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There are several versions of the LM386. Dave Martin's Corsair also used one to series modulate the driver stage of his Corsair. The consensus in our builder's group was that the best modulation came from the LM386-N3 or N4 that was capable of more current output. The typical '386 is the N1, which runs out of gas in these applications. You might have better results with an N3. Worth a try...Good point ff! I used an N3 in my build. The audio is still muted a bit, but perhaps not as much as the other variants. I believe I ordered a handful of the LM386 N3's from Jameco at some point. I simply chose the N3 because it puts out more power. IIRC, Radio Shack only sells the N1.
One thing I would add is a 0.1 uF cap to ground on the V+ side of your 3.3 uH choke. That will work with the choke to help keep RF out of your voltage feed.I added a 100uF capacitor at the RFC as a voltage stabilizer. Would the 0.1uF be enough capacitance? I guess it depends on the stability of the power supply. I've usually used a higher value electrolytic as voltage stabilizers. One structure I've seen in similar circuits, is a 100uF capacitor and a 0.1uF in series at the RFC. I assume there is a reason for this, but it just looks like a 99.9uF capacitor to me.
Also, using two 100K as a divider sets your gate bias point at 6V. In this situation there's no reason to make the IRF amplify linearly. The higher the bias point, the hotter the IRF will run because it will be conducting for longer portions of the cycle. A single gate resistor to ground will set the gate impedance. I normally use one between 1K-10K, but that's just my fetishFor some reason my IRF510 stops working if I just add a single resistor bias on the gate. If I lower the voltage with the divider it starts working. I'm really not sure why this is, as I believe it's suppose to operate with a 12v input. I might edit the divider though to bring the voltage up to +9 or +10v and see if it still operates. Maybe I have some knock off FET's, although I bought them from a reputable source..
And Ole makes great points - use a 0.1 uF coupling cap in the drain circuit followed by a C-L-C LPF. This will improve your output waveform by reducing the harmonic energy.Yes, for sure if I was to use this in any serious way. I've plugged it into the altoids low pass filter I've previously created just to see how it cleans things up, although that filter is for 40m, so it's not really ideal. It is improving the waveform on the harmonics in that range though.
If I was you, so put a 100K resistor direct from Gate to ground. I do not like the gate hangs and 'float'That's a good point. I did originally have a 100K resistor in there for biasing, but was having problems with it distorting the waveform. But, I also changed things from the original circuit I threw on the breadboard, so I'll try adding it in again and see what happens...
and the only book that really tells you how to design isI also recommend Experimental Methods in RF Design by Hayward. The Solid State book I believe is no longer in print, and the Experimental Methods book is the updated version. The nice part is you can pull docs from decades ago and they still apply today. Analog radio is a century old technology, and not much has changed since the early days.
Solid State Design for the Radio Amateur - by Hayward
