I made typical ultrasonic transmitter based on 555 timer, and because needed to boost the signal for long distance I have to drive the transmitter with high voltage (up to 40V from the datasheet). for that I used a NPN transistor that can handle the 40V, The R4 is connected to 40V rather than 12V. Now, I'm having the issue that the transistor can not drive the transmitter load the signal become week and distorted. I have attached the circuit, please provide me with some guidance.\

Thank you all in advance.

 

You need to use a pushpull output or transformer like this example, this will give you the maximum voltage swing....

Use TIP121/126 transistors, or equivalent npn/pnp types.

 

Note that in post #1, the output duty cycle from your 555 circuit is only 9%. This will reduce both the output and the lifetime of the transducer. Consider a 50% duty cycle oscillator. This can be capacitor-coupled into a driver stage with two saturated switches for the max possible output and very little device heating.

ak