JPS5840909A - Gain control circuit - Google Patents

Abstract

PURPOSE:To realize a noncontacting type and to increase the reliability for a gain control circuit, by connecting in series a pair of transistors which are turned on and off contrary to each other and connecting a variable capacitor between the point of series connection of the transistors and the ground to form a variable resistance circuit. CONSTITUTION:A pair of transistors S3 and S4 which are turned on and off contrary to each other are connected in series to each other. A variable capacitor VC is connected between the point of the series connection of S3 and S4 and the ground to form a variable resistance circuit R5. This circuit R5 is put into the feedback circuit of an operational amplifier OP. The capacity of the capacitor VC is varied to change Rf. Thus gain H is varied. The capacitor VC produces no friction nor noise due to said friction since it has no sliding part unlike a variable resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a non-sliding, non-contact type control device. ! This invention relates to a gain control circuit using 0 elements.

Variable resistors are widely used for gain control, but since they are sliding, contact type resistors, they tend to produce noise due to wear of the contact parts when used for a long time.

On the other hand, in a variable capacitor, the fixed and movable electrodes face each other with a gap in between and are non-contact type, so there is little risk of noise generation due to wear and the like, and the reliability is high. The capacitor then functions as a resistor through switching, as seen in switched capacitor filters. The present invention focuses on the above points and attempts to provide a gain control circuit with a long life and high reliability using a variable capacitor.The present invention will now be described in detail with reference to the drawings.

As shown in FIG. 1(a), a changeover switch 8 and a capacitor C are provided, and the switch S is turned to the input IIIIN to charge the voltage V, and then the switch 8 is turned to the output side 0UTK to charge the now charged charge Q. = Discharge CVl, then turn switch 8 to the input side again to charge the capacitor, and repeat the same process.The changeover switch 8 is connected to the transistor S, as shown in Figure 1(b). , S, and these transistors are turned on and off in opposite phases using signals φ and φ that are 180@ phase different from each other. In this way, a discharge current due to the charge CK caused by the input voltage v1 flows to the output side and generates a voltage V, which is the same as if a resistor were connected between the input voltage v1 and the output. When the matching frequency, that is, the frequency of the signals φ and φ is f6, the equivalent resistance R
@q is represented by R@q=1//a·C.

A switched capacitor filter is a filter implemented using a monolithic IC using circuit elements, but in this case, the capacitance C of the capacitor is fixed. In the present invention, the range of resistance values that can be obtained by changing the capacitance C was determined, and the results shown in FIG. 5 were obtained. In this figure, the vertical axis represents the switching frequency fax, and the horizontal axis represents the equivalent resistance Req, both of which are on a logarithmic scale. The diagonal straight line group represents the capacitor capacitance C with IPFe 3pF, sp-...
-... The fe R/q4I properties are shown when various changes are made. As is clear from this graph, f c = 50
If the capacitance IC is changed in the range of 100 to 5 pF as D KHz, the resistance value will be changed in the range of 2OKΩ to 400Ω. A variable resistor in this range can be fully used as a gain adjustment element for amplifiers in electronic equipment such as radios and audio equipment. Also, changing the capacitance within this range can be easily achieved with a rotary variable capacitor called a variable capacitor, or with a variable capacitance diode. Figure 2 shows this variable capacitor V
An equivalent variable resistance element using C is shown. The connections are the same as in FIG. Transistor 8. , S, ii MO5FET
However, this may also be a bipolar transistor.Since the transistor is bi-directional, the input IN and output OUT in FIG. 2 may be reversed. One end of the variable capacitor VC is connected to the ground, but this is because the power supply has the ground as one terminal, so if the power supply is a positive or negative voltage output type, the above one end is connected to its negative voltage line or positive voltage line. becomes.

FIG. 3 shows an embodiment of the present invention @1. OP is an operational amplifier (operational amplifier), and C is a rattan 1 diagram (b
) Input resistances R1, 8, consisting of equivalent resistance elements shown in FIG.

S, VC is a feedback resistor R/ consisting of a variable resistance element shown in FIG. The gain H of this circuit is H=R//R
1, the gain H changes by changing R/ by changing the capacitance of the variable capacitor vc.

And since variable capacitors and resistors do not have sliding parts like variable resistors, there is no fear of wear or noise caused by this.

FIG. 4 shows a second embodiment of the invention. oPl～OPs
is an operational amplifier, each of which is fed back by a resistor, a capacitor, or both, and a resistor is further applied to all of the cascade-connected components. This is Bi-qu
This is called an ad type filter and produces a band-passed output to the terminal OUT.

The gain H of this circuit is expressed as ωo = R H = R1/Rx at the upper resonance frequency, so the input resistance Rx'1r
By changing the resistance value of the equivalent variable resistance element shown in FIG. 2, the gain H can be changed.

As explained above, according to the present invention, since it is a non-contact type, a highly reliable gain control circuit can be obtained.

Automatic gain control is also possible by using a variable capacitance diode as the variable capacitor.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a switched capacitor, Figure llX2 is a circuit diagram of an equivalent variable resistance element that is the capacitor in Figure 1 made variable, and #! 3 and 4 are circuit diagrams showing examples of the present invention, and FIG. 5 is a graph showing the fe R@(l characteristics of the device shown in FIG. 2. In the drawings, 8., S2.6, and 84 are A pair of transistors, V
C is a variable capacitor, and OP* OPI to OP are amplifiers. Figure 1 Figure 2 Figure 3 Figure 4 Procedural amendment (voluntary) October 28, 1980 Commissioner of the Japan Patent Office Shima 1) Haruki Tono1, Indication of the case 1982 Patent H No. 1590822, Invention Name of gain control circuit & relationship with the case of the person making the correction Patent applicant address 1-2-28 Gosho-dori, Hyogo-ku, Kobe, Hyogo Prefecture Name Fujitsu Ten Ltd. Representative: Chapter 4 Funabashi, Agent Address: 101-5 Number of inventions increased by amendment None l Subject of amendment Contents of the amendment in the catalog of attached documents to the application, the detailed explanation of the invention in the specification (a) (1) From the end of page 3 of the specification to the 4 pages, 1 row of rFETs may be used. It is a JfrFET. ”. (2) Supplement the attached power of attorney.

Claims (1)

[Claims] A variable resistance circuit consisting of a pair of transistors connected in series and a variable capacitor connected between the series connection point and ground is inserted into the input circuit or feedback circuit of the amplifier. A gain control circuit characterized by: