The microphone has high sensitivity in the audio range, but in the ultrasonic range, the sensitivity decreases rapidly. The receiver is very sensitive. To prevent overdriving and feedback due to the high sensitivity of the microphone in the audio range, the gain of the amplifier should be low in the audio range and increase with frequency in the ultrasonic range.
The intrinsic noise of the amplifier should be as low as possible. The amplifier below satifies this requirement: The gain at 10 kHz is: 6 dB, at 20 kHz: 24 dB and at 100 kHz: 50 dB.