a trap frequency and an RC circuit where the product decides the cut-off frequency and using a variable capacitor as the integrated circuit capacitance. CONSTITUTION:In selecting resistors R1, R2, R3 of the Twin-T trap circuit 3 and variable capacitors C1, C2, C3 as R1=R2=2R3identicalR and C1=C2=C3/2identicalC, the trap frequency fT is expressed in Equation I. The cut-off frequency fc is expressed in Equation II in the positive feedback 2nd order LPF circuit 4. Thus, a signal outputted to an output pin 7 is the sum of response characteristics illustrated in Curves 8, 9 and a steep frequency characteristic is realized as shown in Curve 10. In changing a variable power supply V2 provided externally to the IC, since the n-channel potential of the variable capacitors C1-C6 is changed altogether via amplifiers K1, K2, a voltage VR across the variable capacitors C1-C6 is changed at the same time in the same rate.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a filter integrated circuit suitable for integrating a filter into a monolithic IC formed on a silicon wafer or the like. [Background of the Invention] Conventionally, in electronic circuits, in order to obtain a desired signal, low-pass, high-pass, and bandpass filters (hereinafter referred to as LPF, HPF, BP) are used.
(abbreviated as F) and phase equalizer, inductance L,
A filter consisting of a capacitor C1 and a resistor R was used. For this reason, integration of electronic circuits (monolithic IC,
As the use of IC (hereinafter abbreviated as "IC") progresses, the cost of these filters is decreasing.

This was a major factor preventing electronic circuits from becoming smaller and lighter. Particularly in portable equipment where mobility is important, compactness and weight reduction are important, and IC-based filters have been desired.

For example, in a magnetic recording/reproducing device, a luminance signal (0 to 3 Ml1
z) is extracted and the luminance signal is subjected to FM modulation. Furthermore, during reproduction, in order to remove unnecessary signals outside the luminance signal band that occur when the FM modulated wave is FM demodulated,
In order to share the PF, the desired characteristics of the LPF described above required a sharp and highly accurate frequency characteristic as shown in FIG.

On the other hand, in ICs, the absolute value accuracy of integrated elements is poor (usually 20-3o%), and integrated resistors.

There has been a drawback in that it is difficult to obtain desired filter characteristics that require precision using integrated capacitors. For this reason,
Conventionally, the above-mentioned LPF includes transformer-coupled inductances L, , L, , L, as shown in FIG.
, A high-order block filter (1 and 2 indicate the block range) consisting of small capacitances C, ,C, ,C, ,C, is used. However, in order to obtain steep frequency characteristics, a high-order filter is required, and in addition, high precision is required, which requires complicated adjustment of the inductances L1 to L4, resulting in high costs and small size. This made it difficult to reduce weight.

[Purpose of the invention]

An object of the present invention is to eliminate the drawbacks of the prior art described above, and to integrate a filter with high precision and steep frequency characteristics into an integrated element (
An object of the present invention is to provide a filter integrated circuit that can be configured with IC capacitors and IC resistors, easily absorb variations in integrated elements, and ensure performance.

[Summary of the invention]

In order to achieve the above-mentioned purpose, we developed a TυLn-T circuit in which the value of the product of the IC capacitor and the IC resistor determines the trap frequency (?L 11 frequency), and a TυLn-T circuit in which the product of the IC capacitance and the IC resistor determines the trap frequency (? A filter integrated circuit combining an RC circuit that determines the fault wave number may be constructed.A varicap is used as the IC capacitance, and the voltage applied to both ends of the varicap is simultaneously adjusted to eliminate variations in the IC capacitance and IC resistance. It is something that absorbs.

[Embodiments of the invention]

An embodiment of the present invention will be explained below. FIG. 3 is a diagram showing an embodiment of the present invention, and FIG. 4 is a diagram explaining the operation of the embodiment shown in FIG.

In FIG. 3, 3 is a T-L--T type trap circuit, 4
is a positive feedback type secondary LPF circuit. Q, , Q, indicate transistors, and the transistors Q, , Q, are constant current Riko 1. Together with the resistor, it forms a back 1 circuit that achieves a high input impedance (the pace of the transistor Q) and a low output impedance (the collector of the transistor Q). Q, , Q, represent transistors;
The transistors Q, ,Q, are constant current sources.■3. Together with the resistor, it constitutes a buffer 1 circuit similar to the above. K, ,
K, are the same differential amplifiers, transistor Qs, constant current source 1. constitutes Emifluoro. C and -C indicate integrated capacitors (varicaps) consisting of PN junctions, and the values of the varicaps change depending on the voltage VR applied to both ends. JLog using, for example, the base-emitter capacitance of a transistor as a varicap
C7= K-α1af (φ+■j) where CI: Pace-emitter junction capacitance C, 2 (0): Base-emitter junction capacitance at bias 00 Vj: Emitter medium pace voltage (diode reverse bias voltage) φ: Built-in Voltage α: voltage dependence coefficient K: '#(C,=(01φα)), and the characteristics shown in FIG. 5 are obtained.

In addition, ``■'' is a stabilized power supply, and V2 is a variable power supply external to the IC. The signal input to the input pin 6 of the ICs is the third
In the circuit shown in the figure, 'l'yA? The signal is outputted to the output pin 7 through the L-T type trap circuit 3 and the positive feedback type secondary filter circuit 4.

Here, the resistance R1 of Turirn-T type trap circuit 3
.

If R, , & and varicap C,,C,,C, are respectively chosen as R1=R1=2'BsmiR3c,=c,=7=C, then the trap frequency fr is fT-2,. It is represented by □ ・・・・・・・・・・・・■ and has the frequency characteristic shown by curve 8 in FIG.

In addition, in the positive feedback type secondary LPF circuit 4, the cutoff layer wave number fc
It has a frequency characteristic shown by curve 9 in FIG. Therefore, the signal output to the output pin 7 can realize a steep frequency characteristic as shown by curve 10 in FIG. 4, which is the sum of the responses shown by curves 8 and 9 in FIG.

Furthermore, since the relative accuracy between integrated elements within the IC can be maintained sufficiently high, if R5 6 is set, the cutoff layer wave number can be expressed as jc =□ 2πCRbsa-.

Therefore, even if the integrated elements (C9) vary, the trap frequencies fT, L and the fault wave number fc will vary at the same ratio, as can be seen from the above equations ① and ②, so as shown by curves 11 and 12 in Fig. 4. The desired characteristic 10 becomes a characteristic simply shifted in frequency.

Therefore, by changing the variable power supply V outside the IC, it is possible to change both the side potentials of the varicaps C and -C to the amplifier via , K. , can change at the same rate at the same time. Therefore, the cut-off wave number and trap frequency determined by the product of integrated resistor R and integrated capacitor C can be adjusted to desired values, and desired characteristics (curve 10 in FIG. 4) can be obtained.

In addition, the voltage of the stabilized power supply vl is determined by the resistor R1 and the transistor Q.
, ,Q, to the base of the varicap C8, then through the resistor R, to the base of the cap Cs, and then through the resistor R, to the varicap C! is applied to the base of

Furthermore, the above voltage is applied to the base of the varicap C1 through the transistors Qs, Qa and the resistor R to the primary resistor R.
- is supplied to the base of the varicap C6.

Here, the base-emitter voltages of the transistors within the same IC can be made almost the same. Therefore, each varicap vR has the DC amplification factor of one transistor.
FE % amplifier has an output voltage of 1°K, Vx+, V
If K2, Vu(cs)=Vx+ (Vl,',',
X(Rt+R++Rs)) ・=■VR(C4)−■に2 (v,−ゑf×(几7十几1+R1+R)ト・・
...■...■ When 'FB is sufficiently large, the term including "FE" on the right side of the above formula is ignored, and 1 for ■ and 2 for ■ become almost the same, and each varicap The same vR is applied,
External variable power supply ■, Vic+ + VK2 can be changed and adjusted at the same time.

FIG. 6 is a diagram showing another embodiment of the present invention, and FIG. 7 is a diagram showing another embodiment of the present invention.
This is a diagram for explaining the operation of the figure, and the same or similar parts as in FIG. 6 are denoted by the same reference numerals. In this embodiment, the transistors Q+++Q+2 constitute a differential amplifier, and the DC voltages at both bases of the transistors Q11 and Qa2 are almost the same.

So now, if the base voltage of transistor Q is α, then Vi (cl) = Vx + - (Va + Si;
) ) ・・・・・・■VR(03) =Vx+ 'V
It is expressed as α......■.

By the way, in the example shown in FIG. 3, when 'FE is not large enough and the term including JFE cannot be ignored, VR has the largest difference between varicap CI and C. In the example above, the above 0 to 0 formula is
If you set =Ra+R*, the difference in VR will be the largest between Varicap C3 and C4 (see the example in Figure 3) If) JF
This has the effect of reducing the error of V due to E.

Further, the capacitance value of the varicap increases as the temperature rises, as shown in FIG. As a result, the tomographic wave number fc and the trap frequency fT will vary slightly, and the desired filter characteristics will have temperature characteristics. On the other hand, in the embodiment shown in FIG.
The voltage on the base side of each varicap has a temperature characteristic of about +2 mV/'c, and the emitter side of each varicap has a temperature characteristic of about +2 mV/'c x 2, for example, using an amplifier with a gain of 2, K. It has a temperature characteristic of approximately +4 mV/'C. Therefore, the applied voltage V of each varicap.

, (approximately +4mV/℃) - (approximately +2mV/℃) = approximately +2
It has a temperature characteristic of mV7''C and reduces the temperature characteristic fluctuation of the capacitance value of each varicap. As shown in Figure 7, VR'(VR' = V
If R10.7V) is Vm, then the capacitance value is C? If L is used and VR' = V m remains, the temperature will rise and 1
When the temperature reaches 00°C, the capacitance value changes to Cm. On the other hand, as the temperature rises, VR' is changed to Vn, and the capacitance value is changed to C? ◇In order to best eliminate the temperature characteristics of the capacitance value of the varicap, it is sufficient to provide the temperature characteristics as a VR as shown in Fig. 7.
It can be seen that the embodiment shown in FIG. 6 can significantly reduce the temperature characteristics of the capacitor.

By the way, the temperature characteristic of the resistance value of integrated resistors is generally +15
It is about 0OFPM/℃. Therefore, if the part indicated by the dotted line 13 in FIG. 6 is changed to the part shown in FIG. Become. Therefore, by operating to compensate for the temperature characteristics of the integrated resistor, the temperature characteristics of the product of C and R that determine the cut-off layer wave number fc and the trap frequency fr can be reduced to the maximum, and the desired filter The temperature characteristics of the characteristics can be significantly reduced.

The present invention has been described above using one stage each of T, A, -T type trap circuits and positive feedback secondary filter circuits, but even if these combinations are changed, the HPF
It is clear that the same effects as shown in the embodiments can be obtained even with a configuration including the above.

〔Effect of the invention〕

As described above, according to the present invention, a filter circuit having a steep frequency characteristic can be integrated with a simple circuit configuration, and highly accurate performance can be achieved with simple adjustment. This greatly contributes to the miniaturization and weight reduction of the simulator circuit, and can further contribute to the miniaturization and weight reduction of v'rit and the like using the electronic circuit.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the characteristics of a conventional external filter, Fig. 2 is a circuit diagram of a conventional external filter, Fig. 3 is a circuit diagram showing an embodiment of the present invention, Figs. 4 and 5. , FIG. 7 is a diagram explaining the present invention in detail, and FIGS. 6 and 8 are diagrams showing other embodiments of the present invention. 3...TyA--'l' type trap circuit, 4...Positive feedback type secondary filter circuit, 5...Integration range, 6...Kukata bin, 7...Output pin, 1o. ... Desired filter characteristics, 11, 12...
・Filter characteristics when there is large variation. Akio Takahashi ¥1 1 ffi Figure 2 Figure 4 Figure 5 <MHz) Figure S (pf = built-in voltage) Figure 8

Claims (1)

[Claims] 1) In an integrated circuit in which circuits having frequency characteristics are integrated, an integrated resistor and an integrated capacitor whose capacitance value changes depending on an applied voltage across the integrated resistor and the integrated resistor are used. The value of the product with the above integrated capacitance determines the trap frequency Tm-
1 circuit and at least one positive feedback filter circuit whose cutoff frequency is determined by the product of the integrated resistor and the integrated capacitor are combined in the same integrated circuit to form a steep frequency characteristic, and the integrated circuit The values of all the integrated capacitors whose multi-frequency characteristics are determined by the voltage applied to the external terminal of the converter are simultaneously changed at the same ratio, and the trap frequency and the cutoff frequency are adjusted by the externally applied voltage. A filter integrated circuit featuring: 2) The voltage applied across the integrated capacitor has a temperature characteristic of 1, thereby reducing the temperature characteristic of the product of the integrated resistor and the integrated capacitor. The filter integrated circuit described.