VK6ZGO...I like the way you talk. Do they teach tube circuits in school any longer? I was taught tubes first, then transistors. We had to build and analyse all kinds of tube circuits before we studied transistors. Then the difference between voltage and current devices could be compared. But the circuit principles are the same. They are probably using terms I haven't even heard of to explain transistor operation now. Oh well.....they were always screechy.

I doubt it,but I've been away from school for many years.
BJTs were pretty new stuff when I started!

We used to have young blokes coming in with their Diplomas,proud as Punch that they hadn't learned anything about "obsolete tubes",only to be confronted by our big old "Brit-beast"----A Marconi TV Transmitter which was all "hollow-state"

I would assure them it wasn't a problem,as they were just "FETs with a fire in 'em!"

 

I've always wondered what constitutes the appropriate value of inductance for an RF choke. It is different at different frequencies. The reactance is given as....

XL = 2*pi*f*L

... meaning the same inductor has a higher reactance at higher frequencies. In many cases a choke is used to bias a transistor's collector but keep the signal off the power source's rail. This is done by XL. What is a high enough XL? 5K? 10K? ..... 100K?

It's not so much a specific value, but a reactance value compared to what you're powering with it. For instance, in a parallel tuned transistor amplfiier fed with an RF choke, the choke should have about 10x the reactance as the tank coil at the frequency of operation. Hope this helps. Check out the ARRL Handbook for more details.
ERic

 

also, you have to avoid self resonance of the choke. that can bring on all kinds of strange problems.

 

I doubt it,but I've been away from school for many years.
BJTs were pretty new stuff when I started!

We used to have young blokes coming in with their Diplomas,proud as Punch that they hadn't learned anything about "obsolete tubes",only to be confronted by our big old "Brit-beast"----A Marconi TV Transmitter which was all "hollow-state"

I would assure them it wasn't a problem,as they were just "FETs with a fire in 'em!"

http://www.youtube.com/watch?v=EzyXMEpq4qw

 

Yes, what size choke/coil you need depends on the frequency. At 19.8khz they can be pretty big.