From: apogosso@tpg.com.au   
      
   "flipper"  wrote in message   
      
   > Ok, while looking at it I see what you were getting at. Loop it 1 and   
   > get the 'gain' by lowering R2. Or, more accurately, with lower R2 we   
   > don't need gain.   
      
   Alex:   
   That is correct. Loop gain increases, output voltage swing of the LM358   
   reduces, which is good, concerning that LM358 has quite a limited slew rate.   
      
   > Soooo, I've taken a little more time to look at it and have some new   
   > versions up. And since I've got a 5V supply I just ran that to the   
   > differential pair bias and adjusted idle to match the slightly lower   
   > voltage.   
      
   Alex:   
   Now when the voltage swing on the base of Q1 is 0.5V, you can reduce bias   
   voltage on the bases of the diff pair furter -- to have more headroom for   
   the modulated RF on the collector.   
      
   > I also did a 'stripped' version taking out the seemingly redundant RF   
   > caps. Spice indicates R3 might be useful to drop Q but I upped it to   
   > 2.2k. Although, I'm not really simulating actual Q on the inductor and   
   > caps up there so it might not be needed in the final.   
      
   Alex:   
   It is a pity to drop Q by a resistor. The resistor wastes precious milliwats   
   of the output power into useless heat. To control Q without shunt resistors   
   you can do the following, as I alredy hinted:   
   - create an LC tank tuned to your desired transmission frequency with   
   relatively large L and relatively small C of about 100pF;   
   - make taps on the inductor, ideally each next tap at about 70% of the   
   number of turns of the previous tap. For instance, first tap at 70% of the   
   total number of turns Nt; second tap at 49% of the Nt, third -- at 35% of   
   Nt, etc. Last tap should be at about 5%. No need to make lower taps;   
   - connect your aerial to the tap which would give optimum Q. The higher the   
   tap, the lower the Q because the antenna loads the tank by its effective   
   radiation resistance;   
   - lastly, connect collector to a highest tap which will still give unclipped   
   modulated signal (monitor by a scope).   
      
   The above procedure will virtually create a good versatile matching circuit.   
   If your antenna is short, do not be surprised to have as much as 200V of Rf   
   on it! If your antenna is a loop running around a room along the skirting   
   board, then probably  it will be connected to low taps, but the current in   
   it might reach 200mA or so.   
      
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