JPS6165513A - Low pass filter - Google Patents

Abstract

PURPOSE:To attain circuit integration with simple circuit constitution by synthesizing a basic signal of an inputted digital signal and its inverting signal with respect to a circuit suitable for a pulse width modulation type D/A converting circuit to provide a filter function. CONSTITUTION:An amplifier 12 is replaced by a resistor 12a and an amplifier 13 consists of a transistor (TR)13a and a resistor 13b. A pulse width modulation signal inputted to an input terminal 11 is outputted to an output terminal 15 in phase via the resistor 12a, and outputted in inverted phase by the TR13a and they are synthesized at a point D via a capacitor 14. Thus, in adjusting the amplification factor of the TR13a by means of a variable resistor 21 so that the pulse width modulation signal outputted via the resistor 12a and the inverted pulse width modulation signal outputted from the TR13a are just cancelled, the filter operation is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a low-pass filter circuit suitable for a pulse width modulation type D/A conversion circuit.

[Prior art and its problems]

Conventionally, the pulse width modulation i D/A conversion circuit is a general KO-
This is done by using a pass filter circuit.

That is, conventionally, a signal (hereinafter abbreviated as a 4-pulse width modulation signal) with different t4 pulse widths (duty) is obtained according to the input desotal signal, and the signal is passed through a low-pass filter circuit whose period is equal to or less than that of the 4-pulse width modulation signal. The signal is extracted as a low-frequency signal or as a low-frequency signal. However, the low-pass filter circuit used in the Knolls width modulation type D/A conversion circuit generally causes a delay, and it is not desirable to utilize this. For example, in order to sufficiently remove ripples, the number of stages of the ronosfilter circuit is required to be large, and the delay time is also large. In addition, it is necessary to configure a circuit using, for example, an active filter for signal processing, which increases the cost and makes it difficult to integrate into an IC.
There was a problem in that it hindered miniaturization.

[Purpose of the invention]

The present invention has been made in view of the above points, and can be integrated into an IC with a simple circuit configuration. The purpose is to provide a bus filter circuit.

[Key points of the invention]

The low-pass filter circuit used in the pulse width modulation type D/A conversion circuit of the present invention has a filter function by synthesizing the basic signal of the input digital signal and its inverted signal. .

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings. First, the basic configuration of the low-pass filter circuit of the present invention will be explained with reference to FIG.

In FIG. 1, reference numeral 11 denotes an input terminal to which a pulse width modulation signal is input. Then, the pulse width modulation signal applied to the input terminal 11 outputs the input signal in the same phase.
The signal is input to the "1" amplifier 12, and is also input to the "-1" amplifier 13, which inverts the phase of the input signal and outputs it. Then, the output of this amplifier 13 is combined with the output of the amplifier 12 through all the capacitors 14, and is output from the output terminal 15. Further, a resistor 16 is connected between this output terminal 15 and ground.

In the above configuration, a pulse width modulated signal 1 (1) with a signal level vl and a pulse period T is now input to input terminal 1, tab, and point A as shown in Fig. 2 (,). Then, this pulse width modulation signal 1 (1) is transmitted to the amplifier 12
The signals are output in the same phase through the amplifier 13, and are output after being phase-inverted by the amplifier 13. Therefore, the output side B of the amplifier 13
At the point, a waveform signal 't (t) obtained by inverting the phase of the input signal is obtained as shown in FIG. 2(b). The DC level of the signal at this point B is the vc point. Therefore, the signal extracted from point B via the condenser 4t, that is, the signal at point 0, has the DC component removed, as shown in FIG. 2(c).
The signal waveform has a level changing between rVi-VcJ and rVeJ with the 0# level as the center. Then, the signal shown in FIG. The signals shown in FIG. 2(,) are combined at point D as shown in FIG. 2(d).

As a result of this synthesis, the mutual noises are canceled and only the DC signal having the signal level of VO is extracted as shown in FIG. 2(,). In the above synthesis, f(t) (f(t) Vc ) =Vc. This VC point indicates that no DC level is transmitted by the capacitor 14, and instead the DC component in the input signal is taken out as the output signal. Therefore, it is shown that when the above-mentioned low-pass filter circuit is used in a Norse width modulation type A/A conversion circuit, the DC level is guaranteed as in normal νA conversion.

Next, a specific embodiment of the present invention will be explained with reference to FIG. In the embodiment shown in FIG. 3, amplifier 12 is connected to resistor 1
2a, and the amplifier 13 is constituted by a transistor 13& and a resistor 13b). Above transistor 1
3m has its base connected to the input terminal 11, and its emitter grounded via the resistor 13b1c. Furthermore, the collector of the transistor 13a is connected to the vc through the variable resistor 21.
c It is connected to the power supply and also the conde.

It is connected to the output terminal 15 via the sensor 14.

In the above configuration, the pulse width modulation signal input to the input terminal 11 is transmitted to the output terminal 1 in the same phase via the resistor 12m.
5, the phase is inverted and outputted by the transistor 131, and the signals are combined at the point D via the capacitor 14. Therefore, the amplification degree of the transistor 13& is adjusted by the variable resistor 21 so that the pulse width modulation signal outputted via the resistor 12hf and the phase-inverted pulse width modulation signal outputted from the transistor 13a are exactly offset. Then, the same filter operation as that described in FIG. 1 is performed.

[Other Embodiments of the Invention] In the embodiment shown in FIG. 1, the outputs of the amplifiers 12 and 13 are combined via the capacitor 14ft, but other embodiments may be used instead of the capacitor 14 as shown in FIG. The filter 22 may also be used for synthesis.

In the embodiment shown in FIG. 1, two amplifiers 12.1
3f is used, however, as shown in FIG. 5, the amplifier 13t may be omitted and the output of the amplifier 12 and the signal obtained via the capacitor 14 may be combined by the subtracter 23.

In this case, the amplifier 121c on the +1j side is omitted, and the -1j side amplifier 121c is omitted.
It is also possible to use the amplifier 13f on the `` side and perform the synthesis using an adder.

〔Effect of the invention〕

As detailed above, according to the present invention, since the filter operation is performed by combining the basic signal of the pulse width modulation signal and its inverted signal, the circuit configuration can be simplified and the delay time can be reduced. Easy to integrate into IC. It is possible to configure this rounding irregularly and to reduce costs.

Further, since the basic signal and its inverted signal are synthesized, even if a disturbance occurs in the input signal waveform, it is canceled out and the output signal waveform is not disturbed.

[Brief explanation of the drawing]

The drawings show an embodiment of the present invention, and FIG. 1 shows the basic circuit configuration, and FIG. 3 and 3 are diagrams showing specific circuit configurations, and FIGS. 4 and 5 are circuit configuration diagrams showing other embodiments of the present invention, respectively. 11...Input terminal, 12.13...Amplifier, 14.
...Capacitor, 15...Output terminal, 22...Filter, 23...Subtractor. Applicant's representative Patent attorney Takehiko Suzue Figure 1 (d) (e) Figure 3, Figure 4 Figure 5, same

Claims (1)

[Claims] In a low-pass filter circuit used in a pulse width modulation type D/A conversion circuit, means for outputting a pulse width modulation signal as a basic signal in the same phase, means for outputting the input signal after inverting the phase thereof, and the above means. 1. A low-pass filter circuit comprising means for synthesizing an output basic signal and its phase inverted signal and outputting only a DC component of an input signal.