JPH05335885A - Impedance variable circuit and low frequency amplifier circuit - Google Patents

Abstract

PURPOSE:To obtain an impedance variable circuit able to change an input impedance by the control of a DC voltage and the low frequency amplifier circuit employing the impedance variable circuit. CONSTITUTION:The circuit is made up of a buffer amplifier 3 being a noninverting amplifier, a VCA 4, an inverting amplifier 5 and a circuit feeding back a signal to an input terminal of the noninverting amplifier at a pre-stage of the inverting amplifier. The input impedance when viewing a point A is varied by varying a gain of the VCA 4 with a DC control voltage of the VCA 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an impedance variable circuit and a low frequency amplification circuit suitable for use in a sound quality adjusting circuit for varying frequency characteristics used in audio equipment and the like.

[0002]

2. Description of the Related Art A sound quality adjusting circuit such as a graphic equalizer is installed in an audio device or the like. FIG. 4 is a circuit diagram showing the basic configuration of the graphic equalizer. Figure 4
Is basically composed of a series LC resonance circuit and an OP amplifier, and its resonance frequency (f = 1 / 2π (L
C) The frequency to be boosted is determined by ^1/2 ), and the boost amount can be adjusted by the variable resistors (R1 to R5). Therefore, when trying to adjust at multiple frequency points, multiple resonant circuits (L1 and C1 to L5 and C
5) is used.

In particular, in order to resonate at a low frequency,
A large capacity capacitor and inductor are required. Therefore, recently, a semiconductor inductor as shown in FIG. 5 has been used. An emitter follower NPN transistor Q1 is used, and resistors R6 and R7 and a capacitor C are used.
6, the resonance frequency f is determined by f = 1 / 2π (C6.C7.R6.R7) ^1/2 .

That is, the graphic equalizer circuit is composed of a basic circuit as shown in FIG.
At the resonance frequency f determined by the C resonance circuit, FIG.
As shown in (b), the variable resistor R is boosted at the point a side and cut at the point c side. As described above, since the resonance frequency of the graphic equalizer is generally determined by the series LC resonance circuit, in order to change the frequency to be adjusted,
A method of using a plurality of resonance circuits and selecting a frequency to be boosted from the plurality of resonance circuits is used.

Further, in the equalizer circuit used for enhancing the bass, the frequency to be emphasized differs depending on the music source to be reproduced. Therefore, several variable resistors are prepared to freely adjust the sound quality, and each variable resistor is adjusted. Therefore, it was necessary to adjust the boost amount. Furthermore, in order to accurately obtain the desired frequency characteristic, more variable resistors are required, and the circuit becomes expensive.

Therefore, a low frequency amplification circuit as shown in FIG. 7 (a) has been considered. This circuit is a twin T-type bandpass filter (hereinafter referred to as BPF) having a resonance frequency in the low range.
1) and an adder circuit 2. The BPF 1 extracts a low-frequency signal, amplifies it, and adds it to the original signal to create a frequency characteristic in which the low-frequency is boosted comprehensively. BPF1 is a resonance circuit, and its resonance frequency f0
Is f0 = 1 / (2π · C8 · RA) where RA = (R8 · R9 · R10 / (R9 + R10))
Determined by ^1/2 . Therefore, as shown in FIG.
The resonance frequency of the BPF 1 can be changed by changing the resistance value of the variable resistor R9.

[0007]

As described above, since the graphic equalizer controls a plurality of variable resistors, it is difficult to easily obtain a desired frequency characteristic by using a remote control (hereinafter, abbreviated as remote controller). Have difficulty. In order to realize this, there is no alternative but to change the variable resistance by a motor or the like, or to prepare some resistances and sequentially switch those resistances by analog switches, which results in cost increase. Further, also in the low-frequency amplification circuit, since the resonance frequency of the BPF 1 is changed by using the variable resistor, it is difficult to change the resonance frequency to a desired frequency by using the remote controller for the reason described above.

As another method of controlling the variable resistance, there is a method of using a transistor or FET (field effect transistor). However, in the case of a transistor, the resistance value between the collector and the emitter changes greatly with a slight change in the base current, and the characteristic is non-linear, making it unsuitable for control of large amplitude AC signals. is there. Also, F
Although the control voltage range of ET is wider than that of the transistor, it is difficult to faithfully change the resistance value because of the problem of non-linearity and the difficulty of applying bias.

[0009]

In order to solve the above-mentioned problems of the prior art, the present invention provides (1) a buffer amplifier, a variable gain amplifier whose gain can be controlled by a DC voltage, an amplifier, and a feedback impedance element. And consists of
Feedback in which the buffer amplifier, the variable gain amplifier, the amplifier, and the feedback impedance element are connected in series, and the variable gain amplifier is disposed at one of two locations between the buffer amplifier and the amplifier, and the feedback impedance element is disposed at the other side. Provided is an impedance variable circuit characterized in that the impedance seen from the input terminal of the buffer amplifier and the input terminal of the amplifier is changed by forming a loop and changing the DC voltage supplied to the variable gain amplifier. And (2) an audio signal is supplied, the bandpass filter including the variable impedance circuit according to claim 1 for extracting the frequency component of the resonance frequency of the resonance circuit, the output signal of the bandpass filter and the audio signal. Consists of an adder circuit that adds and obtains an output signal with enhanced low-frequency components (3) The low-pass amplification circuit according to claim 2, wherein the low-pass amplification circuit according to claim 2 is supplied with an audio signal and extracts a low-pass frequency component, and the low-pass filter. Among the output signals, a frequency detecting circuit that detects the most existing frequency component and outputs a DC voltage corresponding to the frequency is provided, and the DC voltage controls the resistance value of the impedance variable circuit. The present invention provides a low-frequency amplifier circuit characterized in that the resonance frequency is set to the most existing frequency.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention constitutes a feedback amplifier comprising an amplifier and a variable gain amplifier (hereinafter abbreviated as VCA) whose gain can be controlled by a DC voltage, and by varying the DC voltage of VCA, its input impedance is Provided are an impedance variable circuit that utilizes the change and a low-frequency amplification circuit that uses the impedance variable circuit.

The impedance variable circuit of the present invention will be described below with reference to the accompanying drawings. 1 is a circuit diagram showing a first embodiment of the present invention, and FIG. 2 is a circuit diagram showing a second embodiment of the present invention. FIG. 1A shows a variable resistance circuit according to the present invention. The buffer amplifier 3 is composed of a non-inverting amplifier, the VCA 4, the inverting amplifier 5, and a circuit for feeding back the inverting amplifier (amplifier) 5 to the input terminal of the buffer amplifier 3 described above.

In FIG. 1A, if a resistor (feedback impedance element) Rf is used in the feedback circuit and the gain of this entire circuit is a and the feedback amount is f, the input impedance Ri seen from the input terminal A is 1 /. It becomes (1 + af). Here, the feedback amount f is determined by the gain G of the VCA 4 and the gain b of the inverting amplifier 5 inserted in the feedback circuit, and f = bG.

Therefore, a = 1, b = R12 / R11 = 10,
When Rf = 10 k [Ω], the input impedance Ri
When the gain G of the VCA4 is 1, Ri = Rf / (1 + af) = Rf / (1 + abG) = 10k / 11 = 909 [Ω]. If G is 0.1, Ri = Rf / (1 + af) = Rf / (1 + abG) = 10k / 2 = 5k [Ω], and by changing the gain G of VCA4,
The input impedance viewed from the input terminal A can be changed.

FIG. 1B shows a variable capacitance circuit in which the feedback resistance Rf of the variable resistance circuit of FIG. 1A is replaced with a capacitor Cf (feedback impedance element). Similar to the circuit of FIG. 1A, the input impedance seen from the terminal B is 1
It becomes / (1 + af) times. Therefore, due to the mirror effect,
The capacitance of the capacitor Cf seen from the terminal B is (1 + af) times. Here, b = R12 / R11 = 10, Cf = 1 [μ
F], the input capacitance Ci of this variable capacitance circuit is V
When the gain G of CA4 is 1, Ci = Cf.multidot. (1 + af) = Cf.multidot. (1 + abG) = 1.times.11 = 11 [.mu.F].

If G is 0.1, Ci = Cf.multidot. (1 + af) = Cf.multidot. (1 + abG) = 1.times.2 = 2 [.mu.F], and by changing the gain G of VCA4,
The capacitance seen from the terminal B changes, and the input impedance can be changed. In this way, the input impedance can be changed by changing the DC voltage of the VCA 4 and changing the gain G.

Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 2A shows a non-inverting amplifier 6 and a VCA4.
And a variable amplifier circuit composed of a buffer amplifier 7 of the VCA 4 and a circuit for feeding back the output from the buffer amplifier 7 to the input terminal of the preceding non-inverting amplifier 6. The basic circuit of this electronic resistance circuit is shown in FIG. The input impedance Ri seen from the input terminal C of this circuit can be expressed by Ri = R13 (1 + R14 / (R13R15 / (R13 + R15)).

The output of the non-inverting amplifier 6 of this circuit is shown in FIG.
As shown in (a), the VCA4 and the buffer amplifier 7 are connected to its output.
By inserting the control voltage of VCA4, terminal A
The impedance seen from can be changed. VCA
If the gain of 4 is G, then Ri = (R15 / (R13 (1-G) + R15)). R13 (1 + R14 / (R13.R15 / (R13 + R15))), and R = R13 = R14 = R15, 1 ≧ G ≧ 0, when G = 1, Ri = 3R When G = 0, Ri = (3/2) R.

If R15 in FIG. 2 (a) is replaced by a capacitor C, then R15 = 1 / S.multidot.C (where S = j.omega.), And Ri = (1 / ((1-G) R13・ S ・ C + 1)) ・ (R13 + R14 + R13 ・ R14 ・ S ・ C) If R13.S.C (1-G) >> 1, then Ri = R14 / (1-G) + (R13 + R14) /R13.S.C (1-G) and change the gain G of the VCA4. By
It becomes the characteristic of the variable capacitor.

FIG. 3 is a circuit diagram of a low frequency amplification circuit using the variable impedance circuit of the present invention. Figure 7 (a)
The impedance variable circuit of the present invention is used in the portion R9 of the resonance circuit shown in FIG. In FIG. 3, the same parts as those in FIG. 7A are designated by the same reference numerals, and the description thereof will be omitted. In the circuit shown in FIG. 3, the resonance frequency of the BPF 9 can be varied by controlling the VCA 4 with a DC voltage and changing the input impedance of the variable resistance circuit 8. Therefore, the resonance frequency of the low-frequency amplifier circuit can be changed by the DC voltage, and the resonance frequency can be easily controlled by the remote controller or the like. By adding the output of the BPF 9 in FIG. 3 to the input signal, the frequency characteristic in which the resonance frequency is boosted according to the DC voltage applied to the control terminal of the VCA 4 is obtained. Further, the boost level can be varied by varying and adding the level to be added by another VCA 10. As described above, the resonance frequency and the boost amount can be easily changed by changing the DC voltage applied to the VCAs 4, 10.

Further, by using the above-mentioned low frequency amplification circuit,
FIG. 8 shows a circuit capable of detecting the most existing main frequency among the low frequency components of the input audio signal and automatically setting the resonance frequency of the low frequency amplification circuit to the detected frequency components. Will be explained. In addition, in FIG.
Only the block configuration is shown, and the same portions as those in FIG. 3 are denoted by the same reference numerals and the description thereof will be omitted.

In FIG. 8, the audio signal input to the input terminal is frequency-detected through the addition circuit 2 and the BPF 9 of the low-frequency amplifier circuit described in FIG. 3 and the low-pass filter 12 (hereinafter abbreviated as LPF). It is supplied to the circuit 13. L
The PF 12 has a cutoff frequency set to a frequency slightly higher than the maximum value of the variable resonance frequency of the low-frequency amplifier circuit, and detects a signal from which a high frequency component unrelated to bass enhancement is removed. It is supplied to the circuit 13. The frequency detecting circuit 13 detects the most existing main frequency in the low frequency band of the input audio signal and converts the frequency into a DC voltage. This DC voltage is supplied to the control terminal of VCA4 (shown in FIG. 3) of the variable resistance circuit 8. From this DC voltage, the BPF 9 sets the resonance frequency to the main frequency in the low frequency component in the audio signal.

Next, referring to FIG. 9, the frequency detection circuit 13
A specific circuit of will be described. The frequency detection circuit 13 may be a generally used FM detection circuit. In this embodiment, the frequency detection circuit 13 is composed of a limiter circuit 14, a differentiating circuit 15, a rectifying circuit 16, and a basic circuit block of an LPF 17, and performs signal processing thereof. The process will be described with reference to FIG. The limiter circuit 14 limits the amplitude of the input signal as shown in FIG. 10A to obtain a rectangular wave (FIG. 10B).

The differential circuit 15 at the next stage differentiates the rectangular wave to obtain a differential waveform as shown in FIG. This differential waveform is rectified by the rectifier circuit 16 and only the upper or lower waveform is extracted. Whether to select the upper side or the lower side depends on the control characteristic of VCA4 (shown in FIG. 3) of the variable resistance circuit 11 (the characteristic that the gain increases or decreases when the DC voltage increases). Considering the case of the upper side, the pulse waveform of only this upper side (see FIG.
When (d)) is integrated by the LPF 17, a DC voltage is obtained. As shown in FIG. 10E, a high voltage value is obtained when the frequency is high, and a low DC voltage is obtained when the frequency is low. By utilizing this DC voltage, the resonance frequency of the BPF 9 is changed. Although an FM detection circuit that can be easily configured by an OP amplifier is used here, an F detection circuit using a PLL is used.
Of course, it is also possible to use the M detection circuit.

[0024]

As described in detail above, the variable impedance circuit according to claim 1 of the present invention forms a feedback amplifier using an OP amplifier and a VCA, and changes the DC control voltage of the VCA to change the gain of the VCA. Since the input impedance of the feedback amplifier is changed by controlling, the DC voltage that is easy to control by a remote controller,
It has an effect that the impedance can be changed. Further, the low frequency amplification circuit according to claim 2 uses the impedance circuit as a variable resistance portion of a resonance circuit,
The resonance frequency can be varied by controlling the DC voltage, and the low-frequency amplification circuit according to claim 3 detects the most existing main frequency detected by the frequency detection circuit, Since the frequency information is converted into DC voltage information and supplied to the VCA of the low-frequency amplifier circuit according to claim 2 and the resonance frequency is set to its main frequency, the bass enhancement effect can be sufficiently exerted. It has an extremely excellent effect in practical use.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of an impedance variable circuit of the present invention.

FIG. 2 is a circuit diagram showing a second embodiment of the impedance variable circuit of the present invention.

FIG. 3 shows a first example using the impedance variable circuit of the present invention.
FIG. 3 is a circuit diagram showing the low frequency amplification circuit of FIG.

FIG. 4 is a circuit diagram showing a basic configuration of a graphic equalizer.

FIG. 5 is a diagram showing a semiconductor inductance circuit.

FIG. 6 is a diagram showing a boost cut operation of the graphic equalizer.

FIG. 7 is a diagram showing a low-frequency amplifier circuit using a twin T-type BPF.

FIG. 8 is a second view using the variable impedance circuit of the present invention.
FIG. 3 is a circuit diagram showing the low frequency amplification circuit of FIG.

9 is a specific circuit diagram of the frequency detection circuit in FIG.

FIG. 10 is a signal waveform diagram for explaining the operation of the circuit shown in FIG.

[Explanation of symbols]

 3, 7 buffer amplifier 4 variable gain amplifier (VCA) 5 inverting amplifier (amplifier) 6 non-inverting amplifier (amplifier) Rf resistance (feedback impedance element) Cf capacitor (feedback impedance element)

Claims (3)

[Claims] 1. A buffer amplifier, a variable gain amplifier whose gain can be controlled by a DC voltage, an amplifier, and a feedback impedance element, wherein the buffer amplifier, the variable gain amplifier, the amplifier, and the feedback impedance element are connected in series. A feedback loop in which the variable gain amplifier is arranged at one of two positions between the buffer amplifier and the amplifier, and the feedback impedance element is arranged at the other side is formed to vary the DC voltage supplied to the variable gain amplifier. Thus, the impedance variable circuit, which varies the impedance viewed from the input terminal of the buffer amplifier and the input terminal of the amplifier. 2. A bandpass filter which is supplied with an audio signal and extracts a frequency component of a resonance frequency of a resonance circuit including the variable impedance circuit according to claim 1, and an output signal of the bandpass filter and the audio signal are added. And an adder circuit for obtaining an output signal having an enhanced low-frequency component. 3. A low-pass filter to which a voice signal is supplied and which extracts a low-frequency component, and a main frequency that is most present among the output signals of the low-pass filter is detected and a DC voltage corresponding to the detected frequency is output. 3. The low-frequency amplification circuit according to claim 2, further comprising: a frequency detection circuit for controlling the resistance value of the variable impedance circuit by the DC voltage to set the resonance frequency to the most existing frequency. circuit.