Ultrasound Physics

Ultrasound Unipolar Transmitter, Pulser

How it works:
This circuit transmits a short pulse at a low duty factor level, mainly for diagnostic applications. When there is no transmit signal, the MOSFET M1 is off and the capacitor C1 is charged through R1 by HV. When a transmit trigger signal arrives at the gate of M1, it will cause M1 to turn on and C1 will discharge to the load transducer (RL). A negative pulse will be applied on the transducer. The pulse width is determined by the time constant: Τ = RC.

If the trigger pulse width is narrower compared to 5RC, the transmitted pulse width will be determined by the trigger pulse as shown in the above right figure.

R1: provides a charging current to C1 through HV. Its value is determined by the period of the pulse repetition. It has to be small enough to let C1 fully charged before the next transmit. If it is too small, big current will go through it when M1 is turned on, causing C1 not discharge properly to the load, or even the damage of M1.
M1: Three parameters for M1 are important.
- Vds: how much voltage it can handle. This parameter has to be at least 1.2 times higher than your pulse amplitude. For example, if you want to transmit a 100v negative pulse on the transducer, the Vds for M1 should be 120 - 150 v.
- Cgs, tr, tf: Switch speed. These parameters determine how fast M1 can turn on/off. Tr, tf, is determined by Cgs, and testing circuit. With a special designed driving circuit, a faster switch speed can be achieved. Normally, tr/tf should be less than a quarter of your center period. If you excite the circuit at 10MHz, the center period is 100ns, and tr+tf should be less than 50ns.
- RDS(on): the on resistance. It should be less than five percent of the transducer impedance.
C1:
- Voltage tolerance: should be at least 1.5 times higher than HV.
- Capacitance: determined by the pulse width and transducer impedance. The time constant, RC, should be less than 20 percent of the pulse width.