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Describes two custom integrated circuits which were developed for an implantable pulsed Doppler ultrasonic blood flowmeter. Prime design goals were a minimum circuit volume, minimum power consumption, and operation at low supply voltages. The first of the two IC's performs system timing functions and produces the ultrasonic transmit burst. It can deliver up to 40 mW of peak output power at 50 percent efficiency and requires 3.7 mW standby power. The second IC, containing an RF amplifier, mixer and output amplifier, provides 54 dB conversion voltage gain for an 0.8 MHz bandwidth centered at 5.8 MHz, <3 dB noise figure, a dynamic range of 40 dB, and 1 μs recovery time from a 1 V overload. This chip requires 2.7 mW power input.