In general, the ultrasonic diagnostic apparatus contains a transmission circuit for generating a high voltage AC signal in the device, and a high-voltage transmission pulse is transmitted through a cable to excite the ultrasonic transducer in an ultrasonic probe. Furthermore, an ultrasonic wave reflected and returned from a living body is received by the ultrasonic transducer, and a reception signal is likewise transmitted through the cable into the device, separated in a TR separation circuit (transmission/reception separating circuit), transmitted to the reception circuit to be amplified, and then transmitted to an image processor.
The reception signal is generally a very faint signal of about several tens to several hundreds mV. Accordingly, these signals are lowered in signal level due to voltage division caused by parasitic capacitance of the cable, or SNR remarkably deteriorates due to a cable insertion loss or an effect of parasitic resistance of a high-voltage switch. Therefore, it is considered that a preamplifier is provided to a grip end portion in the ultrasonic probe (probe) to amplify a reception signal when the reception signal is transmitted through the cable, whereby an effect of noise can be relatively suppressed.
Patent Document 1 is known as a circuit which has been hitherto devised to provide a preamplifier in an ultrasonic probe.
Furthermore, Patent Document 2 is also an example of a circuit for protecting a preamplifier.