Re: RF power chain mods and improvements..

ajparent1/KB1GMX
 

On Wed, Jul 18, 2018 at 02:42 PM, Nick VK4PLN wrote:
Would you say the 2n2222A were better candidates than 2n5109? that is in regards to flatness of output across the bands?
Where in the world did you get that idea?
Price and availability and poor reading. 
The list was from common and easy that solves many  users needs (good at 80-17M) to exotic
and better (for 20mhz and up) but maybe trickier due to higher FT.

A technical Question/ Learning session if you wouldnt mind Allison/ Others:

So to set the bias of these BJT we are changing a resistor to get a certain amperage.
Lets take the pre-driver #1, Q90, for instance: is the bias set by R81 and R82(voltage divider)? and if so, according to your earlier post we measure the voltage drop across the emitter resistor, and then calculate the mA. Now you state 20-30ma? where does that figure come from? is it different for each BJT we try?
Run Q90 at 30ma and watch it heat up.  Its a ridiculous current to run for a stage that may produce 10mW
and a device that is not designed to run at that current level. (BFR106 datasheet).  I've suggested numbers
for this as UNDER 20ma even with the 3904 (mine with the factory 3904 was running at 33ma!).
To correct the bias, change the base to collector resistor R81. 

The predrivers have to produce a bit more but they are in parallel so 20ma each is fine
Also running more than that is just wasting current to produce heat, not RF.   

The driver in mine (3904s) were idling at 39ma each and the IR temp meter was show them at 145F.
Again its a impractically high current for most devices used in that spot.   Many of the devices have
a sweet spot and its a balance of wasted power as heat and circuit requirements for impedance and 
power out.   So it is both different for each device and also a consideration to keep from cooking it.

However if you think you have a better number...  I'm waiting. 
FYI if you read the whole chain I've repeated myself so many times, I give up.


Allison

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