class a linear or not?
1mw in for 4w out seems too optimistic, what do you think?
Output match is a 4:1 impedance transformer, so with a 50 ohm load, the collector impedace is 12.5 ohms, with a 12V supply, 4W seems on the edge of possible (3W is probably more realistic).
Note that 16dB is 100mW drive for 4W output, not 1mw (Would need 36dB gain)!
It is proably not hugely linear, being as the RF feedback at high frequency is falling off the side of a cliff, but for the stated application (A simple QRP rig) it should get the job done with the addition of a suitable output filter.
If they are available locally, something based on a RD06HVF1 mosfet will likely have more gain, and potentially better linearity, but whatever works.
Regards, Dan.
Thanks Dan,
1mW was a typo, it should be 100mW as you stated. I have successfully tested (try and see) a very low noise crystal oscillator, followed by two bfr96 buffers, which gives a 100mW (even double that with more harmonics) low distortion sinewave at the output. I verified it's low distortion bu obseving the harmonics at the FFT which are well below 40db below the carrier. I do not know if this is a good value, but it is the best I have achieved so far after lots of experimentation with different circuits and transistors.
By goal is NOT to use an output LPF, so that a simple broadband QRP transmitter can be built without the need to switch output filters for different bands. Experimentally I see that if the amps are biased into class-A and they are underdriven, then a very low distortion, low harmonics signal is present at the output. Of course efficiency is not good, considering both power consumption and cost (more stages and power transistors needed, but no need for an output bandpass filter.
Being class-A it consumes maximum power even no input signal is present, so it has all the heatsinking it needs and it can respond to output shorts or open circuit (no and connected).
I have seen from the datasheet that the cheaper 2sc2166 can be used with 100mW input drive. I do not expect full 6W power output, but something like 1W or maybe more when operated in class-A. Can you propose a suitable circuit? or topology to test? Of primary interest are the low distortion signal (very low output harmonics)
Go push/pull in that case, it will get you substantial even harmonic supression.
Emitter degeneration is helpful, both for improved distortion, linearity and bias stability, but it does of course cost you gain. RF feedback is also helpful, usually taken as a sense loop on the DC injection bifilar.
You will need better then neg 40 out of the exciter if pushing a linear stage as -40dBc does not quite meet regs at power levels greter then a watt.
Or you could just go for the 'power opamp' approach, TI & Linear both have CFB opamps capable of that order of power at HF.
Personally, I tend to the view that 5 or 6 switched filters is easier then trying to make EVERYTHING come up just right to allow you to get away without them, especially if you are building more then one radio.
73 M0HCN.
I think that if one is to go for switched filters then class-E is the way to go but this is just a personal approach.
Can you give some examples of opamps and what to look for?
I was looking for information about how low the harmonics should be for a 1-5W system but I could not find any info on this, your help is appreciated.
By the way, most QRP kits and circuits I have tested have a much higher distortion! I have the feeling that these cheap qrp kits sold on the net really do not care about harmonics.
Spurious and out of band emissions limits: http://life.itu.int/radioclub/rr/ap03.htm
Have a look at the "ADSL LINE DRIVER" chips from the usual suspects, some of them will get you a cleanish watt or so.
Class E is great for continious carrier modes, but a pain in the arse for anything carrying linear modulation, worth it for the power savings at 100KW, not worth it at 5W.
I dont do kits, so I cannot comment on the general quality.
Regards, Dan.
Thanks for the spur info, that is what I was looking for!
Just to make sure I have understand the table correctly, for 9 kHz to 30 MHz all spurs (harmonics) must be 40 dB below the carrier and the total allowable level is 50mW max.
Have I understood that correctly?