Making push pull HF amp broadband
I have also made this little broadband linear in the past http://qrp.gr/hfamp/index.htm
I was wondering how can I make the first circuit broadband? I have tried changing the final core to a ferrite one and also removing the variable capacitor with no success.
Any advice would be useful.
When making a broadband amp the choice of the ferrite transformers is importent. So in the case of the first amp, the output transformer is 1+1 :1 ? so feed 100mmV into the output lead and measure at the transistor collectors. you should get 100mV across the HF band, if it drops off at too low a frequency, either the core is not the correct sort for HF, or you have too many turns on the core. The cores we used to use were Mullard FX NNNN, there wer two sorts, one for HF, the other for 20 =70 MHZ (vision IF?). they needed one turn for 50 ohms and worked from 30 MHZ right down to .5 or so. You can expect a little drop off at 30 MHZ as spurious lead inductance comes into play.
Frank
So it is only a matter of the cores used. I suspected it since the transistors can operate to 30MHz and there are really no other components to tune to this circuit. Will the lossy ferrites (like the 43 material) give significantly less output power than the tuned output transformer used in this circuit?
I think I have remembered the cores were FX 2249, material B1 from Mullard, :- http://www.electrojumble.org/techdata.htm and look for the fx 2249.
The wrong core material will result in a "low pass filter" effect where all the RF goes in heating up the core and none is transferred to the secondary.
As I mentioned too many turns results in the self resonance frequency of the transformer being in band, limiting the frequency response.
Believe it or not a proper wideband transformer will have a lower loss then a tuned circuit. But we are only talking about .1 rather then 1 dB.
Frank
Some practical advice can be found here:
http://www.qsl.net/ik4auy/article_4_english.htm