JPH0126565B2 - - Google Patents

Description

[Detailed description of the invention]

(1) Technical Field of the Invention The present invention relates to electronicization of a variable attenuation circuit (hereinafter referred to as an AV circuit). (2) Background of the technology Conventionally, variable attenuators have been prepared by preparing several types of resistance attenuators and obtaining a desired amount of attenuation by combining them. The combination of resistance attenuators was selected by switching straps using solder.
However, in recent years, as devices have become smaller and more semiconductor-based, variable attenuators are also required to be smaller and more semiconductor-based. (3) Prior Art and Problems One of the conventional electronic variable attenuation circuits is shown in FIG. The AV circuit shown in Figure 1 consists of a switched capacitor type integrator in which switched capacitor C _j is controlled by switch T _j that is selectively turned on by the output from the address decoder. . The value of capacitor C _j is selected, for example, as shown in the table below.

[Table] Figure 2 shows a simplified circuit to illustrate the principle of the AV circuit in Figure 1. The transfer function of the circuit in Figure 2 is V _p / _{V i} =-C _i / _{C x} 11 + _{C f /} C , the switch circuits S ₁ , S ₂ , and S ₃ are operated at a clock frequency of 256 kHz, and if 0.86<C _f /C _x <1.15 is selected, the attenuation deviation is maximum in the 0 to 4 kHz band.
It is 0.0048 dB, and the amount of attenuation is determined mostly by C _i /C _x . By the way, in the AV circuit of FIG. 2, the switch circuit _S1 is constituted by a switching transistor as shown in FIG. 3a, _and the capacitor C _x is actually _C ＋C Work as _{a s} . Therefore, an error occurs in the attenuation amount of the circuit shown in FIG. 2 due to the stray capacitor _Cs . By the way, the ones shown in FIGS. 3b and 3c are known as conventional switched capacitors that are not affected by stray capacitors, that is, stray capacitor-free switched capacitors. The switched capacitor shown in FIG. 3b is a positive phase type in which the input and output are not inverted, and the switched capacitor shown in FIG. 3c is a negative phase type in which the input and output are inverted.
An AV circuit using these stray capacitor-free switched capacitors is shown in FIG. The transfer function of the circuit in Figure 4 is V _p /V _i =-C _i /C _x Z / Z + C _f /C _X (Z-1) (2), and the amount of attenuation is determined by C _i /C _x . , this circuit has the disadvantage that the frequency characteristics are inferior to the circuit shown in FIG. 2, and the attenuation deviation is large. (4) Purpose of the Invention In view of the problems of the conventional type described above, the purpose of the present invention is to provide a switched capacitor type variable attenuation circuit which is less affected by stray capacitors and has a small attenuation deviation. be. (5) Structure of the Invention The present invention is constituted by a switched capacitor type integrator that uses a switched capacitor in the input section and the feedback section, and the switched capacitor is selected from several types and inputted. In the electronic variable attenuation circuit, the switched capacitor used in the feedback section is formed of a stray capacitor-free type and a positive phase type, and one of the outputs of the first inverting amplifier for taking out the output signal is There is provided an electronic variable attenuation circuit characterized in that the feedback section is fed back to the feedback section via a second inverting amplifier. (6) Embodiments of the Invention The principle circuit diagram of the electronic variable attenuation circuit according to the present invention is shown in FIG. In the AV circuit of Fig. 5, the switched capacitor C _i in the input section is of the type shown in Fig. 3 b, and the switched capacitor C _x in the feedback section is also of the type shown in Fig. 3 b. It is something.
Therefore, since the switched capacitor C _x is of the positive phase type, the output signal V ₀ is input to C _x via the inverting amplifier OP2 to form a negative feedback loop. In the circuit shown in Fig. 5, both the switched capacitors in the input section and the feedback section are of the stray capacitor-free type and of the positive phase type.
The transfer function is expressed by the above equation (1), and the influence of the stray capacitor is small and the attenuation amount deviation is small. In addition,
FIG. 7 shows the frequency characteristics of the attenuation compared with the circuit of FIG. 4. FIG. 6 shows an electronic system as an embodiment of the present invention.
An AV circuit is shown. In the circuit of Figure 6,
Four types of capacitors C ₁ , C ₂ , C ₃ , and C 4 are available as the switched capacitor C _x in the feedback section, and C ₅ , _{C 6 as} the switched capacitor C _i in the input section.
Eight types of _C12 are available. These capacitors
The capacitance values of C ₁ to _{C 12} are the same as in the circuit of FIG. 1 described above and shown in the table above. The amount of attenuation can be selected by selecting one of these capacitances from C ₁ to C ₄ and C ₅ to _{C 12} using switches T ₁ to _{T 4} and T ₅ to _{T 12} , respectively. Figure 6
In AV circuits, the amount of attenuation is in the range of 0 to 15.5 dB.
Can be selected in 0.5dB steps. (7) Effects of the Invention According to the present invention, there is provided an electronic variable attenuation circuit which is free from the influence of stray capacitors, has excellent frequency characteristics, can accurately and easily select the amount of attenuation, and has small attenuation deviation. obtain.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of one conventional electronic AV circuit, Figure 2 is a circuit diagram showing the basic configuration of the circuit in Figure 1, and Figures 3a, b, and c are circuit diagrams of various switches. A diagram showing the operating principle of the docapacitor, Fig. 4, is the principle circuit of another conventional electronic AV circuit.
FIG. 5 is a basic circuit diagram of an electronic AV circuit according to the present invention, FIG. 6 is a circuit diagram of an electronic AV circuit as an embodiment of the present invention, and FIG. 7 is a diagram of FIGS. 6 is a diagram showing attenuation characteristics of the circuit of FIG. 5. FIG.

Claims (1)

[Claims] 1. In an electronic variable attenuation circuit composed of a switched capacitor type integrator using a switched capacitor in the input section and the feedback section, the switched capacitor can be selected from several types and inserted. , the switched capacitor used in the feedback section is formed of a stray capacitor-free type and a positive phase type, and a part of the output of the first inverting amplifier from which the output signal is taken out is passed through the second inverting amplifier. An electronic variable attenuation circuit characterized in that the feedback is returned to the feedback section.