We show the process of adjusting the Bias current on an amateur radio. See the complement on Alan's you tube channel W2AEW. See link • #185: Back to Basics: ...
THanks for your time to make these videos. That was a great explanation with the graphs to explain the process and reason for setting the bias current.
Nicely done. It is interesting to note the Yaesu FT 747 has the same layout for it's PA. I found this video very helpful in learning about the sub systems of the 747
Very good tutorial on SSB and AM linear biasing. HOWEVER, we should mention that for FM and CW, RTTY, etc. we want non-linear amplification for the higher efficiency it affords, but if cost and space are needed linear will still work and probably no one will even notice. I won't tell anyone. K4OF.
Hi Peter, I am doing the exact same work, but on mine bias current starts on 30mA and rise up to 400mA after 3-4 mins of carrier. Seems that the 2SD880 current regulator thermal drift but the series diodes on top of finals don't counter balance the current because the finals are unloaded and cold of course. So how to set the proper bias current? I'm thinking to do a warm-up of final stage talking on dummy load for 10 mins (so it will warm-up the funals as well) and after suddenly doing the bias adjustment. What's your thoughts? Alex Ve7ada
Why designers did not used 2 pots to adjust bias separately for each transistor back in old times? What prevented them from doing that? Was this limited by technology?
TheLaidukas It is always costs what they have to consider. More components and more complex design makes the radio more costly on production site. Thus engineers very often are oblige to leave the best way.
Hi again, thank you for a good video. If I have a radio with separate BIAS adjustment for each transistor I don't need to have matched pair of transistors? is that correct? As long as I adjust each of them to the same BIAS current?
Mikael Karlsson Hi Mike, yes that is right, as long both set to the same BIAS current. Do not set to the same voltage level, always test current level!!
Tom Smith Hi Fred, yes in any case! Although you would not directly recognised it in FM. There is almost nothing to hear in the modulation but you would transmit more harmonics. You could easily break your local laws as harmonics must fulfil the radio regulations. Harmonics can have an impact on other radio stations not only amateur radio but rather more official stations as well . 73 Peter
Just done this on my Yaesu FT-450D which was always drawing much power without modulation. At the FT-450D you can adjust Predriver 100mA +- 10 mA 2x Driver 2x 500mA +- 50mA 2X Finals with 2x 1A +- 100mA After that you should also adjust the CM Coupler Balance which should be under 100mV at a testpoint. Mine was before adjust 127mA Predriver 751mA 1st Driver 480mA 2nd Driver 1.3A 1st Final 1.6A 2nd Final. The Voltage at the CM Coupler Balance was about 500mV I think this was way off! Is this normal for a 1 Year old device? Finally i got all values well within the normal range and the Signal on the SDR looks much cleaner too now. 73 de DK3JF
jjmtzama The right way is to use current due to the fact that you never know where you end up with voltages. 0,7V can already be 250mA but it can also be 100mA or 380mA. Thus voltages is in this case very imprecise.
Hi, Good tutorial. But if we not have detail to set the bias current...how can we proceed !? I think that it's enough to set 0,7/0,8 Volts between base and emitter of final transistor stage. Let me know if you want !! '73
Hi Alessandro, yes you can't simply go ahead with 0,7V or so. It depends on the circuit itself and thus on the class the amplifier is running on and of course of the used transistors. It is a difference if it is a FET or a bipolar one... That all means you have to analyse the circuit first and then you use the data sheets for the transistors..Hope that helps a bit 73
