Capacitive load driving circuit, electrostatic transducer, method of setting circuit constant, ultrasonic speaker, display device, and directional acoustic system

An electrostatic transducer, comprising:

• an error amplification circuit, including input terminals, that amplifies a difference between an external input signal provided to one input terminal and a negative feedback signal provided from the following negative feedback circuit to the other input terminal;
• a modulation circuit that pulse-modulates the signal outputted from the error amplification circuit;
• a power switching circuit that switches between a power supply voltage and a ground potential or between a positive power supply voltage and a negative power supply voltage;
• a gate driving circuit that generates a gate driving signal to switch-control a switching element configuring the power switching circuit, from a modulated signal outputted from the modulation circuit;
• a low-pass filter, connected to an output side of the power switching circuit, that removes switching carrier components included in an output signal of the power switching circuit;
• a negative feedback circuit, that performs negative feedback from the output terminal of the low-pass filter, to provide the negative feedback signal to the input terminal of the error amplification circuit;
• and
• an output transformer that boosts an output signal of the low-pass filter and has a primary winding connected to an output terminal of the low-pass filter;
• the electrostatic transducer being connected in parallel with a secondary winding of the output transformer and driven as a capacitive load.

A method of setting a circuit constant, comprising:

• amplifying a difference between an external input signal and a negative feedback signal from an output side by an error amplification circuit;
• pulse-modulating the signal outputted from the error amplification circuit by a modulation circuit;
• switching between a power supply voltage and a ground potential or between a positive power supply voltage and a negative power supply voltage by a power switching circuit;
• generating a gate driving signal to switch-control a switching element configuring the power switching circuit, from a modulated signal outputted from the modulation circuit by a gate driving circuit;
• removing switching carrier components included in an output signal of the power switching circuit by a low-pass filter that is connected to an output side of the power switching circuit;
• performing a negative feedback from the output terminal of the low-pass filter to an input terminal of the error amplification circuit by a negative feedback circuit;
• boosting an output signal of the low-pass filter by an output transformer in which a primary winding is connected to an output terminal of the low-pass filter and a secondary winding is connected in parallel with the electrostatic transducer serving as a capacitive load;
• setting an output circuit formed by the low-pass filter, the output transformer, and a load capacitance so as to have two parallel resonance frequencies f1 and f3 and two serial resonance frequencies f2 and f4 (f1<f2<f3<f4), as viewed from an input side;
• and
• setting a constant of each circuit element configuring the output circuit so that the first parallel resonance frequency f1 of the resonance frequencies matches or approximately matches a rated driving frequency or a carrier wave frequency fd of the electrostatic transducer.

An ultrasonic speaker, comprising:

• an electrostatic transducer that is driven according to a signal in an ultrasonic frequency band;
• a modulator that supplies the electrostatic transducer with a modulated signal which is obtained by modulating a carrier wave signal in the ultrasonic frequency band with an acoustic signal in an audible frequency band;
• the electrostatic transducer that configures the ultrasonic speaker including:
• an error amplification circuit, including input terminals, that amplifies a difference between an external input signal provided to one input terminal and a negative feedback signal provided from the following negative feedback circuit to the other input terminal;
• a modulation circuit that pulse-modulates the signal outputted from the error amplification circuit;
• a power switching circuit that switches between a power supply voltage and a ground potential or between a positive power supply voltage and a negative power supply voltage;
• a gate driving circuit that generates a gate driving signal to switch-control a switching element configuring the power switching circuit, from a modulated signal outputted from the modulation circuit;
• a low-pass filter, connected to an output side of the power switching circuit, that removes switching carrier components included in an output signal of the power switching circuit;
• a negative feedback circuit, that negatively feedbacks from the output terminal of the low-pass filter, to provide the negative feedback signal to the input terminal of the error amplification circuit;
• and
• an output transformer that boosts an output signal of the low-pass filter and that has a primary winding connected to an output terminal of the low-pass filter,
• the electrostatic transducer being connected in parallel with a secondary winding of the output transformer and driven as a capacitive load.

A display device, comprising:

• a projection optical system that projects an image on a projection surface;
• and
• an ultrasonic speaker that reproduces a signal sound in an audible frequency band;
• the ultrasonic speaker, including;
• an electrostatic transducer that is driven according to a signal in an ultrasonic frequency band;
• a modulator that supplies the electrostatic transducer with a modulated signal which is obtained by modulating a carrier wave signal in the ultrasonic frequency band with sound signals supplied from an acoustic sound source;
• the electrostatic transducer that configures the ultrasonic speaker including;
• an error amplification circuit, including an input side and an output side, that amplifies a difference between an external input signal provided to the input side and a negative feedback signal generated from the output side;
• a modulation circuit that pulse-modulates the signal outputted from the error amplification circuit;
• a power switching circuit that switches between a power supply voltage and a ground potential or between a positive power supply voltage and a negative power supply voltage;
• a gate driving circuit that generates a gate driving signal to switch-control a switching element configuring the power switching circuit, from a modulated signal outputted from the modulation circuit;
• a low-pass filter, connected to an output side of the power switching circuit, that removes switching carrier components included in an output signal of the power switching circuit;
• a negative feedback circuit, that performs negative feedback from the output terminal of the low-pass filter, to provide the negative feedback signal to the input side of the error amplification circuit;
• and
• an output transformer that boosts an output signal of the low-pass filter and that has a primary winding connected to an output terminal of the low-pass filter;
• the electrostatic transducer being connected in parallel with a secondary winding of the output transformer and driven as a capacitive load.

A directional acoustic system, comprising:

• an ultrasonic speaker that reproduces a signal sound in a first frequency range of sound signals supplied from an acoustic sound source;
• a reproduction speaker that reproduces a signal in a second frequency range of the sound signals supplied from the acoustic sound source;
• the ultrasonic speaker, including;
• and electrostatic transducer that is driven according to a signal in an ultrasonic frequency band;
• a modulator that supplied the electrostatic transducer with a modulated signal which is obtained by modulating a carrier wave signal in the ultrasonic frequency band with the sound signal in the first frequency range of the sound signals supplied from the acoustic sound source;
• the electrostatic transducer that configures the ultrasonic speaker including:
• an error amplification circuit, including input terminals, that amplifies a difference between an external input signal provided to one input terminal and a negative feedback signal provided from the following negative feedback circuit to the other input terminal;
• a modulation circuit that pulse-modulates the signal outputted from the error amplification circuit;
• a power switching circuit that switches between a power supply voltage and a ground potential or between a positive power supply voltage and a negative power supply voltage;
• a gate driving circuit that generates a gate driving signal to switch-control a switching element configuring the power switching circuit, from a modulated signal outputted from the modulation circuit;
• a low-pass filter, connected to an output side of the power switching circuit, that removes switching carrier components included in an output signal of the power switching circuit;
• a negative feedback circuit, that performs negative feedback from the output terminal of the low-pass filter, to provide the negative feedback signal to the input terminal of the error amplification circuit;
• and
• an output transformer that boosts an output signal of the low-pass filter and that has a primary winding connected to an output terminal of the low-pass filter;
• the electrostatic transducer being connected in parallel with a secondary winding of the output transformer and driven as a capacitive load.

A directional acoustic system, comprising:

• an ultrasonic speaker that reproduces a signal sound in a first frequency range of sound signals supplied from an acoustic sound source;
• and
• a reproduction speaker that reproduces a signal sound in a second frequency range of the sound signals supplied from the acoustic sound source;
• the ultrasonic speaker, including:
• an electrostatic transducer that is driven according to a signal in an ultrasonic frequency band;
• and
• a modulator that supplied the electrostatic transducer with a modulated signal which is obtained by modulating a carrier wave signal in the ultrasonic frequency band with the sound signal in the first frequency range of the sound signals supplied from the acoustic sound source;
• and
• the electrostatic transducer configuring the ultrasonic speaker including an output circuit formed by a low-pass filter, an output transformer, and a load electrostatic capacitance that has two parallel resonance frequencies f1 and f3 and two serial resonance frequencies f2 and f4, f1 being less than f2, f2 being less than f3, and f3 being less than f4, as viewed from an input side.