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#36
March 15th 04, 07:43 AM
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Paul Burridge wrote in message . ..
On 14 Mar 2004 10:01:00 -0800, (R.Legg) wrote: C2's small size (3.3pF)is attenuating any 5th harmonic current by 6db into Q2's base biasing network, in both posted versions. Curious. Can you show some figures to back this claim up? (Not that I don't believe you; just that I'd like to see how you arrived at this view). The reactance of the C2 part is almost 3K at the fifth harmonic. Input impedance of the biasing network is 300 ohms - this is about half the small signal input impedance of the 3904 @4mA. Even with bypassed emitter, only 1/3 of C2's AC current will enter the base of Q2. If the resonant circuit used lower L and higher C values, C2 could be increased without as severe an effect as it has here. Biasing the first stage as classC in the second revision is a pretty drastic change from the previous class A revision (100mW). Don't you believe in tiny steps? I've been trying nothing else *but* "tiny steps" for the last few days. There's no harm in the ocassional quantum jump. :-) As previous posters have stated, if the input is squarish then the harmonics are already there. There is a +/- 3% window on all the optimum duty cycles (ie 10, 30, 50, 70, 90%), including risetime, for which the 5th harmonic amplitude is relatively constant, at about 10% of the initial peak amplitude. Note that the 30/70% period is a median quasi-minima for both 3rd and 4th harmonics, possibly reducing LF filtering problems in the first stage, as the 1st and 2nd are farther away. 50% being available, you should stick to it. I don't know if you're doing any actual physical breadboarding. The 100mW power dissipation suggests not. Pre-apps it's time. If this is a physical breadboard, then perhaps you might let us know what you are actually using for your 2uH inductors. You wouldn't want the relatively hefty classA bias to have any effect on them, so there should be a lot of air in their flux path - not a couple of turns on a bead, I hope. RL |
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