JPH06120744A - Digital amplifier distortion reducing circuit - Google Patents

Abstract

PURPOSE:To reduce an overshoot generated accompanied with the switching of a current, in a digital amplifier in which the switching of large current can be, attained at a high speed in the cycle of a PCM, the signal width of the PCM is converted into an amplitude, and a load is driven. CONSTITUTION:The digital amplifier prepares a pulse width modulated signal obtained by converting a pulse code modulated signal, operates the switching of a power source in the cycle of the pulse width modulated signal, converts the width of the pulse width modulated signal into the amplitude, and prepares the large current to be impressed on the load. This amplifier is equipped with a variable resistance element 1 provided before the switching of the load, which changes the resistance, and adjusts the current. Moreover, the circuit is equipped with a resistor 2 provided behind the load, which detects the overshoot of the large current generated at the load, and a control circuit 3 which judges the presence or absence of the generated overshoot, decreases the resistance of the variable resistance element 1 when the overshoot is absent, and increases the resistance when the overshoot is present.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital amplifier for switching a large current at high speed in a cycle of PCM and converting a signal width of the PCM into an amplitude to drive a load. It relates to reducing the overshoot that occurs.

[0002]

2. Description of the Related Art Conventionally, when a digital signal is reproduced by an audio device as such a technique, a pulse code modulation signal (PCM) is converted into a pulse width modulation signal (PWM), and the pulse width modulation is an analog signal. In some cases, the analog signal is converted into an analog signal and is input to an amplifier or the like having a normal linear characteristic to drive a speaker. On the other hand, there is a digital amplifier improved in terms of configuration and performance. In this digital amplifier, a load, for example, a large current flowing through a speaker is switching-controlled at high speed at a rate of ON / OFF time of a pulse width modulation signal converted from a pulse code modulation signal, and a pulse width is converted into a pulse amplitude. .

[0003]

However, in the conventional digital amplifier, resonance occurs when a large current is switched at high speed due to the inductance forming the speaker and the capacitors forming the other circuits. Shoot occurs. This overshoot distorts the waveform of the drive current that drives the speaker, and as a result, the process of delaying the switching time was performed in order to remove the signal waveform distortion and obtain a sufficient effect. There was a problem of sacrificing to some extent.

In view of the above problems, it is an object of the present invention to provide a digital amplifier distortion reduction circuit capable of eliminating overshoot by switching and reducing distortion.

[0005]

In order to solve the above-mentioned problems, the present invention switches the power supply at the cycle of the pulse width modulation signal obtained by converting the pulse code modulation signal and converts the width of the pulse width modulation signal into the amplitude. Then, a variable resistance element, a resistor, and a control circuit are provided in a digital amplifier that forms a large current to be applied to the load.

The variable resistance element uses a transistor as a variable impedance, is provided in a stage before switching to the load, and changes the resistance to adjust the large current. The resistor is provided after the load and detects a large current overshoot generated in the load. The control circuit determines the presence or absence of overshoot generated in the resistor, adjusts the base current of the transistor forming the variable resistance element, and reduces the resistance of the variable resistance element when there is no overshoot. When there is the overshoot, the variable impedance of the transistor is adjusted so as to increase the resistance of the variable resistance element.

[0007]

According to the digital amplifier distortion reduction circuit of the present invention, the variable resistance element is provided before the switching to the load, whereby the resistance is changed to adjust the large current, The resistor is provided in the latter stage of the load so that an overshoot of a large current generated in the load is detected. In the control circuit, the presence or absence of the overshoot generated by the load is detected by the signal detected by the resistor. When there is no overshoot, the resistance of the variable resistance element decreases, and when there is the overshoot, the resistance of the variable resistance element increases. The signal detected by the resistor is fed back to the variable resistance element, and the variable resistor can prevent the output waveform from being distorted due to overshoot generated each time the load is turned on and off, and can improve the distortion characteristic of the digital amplifier. Became.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a digital amplifier distortion reduction circuit according to an embodiment of the present invention. First, the digital amplifier which is the object of the present invention will be described. A digital amplifier 100 shown in this figure includes a PCM / PWM converter 101 for converting a pulse code modulated signal into a pulse width modulated signal, and a PCM
A driver unit 102 that is connected to the PWM conversion unit 101 and drives a switch described later, and switches 103, 104, 105, and 10 that are controlled by the driver unit 102 to form a bridge, one of which is connected to a power source and the other of which is grounded.
6 and a load 107 connected to the switches 103, 104, 105 and 106, which reproduces an electric signal by a large current switched at a high speed, for example, a coil 107 having an inductance ZL constituting a speaker. Here, the switches 103, 104, 105 and 106 are composed of transistors and field effect transistors (FETs). The coil 107 is provided with a low pass filter (not shown).

FIG. 2 is a diagram showing a signal waveform of a main part of the digital amplifier shown in FIG. The pulse code modulation signal is converted into a pulse width modulation signal by the PCM / PWM converter 101 at a carrier cycle as shown in FIG. If the "H (high)" time of this pulse width modulation is long, the ON time of the switch 103 and the like in the driver unit 102 becomes long, and the long pulse is integrated by the low-pass filter provided in the coil 107. As a result, the amplitude of the output signal applied to the load increases. Conversely, "L
When the time length of “(low)” becomes long, the driver unit 102
Then, the off time of the switch 103 and the like becomes long, and the amplitude of the output signal applied to the load becomes small due to the integration of the low-pass filter. In the waveform of the solid line part of this figure (c), the switches 103 and 106 are ON, and the switches 104 and 105 are
Is turned off, and conversely, the waveform of the dotted line portion in FIG. 7C shows that the switches 103 and 106 are off and the switch 1
The case where 04 and 105 are on is shown.

The digital amplifier distortion reduction circuit shown in the figure is provided between a switch 103, 104, 105 and 106 and a power source and a variable resistance element 1 capable of varying resistance, a switch 103, 104, 105 and 106 and A resistor 2 provided between the resistor 2 and the ground, and a control circuit 3 that detects a voltage fluctuation of the resistor 2 and controls the resistance of the variable resistance element 1 are provided.

FIG. 3 is a diagram showing the configuration of the digital amplifier distortion reduction circuit of FIG. The variable resistance element 1 that constitutes the digital amplifier distortion reduction circuit 10 shown in the figure comprises a pnp transistor 11, the emitter of which is connected to the power supply, and the collector of which is connected to the coil 103 which is a load such as the switch 103 and the like. When the voltage VCE across the collector changes, its resistance changes.

The control circuit 3 has a resistor 12, one of which is connected to a power source and the other of which is connected to the base of the transistor 11, and resistors 13 and 14 each of which is connected to the other of the resistor 12 and its resistors. The emitter is the resistor 13
A pnp transistor 15 having its base connected to the other end of the resistor 14, and a plurality of diodes 16 having their anodes connected to the collector of the pnp transistor 15 and their cathodes grounded to form a reference voltage. And its anode is a pnp transistor 1
5 is connected to the base and its cathode is the resistor 2
Connected to.

Next, the operation of this digital amplifier distortion reduction circuit will be described. It is assumed that the base current of the transistor 11 is controlled by the transistor 15 and the large current of the collector of the transistor 11 is controlled. When no overshoot occurs in the coil 107 which is a load, the voltage VCE between the emitter and collector of the transistor 11 is small, that is, the internal resistance between them is small. When an overshoot occurs in the coil 107, a part of this overshoot is fed back to the base of the transistor 11 to increase the voltage VCE between the emitter and collector of the transistor 11, that is, the internal resistance thereof. Thus, the feedback control from the control circuit 3 to the base current of the transistor 11 is performed by the control circuit 3. The reference voltage V of the diode 17 in the control circuit 3
With f, the base voltage of the transistor 15 is adjusted in consideration of the voltage drop Zx I due to the normal current I (without overshoot) flowing through the resistor 2 so that the transistor 15 becomes substantially saturated. The base current of the transistor 11 is controlled by the collector current of the transistor 15 and the transistor 11 becomes substantially saturated, the voltage between the emitter and the collector of the transistor 11 decreases, and the internal resistance of the transistor 11 decreases. A large collector current flows through this. Switch 103
When an overcurrent due to overshoot occurs in the coil 107, which is a load, the voltage across the resistor 2 increases, which reduces the current flowing through the resistor 14 and reduces the voltage between the base and emitter of the transistor 15. As a result, the forward voltage decreases, and the collector current of the transistor 15 decreases. This decrease results in a decrease in the base current of the transistor 11, and accordingly the voltage VBC between the base and collector of the transistor 15 increases. Therefore, the internal resistance of the transistor 15 rapidly increases with the occurrence of overshoot. . Therefore, the collector current of the transistor 15 is reduced. Diode 1
The voltage formed by 6 prevents the breakdown between the base and collector of the transistor 15 due to the increase in voltage when the resistor 2 overshoots.

FIG. 4 is a diagram showing the reduction and decrease of overshoot according to this embodiment. According to this embodiment, this figure (a)
The signal waveform to the load having the overshoot as shown in FIG. 6 becomes a signal waveform without the overshoot as shown in FIG. 3B, and a digital amplifier without distortion can be realized. Note that, in FIG. 1, the digital amplifier distortion reduction circuit according to the present embodiment may be used in the configuration without the switches 104 and 105 in this figure. Further, a case where one switch is provided will be described.

FIG. 5 is a diagram showing a modification of the present embodiment, which is a configuration for detecting overshoot by utilizing the characteristics of a transistor which is a switch, and FIG. 6 is a diagram showing characteristics of the transistor used in FIG. Is. As shown in FIG. 5, a coil 10 that is a load connected to the variable resistance element 1
A switch 108, which is turned on and off by the driver unit 102 and includes an npn transistor, is connected to the other side of the switch 7. The collector of the transistor of this switch 108 is connected to the control circuit 3, and the change in voltage thereof is judged by the control circuit 3. As shown in FIG. 5, when the collector current IC increases, the transistor constituting the switch 108 has a voltage VCE between the collector and the emitter.
Is also increasing. Due to the occurrence of the overshoot IC with respect to the normal collector current IC when the transistor is on, the collector current IC changes greatly, and the voltage VCE between the collector and the emitter increases accordingly, and this voltage change is judged by the control circuit 3. Then, similarly to the above, the resistance of the variable resistance element 1 is controlled to increase.

Although the present invention has been described for the purpose of reducing distortion, it can be applied not only to the speaker but also to the control of the motor as well as the application of limiting the overcurrent.

[0017]

As described above, according to the present invention, the resistance is changed before the switching to the load to adjust the large current, the overshoot of the large current generated in the load is detected, and the overshoot of the overshoot is detected. The detection signal is fed back to judge the presence or absence and reduce the resistance when there is no overshoot, and increase the resistance when there is overshoot, so there is an overshoot that occurs each time the load is turned on and off. It is possible to prevent the output waveform from being distorted and improve the distortion characteristics of the digital amplifier.

[Brief description of drawings]

FIG. 1 is a diagram showing a digital amplifier distortion reduction circuit according to an embodiment of the present invention.

FIG. 2 is a diagram showing a signal waveform of the digital amplifier shown in FIG.

FIG. 3 is a diagram showing a configuration of a digital amplifier distortion reduction circuit of FIG.

FIG. 4 is a diagram showing an effect of reducing overshoot according to the present embodiment.

FIG. 5 is a diagram showing a configuration which is a modification of the present embodiment and detects overshoot by utilizing characteristics of a transistor which is a switch.

FIG. 6 is a diagram showing characteristics of a transistor used in FIG.

[Explanation of symbols]

 1 ... Variable resistance element 2 ... Resistor 3 ... Control circuit 100 ... Digital amplifier 101 ... PCM / PWM converter 103, 104, 105, 106 ... Switch 107 ... Load

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masahiro Babasaki 1-228 Goshodori, Hyogo-ku, Kobe, Hyogo Prefecture Inside Fujitsu Ten Limited

Claims (1)

[Claims] 1. A digital amplifier for switching a power supply at a high speed in a cycle of a pulse width modulation signal, converting a width of the pulse width modulation signal into an amplitude, and forming a large current applied to a load, the digital amplifier comprising: Variable resistance element (1) which is provided in a stage before switching of the above and which adjusts current by changing resistance, and a resistor (2) which is provided in a stage after the load and detects overshoot of a large current generated in the load. The presence or absence of overshoot occurring in the resistor (2) is determined. If there is no overshoot, the resistance of the variable resistance element (1) is reduced, and if there is the overshoot, the variable A digital amplifier distortion reduction circuit comprising a control circuit (3) for increasing the resistance of a resistance element (1).