JPH0522077A - Differential amplifier circuit with built-in low pass filter - Google Patents

Abstract

PURPOSE:To reduce the number of operational amplifiers, to simplify configuration, to reduce signal processing scale and to reduce current consumption. CONSTITUTION:An active low-pass filter(LPF) is composed of resistors R1, R2, R3, R4, R5 and R6 and capacitors C1, C2, C3, C4 and C5 to load positive feedback to an operational amplifier 2 and in this case, the resistance value is made equal respectively for the resistors R1 and R2, resistors R3 and R4 and resistors R5 and R6. Then, the capacity is made equal respectively for the capacitors C1 and C2 and the capacitors C3 and C4, the amplification factor, cut-off frequency and Q value of this circuit are decided, and the value level relation of respective elements is decided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a differential amplifier circuit with a built-in low-pass filter, which is suitable for use in a CD (Compact Disc) audio device.

[0002]

2. Description of the Related Art Recently, a 1-bit digital-analog converter (hereinafter referred to as 1-bit DAC) which is frequently used.
In the differential input circuit in the subsequent stage of the PWM (pulse width modulation) circuit such as, for example, a differential circuit using an operational amplifier and an integrating circuit including a resistor R and a capacitor C to obtain an averaged (integrated) analog signal. A low-pass filter for noise removal that uses an amplifier circuit and an operational amplifier in the same manner (hereinafter referred to as LP if necessary).
(Denoted as F) is generally provided.

[0003]

When the differential amplifier circuit of the above conventional example is used in a CD audio device, at least two operational amplifiers used for the differential amplifier and the LPF are required. In this case, there are inconveniences that the number of parts increases, the configuration becomes complicated, the manufacturing cost increases, and the defect rate in the manufacturing process increases. In addition, the signal processing scale becomes large and the current consumption increases.

The present invention has been made in view of the above points, and the number of operational amplifiers is reduced, the configuration is simplified, the signal processing scale is reduced, and the current consumption is reduced. An object is to provide an amplifier circuit.

[0005]

Means for Solving the Problems One input terminal (T_i1)
The first and second resistors (R₁, R₃) Series circuit
Connected to the non-inverting input terminal of the operational amplifier (2),
1st and 2nd resistor (R₁, R₃) Connection point to the first
Capacitor (C₁Output of the operational amplifier (2) via
And the non-inverting input terminal of the operational amplifier (2) is connected to the
3 resistors (R _Five) And the second capacitor (C₃)
The other input terminal (T_i2)
Is the first resistor (R₁) And the fourth resistance with the same resistance value
Responder (R ₂) And the second resistor (R₃) And the resistance
Equal fifth resistor (R_Four) Through the series circuit
Connected to the inverting input terminal of the operational amplifier (2),
5 resistors (R₂, R_Four) Connection point to the first
Sensor (C₁) And a third capacitor having the same capacitance value as
(C₂) To ground, and the operational amplifier (2) is inverted
The input end is connected to the third resistor (R_Five) With the same resistance
6 resistors (R₆) And the second capacitor (C₃)
And a fourth capacitor (C_Four) Parallel circuit
Connected to the output terminal of the operational amplifier (2) via
Rate, cut-off frequency and Q value are set, and the above resistors and capacitors are
It is characterized in that the size relationship of Densa is decided
Differential amplifier circuit with built-in low-pass filter.

[0006]

According to the differential amplifier circuit with a built-in LPF of the present invention, an active LPF composed of a resistor and a capacitor for positive feedback is provided in the differential amplifier. Amplifiers are not required, the number of operational amplifiers is reduced, the configuration is simplified, and the signal processing scale is reduced, so that current consumption can be reduced.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the LPF built-in differential amplifier circuit of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of the first embodiment. In this example, an input signal (voltage) V _A is supplied to one end of a resistor R ₁ connected to an input terminal T _i1 . One end of the resistor R ₃ is connected to the other end of the resistor R _1, the other end of the resistor R ₃ is connected to the positive input terminal of the operational amplifier 2 (+ non-inverting input terminal). A capacitor C is provided at the connection point between the resistors R ₁ and R _3.
One end of ₁ is connected, and the other end is connected to the output end of the operational amplifier 2. Further, the positive input terminal (+) of the operational amplifier 2 is connected to one ends of a resistor R ₅ and a capacitor C ₃ which are connected in parallel, and the other ends thereof are grounded.

The input signal (voltage) V _B is supplied to one end of the resistor R ₂ connected to the input terminal T _i2 . The resistor R ₂ at the other end to a resistor minus the other end of the resistor R ₄ is an operational amplifier 2 with one end of the R ₄ is connected - is connected to the (inverting input terminal) input. Resistor R ₂ and resistor R
One end of the capacitor C ₂ is connected to the connection point with _4, and the other end is grounded. Further, the negative input terminal of the operational amplifier 2 is connected in parallel with a resistor R ₆ and a capacitor C ₄
Are connected to one end, and the other ends are connected to the output end of the operational amplifier 2. An output signal (voltage) V _O is obtained from this output terminal through the input terminal T _O.

Next, the operation in the first embodiment
explain. Here, each resistor (R₁Through R₆) Conden
Service (C₁To C_Four), R ₁= R₂= R_A, R₃
= R_Four= R_B, R_Five= R₆= R_C, C₁= C₂= C_A,
C ₃= C_Four= C_B, S are multiple variables (Laplace operator)
Then, a node equation is established at each contact in this case, and
When the transfer function is obtained by performing the Lath transform, this transfer function is

[0010]

[Equation 1]

It is represented by This (Equation 1) is expressed by the following general second-order LPF equation.

[0012]

[Equation 2]

In the above equation

[0014]

[Equation 3]

And the Q value is

[0016]

[Equation 4]

[0017]

Thus, the resistors R ₁ and R ₂ , the resistors R ₃ and R _4, and the resistors R ₅ and R ₆ have the same resistance value, and the capacitors C ₁ and C ₂ , a positive feedback active LPF having the same capacitance as the capacitor C ₃ and the capacitor C ₄ is provided in the operational amplifier 2, and the amplification factor, cutoff frequency and Q value of this circuit are determined to determine the magnitude relationship between the respective elements. I have decided.

In this case, the number of operational amplifiers that have conventionally constituted the LPF is reduced, the configuration is simplified, the signal processing scale is reduced, and the current consumption is reduced.

FIG. 2 shows the configuration of the second embodiment. This example is an example in the case of being applied to a CD audio device, for example, from the 1-bit DAC to the input terminal T
PWM plus / minus signals are supplied to _i1 and T _i2 , respectively. The 1-bit DAC uses a PWM (pulse width modulation) method that has a highly accurate resolution in the time axis direction,
An integrating circuit for converting into an analog signal is required. Further, since the quantization noise is formed from the audible frequency to the high frequency while compressing 16-bit data into several bits by using the oversampling technique and the noise shaping technique, an LPF for noise removal is required.

For this reason, as shown in FIG.
The PWM plus / minus signal is added before the example configuration (Fig. 1).
No. is the input terminal T_i1, T_i2Through one end of each
Resistor R₈, Resistor R₉Together with these
Between the other end of the capacitor C_Five(C_X) Connected configurations
The branch circuit 4 is added. In this case, R₈= R₉= R _X
And is composed of a third-order LPF as a whole,
Output terminal T from amplifier 2_OThrough audio output signal
V_AOIs output.

Next, the operation of the second embodiment will be described. The transfer function of this circuit is

[0023]

[Equation 5]

It is represented by Since the cutoff frequency is set higher than the audible frequency, the transfer function in the audible band is

[0025]

[Equation 6]

The gain can be arbitrarily set by the ratio of the sum of R _X , R _A and R _B and R _C.

Thus, for example, when processing the PWM plus / minus signal from the 1-bit DAC in a portable CD audio device, integration and noise removal can be performed respectively. In this case, since the number of elements constituting the circuit is reduced, signal deterioration is less likely to occur, the sound quality is improved, and the current consumption and the number of parts are reduced, which enables downsizing and long-term use, which is convenient for carrying. It will be improved.

[0028]

As can be understood from the above description, according to the differential amplifier circuit with a built-in LPF of the present invention, an active L comprising a resistor and a capacitor in which positive feedback is applied to the differential amplifier.
Since the PF is provided, the number of operational amplifiers is reduced,
This has the effect that the configuration can be simplified, the signal processing scale can be reduced, and the current consumption can be reduced.

In addition, when the LPF built-in differential amplifier circuit is applied to a portable CD audio device or the like, the number of elements constituting the circuit is reduced, so that the signal waveform is less likely to deteriorate and the sound quality is improved. The current consumption and the number of parts are reduced, which enables miniaturization and long-time use, and has the effect of improving the convenience of carrying.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of a first embodiment of a differential amplifier circuit with a built-in LPF of the present invention.

FIG. 2 is a circuit diagram showing a configuration of a second embodiment of an LPF built-in differential amplifier circuit of the present invention.

[Explanation of symbols]

Second operational amplifier _{C 1, C 2, C 3} , C 4, C 5 capacitors _{R 1, R 2, R 3} , R 4, R 5, R 6, R 8, R 9 resistors V _A, V _B input signal (Voltage) _VO output signal (Voltage)

Claims (1)

Claim: What is claimed is: 1. One input terminal is connected to a non-inverting input terminal of an operational amplifier via a series circuit of first and second resistors, and the first and second resistors are connected. Is connected to the output terminal of the operational amplifier via the first capacitor, and the non-inverting input terminal of the operational amplifier is grounded via the parallel circuit of the third resistor and the second capacitor and The input terminal of the operational amplifier is connected through a series circuit of a fourth resistor having the same resistance value as the first resistor and a fifth resistor having the same resistance value as the second resistor. Connect to
The connection point of the fourth and fifth resistors is grounded via a third capacitor having the same capacitance value as the first capacitor, and the inverting input terminal of the operational amplifier is connected to the third resistor and the resistance value. Is connected to the output terminal of the operational amplifier through a parallel circuit of a sixth resistor having the same value and a fourth capacitor having the same value as the second capacitor, and the amplification factor, cutoff frequency and Q value are determined and A differential amplifier circuit with a built-in low-pass filter, wherein the magnitude relationship between each resistor and capacitor is determined.