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Registered Member #2008
Joined: Tue Mar 03 2009, 05:11AM
Location: USA, Frederick, MD
Posts: 118
Thank you for your answers, Mattski. I made mistakes in all 3 questions and now I had somehow answered them myself.
One thing I don't get now is that when you suggested me with a 3 transistor version of the circuit you added a transistor and two resistors into my circuit. 4.7K at the base and 1K to load the transistor. How did you know what values of resistors to put there?
What makes certain resistor values, such as 4.7k, 3.3k, 1k reoccur in every circuit no matter what math may suggest?
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
In this circuit which Avalanche demonstrated since you have a Vin that goes to -3.3V, it doesn't matter what resistors you put because the 1st BJT will be off, so the bipolar pair will be driven with the full 12v. When Vin goes to +3.3V the base currennt will be (3.3-0.7)/4700=550uA. Since typical current gain of small signal BJT (hfe or beta) is about 100, the BJT will try to conduct 55mA. 55mA through 1kOhm will drop 55V, so the collector voltage will drop and this causes the current to reach a steady state value less than 55mA, and the input to the bipolar pair will probably be under 200mV.
You can play around with these numbers a bit and see that even if you switch the resistors, it should still work. So until you optimize for things like high frequency performance or power consumption, you just pick a common 5% resistor that works and is usually in a pile on your workbench.
All about circuits has a section on this type of stuff, searching for online university lecture notes is another good source of info, and Grey and Meyer is a nice reference on transistor circuits, with a chapter specifically on driver circuits.
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Complementary transistor MOSFET driver.
