JPH0946787A - Loudspeaker driving circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To operate a circuit without permitting a large current to flow and to directly apply a digital signal by inputting the signal to a loudspeaker via a pulse width modulation(PWM) waveform generation circuit and an inverter. SOLUTION: One terminal of the loudspeaker 13 is grounded 14, and the other terminal is connected to the output of a first inverter 3. Moreover, the input of the first inverter 3 is connected to the output of the PWM waveform generation circuit 2, and a digital value in accordance with loudspeaker output is inputted from an input terminal 1 to the PWM waveform generation circuit 2. When a PWM waveform is outputted from the first inverter 3, an output impedance is extremely lowered. In this way, since the loudspeaker 13 is driven with a low impedance, the loudspeaker is driven without permitting the large current to flow on the inverter 3, which makes a loudspeaker driving circuit into low power consumption. Moreover, an audio signal is fetched from the loudspeaker 13 by directly inputting the digital signal to a PWM waveform generator 2 without providing a D/A converter.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention provides a circuit configuration for simplifying a speaker drive circuit in a circuit such as a D / A converter accompanied by audio output.

[0002]

2. Description of the Related Art Conventionally, a speaker has been driven by an amplifier as shown in FIG. In FIG. 7, reference numeral 12 is a voltage follower amplifier, which is input from the input terminal 11. A speaker 13 has one terminal grounded 14 and the other terminal connected to the output of the amplifier 12.
In FIG. 7, when an analog signal is applied to the input terminal 11, the analog signal is amplified by the amplifier 12 having a very large input resistance and a very small output resistance to drive the speaker 13.

Generally, since the input impedance of the speaker 13 is as low as 4Ω to 32Ω, a special amplifier 12 having a low output impedance and a high load driving ability is required. However, the CMOS-amplifier output transistor is shown in FIG.
It operates in the saturation region of the transistor characteristics shown in.

In FIG. 8, Isd is a source / drain current, Vsd is a source / drain voltage, Vsg is a source / gate voltage, and FIG. 8 is a source / gate voltage Vsg.
The characteristic of the source / drain current Isd with respect to the source / drain voltage Vsd is shown by using as a parameter. I in FIG.
As shown by the −V characteristic, the drain conductance (gd) of the transistor characteristic in the saturation region (slope of the IV characteristic) is small, and the output impedance of the transistor, which is the reciprocal of gd, is large. In order to reduce the output impedance, it is necessary to flow a large current through the transistor of the output circuit in the amplifier 12 operating in the saturation region.
The conventional speaker drive circuit as described above has a drawback that the current consumption increases. When applied to equipment that converts digital signals to analog signals and outputs to speakers,
The digital signal has once been converted into an analog signal and then input to the input terminal 11 of the circuit shown in FIG. 7 to drive the speaker 13, which has the drawback of requiring a D / A converter. .

FIG. 9 shows an example of a conventional circuit for driving a speaker differentially. The output of an inverting amplifier including a gain 15 amplifier 15 and resistors 16 and 17 is connected to a terminal corresponding to the ground 14 side in FIG. 7, and the input is connected to the input terminal 11. As a result, differential signals are applied to both terminals of the speaker 13. This circuit has exactly the same drawbacks as the conventional example of FIG.

[0006]

As described above, in the conventional circuit shown in FIG. 7, it is necessary to flow a large current through the transistor in order to operate in the saturation region, and it is not possible to directly apply a digital signal. It was not possible, and it was necessary to once convert it into an analog signal with a D / A converter.

Also in the case of the differential type shown in FIG. 9, the same drawbacks as in the circuit shown in FIG. 7 cannot be avoided.

An object of the present invention is to provide a speaker driving circuit which can operate without passing a large current and can directly apply a digital signal.

[0009]

SUMMARY OF THE INVENTION A speaker drive circuit according to the present invention is configured such that one terminal of a primary winding of a speaker is grounded and the other terminal is connected to an output of a first inverter. Is connected to the output of a PWM (pulse width modulation) waveform generation circuit, and a digital signal is input to the input of this PWM waveform generation circuit.

The output of the first inverter is connected to one terminal of the speaker, the output of the second inverter is connected to the other terminal, and the output of the PWM waveform generating circuit is connected to the input of the first inverter. The output of the third inverter is connected to the input of the second inverter, the output of the PWM waveform generating circuit is connected to the input of the third inverter, and the digital signal is input to the input of the PWM waveform generating circuit. It is a thing.

[0011]

In the present invention, when a digital signal is applied to the PWM waveform generation circuit from the input terminal, it is converted into an analog signal by the PWM waveform generation circuit, and the speaker is driven thereby.

[0012]

【Example】

[Embodiment 1] FIG. 1 is a circuit diagram showing a first embodiment of the present invention. One terminal of the speaker 13 is grounded 14 and the other terminal is connected to the output of the first inverter 3. further,
The input of the first inverter 3 is connected to the output of the PWM waveform generating circuit 2, and a digital value corresponding to the speaker output is input from the input terminal 1 to the PWM waveform generating circuit 2.

FIG. 2 shows a more specific circuit configuration of FIG. In FIG. 2, the first inverter 3 is composed of a simple CMOS inverter circuit including a PMOS transistor 31 and an NMOS transistor 32. In addition,
33 indicates a power source.

FIG. 3 shows an example of a PWM output waveform for an input digital value of 2 bits. That is, in FIG.
When the number of bits of the sampling time is 8 bits and the 2-bit digital signals 00, 01, 10, and 11 are input, the waveform output from the PWM waveform generation circuit 2 gradually increases in width only as shown in the figure. It is shown that D-Clock is a digital clock,
PS-Clock indicates a preset clock.

The PWM waveform is a pulse waveform having only two levels, the ground level and the power supply voltage level, in the voltage axis direction,
It is a waveform that changes the pulse width according to the input digital code. When this waveform is output from the first inverter 3, when the inverter output is at the power supply voltage level, the PMO
The voltage between the source and drain of the S-transistor 31 becomes almost 0V, and the PMOS transistor 31 operates in the non-saturation region shown in FIG. 8, so that the output impedance becomes very low. Further, when the inverter output is at the ground level, the voltage between the source and drain of the NMOS transistor 32 becomes almost 0V, and the NMOS transistor 32 operates in the non-saturation region, so that the output impedance becomes very low. As described above, by using the PWM waveform and the first inverter 3, the speaker 13 can be driven with low impedance. Therefore, P, NMOS transistor 3
It is possible to drive the speaker without supplying a large current to the terminals 1 and 32, and it is possible to reduce the power consumption of the speaker drive circuit. Furthermore, when a digital audio signal is converted into a digital signal of about 1 to 3 bits by using a noise shaping circuit or the like, the digital signal is directly PWM-converted.
By inputting to the waveform generator 2, a voice signal can be taken out from the speaker 13 without a D / A converter.

The sampling period is, for example, 1 MHz.
z to 10 MHz, and the number of bits is 2 to 4 bits. As the speaker, various types such as a voice coil type and a condenser type can be adopted. Further, normally, a digital signal is input to the input terminal 1 after passing through the noise shaving circuit.

[Second Embodiment] FIG. 4 is a circuit diagram showing a second embodiment of the present invention. Ground 14 of Example 1 shown in FIG.
The output of the second inverter 4 is connected to the side terminal, the output of the third inverter 5 is connected to the input of the second inverter 4, and the input of the third inverter 5 is connected to the PWM waveform generating circuit 2
It is configured to connect to the output of. The operation is the same as that of the embodiment shown in FIGS. 1 and 2, but the in-phase noise component is canceled and the noise is reduced by adopting a configuration in which both terminals of the speaker 13 are driven by a differential PWM signal.

FIG. 5 is a more specific circuit diagram of FIG. 4, in which the second inverter 4 is similar to the first inverter 3.
It can be configured with a simple CMOS inverter circuit. That is, 41 is a PMOS transistor, 42 is an NMOS transistor, and 43 is a power supply. Since the third inverter 5 is included, the first inverter 3 and the second inverter 4 work differentially.

In FIG. 6, the input of the PWM waveform generating circuit 2 is 2
An example of a circuit in the case of bit is shown. This circuit is a circuit that generates the PWM waveform shown in FIG. PW can be easily obtained by decoding the input digital value, converting the decoded value to parallel to serial, and outputting this to the shift circuit.
It is possible to generate M waveforms.

In FIG. 6, 21 is a decoder, 22 is a preset clock input terminal, 23 is a digital clock input terminal, 24 is a PWM waveform output terminal, 25 is a "1" input terminal, and 26 is a "0". Input terminal 27 is a "0" input terminal, and FF1 to FF4 and FF11 to
The FF 14 is a flip-flop and constitutes a shift circuit. P in each flip-flop is a preset terminal, P
C is the input terminal of the preset clock, D is the input terminal, Q
Is an output terminal, and C is a clock terminal.

The operation will be described. Flip-flops FF1 to FF4 and FF11 to FF at the timing "1" of the preset clock PS-Clock shown in FIG.
14 presets the data of the decoder 21 and the input terminals 25 to 27 in the output terminal Q.

Next, the digital clock D-Clock
At each rising timing of FF1 → FF2 → F
F3 → FF4 → FF11 → FF12 → FF13 → FF1
The operation of sequentially transmitting the contents of each flip-flop in the order of 4 is performed. As a result, the flip-flop F
At the output terminal 24, the decoder outputs given in parallel to the F1 to FF4 and the data at the input terminals 25 to 27 can be converted into a serial waveform. At this time, if the data to be given to the preset terminal P is decoded so that the data to be given in parallel is outputted as PWM waveforms of "00" to "11" in FIG. 3, the PWM waveform shown in FIG. 3 can be easily obtained. It is output to the output terminal 24.

[0023]

As described in detail above, according to the present invention, one terminal of the speaker is grounded, the output of the first inverter is connected to the other terminal, and the input of the first inverter is PWM.
Since the output of the waveform generation circuit is connected and the digital signal is input to the input of the PWM waveform generation circuit, there is an advantage that an amplifier is not required and the speaker can be driven by a simple CMOS inverter. Therefore, in the conventional technique, a large current has to be passed through the amplifier for driving the speaker, but the use of the present invention does not require a large current to flow, and the advantage is that it is possible to reduce the power consumption of the speaker driving circuit. Have Furthermore, in order to configure an amplifier, a capacitive element for performing phase compensation is required, and a capacitive manufacturing process is required in the LSI manufacturing process. However, when the present invention is used, the capacitive element is not necessary, and a normal LSI manufacturing process is required. It has the advantage that it can be manufactured in.

Further, according to the present invention, the output of the first inverter is connected to one terminal of the speaker, the output of the second inverter is connected to the other terminal, and the input of the first inverter is PWM.
The output of the waveform generating circuit is connected, the output of the third inverter is connected to the input of the second inverter, and the output of the PWM waveform generating circuit is connected to the input of the third inverter.
Since a digital signal is input to the input of the M waveform generation circuit, in addition to the above advantages, the speaker uses a differential PWM.
Since it is driven by a signal, the in-phase noise component is canceled out, and the effect of noise reduction is achieved.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a more specific circuit example of the embodiment of FIG.

FIG. 3 is a diagram showing an example of a PWM waveform used in the present invention.

FIG. 4 is a circuit diagram showing a second embodiment of the present invention.

5 is a diagram showing a more specific circuit example of the embodiment of FIG.

FIG. 6 is a diagram showing an example of a PWM waveform generation circuit used in the present invention.

FIG. 7 is a diagram showing an example of a conventional speaker drive circuit.

FIG. 8 is a current-voltage characteristic diagram of a CMOS-amplifier output transistor.

FIG. 9 is a diagram showing a differential type example of a conventional speaker drive circuit.

[Explanation of symbols]

 1 Input Terminal 2 PWM Waveform Generator 3 First Inverter 4 Second Inverter 5 Third Inverter 13 Speaker 14 Ground 31 PMOS Transistor 32 NMOS Transistor 33 Power Supply 41 PMOS Transistor 42 NMOS Transistor 43 Power Supply

Claims (2)

[Claims] 1. A speaker is grounded at one terminal, the output of a first inverter is connected to the other terminal, and the output of a PWM waveform generating circuit is connected to the input of this first inverter. A speaker drive circuit characterized by inputting a digital signal to an input. 2. The speaker has one terminal connected to the output of the first inverter, the other terminal connected to the output of the second inverter, and the input of the first inverter connected to the output of the PWM waveform generating circuit. , The output of the third inverter is connected to the input of the second inverter, the output of the PWM waveform generating circuit is connected to the input of the third inverter, and the digital signal is input to the input of the PWM waveform generating circuit. A speaker drive circuit characterized by: