ultrasonic transmitter H bridge

[reyntjensm]

So i'm trying to design an ultrasonic transceiver... I want to use a H bridge made from a few transistors to double the voltage across the transducer( this is R7 in my schematic since i couldn't find a good spice model for the transducer). The power supply voltage is 12V. My signal source has a 5V square wave at 40 kHz. I have already tried different configurations but i always encounter the same problem: So with a 5V input signal. My H bridge goes from 6v to -12V. As soon as i have a 5,5V input square wave the H bridge goes from 12V to -12V, this is what i want to achieve. I could use an extra transistor to generate a 12V square wave from the 5V square wave. But i don't understand why the circuit only functions as expected with a 5,5V signal and not with a 5V signal? I can't find the error, any suggestions?

[Capernicus]

whats the 100k resistor doing on the output line of the h-bridge?   isnt that where the ultrasound emitter would go?

[Berni]

This arrangement of transistors needs a 0 to 12V input signal to work. That is why the extra transistor is needed shifting the 5V signal to 12V.

Also careful with using this circuit at higher powers since cross conduction could become a issue.

[magic]

When the input is near 6V, both transistors are receiving about 0.5mA base current and conducting 100mA.
It's a crapshoot where the output of the bridge will be.

Furthermore, if you built this circuit IRL, it would surely blow up. Not only on the left side where it is driven with 5V, but possibly also on the right when it crosses 6V during switching.

[jesuscf]

It looks to me that both Q1 and Q4 may be on at the same time.  It is not a good idea to just place resistors between the bases of the NPN and PNP transistors on the sides of the H-bridge and expect to have one transistor cut off and the other saturated with an arbitrary logic voltage.  A better configuration uses two extra transistors as shown in the attached diagram.  Since your source voltage is 12V, you may be able to just use a CMOS inverter IC (like the CD4069) configured as an H-bridge to drive the ultrasonic transducer... it is way simpler and most likely cheaper.