Hey guys i need to design a simple H-bridge that uses vcc=15V to run the a DC motor and use a 4.2V signal to drive the H-bridge forward and backward. and i've attached a pic of what i've tried so far but i've run into the problem of the top PNP transistors staying constantly on. i know it has got to do with the fact that my PNP emitter voltage is higher than my base voltage and as a result the transistor doesn't turn off.
i'm wondering if anyone could give me any advice on what i could do to the circuit to make it work?
you need to build a decent gate-drive (what you think is the problem is correct).
Since you are working form 15V things are alot easier, a comparator (or an OPAMP wired as a comparator) can change yr 4.5V to a 15V signal needed to drive the PNP.
or you could concider putting down a NPN with a pull-up resistor to 15V and using the collector-point as the drive for the top-PNP (it will be slow at turn-on tho, which is itself is a kind of good thing)
Hey thanks guys. I used one of the H-bridge designs that was suggested to Emantra (i've attached it to see if you guys think there are any problems or disadvantages) and it works fine. I also forgot to mention that cost was a huge factor and hence i wasn't able to use the op-amp solution. also another thing is our vcc supply will vary between 15-24V. thanks for all your guys help i really appreciate it.