JP3060611B2 - Differential amplifier circuit with built-in low-pass filter - Google Patents

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 suitable for use in a CD (compact disk) audio device.

[0002]

2. Description of the Related Art Recently, frequently used 1-bit digital-to-analog converters (hereinafter referred to as 1-bit DACs) have been used.
The differential input circuit at the subsequent stage of the PWM (pulse width modulation) circuit includes an integrating circuit including a resistor R and a capacitor C for obtaining an averaged (integrated) analog signal, and a differential circuit using an operational amplifier. An amplifier circuit and a low-pass filter for noise removal using an operational amplifier (hereinafter referred to as LP
F) is generally provided.

[0003]

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

The present invention has been made in view of the above points, and has an excellent differential with a built-in LPF capable of reducing the number of operational amplifiers, simplifying the configuration, reducing the scale of signal processing, and reducing current consumption. It is an object to provide an amplifier circuit.

[0005]

Means for Solving the Problems One input terminal (T)_i1)
To the first and second resistors (R₁, R_ThreeVia a series circuit)
And connected to the non-inverting input terminal of the operational amplifier (2).
The first and second resistors (R₁, R_Three) To the first connector
Capacitor (C₁) Via the output of the operational amplifier (2)
Terminal of the operational amplifier (2).
3 resistors (R _Five) And a second capacitor (C_ThreeAverage)
Ground via the column circuit and the other input terminal (T_i2)
To the first resistor (R₁) And a fourth resistor having the same resistance value.
Anti-arm (R _Two) And the second resistor (R_Three) And the resistance value
An equal fifth resistor (R_Four) Through the series circuit
Connected to the inverting input terminal of the operational amplifier (2).
5 resistors (R_Two, R_Four) Is connected to the first condenser
Sensor (C₁) And a third capacitor having a capacitance value equal to
(C_Two) To ground, and invert the operational amplifier (2)
Connect the input terminal to the third resistor (R_Five) And the resistance value is equal
6 resistors (R₆) And the second capacitor (C_Three)
And a fourth capacitor (C_Four) Parallel circuit
Connected to the output terminal of the operational amplifier (2) through
Rate, cut-off frequency and Q value, and
The feature is to determine the size relationship of DENSA
Low-pass filter built-in differential amplifier circuit.

[0006]

According to the differential amplifier circuit with a built-in LPF of the present invention, the differential amplifier is provided with an active LPF comprising a resistor and a capacitor to which positive feedback is applied. The need for an amplifier is eliminated, the number of operational amplifiers is reduced, the configuration is simplified, the signal processing scale is reduced, and the current consumption can be reduced.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the differential amplifier circuit with a built-in LPF 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, the input signal (voltage) V _A is supplied to the one end connected to the resistor R ₁ to the 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). Capacitor C to a connection point between the resistor R ₁ and the resistor R _3
1 is connected to one end, and the other end is connected to the output terminal of the operational amplifier 2. Further, the arithmetic positive input terminal of the amplifier 2 to the (+), one end of each of the resistor R ₅ and capacitor C ₃ connected in parallel is connected, the other end of each is grounded.

An input signal (voltage) V _B is supplied to one end of a resistor R ₂ connected to an 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
₄ is connected to one end of the capacitor C ₂ to the connection point of the further other end is grounded. A resistor R ₆ and a capacitor C ₄ connected in parallel to the minus input terminal of the operational amplifier 2 are connected.
Are connected to one another, and the other ends are connected to the output terminal of the operational amplifier 2. An output signal (voltage) V _O is obtained from the output terminal through the input terminal T _O.

Next, the operation in the first embodiment will be described.
explain. Here, each resistor (R₁Or R₆) Conden
Sa (C₁Or C_Four) Is R ₁= R_Two= R_A, R_Three
= R_Four= R_B, R_Five= R₆= R_C, C₁= C_Two= C_A,
C _Three= C_Four= C_B, S are multiple variables (Laplace operator)
In this case, a nodal equation is established at each contact point,
When a transfer function is obtained by performing a Lath transform, this transfer function is

[0010]

(Equation 1)

## EQU1 ## 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)

## EQU1 ##

As described above, the resistors R ₁ and R ₂ , the resistors R ₃ and R _{4, the} resistors R ₅ and R ₆ have the same resistance, and the capacitors C ₁ and C ₆ have the same resistance. _2, a positive feedback active LPF is provided to the operational amplifier 2 to the capacitor C ₃ and the capacitor C ₄ and respectively equal capacity, the amplification factor of this circuit, defining a cutoff frequency and Q value, the magnitude value relationship of the elements I have decided.

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

FIG. 2 shows the configuration of the second embodiment. This example is an example in which the present invention is applied to, for example, a CD audio device.
PWM plus / minus signals are supplied to _i1 and _Ti2 , respectively. The 1-bit DAC adopts a PWM (pulse width modulation) method having a highly accurate resolution in the time axis direction.
An integration circuit for converting to an analog signal is required. Also, since the quantization noise is formed from an audible frequency to a high frequency while 16-bit data is compressed 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.
Before the example configuration (Fig. 1), the PWM plus / minus
Signal is input terminal T_i1, T_i2Supplied to one end of each
Resistor R₈, Resistor R₉Along with these
Between the other end of the capacitor C_Five(C_X) Connected product
A dividing 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 the 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)

## EQU2 ## Since the cutoff frequency is set higher than the audio frequency, the transfer function in the audio 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 to R _C.

As described above, for example, when processing a PWM plus / minus signal from a 1-bit DAC in a portable CD audio device, integration and noise removal can be respectively performed. In this case, the number of elements constituting the circuit is reduced, so that signal degradation hardly occurs, the sound quality is improved, the current consumption and the number of components are reduced, and the device can be reduced in size and used for a long time. It will improve.

[0028]

As can be understood from the above description, according to the LPF built-in differential amplifier circuit of the present invention, the active amplifier composed of a resistor and a capacitor that apply positive feedback 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 scale of signal processing can be reduced, and the current consumption can be reduced.

In addition, when the LPF built-in differential amplifying 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 hardly deteriorates and the sound quality is improved. The current consumption and the number of components are reduced, miniaturization and long-term use are possible, and there is an effect that the convenience of carrying is improved.

[Brief description of the 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 the differential amplifier circuit with a built-in LPF according to 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) V _O output signal (voltage)

Claims (1)

(57) [Claims] 1. An input terminal is connected to a non-inverting input terminal of an operational amplifier via a series circuit of first and second resistors, and a connection point of the first and second resistors is connected to a first node. And a non-inverting input terminal of the operational amplifier is grounded through a parallel circuit of a third resistor and a second capacitor, and the other input terminal is connected to the other input terminal of the operational amplifier. 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 are connected to the inverting input terminal of the operational amplifier via a series circuit;
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. Connected to the output terminal of the operational amplifier via a parallel circuit of a sixth resistor having the same value and a fourth capacitor having the same value as the second capacitor, and determining the amplification factor, cutoff frequency and Q value, A differential amplifier circuit with a built-in low-pass filter, wherein the magnitude relationship between each resistor and capacitor is determined.