The three transistors each form a three-stage amplification circuit.
R40 is an ultrasonic pickup, or called detector, sensor, is a piezoelectric element. The role is to change ultrasonic waves into electrical signals.
R3 is the bias resistor of BG2, and its function is to provide bias current to BG2 so that BG2 can amplify the weak signal.
R2 is the collector load resistor of BG2, which converts the signal current into a voltage when the signal current flows. It is also the collector supply resistor, and the power supply (VCC) is added to the collector of BG2 through R3.
C7 is a coupling capacitor, which serves to couple out the amplified signal from BG2, and at the same time isolate the DC from the collector of BG2, preventing BG2 and BG3 from interacting with each other
BG2 is in an amplified state under the action of R3 and R2.
The circuit of BG3 is exactly the same as BG2.
C8, D5 and D6 form a voltage doubling detector circuit, which is actually a rectifier. The role of the BG3 amplified output of the AC signal into DC voltage, and get double the AC DC voltage. C8 also play a role in isolating DC.
BG4 is the last stage of the amplifier circuit, because the signal sent to the BG4 has been stronger, so there is no bias circuit, BG4 here should be used as a switch.
Signal flow/principle of operation:
When R40 receives ultrasonic waves, R40 will ultrasonic signals into voltage signals, this signal voltage is added to the base of the BG2, BG2 will be amplified, amplified from the collector output, coupled to the base of the BG3 through C7, amplified by the BG3, amplified from the collector output, was
C8, D5, D6, the voltage doubling detector circuit into a voltage multiplier. D6, C8, D5, D6, the voltage doubling detector circuit into a DC voltage. The DC signal voltage, which is double that of the AC signal voltage, is added to the base of BG3, which is amplified again and sent to the following circuit by P.
When R40 does not receive the ultrasonic wave, there is no AC signal output from R40, BG2 is in static state, and BG3 is also in static state; there is no DC voltage output from the doubling detector circuit composed of C8, D5 and D6, and BG4 is in the unbiased state, which of course means it is in the cut-off state, and there is no signal output.
According to the principle of this figure, BG4 is working in a switching mode, when R40 detects the ultrasonic wave, BG4 saturation, C-E is equal to a short circuit, when R40 does not detect the ultrasonic wave, BG4 cutoff, C-E is equal to an open circuit.