JPS5840935A - High frequency tuning circuit - Google Patents

Abstract

PURPOSE:To avoid a change of an amplifying band regardless of a change of tuning frequency and to obtain flat noise characteristics, by having a longitudinal connection among the 1st, 2nd and 3rd operational amplifiers and giving a feedback to the 1st operational amplifier from the 3rd one. CONSTITUTION:The 1st operational amplifier OP1 contains a feedback circuit having a parallel circuit of an input resistance R4, a resistance R1 and a capacitor C1. The 2nd operational amplifier OP2 has a feedback circuit containing an input resistance R2 and a capacitor C2. The 3rd operational amplifier OP3 has a feedback circuit including an input resistance R5 and a resistance R6. The output terminal of the amplifier OP3 is connected to the input terminal of the amplifier OP1 via a resistance R3. A relation of R2=R3=R0 is satisfied between the resistances R2 and R3, and C1=C2=C0 is satisfied between capacitors C1 and C2. The C0, R1 and R4 are set constant respectively, while R5 and R6 are set varible. As a result, the gain exists even in the band of 100MHz to ensure the satisfactory application for a radio high frequency tuning amplifier.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high frequency tuning circuit in which the amplification band does not change even if the tuning frequency changes.

In a receiving device that has a relatively wide receiving frequency band in the high frequency band, such as a muy-wave broadcast band receiver, Qx tends to be constant, so the amplification band is narrow in the lower limit frequency band and widen in the upper limit frequency band (the gain is in the former (the latter is large - the latter is small), the fidelity and noise characteristics will fluctuate.0 The present invention improves this and provides a high frequency tuning circuit that maintains the amplification band constant even if the center frequency changes over a wide RIF band. This will be explained next with reference to the drawings.

Circuits that keep the band constant and only change the center frequency are known as low-frequency filter circuits. FIG. 1 is an example of this, in which an operational amplifier OP, a resistor RaxR@, and a fundensor Ca and Cb are connected as shown, and the filter has the characteristics of a 1kn bandpass filter.

That is, roughly speaking, the capacitor cb is a bypass element, the capacitor Ca is a high suppression element, and if the time constant created with the resistors R to Rg and the overall frequency are appropriately selected, a band pass can be achieved. In this circuit, by changing the resistance of the resistor Rh as a variable resistor, the band is constant, the gain is constant, and only the center frequency can be changed.
There is a condition of Rb, which is difficult to achieve in the RF band. In other words, the operation becomes unstable or worsens to 8. Ra) If the Rh conditions are not fully satisfied,
The above band, constant gain, and only change in center frequency hold true only approximately.

FIG. 2 shows a circuit called a bi-quad type filter circuit, in which operational amplifiers OP1 to OP8, resistors to R capacitor C1ock are connected as shown. The analysis results of this circuit are described in literature such as "Design of Active Filter" (Electronic Science Series 52.pp147), but the general formula T (S
) is represented by . On the other hand, in the circuit of FIG. 2, the output voltage V! with respect to the input voltage v1! When we search for , we get . Therefore, the following equation holds.

Here, if we set Rt=R Hatake=R・, C,=Cnomiya=C・, then
The gain H1 center angular frequency ω· and the bandwidth Δω are as follows.

From these equations (4), (5), and (+5), C
If R1 and R are kept constant and R1 and - are changed (in the conventional circuit, 1 = - = -), Δω and H are constant and only ω can be changed. The ratio of resistors R1 and R6 affects ω, so one can be fixed while the other is varied, but if one is varied so that when the other increases, the other decreases, so to speak, a push-nibble type operation occurs. , the central angular frequency object is sensitive and can be changed significantly. Figure 3 shows the adjustment part. The resistor R15R4 is configured as a -tiJ variable resistor, one end a of which is connected to the front stage amplifier OP.

The other end is connected to the output end of the stage amplifier OP, and to the input end of the first stage amplifier OP1 via the feedback resistor Rs, and the slider C is connected to the input end of the stage amplifier OP. be done. If the slider C is moved to the left in the drawing, 8·/R becomes larger, and if the slider C is moved to the right, R and /R1 become smaller.

As shown in the above equation (5), Ra/R1 is only effective as the square root of ω, and the conventional circuit is also similar in that it is effective only as the square root. In the present circuit, there are two variable elements, ``bird'' and ``-'', so by increasing or decreasing these in the opposite direction, it is possible to make the effect on ω· stronger than on K. In addition to ordinary resistors, equivalent resistances (switched capacitors) found in switched capacitor filters can be used for these resistors R, , R.

The circuit shown in FIG. 2, set as described above, has a gain even in the 100 MHz band, and can be used satisfactorily as a radio frequency tuned amplifier. Since only ωO changes (see Q4 for details) and H1Δω remains constant, excellent amplification characteristics are exhibited over a wide broadcasting frequency band with little risk of noise contamination. Furthermore, this circuit does not use any inductance and is composed only of an operational amplifier, R, and C, so it can be easily integrated into an IC.

As explained above, according to the present invention, it is possible to obtain an excellent tuned amplifier circuit that is applied to a high frequency circuit such as an AM receiver and has flat sensitivity, fidelity, and noise characteristics over the entire reception band.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional example, FIG. 2 is a circuit diagram showing a high frequency tuning circuit to which the present invention is applied, and FIG. 3 is a circuit diagram showing essential parts of the present invention. In the drawing, OF is the first operational amplifier, OP! is the second operational amplifier, and OF8 is the third operational amplifier. Figure 1 Figure 2 Figure 3 1 Criticism 1986-4e935 (3) Procedural Amendment W (Voluntary) October 28, 1980 Director General of the Patent Office Haruki Shimakawa 1, Indication of Case 1988 Patent Application No. 139083 No. 2, Name of the invention Relationship with the High Frequency Tuning Circuit 5 Amendment Case Patent Applicant Address 1-2-28 Gosho-dori, Hyogo-ku, Kobe, Hyogo Prefecture Name Fujitsu Ten Ltd. Representative Funabashi Chapter 4 Agent 〒101 5, Date of amendment order: None & Number of inventions to be increased (by amendment) None: Target of amendment
Contents of amendments to the list of documents attached to the application, the scope of claims of Mei #I Homare, and the detailed description of the invention Attachment instructions (1) Lines 5 to 16 of page 1 of the specification The claims are amended as follows. "A first operational amplifier having an input resistor R4 and a feedback circuit consisting of a parallel circuit of a resistor R1 and a capacitor C1, a second operational amplifier having an input resistor R2 and a feedback circuit consisting of a capacitor C2, and a human resistor. is, and a third circuit having a feedback circuit consisting of a resistor R6.
operational amplifiers are connected in cascade, and sI3 is connected by resistor R3.
A circuit circuit in which feedback is applied from the output terminal of the operational term g unit to the input terminal of the first operational amplifier, and the resistor Rs e Ra is set to R
Capacitor Cs e Cs k C
A frequency tuning circuit characterized in that l=C5=Co, CO, R1+Ra are constant, and the ratio &/Rs is variable. ” (2) Insert K [second-order band-pass filter] next to “Although,” on page 3, line 12 of the same page. (3) r100MHzJir number MHzJ on page 6, line 2
Correct to. (4) Supplement the attached power of attorney.

Claims (1)

[Claims] A first operational amplifier having a feedback circuit composed of an input resistor R4 and a parallel circuit of a resistor and a capacitor C1, and a second operational amplifier having a feedback circuit composed of an input resistor and a capacitor C1. A third operational amplifier having a feedback circuit consisting of a resistor R6 is connected in cascade with the amplifier and the input resistor, and the output terminal of the third operational amplifier is connected to the input terminal of the first operational amplifier via the resistor. A feedback circuit is provided, the resistor -1Rs is R1=R, and a capacitor cl. Let C-rich be C,=C,=C・, and C・*R1*R
A high frequency tuning circuit characterized in that the ratio R6/& is variable while the ratio R6/& is constant.