On 10/19/2011 2:08 PM, Michael J. Mahon wrote:
Wow! That seems inordinately beefy for a video output transistor. ;-). 10W? And 300v seems like a lot for a transistor stage that only needs to drive a CRT grid/cathode... Sounds like a significant overdesign.
Not as much as one would think. A voltage of 200 V is quite typical for driving a CRT. In order to have good frequency response you need to use a relatively low load resistor - say 10K. So we have 20 mA of current and up to 4 W heating up the transistor. Add a little safety margin and you get a circuit that works for 25 years till someone makes a short :)
So it's a bit of an overkill, but reasonable. Medium power transistors are cheap.
-Alex.